Figure 7
COH-diagrams for fluids illustrating the graphite saturation surface (a) and with calculated graphite-saturation surface and oxygen fugacitiy values and isopleths for 2 GPa, 900°C (b) and 2 GPa, 700°C (c). The solid line traversing the diagram represents the graphite-saturation surface, possible fluid compositions plot below. The CCO-buffer locates per definition where the graphite-saturation surface intersects the C-O side of the triangle. Note that in nature, we do not expect fluid compositions with significantly more oxygen than the H2O-CO2 tie-line, rendering the field of oxidized geological fluid compositions extremely narrow, in fact graphically almost not resolvable at 700°C. The red numbers give oxygen fugacities (in log-units) on the graphite saturation surface (a C = 1), the blue numbers on the H2O-CO2 tie-line approximated by a C-activity of 10-6, the green numbers those between H and H2O. Almost the entire width of fluid compositions on the graphite-saturation surface spans a relatively small range in oxygen fugacities equivalent to CCO to IW. More reducing fluids are all on the H-rich side of possible fluid compositions, essentially within a X O fluid of 0.01. The star in (b) indicates a putative fluid from an organic rich sediment that also has some CO2; to drive this composition to the reducing side, CO2 has to be removed from the fluid in carbonates. Magnesite and dolomite are only stable to oxygen fugacities of the H2O-maximum (the inflection in the graphite-saturation surface), but calcite until an approximate logf O2 of -15 (900°C) or -19 (700°C). Fluids from carbonate-free organic-rich sediments are expected to lie on the graphite/diamond saturation surface on the H-side of the H2O-C tie-line, as indicated by the star in (c). These fluids move away from H2O, on the graphite-saturation surface co-precipitating graphite or diamond, if reaction with peridotite produces hydrous phases (phase A at high pressure, serpentine or brucite at low pressure). Eventually such fluids would become more reduced than the IW buffer and would possible reach a H:O ratio of >1000 necessary to precipitate SiC. Note that there is no direct proportionality between oxygen fugacity and X O fluid.