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

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.%).

Fluorine and chlorine behaviour during progressive dehydration melting: Consequences for granite geochemistry and metallogeny

“…() that avoidance between F and Al affects the local distribution of cations in the octahedral sheet, but does not determine the F content of the biotite. Given the reports that Ti solubility in biotite increases with temperature and stabilizes biotite to UHT (Dooley & Patino Douce, ; Forbes & Flower, ; Guidotti, ; Tareen et al., ; Tronnes et al., ), we expected to see more Ti in biotite that had been subjected to higher grade metamorphism. There also appears to be no systematic relationship between Ti and Mg (Figure d).…”

“…Muscovite also plays an important role in melt production in the mid crust, but becomes unstable as temperatures and pressures increase towards lower crustal conditions. Biotite breaks down over a broad temperature interval, controlled by partitioning of solid solution elements between residual biotite, peritectic minerals and melt; notably, Ti enrichment is thought to stabilize biotite to higher melting temperatures (Dooley & Patino Douce, ; Forbes & Flower, ; Guidotti, ; Tareen, Prasad, Basavalingu, & Ganesha, ; Tronnes, Edgar, & Arima, ). It is also thought that substitution of F into the OH site increases its stability to higher temperatures (e.g.…”

On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings

“…The main reason for the complexity of choosing a relevant composition is because of open system behaviour, particularly drainage of melt from suprasolidus crust, which is a major process of granulite facies metamorphism (e.g. von Platen, 1965;Mehnert, 1968;Brown and Fyfe, 1970;Fyfe, 1973a,b;McCarthy, 1976;White and Chappell, 1977;O'Hara et al, 1978;Thompson, 1982;Powell, 1983;Ellis and Thompson, 1986;Clemens and Vielzeuf, 1987;Le Breton and Thompson, 1988;Vielzeuf and Holloway, 1988;Waters, 1988;Peterson and Newton, 1989;Clemens, 1990;Powell and Downes, 1990;Patiño Douce and Johnston, 1991;Peterson et al, 1991;Stevens and Clemens, 1993;Brown, 1994Brown, , 2004Brown, , 2008aSawyer, 1994Sawyer, , 2001Sawyer, , 2014Vielzeuf and Montel, 1994;Gardien et al, 1995;Beard, 1995, 1996;Stevens et al, 1997;Tareen et al, 1998;Gardien et al, 2000;Tareen and Ganesha, 2001;Nair and Chacko, 2002;Powell, 2002, 2010;Guernina and Sawyer, 2003;Johnson and Brown, 2005;Johnson et al, 2010Sawyer et al, ...…”

Halogen-dominant mineralization at Mt. Calvario dome (Mt. Etna) as a response of volatile flushing into the magma plumbing system

“…In this regard, understanding of the differences in the physical and/or chemical conditions leading to crystallization of these two phases could be a relevant issue to evaluate the relative role of P-T-X on hydrothermal fluids in developing diverse assemblages. Phase equilibria of phlogopite in the presence of fluorine have been widely investigated (e.g., Shell and Ivey 1969;Munoz and Ludington 1974;Foley et al 1986;Vukadinovic and Edgar 1993;Foley 1996, 2000;Tareen et al 1995Tareen et al , 1998Veksler et al 1998;Pichavant 1999, 2000). Foley et al (1986), in particular, verified experimentally that the thermal stability of fluorine-rich phlogopite in the system KAlSiO 4 -Mg 2 SiO 4 -SiO 2 -F 2 O -1 is enhanced up to ~300 °C at high pressure by substitution of fluorine for OH groups.…”