Calculation of C-O-H-S equilibria at constant bulk composition: some petrologic implications.

Geologically reasonable amounts of fluids in the C-O-H-S system can control oxidation and sulphidation states of mineral assemblages in both closed and open systems. Examples of fluid buffering are given for graphitic metamorphic rocks, crystallizing volatile-rich felsic magmas, and skarns. The effect is likely to be most important for the last example where sequences of retrograde assemblages are compatible with the cooling of a single fluid. -K.F.