TY - JOUR
AU - Connolly, J.
AB - Carbon-saturated C−O−H (GCOH) fluids have only one compositional degree of freedom. This degree of freedom is specified by the variable X
o that expresses the atomic fraction of oxygen relative to oxygen+hydrogen. The only valid constraint on the maximum in the activity of GCOH fluid species is related to the bulk composition of the fluid, as can be expressed by X
o. In fluid-saturated graphitic rocks, mineral devolatilization reactions are the dominant factor is determining the redox state of the metamorphic environment. X
o is directly proportional to the fo2 of GCOH fluid, and because its value can only be affected by fluid-rock interaction, it is an ideal measure of the redox character and composition of GCOH fluid. Phase diagrams as a function of X
o are analogous to the P-T-Xco2 diagrams used for binary H2O−CO2 fluids; this analogy can be made rigorously if the C−O−H fluid composition is projected through carbon into the O−H subcomposition. After projection, the fluid is described as a binary fluid with the components O and H, and the compositional variable X
o. Description of GCOH fluids in this manner facilitates construction of phase diagram projections that define the P-T stability of mineral assemblages for all possible fluid compositions as well as fluid-absent conditions. In comparison to phase diagrams with variables based on the properties of fluid species, P-T-X
o diagrams more clearly constraint accessible fluid compositions 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 the stability of Fe−Ti oxides in graphitic metapelites and phase relations in metamorphosed iron-formations. With regard to the latter, the stability of the assemblage qtz+mag+gph has been a source of controversy. Both the calculated C−O−H−FeO−TiO2−SiO2 system petrogenetic grid and natural examples, suggest that this assemblage has a large P-T stability field. Discrepancies between earlier C−O−H−FeO−SiO2 system phase 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 generating hydrogen-rich, fO2, 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 approach can be extended to the treatment of carbon-undersaturated systems, and provides a simple means of understanding metasomatic processes of graphite precipitation.
TI - Phase diagram methods for graphitic rocks and application to the system C−O−H−FeO−TiO2−SiO2
JF - Contributions to Mineralogy and Petrology
DO - 10.1007/BF00310720
DA - 2004-08-24
UR - https://www.deepdyve.com/lp/springer-journals/phase-diagram-methods-for-graphitic-rocks-and-application-to-the-IzQU1wOhJL
SP - 94
EP - 116
VL - 119
IS - 1
DP - DeepDyve
ER -