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Contributions to Mineralogy and Petrology

, Volume 119, Issue 1, pp 94–116 | Cite as

Phase diagram methods for graphitic rocks and application to the system C−O−H−FeO−TiO2−SiO2

  • J. A. D. Connolly
Article

Abstract

Carbon-saturated C−O−H (GCOH) fluids have only one compositional degree of freedom. This degree of freedom is specified by the variable Xo 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 Xo. In fluid-saturated graphitic rocks, mineral devolatilization reactions are the dominant factor is determining the redox state of the metamorphic environment. Xo 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 Xo 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 Xo. 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-Xo diagrams more clearly constraint accessible fluid compositions and fluid evolution paths. Calculated P-T-Xo 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-Xo 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-Xo 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.

Keywords

Fluid Composition SiO2 System Graphite Precipitation Petrogenetic Grid Fluid Species 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Springer-Verlag 1995

Authors and Affiliations

  • J. A. D. Connolly
    • 1
  1. 1.Institute for Mineralogy and PetrographySwiss Federal Institute of TechnologyZürichSwitzerland

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