(revised 09/04/2003)

Comments and Questions

Dr. Dipak C. Pal (Lecturer, Department of Geological Sciences, Jadavpur University, Kolkata 700 032, India) has submitted two questions that stem from his studies of pegmatites in the Bastar-Malkangiri Tin Belt, central India. Both questions pertain to the role and evidence for a dominantly aqueous vapor phase in the internal evolution of pegmatites. For the other question see http://www.minsocam.org/MSA/Special/Pig/PIG_CQ/PIG_CQ_pal_open.html. This is the web page for his first question. These questions, especially #1, address the nature of the pegmatite-forming process. Dr. Pal has requested responses that will clarify the issues he has brought up. Please feel free to comment on either or both questions.

Question #1: Vapour saturation in pegmatite - when does it take place and what is the evidence?

Although the importance of a separate (dominantly aqueous) vapor phase in forming pegmatitic textures has been emphasized by Jahns and Burnham (1969), London et al. (1989) maintained that the presence of a separate vapor is not as important as it was previously thought in forming pegmatitic texture. As a matter of fact, many workers now believe that the presence of separate vapour phase may be casual and not causal in the formation of pegmatitic texture. Considering the extreme solubility of water in pegmatitic melt enriched in F, B, etc., even at very low pressure (mentioned in London, 1996, Thomas et al, 2000, 2003), is it possible for a pegmatitic melt to remain water-undersaturated throughout its entire history of crystallization? If not, when and at what stage can such water saturation really take place? As I understand, the answer may not be straightforward, as it will possibly depend on the initial melt composition, its depth of emplacement, and the percentage of melt crystallized. Then, what is the compelling evidence in natural pegmatites that can unequivocally prove that the pegmatitic melt became water-saturated?

Rainer Thomas replies:

The first part of the question was answered by David London (see http://www.geocities.com/oklahomamgs/London/Pegmatite2.html:

As the pegmatite-forming melt rises toward the earth's surface, the pressure imposed on the magma by its surrounding rocks decreases. As the pressure drops, the amount of H2O and CO2 that can dissolve in the melt decreases until the point where the melt can no longer contain all the dissolved gases, and bubbles exsolve from the melt. This process, which geologists term "first boiling", is analogous to the formation of bubbles in carbonated beverages when the top is popped, and pressure is released.

As the pegmatite forming melt crystallizes, the minerals that form - mostly quartz and feldspars - do not contain H2O or CO2. As their mineral-forming constituents are removed from the melt, the concentration of excluded components, e.g., gases, increases again to a point where the melt can no longer hold all of its dissolved gas, and bubbles form. Geologists refer to this as "second boiling".

The second part of the question can be answered, very briefly, with the characteristic appearance of melt and fluid inclusions formed by immiscibility. Evidence of such immiscibility is the occurrence of volatile-rich melt inclusions, fluid inclusions and a mixture of the two in a single growth zone. Because of their large density contrast and the different wetting behavior it is not necessary that, for example, the two endmember phases appear in the same proportions. Therefore it is often quite difficult to recognize coexisting inclusion couples as such.

Recently, I have observed in very different pegmatites silicate melt inclusions, which show super-critical behavior at heating. The occurrence of "super-critical" silicate-rich melt or fluid inclusions is also an indication of the saturation of the pegmatite-forming melt with water. By a small pressure drop, the system can boil and we then observe a "quasi-coexistence" of melt, fluid and vapor trapped as different inclusions in a single growth zone. Often the relationship of the different inclusion types cannot be recognized clearly. This applies particularly to very large pegmatites. Here, the large contrasts in viscosity and density of the different fluid/melt phases also resulted in a spatial separation of these phases, and therefore the relationships among them and the growing crystals become very complex.

References

Sowerby, J.R., Keppler H. (2002) The effect of fluorine, boron and excess sodium on the critical curve in the albite-H2O system. Contrib Mineral Petrol 143: 32-37

Thomas, R., Webster, J.D., and Heinrich, W. (2000) Melt inclusions in pegmatite quartz: complete miscibility between silicate melts and hydrous fluids at low pressure. Contributions to Mineralogy and Petrology 139, 394­401

Thomas, R., Förster, H.-J. and Heinrich W. (2003) The behaviour of boron in a peraluminous granite-pegmatite system and associated hydrothermal solutions: a melt and fluid inclusion study. Contrib. Mineral. Petrol. 144: 457-472

Thomas, R. and Veksler, I. (2002) Formation of granite pegmatites in the light of melt and fluid inclusion studies and new and old experimental work. Mineralogical Society of Poland, Special Papers, Vol. 20, 50th Anniversary of the Faculty of Geology of the Warsaw University, Mineralogical Sciences, 44-48

Veksler, I. V. and Thomas, R. (2002) An experimental study of B-, P- and F-rich synthetic granite pegmatite at 0.1 and 0.2 GPa. Contrib. Mineral. Petrol. 143: 673-683

Veksler, I.V., Thomas, R., and Schmidt, C. (2002) Experimental evidence of three coexisting immiscible fluids in synthetic granite pegmatite. American Mineralogist, Vol 87, 775-779

You can submit questions and comments in electronic formats only to

Dr. David London at dlondon@ou.edu