C‐O‐H‐S fluid composition and oxygen fugacity in graphitic metapelites

摘要: C-0-H fluid produced by the equilibration of HzO and excess graphite must maintain atomic H/O ratio water, 2: 1. This constraint implies that all thermodynamic properties are uniquely determined at isobaric-isothermal conditions. The O,, H20 CO, fugacities (J& fHto fC%) such fluids have been estimated from equations state fit as a function pressure temperature. These can be taken characteristic for graphitic metamorphic systems in which dominant source is dehydration, e.g. pelitic lithologies. Because there no compositional degrees freedom graphite-saturated entirely variance dehydration process not increased comparison with non-graphitic systems. Thus, 'buffering' equilibria, common petrological model, requires redox reactions, decarbonation reactions or external, H/O#2, sources perturb evolution system. Such perturbations likely to significant metapelitic environments, but their tendency will increase fo, phase. At high grades, pyrite desulphidation may cause substantial reduction fHz0 slight increases info, fcs relative sulphur-free fluid. low grade, sulphur solubility =s 2 so decomposition occur sulphurconserving iron depletion silicates, feature sulphidic pelites. With increasing temperature solubility, driven infiltration H,O-rich fluids. absence magnetite assemblages carbonate + aluminosilicate pyrrhotite ilmenite most metapelites consistent an = model GCOH(S) For rocks inapplicable, phase diagram variable defines more useful than conventional