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Stability of F-Ti-phlogopite in the system phlogopite–sillimanite–quartz: an experimental study of dehydration melting in H2O-saturated and undersaturated conditions

Published online by Cambridge University Press:  05 July 2018

J. A. K. Tareen ,
A. V. Keshava Prasad ,
B. Basavalingu  and
A. V. Ganesha
J. A. K. Tareen
Affiliation:
Department of Geology, University of Mysore, Manasagangotri, Mysore - 570 006, India
A. V. Keshava Prasad
Affiliation:
Department of Geology, University of Mysore, Manasagangotri, Mysore - 570 006, India
B. Basavalingu
Affiliation:
Department of Geology, University of Mysore, Manasagangotri, Mysore - 570 006, India
A. V. Ganesha
Affiliation:
Department of Geology, University of Mysore, Manasagangotri, Mysore - 570 006, India
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Abstract

Melt generation during granulite-grade metamorphism is believed to be controlled by the stability temperatures of biotite, whose breakdown provides H2O and controls fluid-absent melting in the lower crust. In a simple KMASH system, the restite minerals crystallising due to incongruent melting of phlogopite depend upon the bulk composition. In an alumina-poor and silica-rich portion of the system (Phl + Qtz), enstatite appears with the melt, while in an alumina-rich system (Phl + Sil + Qtz) cordierite appears first instead of enstatite. Since the temperature of biotite stability is believed to be strongly controlled by its F and Ti content, it will have significant effect on the fluid-absent melting reactions during granulite-grade metamorphism of mica-containing granites as well as pelitic rocks in the deeper crust.

To understand such effects in an aluminous portion of the KMASH system, experiments were performed (between 850 and 1100°C and at 7, 10 and 12 kbar) with bulk composition containing 2Phl-6Sil- 9Qtz, where natural phlogopite with F/(F+OH) = 0.39 and Mg/(Mg+Fe) = 0.96 was used. In runs with this charge and containing 5 wt.% of excess water, cordierite appeared around 920°C at 7 kbar and 990°C at 12 kbar, and it disappeared at about 1080°C with the appearance of 221 sapphirine. In fluid-absent runs, these boundaries marginally shift to higher temperatures (30-50°C). The enstatite which was distinctly absent in H2O-saturated runs, crystallises in the high-temperature sapphirine field with up to 12 wt.% Al2O3 in H2O-undersaturated runs. The enstatite formation with cordierite is perhaps inhibited due to the Al consumption by cordierite and instability of Al-free enstatite at temperatures of cordierite stability. Re-equilibrated phlogopite persists in both the cordierite and sapphirine fields. The temperatures of the beginning of phlogopite breakdown are about 100-140°C above those reported for reaction Phl + Qtz → En + Sa + L (Vielzeuf and Clemens, 1992) with F and Ti-free phlogopite, but are ≈50–100°C lower than the temperatures reported (Tareen et al., 1995; Dooley and Patino Douce, 1996) for the same reaction containing F- and Ti- bearing phlogopite. The combined effect of the F and Ti content in phlogopite on its stability temperatures in the KMASH system has been found to be additive in relation to those containing only F or Ti. H2O-saturated runs produced per-aluminous melts with ≈27 wt.% Al2O3 in the cordierite field and ≈23% Al2O3 in the sapphirine field. The H2O-undersaturated runs produced melts rich in K2O (≈10 wt.%), SiO2 (72.5 wt.%) and relatively poor Al2O3 (12 wt.%).

Keywords

fluorinetitaniumsillimanitedehydration melting
Type
Research Article
Information
Mineralogical Magazine , Volume 62 , Issue 3 , June 1998 , pp. 373 - 380
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1998

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