Phase relations and mineral chemistry involving the phases garnet (Gt), spinel (Sp), hypersthene (Hy), sapphirine (Sa), cordierite
(Cd), sillimanite (Sil) and quartz (Qz) have been experimentally determined in the system FMAS (FeO−MgO−Al
2O
2−SiO
2) under low fO
2 and for various H
2O/CO
2 conditions. Several compositions were studied with 100 (Mg/Mg+Fe) ratio ranging from 64 to 87 with excess quartz and sillimanite.
Our data do not show any differences in Gt−Cd stability and composition as a function of H
2O, CO
2 and H
2O−CO
2 (±CH
4) content, in good agreement with a previous experimental study at lower temperature (Aranovich and Podlesskii 1983). At 1,000°
C and 11 kbar, under CO
2-saturated conditions, cordierite grew from a crystalline mix unseeded with cordierite. Thus, under water-absent conditions,
cordierite will have a high-
P stability field in the presence of CO
2. If water has a pressure stabilizing effect on cordierite, then our results would indicate that the effects of H
2O and CO
2 are of the same magnitude at high temperature. Our data support the theoretical
P-T grid proposed by Hensen (1986) for high-
T metapelites and are largely consistent with the high-temperature experimental data of Hensen and Green (1973). The univariant
boundary Gt+Cd=Hy+Sil+Qz, which marks the disappearance of Hy−Sil−Qz assemblages, has a negative d
P/d
T slope above 1,000° C and a positive one below this temperature. Extrapolation of our data to iron-free systems shows that
the high-
P breakdown limit of Mg-cordierite has a negative slope in the range 1,025–1,300° C and probably positive below 1,000° C. This
indicates a maximum of stability for Mg-cordierite at around 1,000° C and 13 kbar. Because of the curvature of the univariant
reactions En+Sil=Py+Qz, Mg−Cd=En+Sil+Qz and Gt+Cd=Hy+Sil+Qz, the iron-free invariant point involving the phases Py, En, Cd,
Sil and Qz probably does not exist. Sapphirine—Qz-bearing assemblages are stable only at temperatures above 1,050° C. At 1,075°
C, the joint Gt−Sa is stable up to 11 kbar. At higher pressure, garnet, sapphirine and quartz react according to the reaction
Gt+Sa+Qz=Hy+Sil. Reequilibrated sapphirines are more aluminous than the theoretical endmember Mg
2Al
4SiO
10 due to AlAl=MgSi substitutions [100(Al
2O
3/Al
2O
3+FeO+MgO) in experimental sapphirines ranges from 50.5 to 52.2]. Sapphirine in the assemblage Sa−Cd−Sil−Qz shows a decrease
in Al content with decreasing temperature and pressure, such that the alumina isopleths for sapphirine have a slight negative
d
P/d
T slope. A similar decrease in Al content of sapphirine with temperature is also observed in Sa−Sil−Qz assemblages.
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