Tareen, J. A. K. and Prasad, A. V. K. and Basavalingu, B. and Ganesha, A. V. (1998) Stability of F-Ti-phlogopite in the system phlogopite-sillimanite-quartz: an experimental study of dehydration melting in H2O-saturated and undersaturated conditions. Mineralogical Magazine, 62 (3). pp. 373-380.
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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.pdf Restricted to Registered users only Download (571kB) | Request a copy |
Abstract
Melt generation during pranulite-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 degrees 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 degrees C at 7 kbar and 990 degrees C at 12 kbar, and it disappeared at about 1080 degrees C with the appearance of 221 sapphirine. In fluid-absent runs, these boundaries marginally shift to higher temperatures (30-50 degrees 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 about 100-140 degrees C above those reported for reaction Phl + Qtz —> En + Sa + L (Vielzeuf and Clemens, 1992) with F and Ti-free phlogopite, but are approximate to 50-100 degrees 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 approximate to 27 wt.% Al2O3 in the cordierite field and approximate to 23% Al2O3 in the sapphirine held. The H2O-undersaturated runs produced melts rich in K2O (approximate to 10 wt.%), SiO2 (72.5 wt.%) and relatively poor Al2O3 (12 wt %).
| Item Type: | Article |
|---|---|
| Subjects: | F Earth Science > Geology |
| Divisions: | Department of > Earth Science |
| Depositing User: | Users 23 not found. |
| Date Deposited: | 07 Jun 2021 04:49 |
| Last Modified: | 30 May 2022 10:27 |
| URI: | http://eprints.uni-mysore.ac.in/id/eprint/16713 |
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