Metastable equilibrium in the C-H-O system: Graphite deposition in crustal fluids

Abstract

The presence of graphite in natural environments is linked to the redox and thermal conditions of C-H-O fluid/graphite equilibrium in hydrothermal veins and metasomatic contacts. A time-series experimental study was performed to investigate the graphite undersaturated C-H-O system at 600 °C and 1000 MPa, and with f O2 ranging from highly reducing (10 −23 ) to highly oxidizing (10 4 ). A nonvolatile intermediate carbon phase exhibiting the Raman spectral features of poorly ordered graphite was formed as the system evolves toward equilibrium as a function of run duration. The thermometric empirical expressions using the G and D bands in the spectra of graphite failed to accurately estimate the experimental temperature. Thus, the existing Raman geothermometers appear inadequate to address graphite formation under conditions of metastable equilibria and to account for kinetic effects such as, for example, the degree of crystallinity. The presence of poorly ordered graphitic carbon at all the redox conditions investigated suggests that the disordered structure of the mineral attains an extensive thermodynamic stability field, and that it may be more readily deposited than crystalline graphite. Metastable graphitic carbon could, therefore, function as a precursor and substrate for the formation of the well-ordered phase. Such metastable graphite may provide an intermediate state that facilitates subduction of carbonaceous material, while imposing constraints on the formation mechanisms and the 13 C/ 12 C isotopic systematics of deep seated carbonaceous fluids and minerals such as diamonds.

Ecological Succession of Sulfur-Oxidizing Epsilon- and Gammaproteobacteria During Colonization of a Shallow-Water Gas Vent

“…Twenty milliliters of gas were collected and injected into 25 ml stoppered borosilicate vials containing 4 ml of 6% (w/v) AgNO 3 , 25% (w/v) H 3 PO 4 and 20 ml Argon grade zero. The vials were pretreated by following protocols discussed elsewhere (Foustoukos, 2012). The precipitated Ag 2 S and gases in the headspace was measured and analyzed.…”

The chemical behavior of fluids released during deep subduction based on fluid inclusions

“…In slab rocks, carbon is present in a variety of forms, including carbonate minerals and organic matter, derived from altered oceanic crust and marine sediments. Part of the carbon originally present in crustal lithologies is liberated as CO 2 , during low-grade metamorphic decarbonation reactions (e.g., Kerrick and Connolly 2001), or fixed as graphite (Foustoukos 2012;Galvez et al 2013). At greater sub-arc depths, coupled thermal and thermodynamic models strongly suggest that carbonate minerals do not react with silicate phases liberating CO 2 , and are stable along most subduction geothermal gradients (Connolly 2005;Gorman et al 2006;Molina and Poli 2008).…”

Laboratory Simulations of Abiotic Hydrocarbon Formation in Earth's Deep Subsurface

“…1), may not be as rapidly reactive as the graphite employed in the experiments. Experimental studies performed at metamorphic conditions in the crust (up to 725 °C and 1 GPa) have indicated that disordered forms of graphite can persist metastably in contact with C-O-H fluids, at least on the timescales of the experiments (Ziegenbein and Johannes 1980;Pasteris and Chou 1998;Foustoukos 2012). Even if such metastable forms of graphite persist at the higher temperatures and longer residence times of the upper mantle, however, it is not likely to have a large impact on the speciation of carbon in fluids in equilibrium with the graphite, since the thermodynamic 1 Figure 2.…”