PHASE-DIAGRAM METHODS FOR GRAPHITIC ROCKS AND APPLICATION TO THE SYSTEM C-O-H-FEO-TIO2SIO2

Carbon-saturated C-O-H (GCOH) fluids have only one compositional degre e of freedom. This degree of freedom is specified by the variable X(O) that expresses the atomic fraction of oxygen relative to oxygen + hyd rogen. The only valid constraint on the maximum in the activity of GCO H fluid species is related to the bulk composition of the fluid, as ca n be expressed by X(O). In fluid-saturated graphitic rocks, mineral de volatilization reactions are the dominant factor is determining the re dox state of the metamorphic environment. X(O) is directly proportiona l to the f(O2) of GCOH fluid, and because its value can only be affect ed by fluid-rock interaction, it is an ideal measure of the redox char acter and composition of GCOH fluid. Phase diagrams as a function of X (O) are analogous to the P-T-X(CO2) diagrams used for binary H2O-CO2 f luids; this analogy can be made rigorously if the C-O-H fluid composit ion is projected through carbon into the O-H subcomposition. After pro jection, the fluid is described as a binary fluid with the components O and H, and the compositional variable X(O). Description of GCOH flui ds in this manner facilitates construction of phase diagram projection s that define the P-T stability of mineral assemblages for all possibl e fluid compositions as well as fluid-absent conditions. In comparison to phase diagrams with variables based on the properties of fluid spe cies, P-T-X(O) diagrams more clearly constraint accessible fluid compo sitions and fluid evolution paths. Calculated P-T-X(O) projections are presented for the C-O-H-FeO-TiO2-SiO2 system, a limiting model for th e stability of Fe-Ti oxides in graphitic metapelites and phase relatio ns in metamorphosed iron-formations. With regard to the latter, the st ability of the assemblage qtz + mag + gph has been a source of controv ersy. Both the calculated C-O-H-FeO-TiO2-SiO2 system petrogenetic grid and natural examples, suggest that this assemblage has a large P-T st ability field. Discrepancies between earlier C-O-H-FeO-SiO2 system pha se diagram topologies are reconciled by the qtz + mag + gph = sid + fa phase field, a barometric indicator for metamorphosed-iron formations . A more general implication of calculated P-T-X(O) phase relation is that few inorganic mineral-fluid equilibria appear to be capable of ge nerating hydrogen-rich, X(O) <1/3, GCOH fluids at crustal metamorphic conditions. The utility of P-T-X(O) diagrams derives from the use of a true compositional variable to describe fluid composition, this appro ach can be extended to the treatment of carbon-undersaturated sytems, and provides a simple means of understanding metasomatic processes of graphite precipitation.