American Mineralogist
American Mineralogist: Journal of Earth and Planetary Science:
All our papers are special, so each month, the American Mineralogist editors will share highlights on each. We hope this information is enjoyable and useful.
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November-December, 2018
Highlights & Breakthroughs
Low S in the Lunar Mantle…
…but a Lot More S in the Lunar Core
New High P Record for Granulite in U.S.
Counting Conduits
Fluorine, for Stronger Teeth and Bones
How to Make Rocks (And Estimate Magmatic Ascent Rates Too)
Stressed out!
Editors Selections, October, 2018
Highlights & Breakthroughs
Special Collection: Planetary Processes as Revealed by Sulfides and Chalcophile Elements
Articles
Letter
Editors Selections, September, 2018
Highlights & Breakthroughs
On page 1353 of this issue, Aleksandr Stepanov reviews the results of new experiments by Van Lichtervelde (p. 1401, this issue), who are able to reproduce the textures of columbite-group minerals (CGMs) using fluid-rich and highly super-saturated conditions. As noted by Stepanov, nominally “fluxing elements” such as F and P, have little impact on the occurrence of zoning in CGMs. The experiments show that while saturation with respect to CGMs is controlled by Ta contents in associate fluid/liquid phases, the precipitating CGMs still yield compositions that are far displaced from equilibrium and that Nb-Ta fractionation could depend heavily on crystallization kinetics.
Special Collection: Isotopes, Minerals, and Petrology: Honoring John Valley
Super heavy Neoproterozoic Pyrite (Sulfur) From Non-Biologic Processes
On page 1362 of this issue, Cui et al. examine elevated δ34S in pyrite in Neoproterozoic sedimentary deposits in southern China. So-called “super heavy” pyrite, enriched in 34S, appears to be associated with de-glaciation events and has been linked to changes in the isotopic composition of syn-glacial sea-water sulfates, which are then transferred to pyrite by microbial sulfate reduction processes that yield heavy pyrite. The new observations indicate significant grain-scale heterogeneity in S isotopes and textural patterns that indicate a late-stage diagenetic (non-biogenic) origin for super heavy pyrite. The authors suggest that super heavy pyrite forms by thermochemical, rather than microbial sulfate reduction and that SEM-SIMS studies are needed to differentiate the two.
Articles
Fe oxidation state of silicate glasses using electron probe microanalysis
On pages 1445 and 1473 of this issue, Zhang et al. and Hughes et al. present new techniques for the determination of Fe oxidation state in silicate glasses using the electron microprobe. Zhang et al. show that by using a garnet calibration curve the EPMA-derived Fe oxidation state of silicate glasses matches well those obtained using wet chemistry, while Hughes et al. use silicate glasses for calibration. Both groups analyzed some glasses with consistent results. Both studies find that beam damage is a significant issue, which is controlled by beam conditions, water, and SiO2 and FeO concentrations. Zhang et al. mitigate this by moving the sample continuously at a constant speed during analysis, whereas Hughes et al. use a time-dependent intensity correction to allow the analysis of small areas of glass, such as melt inclusions.
Removing As from water
On page 1497 of this issue, Yang et al. report on a new mineral, Segerstromite, which is the first naturally occurring phase to contains the hydrated arsenite, As3+(OH)3, and compared to the arsenate analog of apatite, johnbaumite, this new phase may be an even more useful means to remove As from contaminated water, at least in Ca-bearing systems.
Shocking news on the origin of tissinite in meteorites
On page 1516 of this issue, Rucks et al. discuss the genesis of tissinite, a meteoritic vacancy-rich clinopyroxene; to date, its origin has been unclear, and often assumed to be due to shock transformation of crystalline plagioclase. Using spike heating in a multi-anvil press to simulate a shock event, the authors show that tissinite does not form directly from crystalline plagioclase. Instead, it forms at temperatures over 900 Celsius during decompression from a Ca-rich amorphous plagioclase phase (“maskelynite”) that itself likely formed during a shock event.
Editors Selections, August, 2018
Highlights & Breakthroughs
The
future of Mineralogy
On page 1173
of the August issue, Nobuyoshi Miyajima gives an
overview of advances in electron channeling spectroscopy from TEM. The key data
from the technique are site occupancies, especially in minerals that are
texturally complex, as illustrated in the subject article, Igami et
al. (2018). This is an effectively untapped area of rock-forming minerals,
and perhaps it is only the technical challenge that prevents the technique from
being more widely used. The level of precision in our modeling efforts, from
phase transitions to thermometry or hygrometry could potentially increase by
orders of magnitude if we better understood intra-crystalline partitioning in
natural and experimental systems. And when we greatly increase precision in
measuring –- well, pretty much anything –- we often discover very new and exciting things.
Regular
Articles
Transporting
H2Or across the Transition Zone
On page 1221 of the August issue, Kakizawa et al. present new experiments on the stability of
superhydrous phase B at mantle transition zone P-T
conditions. They find that up to 1600oC, Al2O3
and H2O contents in superhydrous phase B
increase, at the expense of MgO and SiO2,
but above such temperatures, MgO and Al2O3
increase at the expense of SiO2 and H2O. The authors
suggest that Al contents are a key factor in affecting the stability of superhydrous phase B, and it may be stable more so within
Al-rich subducted crust rather than peridotite,
perhaps providing a mechanism for keeping H2O dissolved within
crystalline phases during subduction.
Where did all the Beryllium come from?
On page 1228 of the August issue, Dailey
et al. examine the petrology and geochemistry of the extraordinarily Be-rich
rhyolites of Spor Mountian,
western Utah. They find that the host rhyolitic magmas erupted at relatively
low temperatures of 682-718oC and that these low-T conditions were
accompanied by high F and elevated water that depolymerized the melt. In
concert, these conditions led to especially low mineral/melt partition
coefficients for a range of trace elements, including Be.
The authors posit that mafic magmas mixed with a 25% partial melt of ambient
crust, that the product magma was later 75% crystallized -- and that later
hydrothermal alteration would complete the Be-enrichment process.
A New Age For the Inner Core
On page 1271 of the August issue, Gomi et al. examine equations of state for FeHx compounds that are thought to represent the
mechanisms by which H is incorporated into the solid core. They find that both
magnetism and the solution of H into the core affect electrical resistivity and
thermal conductivity. From the latter, and using phase solution models in the
Fe-Si-H system to estimate temperatures of the inner core/outer core and
core/mantle boundaries, they calculate that for a core. For a core of composition
Fe1-ySiyHx, the inner core age is between 0.49
and 0.86 Ga, as H contents respectively vary from x =
0 to x = 0.7.
Editors Selections, July, 2018
Highlights & Breakthroughs
On page 1009
of the July issue, Konstantinos Demadis
provides an overview of Wysokowski et al. (on page 665 of the May issue). The
contribution is as much as anything, a look forward, posing fascinating and
still unanswered questions about biominerals. For
example, biosilicifiers are not uncommon, and yet why
is it that some organisms choose (let alone are even able) to concentrate
molecules that occur at remarkably low concentrations? And how are they able to
do so at efficiencies that exceed anything we have created in the lab?
Regular
Articles
Fe-oxide reaction controls on groundwater
contaminants
On page 1021 of the July
issue, Voelz et al. examine the surface area-normalized reactivities of hematite as iron (II) bound on the
mineral surface reacts with fluids containing chloronitrobenzene,
a model for several pollutant classes like pesticides, dyes, and munitions.
Their experiments indicate that reactive sites on hematite change as a function
time, pH, and extent of reaction. Additionally, reaction conditions influence
whether the concurrent oxidative mineral growth results in hematite particle
growth, nucleation of goethite on the hematite mineral surface, or a mix of
both. These factors must be quantitatively modeled to understand the fate of
contaminants in Fe-oxide bearing groundwater systems.
New Minerals, Anyone?
On page 1080 of the July
issue, Liu et al. present the latest installment of
predictions of mineral diversity, in this case involving V as an essential
structural constituent. Applying the novel Large Number of Rare Events model,
the authors suggest that Earth’s crust should contain 307±30 vanadium minerals,
and hence that 88 species are yet to be discovered, most of which are expected
to have formed in sedimentary or hydrothermal environments, rather than in high
T igneous or metamorphic systems. The Editors anxiously await manuscripts from
mineralogists specializing in new mineral discoveries that test the hypothesis.
Limits on
Si in the Core
On page
1161 of the July issue, Helffrich et al. (in an
Open Access paper) examine the simultaneous solubility of both O and Si in
Earth’s core. They illustrate how O and Si solubilities
are negatively correlated, at least at the pressure-temperature conditions of
core formation, and find that Si in the core must fall between 0.4 and 3.1 wt%. They
also show how their analysis of core-derived Si affect estimates of core Si
isotope ratios, which could be constrained, might be used to detect core-mantle
interaction in the highly uncertain (and unlikely?) case that the bulk Earth is
chondritic, if carbonaceous chondrites
represent bulk Earth Si isotopically.
Editors Selections, June, 2018
Highlights & Breakthroughs
Crystallography
on Mars
On page
837 of this issue, Michael Velbel provides an overview (in a Highlights and
Breakthrough paper) of two papers that appear in this issue, Morrison et al.
(2018) on pages 848 and 857 of this issue. It will be no surprise to readers of
this journal that analyses of crystal structures would be an essential
component to the exploration of a new planet; Morrison et al. present the
results of such, making use of the CheMin XRD on the Mars rover, Curiosity. Their crystallographic data and unit-cell
calibrations are revising our understanding of mineral compositions on the Martian
surface, with the potential of adding much precision to ideas of Martian sedimentary
provenance, stratigraphy, diagenesis, and ultimate magmatic origin of
individual mineral grains.
Articles
You
can Rely on Al in Qz
On page 839
of this issue, Tailby et al present new experimental results that show very
slow diffusion rates of Al in quartz—slow enough that Al contents
obtained during crystal growth will be almost invariably preserved above the tens-of-micrometers
scale, irrespective of time. Diffusivities of Ti are similarly low, which means
that zoning patterns of either element are quite likely to preserve the
conditions of crystallization. If these elements preserve temperatures and
cooling rates, they might also allow us to estimate crystal growth rates for
quartz in natural systems, perhaps with a precision that is lacking for other minerals.
Mineral Chemistry from Crystallography
On page 848 of this issue,
Morrison et al. present sets of equations that provide stunningly precise
estimates of mineral compositions as calculated from X-ray diffraction data. As
noted in Velbel’s Highlights and Breakthroughs article (p. 837, see above),
this work was an important step in adding considerable precision to estimates
of mineral compositions on Mars. But of course, there is nothing special about
a martian origin, and these equations can and should be applied to terrestrial
systems as well. This study may allow for much more cost-effective estimates of
rock-forming mineral compositions compared to use of the electron microprobe,
with the added benefit of having crystallographic information in the
process.
A One-Mineral Tour of Arc Magmatism
On page 899 of this issue, Tecchiato
et al. use the major and trace element contents and isotope ratios of
clinopyroxene to trace magmatic transport and storage at the Capo Maragiu
Volcanic District in Sardinia. The authors identify three types of
clinopyroxenes, based on texture and composition, that track magma
compositional evolution from Mg#(liq) 70 to Mg#(liq) 40. Especially intriguing
are isotopic patterns with respect to Cpx Mg#, which allow the authors to show
that, at least in this system, only the most mafic magmas are affected by
assimilation of crustal components and that extensive and subsequent cooling
and crystallization occurs as nearly a closed system.
Accelerating Supereruptions
On page 952 of this issue, Myers
et al. obtain ascent rates for three different supereruptions from measurements
of H2O and CO2 in re-entrants (melt inclusions that
connect to enclosing glass) in quartz. Their case studies are from eastern
California (Bishop Tuff), Yellowstone (Huckleberry Ridge Tuff), and New Zealand
(Oruanui), and the ascent rates vary from 0.06 to 13 m/s with the lowest rates
correlating to field relationships that indicate pauses in eruptive output. They
find lower re-entrant H2O and CO2 contents compared to that
of melt inclusions and model the contrasts as representing an accelerating of
the magma during decompression, as magmas migrate from a storage chamber to an
active conduit (on the order of hours to days).
Eckermannite replaces Glaucophane at High P
On page 989 of this issue, Howe et
al. present the results of new experiments that indicate that above 40 kbar,
glaucophane is likely to breakdown into an amphibole that is a solid solution
mix of eckermannite (80%) + ktaophorite (15%) + Mg-winchite (5%). The authors show
at least two different pathways by which the new amphibole may be formed, the
most likely of which may involve the breakdown of jadeite + talc, with the consequent
production of eckermannite, pyrope, coesite, and water. Because of the high
Na/Al ratio of eckermannite, this phase is possibly stable in any system where
jadeite and talc occur as precursors and should replace glaucophane in
metabasite systems above 40 kbar.
Letters
High-Pressure Humites
On page 1002 of this issue, Luoni
et al. make use of thermodynamic models and recent experiments to quantify the
otherwise ultra-high pressure (UHP) metamorphic paths of humite-bearing
serpentinites. In the process, the authors show that UHP metamorphism in the
western Alps was both polybaric and diachronous. This would appear to require a
“mosaic of tectonometamorphic units” that accreted over a range of depths and
times and suggests that similarly dismembered ophiolites in other parts of the
globe may represent composites of multiple slices of lithosphere, rather than coherent
lithospheric sections.
Editors Selections, May, 2018
Review
Biominerals– A Review
On page 665
of this issue, Wysokowski et al. provide a comprehensive
review of biomineralization processes, describing a range of mineral types and biological
processes and structural diversity, with an emphasis on biosilica
in viruses, bacteria, plants, diatoms, and sponges. Their work also illustrates
how the structures and functions of biosilicifiers
can inspire new forms of artificial biomineralization with far-ranging technological
applications, including biomimicry.
Articles
No
Si in the Core?
On page 742
of this issue, Tateno et al. present new, high-pressure
experimental results in the system Fe-Si-S. They find that crystalline metallic
Fe is enriched in Si relative to S, compared to co-existing liquids. And so, with
partial crystallization, especially at inner-core pressures (330 GPa), the solid,
inner core is enriched in Si relative to the outer, liquid core. This finding may
preclude Si as an important light alloying element if the 4.5% jump in density across the inner/outer
core boundary requires an inner core that contains less Si than a presumably equilibrated
liquid outer core. The authors find similar reasons to reject core compositions
in the systems Fe-Si-C and Fe-Si-O, although they cannot exclude liquids in the
system Fe-Si-H. It is not yet clear phase topologies within a more complex system
(e.g., Fe-Si-O-S) might yet still allow a core that is Si-enriched, but if the core
is a ternary system, then a Fe-Si-H ternary would be the only Si-bearing ternary
that could explain the inner/outer core density change.
An Unexpected
Driving Mechanism for the Huckleberry Ridge Tuff
On page 757 of this issue Swallow et
al. examine mafic materials from the 2.08 Ma Huckleberry Ridge Tuff (HRT), the first
and largest of the Yellowstone Plateau caldera-forming eruptions, and find a surprising
result. The mafic materials that were involved in the HRT magmatic system are quite
similar to much more recently erupted materials (ca. 5-10
ka) at the Craters of the Moon lava field in ID, and which also occur just west
of the HRT caldera. These results yield a new perspective on the diversity and roles
of various mafic magma inputs that likely provide the necessary thermal input to
drive eruptions in the Yellowstone region.
A New EOS
for Stishovite and CaCl2-structured
SiO2
On page 792
of this issue Fischer et al. conduct new high-pressure
experiments to determine the equations of state for stishovite, and CaCl2-structured
SiO2, a higher-P polymorph that, as shown by this new study, is stable
at >68 or >78 GPa, along with expected
subduction and ambient geotherms,
respectively. The new EOSs also show that stishovite will indeed be denser than ambient mantle but the CaCl2
polymorph is likely to be buoyant relative to a pyrolite
lower mantle, and that if Si is exsolved from the core into the mantle, the lowermost
mantle might become locally saturated in SiO2, and in such a case, this
SiO2 would add a degree of compositional buoyancy to its enclosing material.
Errors When
Using V to Estimate Mantle fO2.
Letters
Monazite, heal thyself
On page 824, Seydoux-Guillaume
et al. report a study using ion bombardment of LaPO4 monazite aims to understand
why this mineral is never found amorphized. Simultaneous
and sequential irradiations using Au and He ions at energies designed to simulate
the recoil from nuclei undergoing alpha decay and the electronic energy loss of
the alpha particle moving through the structure. This study shows that it is the
latter that prevents amorphization in this mineral. This understanding is for predicting
nuclear waste form performance and has implications
for the application of geochronology and thermochronology
in monazite.
Editors Selections, April, 2018
Invited Centennial Article
XANES Analyses of Ferric/Ferrous Ratios of
Basaltic Glasses
On page 489 of this issue,
Cottrell et al. show that ferric/ferrous ratios of hydrous basaltic glasses are
increased by exposure to synchrotron radiation. They find that XANES spectra
are affected by radiation does, total water content, and initial ferric/ferrous
ratio. The mechanism of radiative oxidation appears
to involve the loss of H from water-bearing samples. The authors provide
analytical strategies so as to obtain accurate ferric/ferrous ratios from XANES
spectra of hydrous basalts.
Vermiculite Insulation Analyses Made Quick and
Easy
On page 571 of this issue Swayze
et al. use portable reflectance spectroscopy to characterize the source and
type of expanded vermiculite samples in various materials, ranging from attic
insulation to potting soil. By applying multiple analytical methods they are
able to show that in situ reflectance spectroscopy may be a powerful tool for
reconnaissance or preliminary investigations, to determine whether such
materials may be derived form a source known to contain asbestiform
materials. Their methods do not identify materials as asbestiform-bearing
(so our lead-in is misleading), but their work shows that the five ore sources
investigated are sufficiently distinct in mineralogy and mineral chemistry that
it might be possible to identify a source material from in situ analyses.
Sequestering Cesium
On page 623 of this issue, Osuna et al. synthesize a new type of high-Na mica that may
provide a more effective barrier and absorber of radioactive waste compared to bentonite. Their work shows that natural materials can be
used as a model to create synthetic analogs, with more targeted and desirable
qualities. In this case, they show that their Na-mica-n material can absorb
large fractions of Cs from low-Cs solutions; although efficiency appears to
decrease dramatically with increasing Cs in solution the partitioning of Cs
between mineral and fluid is always > 1.
A New Feldspar Barometer
On page 600 of this issue, Befus et al. show that certain vibrational modes of
feldspars, visible in Raman spectra, are sensitive to pressure. Their experiments
are performed at ambient temperatures and the spectroscopic measurements are
made at the pressures of interest. But their analysis of bulk moduli indicate
that Ab-rich feldspars that occur as inclusions may
preserve pressures of formation of natural systems, perhaps with some
independence of temperature. These
experiments indicate that Raman spectroscopy may prove to be a more valuable
tool for comparison to some highly T-dependent feldspar barometers based on
chemical equilibrium.
A Model for the Control On Erupted Lava
Compositions
On page 565 of this issue, Conway
et al. provide a comprehensive geochronological and
geochemical study of the long-lived (200 ka) Ruapehu Volcano, in New Zealand. Of their varied findings,
one in particular involves a pronounced temporal shift in the kinds of
materials erupted. Felsic samples yield evidence of a peak in crustal
assimilation at 50-35 ka, coincident with the
eruption of Mg-rich andesites, and so presumably, a
peak in heat transfer from the mantle to the crust. Evolved
magmas from later eruptives acquire their compositions
through crystallization differentiation and, to a lesser degree than the
preceding magmas, crustal assimilation, presumably because mantle-to-crust heat
fluxes are lower, and because low-T fusion fractions of the crust are
exhausted. In this model, it is not
density filtering, but rather thermal maturity that determines the kinds of
materials erupted from a given volcano.
Time Between Magma Mixing and Eruption
On page 582 of this issue, Oeser et al. use Fe-Mg
chemical and isotopic zoning of olivine grains to determine the time between
magma mixing and eruption at Irazu volcano in Costa
Rica. Furthermore, their combination of major and trace element and isotopic
analyses allow them (a) to estimate diffusivities of some trace elements (Cr,
Al, V) relative to Fe-Mg diffusion in olivine, and (b) to identify three
populations of olivine crystals, that appear
to derive from near the Moho, at a range of lower- to
mid-crustal levels, and from a shallow staging reservoir. Diffusion profiles
and a step-wise approach to modeling multi-stage histories, yield time scales
of 100-1750 days between entrainment of the diversely zoned crystals and
eruption. Most crystals yield times scales of a few months to two years.
Apatite Reaches Maturity
On page 550 of this issue, Drouet et al. discuss the results ofcomplementary experiments that mimic the growth and
reactivity of biogenic apatite in cases such as >bone remodeling. They underline that incipient nano-scale
apatite crystalsare non-stoichiometric
and coated by a highly reactive hydrated
layer that is non-apatitic and contains abundant
mobile ions. These proto-bone materials
evolve in solution toward stoichiometric apatite, which is shown to be more thermodynamically stable, but also less
reactive. A key finding is that the growth, stability, and
reactivity of nanocrystalline apatite are closely
related to this "surface hydrated ionic domain", not quite solution,
and certainly not crystalline, but more
like an interphase on the nanocrystals that separates their apatitic
core from the solution from which it precipitates.
Editors Selections, March, 2018
Highlights and Breakthroughs
Outer
Planet and Early Earth Organic Minerals
On page 341
of this issue, Hazen provides an overview of a study by Maynard-Casey et al.,
of minerals on Titan (on page 343 of this issue), a satellite of Saturn.
Maynard-Casey et al. examine a database containing about 800,000 organic
molecular structures and find about a hundred that should be stable near Titan’s
surface environments given its atmospheric composition and thermal conditions.
Their hope is that an identification and systematization of Titan near-surface
mineralogy will help refine out hypotheses about the nature of that distant
planetary body. As Hazen points out, their work may further inspire new work
closer to home, i.e., the search for possible “extinct” organic mineral species
that might have ruled an ironically pre-biotic Earth. One strand of thought is
that some such minerals, overlooked in current mineral evolution models, may
have acted as pre-biotic pathways to RNA.
Articles
The
Meaning of Oscillatory Zoning in Accessory Minerals
On page 355
of this issue, Melnik and Bindeman provide a new model to quantify diffusion-controlled
growth for zircons from silicate melt. Their model can be applied to any
accessory phase, but in this case provides an explanation for the oscillatory
zoning of some zircon grains in felsic rocks. For example, they find that small
temperature changes (<10 °C) are unlikely to cause
dissolution, but affect growth rates sufficient to cause enrichments or
depletions in trace elements (Y, Hf, REE) by a factor of two. They also predict that such T-controlled
growth patterns will be subdued at >850 °C, which
explains the rarity of oscillatory-zoned zircon in dry, high-T rhyolites. Their
modeling also indicates that crystal zoning might also record the response of a
system to minor pressure variations (35-50 bars, perhaps due to recharge,
partial eruption of a pluton, etc.) as such variations cause shifts in water
solubility. The illustrated wavelengths of the two end-member controls (T vs.
water solubility) appear different, and so mapping of wavelengths may help
determine which if either processed acts as a control on zircon growth.
An
Oxybarometer for Arc Magmas
On page 369
of this issue, Shishkina et al. present new experiments and a new calibration for
fO2-sensitive partitioning
of V between Olivine and silicate melts, here extended to lower temperatures
(1025-1150 °C) and hydrous conditions
(0.6–6.5 wt% H2O) that characterize arc magmatism. Their new
model can be used for arc systems generally, its use being illustrated at
Mutnovsky volcano of Kamchatka, where the authors find highly oxidizing
conditions from olivine melt inclusions, ranging from 1.9 to 2.3 log units
above QFM. Perhaps a key result is that their diffusion modeling indicates much
slower rates of re-equilibration of V relative to H2O, which implies
that V partitioning may provide a more reliable record of magmatic fO2 conditions.
Making
Sapphires and Rubies by Partial Melting
On page 469
of this issue, Palke et al. provide a far-ranging and seemingly crucial study
of sapphires and rubies from the U.S. and Thialand/Cambodia. Their trace
element data and oxygen isotope data cast doubt on trace-element discriminant
diagrams that purport to separate metamorphic from magmatically derived rubies
and sapphires. Instead, they find that rubies and sapphires from their two
localities, which plot solidly in a metamorphic field, are in fact magmatic in
origin. The authors make use of the Fo-An-Q ternary at 2 GPa to illustrate their
genesis, showing that corundum can form by partial melting of corundum-absent anorthosite
protoliths, through peritectic melting reactions. The same phase relations
further illustrate how even minor contrasts in bulk composition lead to
different mineral inclusion assemblages (and by implication, significant
contrasts in trace element contents for the resulting gem minerals).
Age
Dating Hydrothermal Alteration (and late Stage Granite emplacement?)
On page 480
of this issue Capatani et al. describe a rare association of zircon and
aeschynite precipitated on anatase, found in miarolitic cavities in a
granophyre bulk composition host rock. The association, and the author’s trace
element and crystal-chemical study indicates that the association formed at low
temperatures, of ca. 250 °C, which is not untypical
for anatase, but is lower than the ca. 300-6000 °C range
often attributed to hydrothermal zircon precipitation. Their model is that the
zircon that nucleates on anatase formed by the dissolution of primary
hydrothermal zircons, with re-deposition at lower temperatures. Their finding
opens the possibility for age dating hydrothermal activity over a much wider T
range, and by extension, providing dates on the final emplacement of granites
and expelled fluids.
Editors Selections, February 2018
Invited Centennial Article
Plate
Tectonics Starts at 2.8 Ga, According to The Metamorphic Rock Record
On page 181 of this issue, Brown
and Johnson make the utterly compelling case that contrasts in geodynamic
regimes are warranted from the metamorphic rock record. From that record they
extract apparent thermal gradients that are then compared to age. They find
what they describe as three “cycles”—although these appear to be
non-repeating, and so might better be described as evolutionary
stages—where peaks in T/P correspond with supercontinent assembly. More
interesting still are the implications for the start of plate tectonics.
Although Brown and Johnson acknowledge that high pressure–low temperature
metamorphism occurs sporadically at best prior to 0.8 Ga, they suggest that
paired metamorphic belts are common after 2.8 Ga, which may indicate a
transition from a deformable stagnant lid to a plate tectonic regime at 2.8 Ga.
Articles
Forcing
Water into Chabazite
On page 207
of this issue, Kong et al. examine the structure of chabazite
of various compositions to 5 GPa, not because natural
samples exist at such pressures, but rather because of prior hints that
elevated pressure may enhance their use as microporous filters. The find an
anomalous 80% increase in the bulk modulus of their natural chabazite
structure when pressurized with water. They infer that the increase in bulk
modulus, which is not viewed when the P medium is a non-penetrating fluid, is
due to water molecules being forced into the structure. Yet to be determined is
whether this property will have practical applications.
Making High-Mg
Andesite (HMA) at Shasta
On
page 216 of this issue, Streck and Leeman weigh in on a debate on the origin of
high magnesian andesite (HMA), in this case using
samples from the Whaleback satellite vent near Mount Shasta Volcano, in the
southern Cascades. They demonstrate that
minerals in these HMA lavas comprise a disequilibrium assemblage that is
derived by mixing of three components: dacite and
basalt magmas, and disaggregated ultramafic country-rock material, the latter
of which lends the high Mg content to the HMA.
This paper stresses the importance of magma mixing in producing HMA at
Shasta, and by extension other similar occurrences. This conclusion precludes
origin of the HMA directly from the mantle as a ‘primitive’ magma, and casts
doubt on its importance to crustal growth in this particular setting and in
general. In contrast, the dominant
mantle contribution is basaltic magma that provides both material and heat to
remobilize pre-existing crustal materials and mix with resulting partial melts
to produce ‘andesitic’ crust.
Apatite-melt
Partitioning of Volatile Elements
On page 260 of this issue,
Riker et al. present a new experimental study of OH, C, and halogen
partitioning between apatite and co-existing silicate liquids. They find that
partition coefficients are affected largely by temperature and only minimally
by pressure and melt composition. They also find an interesting structural
control on C partitioning in that in halogen-free systems, C occupies channel
sites, along with H, and large amounts of C can be absorbed by the structure;
but with the introduction of halogens, C tends also to substitute for P, and
total C contents are greatly reduced for a given melt CO2 content in magamtic systems. These results will be especially useful
for modeling C contents in magmatic systems.
Tourmalines!
On page 298
of this issue, Ferdinando Bosi provides a new
structural analysis of tourmaline, derived from several decades of existing
structural studies. This new synthesis makes use of recently described
end-member compositions that have especially informed the cation exchange
relationships of various trivalent cations. The new synthesis identifies some
discrepancies in how certain tourmaline compositions are classified; although
it is unclear whether the discrepancies are minor or common, a new proposal for
the assignment of Al is intended to provide consistency to tourmaline
descriptions.
Mineral
Matters
Apatite,
Archeology and Mastodons
On page 324
of this issue, Matt Kohn introduces a new article type, Mineral Matters. These are intended to inform the public (aimed at
high school level or above) about how minerals, and Mineralogy, are important
for understanding the world around us. Perhaps no better mineral than apatite
could serve as an inaugural topic, as Matt nicely illustrates how Sr isotopes in such can be used like tree rings, to inform
us about mineral growth history. In the case studies presented here, that
history reflects paleo-biological conditions for human migration patterns in
Europe and Mastodon migrations in N. America. We hope that teachers and
students find these useful.
Editors Selections, December 2017 and January
2018
Invited
Centennial Article
Number three
On page 1 of the January
2018 (vol. 103, 1) issue Douglas Rumble provides a review of 17O, and
its fractionation from its sister O isotopes on Earth. Rumble shows that O produced by photosynthesis never reaches 18O-16O
or 17O-16O isotope exchange equilibrium with seawater. Furthermore,
the molecules 17O18O and 18O18O produced
by photosynthesis do not equilibrate at the temperatures prevalent during photosynthesis. The fractionation may result from the parent water
molecules, in a protein called Photosystem II, being situated in energetically distinct
sites. Rumble advocates for collaboration between isotope biogeochemists
and structural biologists to determine the mechanisms of metabolic isotope exchange
in relation to the atomic structure of enzyme catalysis in proteins.
Pushing
Down on Me, Pushing Down on You
On page 69 of the
January 2018 (vol. 103, 1), Anzolini et al. examine CaSiO3-walstromite,
the most common of Ca-silicate inclusions in putative ultra-deep diamonds, with
a presumed CaSiO3-perovskite precursor. These authors use Raman barometry,
ab initio methods, and quantitative models (e.g., see
Angel et al., Notable paper of Oct. 2014 p. 2146 of that issue) to obtain the entrapment
pressure of a CaSiO3 inclusion. By assuming entrapment temperatures of
1200-2000 K, entrapment pressures range from 8.1 to 9.3 GPa,
which they translate to depths of 240-280 km. These are minimum pressure estimates
as the diamond host contains some cracks, and has undergone an unknown amount of
plastic deformation. The authors suggest that these pressures are clearly deep and
most likely sub-lithospheric and so are not inconsistent with an ultra-deep origin
for these diamonds.
A New Model
for MVT Sulfide Deposits
On page 91 of the
January 2018 (vol. 103, 1), Zhou et al. provide a detailed geochemical and structural
study of a Mississippi Valley Type (MVT) Zn-Pb sulfide
deposit in southern China. The authors use a combination of Pb,
C, O, and S isotope ratios yield a new model to explain the occurrence of sulfides
in early Cambrian carbonate host rocks. The authors suggest, for example, that Pb was sourced from metamorphic rocks that form the lower crust
and that O isotopes from associated carbonates indicates
a mixture of metamorphic fluids and host carbonates. Their model is that deep-crustal
fluids were transported into overlying carbonate causing reduction of S and rapid
precipitation of sulfides. Large sulfide crystals were precipitated by cyclic dissolution re-precipitation reactions, as controlled by water-rock interactions that
can drive CO2 degassing.
Novel Bonding
in Deep Earth Carbonates
On page 171 of the
January 2018 (vol. 103, 1) Vennari and Williams present
new experimental evidence for coordination changes in C, at pressures of 63-86 GPa. In this pressure interval, they find that C may bond to
an additional oxygen in some parts of a high P structure
(called dolomite III), a coordination they refer to as "3+1". A fourfold
coordination would, of course, weaken the C-O bond as it would have a lower electrostatic
valence, and with a more complex C-bonding environment, the authors argue that dolomite
III may be able to house a wide range of elements that would be incompatible in
ambient silicate phases. Yet another implication is that the dolomite III/IV transition
may have a positive Clapeyron slope that would stabilize
dolomite III at high temperature in Earth's lower mantle. But we still lack phase
equilibrium evidence that carbonates exist in the deep mantle, nor observational
suggestions that the effects of putative carbonates are observable in volcanic systems.
Future work must surely test the relevance of these studies.
Highlights
and Breakthroughs
Under Pressure
On page 2349 of the
December 2017 (vol. 102, 12) issue Ziberna et al. present
a new calibration of several equilibria whose intersections are sensitive to pressure.
The value in these findings is that pressure is such a crucial geologic parameter
to determine, and yet among condensed phases volume changes are so small across
most reactions that viable barometers are quite rare. The new models presented in
this work yield pressure estimates that have errors of ±1-2 kbar,
which is about at the limit of precision for condensed phase equilibria and is certainly
more than precise enough to place the target rock types (mafic and ultramafic rocks)
to differentiate whether a given set of mineral assemblages form in the upper, middle,
or lower crust, or upper mantle.
Volcanic Pressure-Time
Paths, from Nanolites
On page 2367 of the
December 2017 (vol. 102, 12) issue, Mujin et al. investigate
microlites and nanometer-scale crystals that they term
nanolites, from the 2011 eruption at Shinmoedake volcano, whose crystal size distributions may reveal
fragmentation and re-welding of magmatic materials during eruption. Their FE-SEM
and TEM study allow them to study crystals at a stunningly small scale and identify
gaps in growth patterns that would appear to record magmatic events at very, very
short time scales. In their model, they attempt to describe nano-scale
textural and mineralogical patterns as pressure-time paths, that
in turn affect magma dehydration and temperature changes, which can affect (spur
or inhibit) crystal nucleation and growth.
The History
of a Pluton in a Crystal
On page 2390 of the
December 2017 (vol. 102, 12) issue Barnes et al. present new major and trace element
data involving zoning profiles of amphiboles from arc-related plutons. They find
core-to-rim patterns that record monotonic cooling, and associated changes in trace
elements that record the progressive saturation and in some cases the later dissolution,
of both major and accessory phases. The key advance is the recognition that amphiboles
are saturated over a wide enough T range to record the saturation of a wide range
of phases, including zircon and biotite. And because of its complexity, amphiboles
provide information about the P-T conditions and the changing magma compositions
from which various minerals form as a pluton or batholith is assembled.
Melts: retained
by and expelled from crystal mushes
On page 2467 of the
December 2017 (vol. 102, 12) issue, Fiedrich et al. use
a range of methods, including whole rock and mineral compositions, mass balance,
and cathodoluminesence imaging, to quantify that amount
of melt that is trapped between crystals, in various units of the Adamello batholith of northern Italy. Their goal is to calculate
“crystallized liquid fractions” or CLFs, which should approximate the melt that
is trapped when magmas reach a critical crystallinity
so as to form a rigid (melt-trapping) framework. They find that the amounts of trapped
liquid vary widely, ranging from 7-70%, and tend to be lowest for systems with strong
CPO, which indicates that syn-magmatic deformation may
be an efficient mechanism by which melts are expelled from an otherwise rigid crystal-liquid
mush.
As in Barite
On page 2512 of the
December 2017 (vol. 102, 12) issue, Ma et al. investigate the conditions under which
As(V) partitions into Barite. They find that As may be readily incorporated into barite and at pH>5 can
from a stable Ba-Arsenate phase. Their work illustrates the conditions under which
aqueous As may be fixed to a crystalline phase, depending upon the concentrations and pH conditions, that may be of crucial aid to
hazard assessments and remediation efforts.
Editors Selections,
November 2017
Highlights & Breakthroughs
Rutile is the Key
On page 2153 of this issue, Alicia
Cruz-Uribe provides an overview of Guo et al. (2017; p. 2268 of
this issue), who investigate rutile grains that form at the rims of Fe-Ti
oxides in greenschist facies metamorphic rocks. Associated mineral and inferred
reactions indicate that rutile forms by the action of highly oxidized fluids,
approaching nearly 4 log units above QFM, during retrograde metamorphism. As
Cruz-Uribe notes, this study illustrates a how rutile may be a record of elevated
fO2 fluids at subduction
zones. The possibility is that rutile is forming as the rocks intercept high fO2 fluids that are driven
off the slab, or are otherwise connected in some way to subduction zone
magmatism. These rutile grains, though, would not be responsible for the high field
strength element signatures of arc magmas.
Volcanoes & Plutons: disconnected
On page 2154 of this issue, Calvin
Miller reviews the granite controversy of the prior century, and new questions
that have evolved since. The context involves new findings by Tang et al. (2017; p. 2190 of
this issue) who use zircon compositions from volcanic and intrusive felsic
rocks from Hong Kong, to test ideas of volcano-plutonic connections. Their work
indicates that at least at Hong Kong, plutons are not the residues of felsic
eruptions. Instead, felsic volcanic chambers are nearly completely evacuated,
and intrusive rocks are evolutionary analogs intruded when conditions for
eruption were unfavorable. Though closely related in space and time, the felsic
rocks of this region are not co-magmatic, but generated independently and
erupted, or not, as structural conditions allow.
Reviews
No Fe-Ti Oxide Magmas
On
page 2157 of this issue,
Lindsley and Epler present new experimental data to re-examine the genesis of massive Fe-Ti
oxide bodies that occur mainly in association with
anorthosites, often as dikes. At issue is whether such oxide-rich bodies are
crystalline residues of a silicate melt, or were melts in and of
themselves, possibly formed along an oxide-silicate melt solvus (as an
immiscible melt). These authors conclude that such oxide bodies have bulk
compositions that cannot occur as melts at
geologically reasonable temperatures. More likely the oxide-rich
bodies were intruded as crystal-rich mushes, perhaps
lubricated by small amounts of a silicate melt. They also
imply that some "jotunite"-like rocks (having low Si and high Fe and
Ti) are these very same lubricating silicate melts, apparently acquiring their
low Si and high Fe and Ti by dissolving some of their otherwise mechanically
associated Fe-Ti oxides.
Review &
Special Collection: Biomaterials—Mineralogy Meets Medicine
Orthophosphates: What can’t
they do?
On page 2170
of this issue, Robert Heimann reviews a class of orthophosphates with NASICON
structure, which nicely illustrate how fundamental concepts of mineralogy are
pivotal in the search for superior bioceramics used in a variety of medical
applications. Heimann proposes that Ca(Ti,Zr) hexaorthophosphates have bone
growth-mediating characteristics that are particularly well suited for use as coatings
on metallic implants (in knee or hip replacements) so as to aid bio-integration
of the foreign materials, and, based on the solid-state ionic conductivity of
these compounds, proposes a new device that is expected to electrically
stimulate bone growth. While the biological uses are emphasized, the author
suggests that these same structures should also be useful for storage of
radioactive waste or as electrodes in molten Na-ion batteries, among other
applications.
Articles
Fables of the Reconstruction
(of Liquids Using Amphibole)
On page 2254 of
this issue, Shimizu et al. develop parameterized
lattice-strain models predictive
of the partitioning of REE into amphibole. They find that REE partition coefficients
are highly sensitive to the amphibole major element compositions--with order of
magnitude variations accompanying arc magma genesis. Their new
mineral-composition model allows one to reconstruct equilibrium
liquid REE concentrations from amphibole compositions alone, provided that
temperature is known, and a new
thermometer is presented as
well. Application of these new models reveals a greater role for amphibole
fractionation of arc magmas, relative to clinopyroxene, and that REE-rich
amphiboles are likely records of particularly low-T
amphibole crystallization.
Fast Moving Dunites (from 150 km)
On page 2295 of this issue, Su et al. describe magnesite + aragonite intergrowths within the Sulu UHP terrane of eastern China, that form as breakdown products of dolomite, at >5 GPa. The authors infer that the precursor dolomite formed from a metasomatic melt, perhaps within the uppermost part of the mantle. In any case, because the breakdown reaction of dolomite is not sensitive to T, it seems that the carbonates were indeed formed at very high pressure, and the lack a retrograde reaction of magnesite + aragonite back to dolomite further indicates that the exhumation was rapid or dry or both. The authors infer that other dunites may harbor the very same clues of a UHP history, if similar textures and assemblages may be found.
Editors Selections,
October 2017
Highlights & Breakthroughs
Precipitation Conditions of Oxides in Diamonds
On page 1969 of this issue,
Dongzhou Zhang provides an overview of Uenver-Thiele et al. (page 2054 of this issue) who
explore the phase relationships of starting compositions having MgFe2O4
and Mg0.5Fe2+0.5Fe23+O4
stoichiometries. These experiments help delimit the precipitation conditions of
magnetite inclusions in diamonds, by taking advantage of textural clues that
might indicate the presence of certain precursor phases, such as ferropericlase
or one of a few “unconventional oxides” that may be stable at high pressure. Their
experimental work also shows that otherwise simple oxides might exhibit rather
great stoichiometric variety at elevated P and T.
Articles
Dynamics of Magmatic Processes
10 Days for Pre-Eruption Magma Assembly at Laki
On page 2007 of this issue,
Neave et al. use the textures and zoning patterns of plagioclase crystals to
examine the disaggregation of a liquid-crystal mush prior to eruption. They
find that most microcrysts grow just after macrocrysts of a mush are entrained
on a final path to eruption. The result is that the seemingly homogeneously
mixed magmas of the Laki eruptions are the result of multiple disaggregation
and mixing events. The authors calculate, for example, that the 15 km3
of erupted material of the 1783-1784 extrusives were assembled in batches of eruptible magma no greater than
about 1.5-2 km3, with magmas being assembled from mush systems about
10 days prior to eruption. These multiple mixing events also yielded similar
magmas, perhaps indicating that the eruption triggering mechanisms are
threshold dependent, although those thresholds remain undiscovered.
Making Bubbles
On page 2022 of this issue Masotta
and Keppler present a new experimental assembly that allows bubble nucleation
and growth experiments at elevated P-T
conditions (up to 850oC and 2 kbar), along controlled P-T-time paths. Their preliminary
experiments, on a haplogranite (or simple, synthetic granite) system appears to
show that bubble nucleation in a closed system will end when the average
distance between bubbles is equal or less than the mean diffusion distance of
water molecules. (But then how does a water molecule at a distance greater than
the mean know that system-wide the mean has been reached?) A potentially
significant result is that magma fragmentation (e.g., vapor phases are >70%
by volume) appears to occur at 0.2 kbar, regardless of decompression rate. In
any case, their new experimental assembly should aid investigations of
late-stage volcanic processes.
Does Hydrous Phase B Exist in Earth’s Mantle?
On page 2032 of this issue,
Kojitani et al. experimentally investigate the stability of Mg14Si5O24,
the so-called “anhydrous phase B” found in some high P experiments in simple
silicate systems. In this work, Kojitani et al. suggest that this phase can
form, by reaction of forsterite and periclase, at pressures that might explain
the seismic X-discontinuity, which occurs at depths of 250-350 km. Their work
confirms earlier suggestions that the reaction to form anhydrous phase B should
only occur in the presence of fluids, when such fluids dissolve Si so as to leave
a relatively Si depleted residue. Such conditions furthermore allow this phase
to be stable at lower pressures. If this hypothesis is valid, anhydrous phase B
might then affect the fluid-rock and melt-rock partitioning of trace elements
in subduction zones.
More on Magma Fragmentation -- Lunar Eruptions
On page 2045 of this
issue, Rutherford et al. use the volatile contents of lunar orange picritic
glasses to reconstruct their eruption history. That history begins with melt
segregation from a melt-rich mantle source at 500 km and vapor (CO-rich, but
including S and H) saturation at somewhere between 4 and 50 km below the
surface (or possibly as deep as 500 km in some scenarios), which then
accelerates magma ascent. By the time these magmas reach depths of 300-600 m,
the residual magmas have lost nearly all their volatiles to the gas phase, and
in the new model, these depths would also represent the point at which the
magmas became fragmented (having >70% bubbles by volume).
Letter
Hidden chromium
On page 2142 of this issue,
Schindler et al. identify a potentially new source of Cr in rock samples:
Chromium (IIII) occurs as inclusions of chromite nanoparticles within silicates
rather than being incorporated into their structures. Transport models of Cr in
the environment may need to take into account the release and transport of
these nanoparticles during weathering rather than transport of chromium as
aqueous species, which could have important implications for environmental
modelling and risk assessments in chromium-rich regions.
Editors Selections,
September 2017
Highlights & Breakthroughs
A Metallic Core Source for Nitrides in Diamonds?
On page 1769 of this issue,
Zedgenizov and Litasov review Kaminsky
and Wirth’s paper (in the August issue) regarding new data on nitride
inclusions in diamonds. As noted in a prior summary, this work describes phases
that are included in so-called “superdeep” diamonds; the nitrides inclusions may
be derived by contamination of the diamond source region by metallic material
from the core. Zedgenizov and Litasov here suggest that the contamination of
the diamond source might have occurred at pressure conditions as low as 5-10
GPa, well short of the 120 GPa near the core-mantle boundary. But a metallic
source for these inclusions is still implied; this leaves open the need for
trace element and isotopic studies of such inclusions, to determine if any
might represent mantle traces of an otherwise long-segregated core.
Special Section: Geochemical Transport
Processes in the Crust and Mantle
The Transport of Rare Earth (and other) Elements in
Metamorphic Systems
On page 1796 of this issue, Jay
Ague examines the dissolution and transport of rare earths and other elements
in the fluids generated in metamorphic systems. He finds that despite their
trivalent character, REE are indeed quite mobile in high flux environments (more
so than other high field strength elements) and are fractionated in the process;
in contrast Th and Zr are relatively immobile. REE mobility appears to be
enhanced in “extreme” environments, such as hydrothermal systems connected to
magma emplacement or any system where supercritical fluids are developed. Ague
proposes a diverse array of interesting and important implications, including
how high field strength elements may be sufficiently mobile so as to sometimes negate
certain tectonic discrimination diagrams; or using mass balance considerations to
distinguish how CO2 is released by metamorphic reactions.
Special Section: Water in Nominally Hydrous
and Anhydrous Minerals
The Subsolidus Partitioning of H in Peridotites
On page 1822 of this issue,
Demouchy et al. present new data on how H is partitioned amongst the nominally
anhydrous minerals in a garnet-lherzolite, experimentally equilibrated at 1100oC
at 3 GPa. A key finding is that their partition coefficients for the pyroxenes
relative to olivine tend to be much lower than those derived from studies where
melt is present. The authors suggest that in some studies, high values for
Pyx/Ol partition coefficients may be related to H loss in Ol. But clearly, new
studies that show co-variations of H in pyroxenes and olivine are needed to
better understand how water is stored, at least in the sub-solidus mantle, and
perhaps under partial melting conditions as well.
Articles
On page 1922 of this issue, Tao et al.
compare the structures and compositions of a Al-10 Å phase and the K-bearing
micas muscovite and phlogopite. Their worked is sparked by studies of the
Mg-bearing 10 Å phase, found in many hydrous high pressure experimental studies,
and thought to be an important carrier of water into the deep mantle via
subduction. This new work indicates that an analogous K-bearing, Al-10 Å has at
definite micro-solid solution relationships with more common K-bearing
phyllosilicates. The results are important for two reasons. First the solid
solution relationships provide a possible link between high- and low-P phase
assemblages, and these linkages may act as steps on a downward ladder, so that
water might eventually be partitioned into a high-P 10 Å phase, of one sort or
another. The authors also point out that if this K-bearing downward-pointed
ladder is operative, then the global K and water cycles are linked.
Letter
Pressures of Inclusion
On page 1957 of this issue, Angel et al.
present new software that can be used to more accurately determine the P-T
paths of mineral-in-mineral inclusions. Their new programs do this by taking
into account the differing equations of state (EoS) for inclusions and hosts,
and by allowing the user to input customized EoS. Their approach uses an
“isomeke”, which is a curve in P-T space that provides the locus of point where
the fractional volume change of the host, induced by some external change of
pressure, yields a comparable fractional volume change in an inclusion. Among
the intriguing results is that quartz included in garnet may experience
pressures along a prograde metamorphic path that are 30-40% lower than a host
garnet; in contrast, pressures experienced by rutile in the same garnet may
differ by <5%, due to the similar EoS for these materials.
Editors Selections,
August 2017
Invited Centennial Article
Predicting new minerals
On
page 1573 of this issue Grew
et al. use B-bearing minerals to examine how the Large Numbers of Rare Events
(LNRE) model has fared in predicting the total number of
as-yet-to-be-discovered minerals. The LNRE model was used to predict that
perhaps as few as 1600 new minerals were to be discovered (Hazen et al. 2015).
But the LNRE approach assumes a fixed rule set, and so does not account for new
technological methods that might be applied to discover new minerals (let alone
shifts in the definition of a mineral); potentially leading to
under-estimates of natural mineral diversity. Thus the LNRE model yields a
predicted total of ca. 500 B mineral species (ca. 200 as yet to be discovered).
The authors also note that as few as 19% of all B minerals were known as
synthetic analogs prior to their discovery. However, what appears to be lacking
in the LNRE approach is intent. For example, does the low fraction of
pre-synthesized B minerals represent limits on our ability to create new
compounds or a lack of compelling reasons to take on an apparently mindless and
unproductive task? The highs and lulls of new mineral descriptions may be
related to intent as well, but these issues do not blunt the authors’
fundamental contention that surprises are ahead.
Outlooks in Earth & Planetary Materials
Predicting new mineral
associations
On page 1588 of this
issue, Morrison et al. introduce Network Analysis as a means to explore
possible genetic relationships between minerals. The use of Network Analysis
will already be familiar to Am Min readers, mostly from their wide use in sociology;like
mapping social media networks or predict voting behavior. Applied to
mineralogy, the idea is that Network Analysis may reveal new associations that
are characteristic of a time, place, or conditions of mineral formation. One
might immediately respond that we have a governing
theory--thermodynamics--that obviates the need for a statistical
approach. The promise of Network Analysis, though, is to examine very large
data sets to predict mineral occurrence in rare minerals that nonetheless might
characterize an environment, an era, or a tectonic setting. These authors use
Cu-bearing minerals as a striking example of how network analysis can be
employed to examine hundreds of species at thousands of localities, and in the
process reveal how Cu-mineral diversity has increased in both magnitude and
kind since Archean.
Articles
Why Cu-Au-Mo Here, And Not There?
On page 1597 of this issue Olson
et al. examine the conditions of Cu, Au, and Mo mineralization in the Pebble
Porphyry deposits of southwest Alaska, by examining the precursor and host
granite plutons and dikes. The authors find that the very high degree of
enrichment in Cu and Au are related to the differentiation of mafic, hydrous,
calc-alkaline, and alkalic magmas under strongly oxidizing conditions, the
latter of which inhibit the saturation of sulfide melts, and so promote the
retention of Cu and Au in increasingly sulfate rich magmas. The authors also
hypothesis that certain trace element enrichments may have occurred in a
so-called “MASH” zone where primitive magmas may have interacted with
sulfide-rich cumulates in a lower crust setting, although the depths of
differentiation and emplacement were not estimated.
More N in Earth’s mantle
On page 1667 of this issue
Kaminsky and Wirth report on a new set of nitride minerals (Fe2N, Fe3N,
and Fe9(N,C)4) found as inclusions in what appear to be a
lower mantle-derived diamond. The authors infer that the nitride minerals derive
from the core-mantle boundary, having formed within the core itself, apparently
based on the association of Fe7C3 in other inclusions in
the same diamond and the presumed usefulness of high P variants of Fe7C3 and Fe7N3
to explain some seismic anomalies of the solid core. However, these same
inclusions carry carbonatite-association minerals. Thus, it is not clear
whether we must accept all the components that occur in such inclusions as
being necessarily derived from the core/mantle boundary or as indices of a more
shallow provenance for all of the included phases, in what may be a highly
reduced domain in the upper mantle; which would appear to be required in
any case to generate those diamonds presumed to form in the uppermost mantle.
Fe-rich magnesiowustite may explain Ultra Low Velocity
Zones at the base of the mantle
On page 1709 of this issue,
Finklestein et al. conduct high P
experiments on a magnesiouwustite with 78 mol% Fe, to determine its elastic
properties. These experiments are motivated by the hypothesis that Ultra Low
Velocity Zones (ULVZs) at the base of Earth’s mantle might reflect the
partitioning of Fe into oxides phases relative to ambient bridgmanite (or
putative post-peroviskite phases). Their work confirms, at least at low
temperatures, that the bulk modulus (K)
of mw decreases with increased iron content, and that the first pressure
derivative (K’) is unchanged with
composition. Two pressing questions emerge from this work: first, do these
elastic property relationships hold at elevated temperature; second, could
ULVZs be explained solely by the way Fe partitions between mw and high pressure
silicate phases or is an absolute enrichment in bulk Fe contents needed to
obtain observed ULVZ anomalies?
Letters
Fe/Mn ratios in our planetary neighborhood
On page 1759 of this issue, Papike
et al. compare the compositions of olivine and pyroxene from Earth, Moon, Mars,
and Angrite meteorites. They conclude that Fe/Mn ratios of these minerals are
similar enough between Angrites and Earth to require that the Angrite parent
body was initially coagulated in some proximity to Earth. The idea is based on
the observation that the Fe contents of the silicate portions of the
terrestrial planets decreases with increasing distance from the sun. This
result is in distinction with Sossi et al. (2016: EPSL) who suggest that Fe/Mn
ratios are not only higher than for Earth within the error of their bulk
measurements, but that Fe/Mn ratios are meaningfully and positively correlated
with Fe isotope ratios, although these authors come to the same conclusion,
i.e., that inter-planetary contrasts in Fe/Mn are controlled not just by variation
in core mass and Fe segregation into a metallic phase, but also by the loss of
the more volatile Mn.
Editors Selections,
July 2017
Editorial
A New High Journal Impact Factor (Not really) and How Rankings Reflect Values
On
page 1369 of this issue, we show our failed prediction of a much higher JIF (it went up, but only by 0.1 from last year), relating JIFs to college rankings, and discuss how these and other rankings reflect values. The failed prediction lends support to our proposal that journals adopt a variant of the JIF (introduced in our editorial) that is more easily reproduced. Average citation rates have in fact increased significantly at Am Min, and as we show, some Am Min papers can acquire vastly more citations than comparably themed papers in Science or Nature. But purveyors of citation statistics have no incentive to transparency, and our discrepant calculations should oblige Editors and journal critics alike to consider independent estimates and indices. (And hats off to JGR editors for joining the bandwagon.)
Highlights and Breakthroughs
Sapphire – Blue with
Inclusions
On
page 1373 of this issue, Lin Sutherland provides a review of Palke and Breeding’s new study of needle-like rutile in sapphire from several localities, such inclusions being the cause of “silk” and “star” varieties of the gemstone. Palke and Breeding find that rutile (and possibly other Fe-Ti oxides) is not necessarily formed by exsolution. In this interpretation, rutile-bearing sapphires are not indicative of initially high temperatures of crystallization; they also indicate that the trace elements contained within the inclusions may help to characterize both geologic and geographic origin. Additionally, this new work lends support to the idea that blue-colored sapphires may acquire their optical characteristics from Ti-Fe charge transfer within the host sapphire.
Invited Centennial Articles
Explaining Radon Loss in
Minerals
On
page 1375 (temporary link) of this issue Krupp et al. measure Rn loss rates for a variety of minerals from various geologic settings and show how such loss rates vary with a number of interesting factors, including grain size, mineral density, temperature, U and Th concentrations, and mineral melting points. They find that Rn emission rates are greatest for minerals with low density and low melting points, and they show that when fission tracks are annealed, Rn loss rates are reduced. The authors thus suggest that Rn may readily diffuse along nuclear tracks. The study does not apply multivariate methods so as to untangle these effects, but this work nonetheless shows that mineralogy, environment, and geologic history can each have a significant effect on Rn concentrations in the atmosphere and groundwaters, and may explain discordant dates in some U-Pb systems.
Articles
Mineralogy of Durable
Concrete from Ancient Rome
On page 1435 of this issue, Jackson et al. present yet another American Mineralogist publication that has garnered very widespread attention in popular news media. In this work, the authors examine phillipsite and Al-tobermorite: mineral cements in certain Roman marine concrete. The ancient concrete is formed by the interaction of seawater with “Pozzolan”, a silica-rich material (often derived from volcanic ash) that upon reaction with seawater produces a highly durable concrete, perhaps more durable than limestone-based Portland derivatives. This new work shows that the growth of zeolite (phillipsite) and Al-tobermorite during seawater-volcanic ash reactions may be key to providing at least some of the structural resiliency of these concrete structures formed in the harbors of ancient Rome. Their work should also aid the identification of natural pozzolans that should optimize the growth of these phases, and so yield durable structures.
Zeolite Dehydration and Extra-Framework
Cations
On
page 1462 of this issue, Lee et al. present the results of dehydration experiments involving natrolite, containing extra-framework cations (EFC) of various size and charge. They find that dehydration temperatures are inversely proportional to EFC size, which might not be unexpected given a possible Coulomb’s Law effect on bonding strength. The authors also find that during dehydration, EFCs migrate from the middle to the edges of the open channels that characterize zeolite structures, which then causes a change in channel orientations relative to crystallographic axes. The authors suggest that EFC type has a greater influence over the stability of zeolite structures than degree of dehydration, with stability increasing with decreasing EFC radius.
What Happens When Lightning Hits Rock?
On page 1470 of this issue, Elmi et al. present a study of fulgurites from a lightning strike of granite exposed at Mt. Mottarone, Baveno (Italy). Fulgurites have long been a curiosity--and perhaps little more--but in this study, the authors delve into the thermal conditions and compositional changes that occur upon lightning-induced partial melting of granite. Their work identifies a mineralogy and energetic conditions that indicate temperatures of ca. 1700 °C, if the system approached equilibrium. They also find that voids in the fulgurite structure result from the degassing of burned organic matter. Beyond the curio aspect, this work tells us that fulgurites may trap ambient gases and reveals much about how materials are affected by lightning strikes, which may then lead to better mitigation of thermal shocks associated with such.
Pyroxenes Stop Subducted
Slabs Cold
On page 1516 of this issue, Xu et al., to better understand the conditions under which subducted slabs may stall in the upper mantle, conduct experiments to measure the physical properties of natural augite to 27 GPa and 700 K. Existing studies of pyroxenes focus on idealized end-member compositions, or involve measurements at high P or high T, but not both, forcing modelers to rely on mixing relationships and extrapolate equations of state. This work shows that natural augite-composition clinopyroxenes, if metastably preserved in a sinking plate, do indeed have a sufficiently low density so as to inhibit subduction beyond transition zone depths. Their modeling efforts indicate that natural augite has sufficiently high Fe so as to cause slabs to sink above transition zone depths, but that at the base of the transition zone, if temperatures remain cool, metastable augite may cause slabs to become neutrally buoyant, at least relative to PREM densities.
Carbonate-Silicate Liquid Immiscibility Explained
On page 1561 of this issue, Morizet et al. examine the structural characteristics of carbonate-rich, low-Si melts, analogous to kimberlite and melilitite bulk compositions. Their NMR study reveals that carbonate dissolves in such de-polymerized melts so as to form Free Ionic Clusters (FIC), instead of network-forming complexes. These FIC are composed of carbonate units bonded to network-modifying cations that donate most or all of their charge to the carbonate complex, as opposed to neighboring silicate structural units (analogous to what may be deduced of mineral structures using Pauling’s Rules). The FIC are thus nearly completely disconnected from ambient silicate melt constituents, and thus may form the nuclei upon which immiscible carbonate melts exsolve from a silicate solution.
Editors Selections,
June 2017
Actinides in Geology
Storing U in Opal
On
page 1154 of this issue,
Schindler et al., examine the crystal structural controls on the incorporation
of U into opal. These authors build on prior work that shows that U may be
structurally bound by amorphous Si for millions of years. Their new study shows
that U is most likely to be retained along fibers or grain boundaries of opal as
various domains are precipitated or transformed by various dissolution-re-precipitation
reactions. The U complexes are apparently especially mobile along the
boundaries separating domains of opal-CT, a variety that contains microcrystalline
cristobalite and tridymite (compared to opal-A, an aggregate of non-crystalline
silica). The U so captured may be released as opal transforms to
microcrystalline quartz.
Dynamics of Magmatic Processes
A Tipping Point for the
Eyjafjallajökull 2010 Eruption
On
page 1173 of this issue,
Laeger et al. present new analyses of pumice and minerals contained in tephra
deposits erupted on May 18 and 22, from the Eyjafjallajökull 2010 eruption in Iceland.
Their major and trace element data allow these authors to identify two distinct
episodes of mixing, between a mafic recharge basalt magma, and two magma
compositions (a trachyandesite and a rhyolite) that geophysical evidence
indicates were intruded and stored as sills starting in 1994. Perhaps most
interesting is that their merger of mineralogical and geophysical data appears
to show that the arrival in 2010 of what is termed “BAS 1” (a basalt
composition) signaled what Colin Wilson would call a “tipping point”, sending
this Icelandic magma plumbing system from a state where the crust can absorb all
those magmas delivered from the mantle, into a state of instability, where new
batches of mantle-derived magma, in mixed form, are erupted.
Articles
Depth of Spin Crossover of Fe3+
On
page 1263 of this issue Sinmyo
et al. use Mössbauer spectroscopy to measure the valence and spin state of Fe
in bridgmanite, at elevated P and ambient T. Prior work has indicated that Fe3+
in bridgmanite may transition from a high spin to an intermediate or low spin
state as pressure increases, which may in turn be measurable by a resulting
change in magnetic character. The authors find that the spin transition in
bridgmanite occurs at 900 km (35 GPa) in a pyrolite mantle, and 1200 km (50 GPa)
in MORB-like domains, with the transition occurring at greater depths still at
higher temperature. And the transition will, of course, be quite sensitive to fO2. Their results suggest
that the spin crossover will not yield detectable contrasts in elastic
properties, but might be observed in geomagnetic surveys. This work implies an
urgency to measure the depth of a spin crossover in Earth’s mantle by
geophysical means, which may then be as helpful for understanding the T and fO2 conditions of the deep mantle, as seismic velocities
were in determining the existence of bridgmanite itself.
Resetting Pb in Seismically
Deformed Zircon
On
page 1311 of this issue,
Kovaleva and Klötzli examine zircons in mylonites from the Ivrea-Verbano zone
of northern Italy to determine to how zircon trace element and isotope
compositions may vary within lattice defects that are generated by seismic
activity. They find that post-growth deformation bands can both gain and lose
certain trace elements (e.g., Hf, Ti, and P) relative to undeformed parts of
the same grain and that these same deformation bands appear to undergo
systematic Pb loss. The greatest Pb loss appears to be associated with planar
deformation bands that occur at the highest angle relative to an unstrained
host lattice. Such Pb loss should lead to the possibility of using coupled
structural and isotopic studies of zircon to age date mylonite formation events
other any other deformation process that affects zircon microstructures.
Liquid Compositions As
Predicted From Amphibole
On page 1353 of this issue, Zhang
et al. provide new “chemometric” equations that may be used to reconstruct a
silicate liquid using an amphibole composition. Amphiboles are a common phase
in arc-related igneous rocks, and because of their compositional complexity,
they would appear to offer a powerful tool for determining silicate liquid
compositions, which at most arc volcanoes are often highly obscured due to the
very common and efficient mixing of diverse magma types. This study revisits an
important earlier attempt by Ridolfi and Renzulli (2012) to use an amphibole
composition to infer the composition of the silicate liquid in crystallized
from. This new work focuses on basanitic to rhyolitic melt compositions, and is
accompanied by an interesting test: their new models can successfully predict
matric glass compositions in selected ignimbrite samples when using coexisting
amphibole rim compositions as input. Their application to natural systems also
shows that the variety of liquid compositions, at least at one volcano, may be
much greater than one might infer from the bulk composition erupted products
alone.
Editors Selections,
May 2017
Highlights and Breakthroughs
Anthropogenic Minerals
On page 925 of this issue, Peter Heaney provides a provocative overview of the immensely popular article by Hazen et al. (2017; this volume) regarding minerals that have an anthropogenic parentage. As Heaney notes, textbooks have largely given up on the notion that minerals form only by inorganic processes; he then asks whether we are “prepared to take the next step”: to accept synthetic crystals as minerals (as already proposed by Zalasiewicz et al. 2014). Hazen et al. do not require such in their definition of an Anthropocene epoch they attempt to play by IMA rules. Some of the most intriguing arguments appear to begin with how hypothetical visitors to, or evolved species on, a late Earth might classify crystalline structures. What is technology at present is archeology to a future age. What is uniquely anthropogenic now, may be a primitive step along a biogenic continuum to some future species. As our language evolves over centuries or even decades, it is sure to vary across species and millennia.
Diamondiferous Clues to a Non-chondritic
Lower Mantle
On page 927 of this issue, William Bassett reviews a new work by Kaminsky and Lin (2017; this volume) who speculate on the composition of Earth’s lower mantle, based on the compositions of minerals included in diamonds and their synthetic analogs as produced in high pressure experiments. In the new work, Kaminsky and Lin (2017) examine the co-existing compositions of ferropericlase and bridgmanite (the latter of which is quenched to an orthopyroxene structure, but is presumed to preserve the composition of the high pressure, proto-mineral phase). They find that in some inclusions, the total amounts of Fe in both phases are high relative to what might be expected had they equilibrated from a pyrolite bulk mantle composition. And yet another set of inclusions contain lower-than-expected Ni. These observations lead Kaminsky and Lin to infer that the lowermost mantle may contain an Fe-rich zone that might even be saturated in a metallic Fe phase that may retain Ni. This suggestion carries with it the implication that mantle plumes from the lowermost mantle should yield volcanic rocks that are enriched in Fe, but depleted in Ni and other siderophile elements.
Defining
Chemical Bonds
On page 928 of this
issue I. David Brown discusses a new paper by Bickmore
et al. (2017; this volume), which attempts to connect bond energy to
electrostatic characteristics, in this case electronegativity. The work is
inspired by the observation that bond energies vary greatly across bond types.
Perhaps the most intriguing implication is one left unsaid by the authors: they
provide a possible fifth explanation for the rarity of many minerals, as
enumerated by Hazen and Ausubel (2016; Am Min). Those
authors suggested that rarity is typical for the more than 5000 minerals yet
named, due to restricted P-T-X stability,
planetary constraints, ephemeral stable conditions and collection bias. Bickmore et al. (2017) provide, if
not an additional cause for rarity, then at least a possible
rationale for ephemeral P-T-X
stability, by postulating that peroxides are rare but persuflides
are common. Peroxides are built upon O-O bonds, which have intrinsically
less energy per bond valence than more ionic M-O bonds.
Not So Deep
Diamond Inclusions?
On page 929 of this issue, Andrew Thomson reflects on some new and important observations by Uenver-Thiele et al. (2017) regarding the putative depths of mantle-derived diamonds. Some diamonds are thought to derive from depths of at least >800 km, on the basis of their containing magnesiowüsite (Mg,FeO), a phase that is not expected to be stable under subsolidus conditions in a pyrolite bulk composition (but is stable to atmospheric pressure in other bulk compositions). Even greater depths are suggested where some magnesioferrite phases occur as exsolution lamellae within Mg,FeO—but such inferences have not verified by experiments at ambient pressure. Uenver-Thiele et al. (2017) conduct the needed experiments and ironically find that such exsolution relationships are likely to occur only at P<10 GPa. Their new work does not preclude entrapment of inclusions at greater depths, but indicates that the exsolution textures in question represent an upper mantle phenomenon.
The Genesis of
Arc Magmas
On page 931 of this issue, Christy Till reviews
the conditions under which arc magmas are generated in the mantle wedge. She
finds that while published P-T
estimates of arc magma genesis vary greatly, the largest source of variation in
these estimates is differences in their calculation methodology. Once calculated with a consistent
approach, wet primitive arc magmas reveal origins via water-saturated partial
melting at 20-35 kbar, with subsequent equilibration
(prior to transport to the crust) over relatively narrow P and T intervals of 8-19
kbar and 1075-1300 °C. Anhydrous arc magmas reveal similar
depths of mantle equilibration en route to the surface but at higher
temperatures (1290-1450 °C), facilitated by channelized rather than porous
flow. Till finds that these processes are sufficient to explain a rather wide
range of arc magmas, from boninites to high Mg
andesite to hydrous calc-alkaline basalt, leaving a lherzolite residue for the latter and a harzburgite
residue for the former two. More importantly, though, Till is able to show how
these P-T conditions can be mapped into
different and distinct parts of the mantle wedge, which means that various
volcanic composition suites may be used to describe the evolution of an ancient
or evolving arc system.
Little Water is
Transported Into the Mantle By Subuducting Basalt
On page 975 of this issue, Schmädicke and Gose measure water contents in both garnet and omphacite in coesite-bearing eclogite. A key inference from their work is that molecular water in these phases is secondary, related to fluids that come into contact with the system following peak metamorphism. (The opposing possibility is that molecular water represents structural water that is exsolved upon decompression). In addition, molecular water may even secondarily enhance the content of structural water. In this case, only structural water of samples devoid of molecular water is useful for estimating water budgets in a downgoing basaltic slab, and these authors thus infer that such contents are quite low, on the order of 300-650 ppm, with most pre- and syn-metamorphic water being released to hanging wall rocks (e.g., the mantle wedge or exhuming high-pressure slices).
Remaking Zircon
On page 1066 of this issue Pidgeon et al. examine the recrystallization of radiation-damaged zircon. The recrystallization process first involves the formation of tetragonal zirconia, which then reacts with silica to recreate zircon. Their experiments indicate, however, that such recrystallization is highly unlikely in natural systems, and Pb and He loss would thus be permanent. They further note that recrystallization would be no more likely in synthetic zircons that are used to sequester radioactive waste, in which case radionuclides would be not be protected against reactions with fluids.
Letters
Animal, Vegetable, Mineral
On page 1129 of this issue, Hummer et al. provide
the structure of one of the very few known organic minerals, in this case, a
naturally crystalline geoporphyrin (porphyrins being
a type of organic compound) called abselonite. This
and related compounds are believed to be due to the breakdown under diagenesis
of chlorophyll a, and the transformation to abselonite
involves a highly specific change of cations from Mg to Ni(II).
Editors Selections,
April, 2017
Review
How to Dissolve
Kidney Stones
On page 701 of this
issue, Hill et al. present a new computer model, based on current thermodynamic
measurements, that can be used to predict the solubility of Ca phosphates in
body fluids. As noted by the authors, the human body must in places remain supersaturated in
hydroxyapatite for healthy bones and teeth but that oversaturation in
the kidneys can lead to the development of unwanted stones. This new model
attempts to predict mineral saturation under some of the various complex fluids
of the human body, and so better inform the conditions that lead to kidney
stone development, and thus better inform preventative measures.
Articles
Found: Magmatic
graphite
On page 728 of this issue, Perez-Soba et al. present a case for what they argue is the first occurrence of graphite (in a granite host) that is of magmatic origin. Their study focuses peraluminous granites of the Variscan Belt, and three different types of magmatic apatite contained in these. Because peraluminous compositions have a high solubility for apatite, these minerals tend to record late stage processes. They find that some fluorapatite grains of apparent magmatic origin contain needle-like inclusions of graphite. They interpret the graphite inclusions as forming at a very late stage of crystallization, where the fluorapatite is in equilibrium with a fluid phase and two immiscible (perphosphorous and peraluminous composition) magmatic liquids and they suggest that a C-rich system may induce liquid immiscibility. These intriguing results point to the usefulness of apatite as a means to investigate the P-T conditions of very late-stage magmatic processes….
An Apatite
Barometer
On
page 743 of this issue, Ashley et al.
calibrate an equation of state for complex apatite solid solutions. They find
that their equation of state may provide a powerful tool to obtain P estimates
from apatite included in garnet. They find that garnet is an ideal host in that
this isotropic mineral also has a very high bulk modulus, and so more ably
preserves the stress state of included minerals. Their tests further indicate
that the pressures recorded by apatite are not highly sensitive to T. Apatite
inclusions in garnet may this provide an accurate barometer in whatever systems
they may be found, and may be especially useful when P-sensitive exchange
equilibria are absent.
Special
Collection: Olivine
A Precise Ni-in-Olivine
Thermometer
On page 750 of this issue, Pu et al. calibrate a new Ol-liquid thermometer, based on the partitioning of Ni between Ol and co-existing silicate melt. The thermometer is calibrated with some several limitations: it uses experimental data collected at 1 bar only, of liquids saturated only with Olivine, containing ≤0.1 wt% NiO. But the resulting experimental calibration data set still spans a wide range of T (1170-1650 °C) and liquid compositions (37-66 wt% SiO2), and the resulting model ably predicts T from experiments performed at 10 kbar, and so may have an advantage over Mg-exchange based thermometers in requiring no knowledge of P (or rather to allow P to be estimated, at least at P<10 kbar, by determining the P at which the Ni and Mg exchange thermometers yield identical temperatures). Finally, their study also hints at the possibility that their Ni exchange thermometer may also be only slightly dependent on magmatic water (H2O, more appropriate than water) contents, thus offering the promise of a highly versatile thermometer that requires minimal input parameters for application.
Special
Collection: Dynamics of Magmatic Processes
Water from Recharge Magmas
May Help Expel Felsic Melt from Mush
On page 766 of this issue, Pistone et al.
conduct rather clever experiments to examine the role of recharge magmas in
aiding the expulsion of melt from overlying felsic mush systems. Their
experiments, which place aphyric, hydrous
andesite below a dacite mush, show a significant transfer of water from the
andesite to the dacite, and in increase in the SiO2 contents of dacitic mush
glasses as the addition of water apparently drives the dissolution of quartz in
the mush. The authors suggest that mush rejuvenation occurs in two stages, the
first dominated by heat transfer and the second by water transfer, from the
mafic recharge magma to the felsic mush. Both these processes lead to an
increase in melt fraction in the dacitic mush, and a decrease in melt density
and viscosity, which are expected to aid felsic melt extraction.
Determining Impact Histories
on Ancient & Weathered Planetary Surfaces
On page 813 of this issue Montalvo et al. survey
some 11,000 zircon grains recovered from sedimentary deposits that may have the
Verdefort impact site as a possible provenance. From that
survey, the authors recovered three zircon grains that show tell-tale signs of
shock-induced formation ({112}
twins), with age dates of ca. 3 Ga that are consistent with the bedrock ages of
the impact site. This work reveals the possibility for determining impact
locations (and apparently maximum ages for impact) by examining the detrital
zircon record.
A Non-Pyrolitic, Fe-rich
Lower Mantle?
On
page 824 of this issue, Kaminsky and Lin compare
and find discrepancies between ferropericlase (fPer) and nominal bridgmanite (Bridg)
(transformed to enstatite, so not true Bridg) compositions that occur as
inclusions in diamonds, and as obtained from experiments. Some experiments are
performed on non-Al bearing bulk compositions, and this work implicitly shows
why experiments performed on natural compositions are so crucial. In any case,
the authors find that some natural fPer have both high Fe and low Ni, from
which the authors infer equilibration with a Fe-Ni alloy, a phase that
presumably only inhabits the deepest mantle. They also find that certain nominal
Bridg grains exhibit high Al, which is also inferred to be a lowermost mantle
signature, on the expectation that Al-in-Bridg increases with increased P. The
highest Al-in-Bridg crystals do not coexist with the lowest Ni-in-fPer grains,
so the precise interpretation is unclear, but these compositions may still
indicate important heterogeneity in the lower mantle. And while pyrolite is a
highly flexible term (intended only to indicate rough proportions of basalt and
peridotite), this study may still lead to the identification of distinct mantle
domains—which if real, should connect to ocean island basalt compositions,
or at least those exhibiting high mantle potential temperatures.
Wavellite as a Near-Surface
Sink for Fluorine
On
page 909 of this issue, Kampf et al.
describe the new mineral, fluorwavellite, and its solid solution behavior
relative to the non-fluorinated wavellite. In surveying wavellite from various
localities, the authors find complete solid solution between the two
end-members, but no connection between F content and mode of occurrence (e.g.,
F contents are not different for minerals obtained from pegmatites, ore deposits,
or hydrothermally altered systems). The authors thus infer that F contents are
controlled by the activities of Al, P, and F, and perhaps also by pH, since
wavellite-fluorwavellite are highly soluble only under the uncommon conditions
of pH>9. At lower pH, the authors suggest that wavellite-fluorwavellite is
an important near-surface sink for F.
Editors Selections, March 2017
Special Collection: Olivine
Ni-in-Ol at Hawaii is Unrelated to Mantle Source
On page 507 of this issue Lynn et all examine NiO contents of high forsterite (Fo88) olivine crystals from various Hawaiian lavas (from Koolau, Kilauea, Loihi, Mauna Kea, and Mauna Loa). They find that over a 2.5X variation at individual volcanoes, Ni-in-Ol contents have no relationship to either host whole rock compositions, and that inter-island whole rock Ni contents strongly overlap and are not correlated to Ni contents in Ol. These authors show that variations in Ni-in-Ol at high Fo can be explained by very minor variations in the Ni contents of parental magmas (0.09-0.11 wt% NiO), and that observed ranges in Ni-in-Ol can be explained by a combination of fractional crystallization and magma mixing, and that to the extent that Ol may record mantle mineralogy, large, oriented crystals are needed to correct for intra-crystalline diffusion.
Special Collection: Apatite: A common mineral, uncommonly versatile
A Potential Oxybarometer Based on S in Apatite
On page 548 of this issue, Konecke et al. examine the oxidation states of S in natural apatite and experimentally equilibrated apatite + liquid pairs. Their work reveals perhaps the first report of a mineral that can incorporate three oxidation states of S (S2-, S4+, and S6+), which apply to experimental systems at oxygen fugacities ranging from FMQ to FMQ+3. These results show the potential for calibrating S6+/S4+ and or S4+/S2- ratios as oxybarometers for apatite saturated systems, although such a function is not calibrated, it probably can be from the data presented. The authors also note that the partitioning of various oxidation states of S may, in part, explain the non-Henrian behavior S portioning in apatite.
Outlooks in Earth and Planetary Minerals
Minerals Help to Define the Anthropocene
Most Am Min readers will know that some 400 news organizations have already covered the work of Hazen et al. (2017), which appears on page 595 of this issue. Hazen et al. show that 200 new minerals that occur exclusively or primarily because of human activities, may leave a decipherable stratigraphic signature, visible to future mineralogists. Some durable crystalline materials might not be classified as minerals today, as they are intentional products of engineering or social activities (Portland cement, semiconductors, various abrasives, etc.) rather than products of nature. Other substances at the boundary of natural and human activities occur because of human influences on near-surface conditions. Future mineralogists may be unable to decipher whether such substances were intentionally manufactured. Nevertheless, they are so ubiquitously distributed across Earth's surface that they will leave a robust mineral-like signature of what may be termed the Anthropocene.
Articles
Magnesioferrite Delimits Diamond P-T conditions
On page 632 of this issue Uenver-Thiele et al. present new experimental studies of the stability field and breakdown reactions of magnesioferrite (MgFe2O4). They find that above 8 GPa and 1000 degrees C it breaks down into Fe2O3 and MgO, and at yet higher temperatures, the reaction involves an unquenchable phase with an approximate stoichiometry of Mg5Fe2O8 or Mg4Fe2O7. Their results may shed light on magnesioferrite inclusions in diamond (often included in (Mg,Fe)O). Either such inclusions formed within the upper mantle, or if a lower mantle genesis is inferred, say from other phases, then the magnesioferrite must have formed from some other precursor phase. LettersMelanophlogite: A New Target For Investigating Life on Mars
On page 686 of this issue, Lazzeri et al. study the isotopic ratios of N and C in the silica clathrate mineral, melanophlogite (from Italy and California). This mineral, which apparently likes Mediterranean climates, also appears to occur in organic-rich settings and may thus preserve records of the biogeochemical cycling of N and C at or near Earth's surface. In their samples, the authors find N isotope ratios influenced by isotopic exchange with organic matter, and so suggest that the N was previously biologically processed. They further suggest that, if melanophlogite or similar microporous phases were to be found on Mars, their N isotopic compositions could be valuable as tests for modern or ancient biological processes.
Alien Alloys
On page 690 of this issue, Ma et al. report three new minerals from the Khatyrka CV3 carbonaceous chondrite. These minerals are alloys in the system Al-Fe-Cu: hollisterite, kryachkoite, and stolperite. This meteorite continues to reveal new surprises, being the host of icosahedrite and decagonite, the first two quasicrystalline minerals.
Editors Selections, February, 2017
Highlights & Breakthroughs
A Rover With a View (to the Early Martian Surface)
On page 233 of this issue Joshua Bandfield provides a perspective on the new mineralogical study by Ruff and Hamilton et al. p. 235 of this volume) on the pre-3.7 Ga Columbian Hills. Their study compares the mineralogy of two ancient surfaces, one more greatly affected by hydrous weathering than the other. They find that Martian weathering, at least in this one region, involved little cation removal, and so little in the way of quarts or phyollosilicates, or the production of amorphous Si phases such as opal. As Bandifeld notes, the attention drawn to Martian phyllosilicates and various hydrous amorphous Si phases is out of proportion to their scarcity on the Martian surface. Instead, the style of weather implied by the mineralogic study of Ruff and Hamilton, is one quite foreign to Earth where even in its coldest, driest parts, phyllosilicates and amorphous Si phases are common. On Mars, then, despite the evidence for aqueous erosion, the dominant weathering theme may be one of acid fog, however apparently contradictory these erosion and weathering themes may be.
Special Collection: Geology and Geobiology of Lassen National Park
Mantle or Crust Explains high d18O?
On page 252 of this issue, Underwood and Clynne present new d18O data on Ol, CPx, and Plag grains derived from mafic lavas of the Lassen Volcanic Center of the southern Cascades. Their investigation plays into the larger question of whether elevated d18O in whole rocks indicates contamination of such by a crustal source, or partial melting of a heterogeneous mantle. They find that d18O is uncorrelated to radiogenic isotopic ratios (Sr, Nd, and Hf) as well as trace element proxies for subducted fluid inputs (Sr/P), but isotopic ratios allow that the range of observed d18O may be mantle derived. Clearly the next step is to compare d18O in olivine to forsterite contents, which should provide a near-decisive test of whether high d18O ratios reflect high-T mantle or low-T crustal processes.
Special Collection: Dynamics of Magmatic Processes
Phenocrysts and Groundmass Age Dates Delimit Magma Storage Times
On page 262 of this issue Casalini et al. present new isotopic and additional geochemical data on the Ischia Volcano of southern Italy. Most fascinating is their Figure 10, where they find different 87Sr/86Sr ratios for groundmass and phenocryst phases, which based on analyzed Rb/Sr ratios permit to calculate crystallization intervals, which is obtained when groundmass and phenocrysts are nominally in isotopic equilibrium. The mineral residence times are then inferred from K-Ar-derived eruption ages. Their method yields time scales of magma storage and crystallization that range from tens to hundreds of thousands of years, with the most evolved magmas being stored in isolated pockets at cool temperatures, of ca. 750 C. These time scale estimates at Ischia provide the first radiogenic isotope probe of the novel approach of Cashman and Giordano (2014).
Special Collection: Water in Nominally Anhydrous Minerals
How OH- is incorporated into Olivine
On page 302 of this issue, Blanchard et al. re-examine the nature of water solubility in olivine (as OH-). Through a range of spectroscopic and computational methods the authors show that OH- solubility is controlled less by total Fe contents than by the total amounts of tri- and tetra-valent cations. Their work indicates that prior studies of OH- solubility can be re-interpreted so as to indicate that defect sites populated by OH- are associated with Ti4+ and Fe3+ in particular, and that H is dissolved mostly as a hydrogarnet species, where 4H occupy a tetrahedral vacancy and only more rarely as 2H occupying an octahedral vacancy.
Articles
CaSiO3-Perovskite: The Engine that Powers Mantle Convection
On page 321 of this issue Perry et al. use ab initio calculations to investigate the solubility of Th, and U into CaSiO3-perovskite with and without Al. They find that enthalpies of solution favor the dissolution of Th4+, and especially U4+, as coupled substitutions with Al3+. Their work suggests that subducting slabs may provide a key mechanism for transporting heat producing elements into the deep mantle. Subducting slabs host Ca-Al rich minerals that promote the storage and transport of U and Th leading to the formation of Al-bearing CaSiO3-perovskite as slabs penetrate the lower mantle. The authors also suggest that their findings may help the recipes for synroc compositions so as to optimize their chemistry for U and Th dissolution into perovskite-like structures.
Near-surface kerogen in deep-seated diamonds?
On page 391 of this issue, Childress and Jacobsen investigate the stability of kerogen at simultaneously high P and moderate T conditions. They find that increases in pressure allow an increase in the thermal stability of kerogen such that the coolest portions of subducted slabs can provide a pathway for carbon into the mantle. Their work provides a possible mechanism to explain recent findings of apparently organic carbon in some deep-seated diamonds.
Igneous or metamorphic hornblende?
On page 436 of this issue, Challener and Glazner, in a stunningly colorful contribution (pictorially, not in prose), examine hornblende compositions from the Half Dome Granodiorite of Yosemite National Park, California. Some of the crystals they examine are quite large, ranging to 2 cm in length, and are euhedral and un-altered in appearance. And yet, these authors suggest that these crystals record mostly a metamorphic history, rather than igneous one. In their interpretation, igneous crystals of the same size and shape were metamorphosed to greenschist facies conditions. Their evidence includes the bulk Hbl compositions, which are equivalent to hornblende plus a small proportion of biotite. They suggest that the original, igneous Hbl reacted with biotite inclusions, and perhaps quartz (rare as an inclusion, presumably due to reaction), to produce magnetite, alkali feldspars and clinozoisite, which are common as inclusions. Their interpretation is supported by exsoluiton textures indicative of low temperature equilibration.
P-T Conditions at the White Sands Nuclear Detonation Site
On page 445 of this issue, Lussier et al. examine the glassy arkosic sandstone called Trinitite, a synthetic product of 16 July 1945 nuclear test at the White Sands Proving Grounds in New Mexico. As noted by the authors, Trinitite has mostly been studied as a curiosity, but renewed interest involves its use as a test case for future forensic studies that may be needed to uncover bomb design, fuel material, etc., and trace these to a user of such. They find that quartz and zircon are both reliable recorders of blast P-T conditions and that nanoscale analyses of zircon grain (featuring remarkably dendritic rims — well worth a look at their Fig. 1) record the changing P-T-t conditions as a blast evolves. In this particular case, the authors infer T of >1500 C and P <10 GPa, the latter being quite below that of a meteorite impact.
Editors Selections, January, 2017
Highlights & Breakthroughs
Periodic Arcs
On page 1 of this issue, P.G. DeCelles reviews the new work of Kirsch et al., published on page 2133 in Am Min in 2016. DeCelles notes that while the rate at which magmas are produced in arcs is stubbornly uncorrelated with orthogonal convergence rates, Kirsch et al. provide an opening for a detectable tectonic control on magmatic addition rates at arcs. This results, in part, through the observations by Kirsch et al. of synchronicity of some (but not all) magmatic flare-ups throughout the Cordillera. As DeCelles notes, though, the mappable and datable effects of plate convergence are subject to many more factors than plate convergence rate. Implied is that convergence rates may provide an ultimate cause of magmatism and upper plate deformation, but are separated from intervening, and highly localized controls and conditions such that causative forces are well hiddenand may remain so absent detailed and comprehensive field and petrologic studies.
Invited Centennial Articles
Predicting Mining Accidents
On page 3 of this issue, Ulrich Bismayer provides an overview of a paper that we highlighted from last months issue: Jiang et al.s acoustic emission experiments on sandstone and coal lithologies. The larger sample size in these experiments allowed the investigators to detect a temporal transition. Early acoustic emissions are randomly scattered about the experimental volume and appear to be random with respect to both time and space. But later acoustic events cluster along what will prove to be collapse planes. And as noted earlier, the energy of these two event systems follow a power law, with a distinct exponent for the random and spatially correlated cases. These provide a means to predict failure events.
Invited Centennial Articles
Why Arc Lavas Contain High LILE
On page 5 of this issue, Hans Keppler examines melt inclusion compositions from primitive arc lavas. His review reveals that fluid mobile elements in arc lavas are, perhaps unexpectedly, controlled by fluids. These elements, which include the large ion lithophile elements (LILE), light rare earth elements (LREE), and U, are correlated with Cl when both Cl and the element of interest are normalized to H2O. This correlation with Cl indicates that the classic enrichment of arc lavas in LILE, LREE, and U are not controlled by subducted sediment inputs or partial melting of subducted crust, since Cl does not affect mineral/melt partitioning. Keppler concludes that (Cl-bearing) fluids are the primary carriers of LILE, LREE, and U into arc magmatic systems, and that Ce/H2O ratios are a proxy for fluid inputs, not slab T.
Articles
Carbon in the Lunar Core
On page 92 of this issue, Steenstra et al. suggest that Carbon is the major light-alloying element in the lunar core. Like Earths core, there is a recognition that the Moons metallic portion has sufficiently low density to require an admixture of elements other than Fe and Ni. And like Earth, sulfur has been a leading candidate of a light-alloying element as it readily dissolves into metallic liquids. These authors use existing estimates of the bulk silicate Moon, and recent experimental work that describes metal/silicate partition, to show that C may compete successfully with S during lunar core formation. These authors contend that C is sufficiently siderophile to allow up to 4.8 wt% C in the lunar core. This work further indicates a close similarity between the bulk silicate portions of Earth and its Moon, and a lack of devolatilization during the Moons formation.
The Ecology of Cobalt
On page 108 of this issue Hazen et al. examine the temporal and spatial distributions of Co-bearing minerals (66 distinct species; >3000 species-locality pairs). Their work indicates that Co-bearing minerals follow a Large Number of Rare Events (LNRE) distribution, which is plotted as a ranking of minerals according to the numbers of localities at which they are found. At the top of such a list (if applied to all minerals), would be quartz, which is found at 45,000 localities; 22% of all minerals are found only at one locality. Hazen et al have previously shown that an LNRE distribution describes minerals as a whole, and minerals characterized by elements that are concentrated (e.g., C), rather than dispersed. Here, by examining Co, Hazen et al. now show that the LNRE distribution also applies to elements, even if they occur not just in concentrated form, but are also dispersed in wide ranging solid solutions. These authors thus show that LNRE distributions can be used to predict how many minerals are yet to be discovered; the editors of Am Min anxiously await the new mineral descriptions that test this hypothesis.
Apatite Mediates Body Fluid Compositions
On page 149 of this issue, Michael Fleet, as a perfect follow-up to Jills Pasteris review in last months issue, investigates the nature of Na and carbonate substitutions in hydroxylapatite; the author finds that these species substitute in a significant way within the hdyroxylapatite c-axis structural channel. A key implication of this finding is that the hydroxylapatite c-axis structural channel may be the key means by which body fluids interact with nanocrystalline bone materials, and so mediate acid-base reactions in biologic systems.
The State of Carbonates in the Deep Mantle
On page 210 of this issue, Solomatova and Asimow calculate crystal structures and relative enthalpies of high-pressure forms of dolomite. They find that a monoclinic dolomite phase has a lower energy compared to other candidate structures, at pressures ranging from 15 to 80 GPa. Their work does not delimit the conditions on which such a carbonate might decompose to other phases, but identifies a potentially important phase for understanding the global C cycle. Their study clearly points to the need for new experiments exploring the structural and phase equilibrium stabilities of comparable Fe- and Mn-bearing carbonate phases.
Ti in the Lower Mantle
On page 227 of this issue, Bindi et al. report a new Ti-bearing bridgmanite-type structure synthesized at transition zone P-T conditions (20 GPa, 1600 C). Their study indicates that Ti may stabilize bridgmanite-like structures at lower pressures and provide clues as to how Ti and other elements are housed within the lower mantle. As these authors note, natural Ti contents are too low to stabilize this new phase in the lower mantle, but it might be stable in certain localized Ti-rich environments. Although not highlighted by the authors, a yet more important implication is that such a component within bridgmanite may be important for explaining high Ti contents in many ocean island volcanic rocks thought to form as lower mantle thermal plumes. This new phase might either provide the source, or control the mineral melt partitioning of Ti and so may be central to understanding what appear to be lower mantle Ti enrichments.
Editors Selections,
December 2016
Invited Centennial Articles
Predicting Trace Element
Partitioning Behavior
On page 2577 of this issue, Sun-ichiro Karato provides a review of the physical basis
for contrasts in partition coefficients. This review attempts to explain a
suite of experimental observations, which include the now-familiar Onuma
diagrams, so well developed by Blundy and Wood and others, which show how
partition coefficients vary with an elements size and charge, but the physical
reasons for the dependence of element partitioning on the size of element have
been unclear particularly for noble gas elements. Karato developed new models
of element partitioning using the models of point defects in minerals and the
hard sphere model of silicate melts. Karatos model provides a physical explanation,
for example, as to why some phases partition noble gases in proportion to their
ionic radii (bridgemanite), while other silicate phases (Ol, Cpx) do not.
Karato finds that his models may help better describe and predict partitioning
behavior. But Karato also concludes that no physical model can yet
satisfactorily predict P-T
dependencies of trace element partitioning, and hence that there is still no
substitute for a thermodynamic description of partitioning behavior.
Mineralogists at the Forefront of Human Health
On page 2594 of this issue,
Jill Pasteris demonstrates why the demarcation between biogenic, synthetic, and
inorganic phases is not a simple one, and may erect unfruitful barriers at
least in some sub-disciplines of medicine, mineralogy, and materials science.
Here, Pasteris examines apatitic biomaterials, which are defined as the
synthetic forms of hydroxylapatite (usually nanocrystalline, in some cases
inter-bonded with organic molecules) that are used to replace natural bone and
tooth materials. Her review illustrates the various ways in which biomaterials
are structured and synthesized, with some fascinating insights into how subtle
variations in synthesis conditions can tailor the required properties of a
mineral to a given biologic function and determine how successfully such
materials will operate when implanted in a human system. The take-home message
of this review is that mineralogists have much to offer to such research, and
argues that biomaterials should be pursued as a major sub-discipline of mineralogical
research. As editors, we happily await the continued publication of papers in
this field.
Articles
Immobilizing Radionuclides With Apatite
On page
2611 of this issue, Rigali et al. review the various ways in which apatite
can be used to isolate a wide range of radionuclides from the near-surface
environment. These means of radionuclide neutralization include the familiar
modes of surface adsorption and partitioning of species into apatite
structures. Rigali et al. also review what may be less familiar mechanisms,
such as dissolution/(re-)precipitation reactions that are now being used to remediate
contaminated groundwater or act as semi-permeable membranes. For example, some
recent studies have shown that apatite can dissolve in the presence of
U-bearing fluids to re-precipitate as U-phosphate or U-carbonate, and that the
addition of hydroxyapatite to contaminated soils may reduce U concentrations in
pore waters to levels deemed safe for drinking.
A Depth Continuum of Water Release During Subduction
On page
2645 of this issue Gemmi et al. employ cutting edge analytical techniques
to determine the structures of two important candidates for carriers of water
into the deep mantle: the 11.5 angstrom phase, Mg6Al(OH)7(SiO4)2,
and the HySo phase, Mg3Al(OH)3(Si2O7).
These phases can form by the breakdown of chlinochlore and so may carry water
to depths beyond clinochlore and chlorite breakdown. These phases lack the
H-bonded, infinite tetrahedral sheets structure of precursor silicates. The
authors find structures with reduced Si-O-Si interconnections and much higher
density. Thus, these high-density phases, which can contain between 8-13 wt% H2O,
are expected to be stable to much greater depths.
Evidence for Mantle Global Warming?
On page
2768 of this issue, Ganne et al. present an analysis of global magmatic
temperatures from published data that span the temporal range of 600 Ma to
present. Their most dramatic finding is that magmatic temperatures, as measured
from whole rock and mineral compositions, record a maximum that falls between
325-125 Ma; these ages are the bookends of the lifespan of Pangea. This time
period also coincides with a peak in mantle potential temperature. The authors
suggest that these findings support numerical models (e.g., Coltice et al. 2009;
Van Avendonk et al. 2016) whereby supercontinent formation results in both
thermal insulation, and a disruption of mantle convection, such that increased
temperatures temporarily influence supercontinent volcanism. Coltice et al.
predict that supercontinent-induced heating should be <100 C; Ganne et al.
identify some key targets for high precision thermometry, as a test of the
Coltice et al. model.
Predicting Mining Accidents, Building Collapse, Etc.
On page
2751 of this issue Jiang et al. provide an update of prior work that
indicated that acoustic emissions presage mine collapse. In this new work, the
authors present experimental results that confirm that acoustic emissions
increase just prior to the collapse of cavities in sandstone and coal. The
energy released by such acoustic emissions can be described by a power law,
with slightly different exponents for different materials, but the exponents
also change with time. A key result then is that collapse of a mine shaft, or
bridge or building, may be presaged by both acoustic emissions and their
energies. Another fascinating result is that cavity collapse yields a power law
with an exponent greater than that associated with crack propagation, the
latter being associated with micro-faults and earthquakes.
A New Hygrometer and Shallow Magma Accelerations at Etna
On page
2774 of this issue, in a Letter, Perinelli et al. re-calibrate their
clinopyroxene-based hygrometer. The original, and new model, are applicable to
trachyte or hawaiiite-type basalts. But while magmatically restrictive, the
model predicts water contents without precise knowledge of liquid composition,
relying on pyroxene components and the P-T
conditions of crystallization. They find that at Mt Etna, magmas begin to
dehydrate mostly at a <400 MPa and lose most of their water at pressure of
<100 MPa. This result corroborates inferences form melt inclusions, and it indicates
that eruption triggering, and magma transport acceleration due to dehydration,
are mostly relatively shallow processes, at least in the Etnean plumbing system.