Experimental investigation of the redox conditions at which carbonate minerals and melts transform to graphite or diamond in Earths mantle
Final Report Abstract
The oxygen fugacity at which carbonate melts are reduced to either graphite or diamond has been measured in eclogitic rock compositions in experiments performed between 3 and 6 GPa and temperatures between 800-1300 °C. The oxygen fugacity in each experiment was measured using a sliding Ir-Fe alloy redox sensor. At temperatures below 1000 °C inconsistent results are likely evidence that equilibrium cannot be achieved when the carbonate phase is dominantly solid. Oxygen fugacities normalized to the FMQ buffer decrease with temperature from -1 to -2 log units at 3 GPa, most likely as a result of dilution of the carbonate liquid with silicate as temperatures increase. The oxygen fugacity buffered by near solidus carbonate melt decreases with pressure but also shows a similar decrease with temperature at 6 GPa. Determinations of the oxygen fugacity buffered by the coexistence of graphite and carbonate melt in eclogites are approximately 1 log unit below thermodynamic calculations for a similar redox buffering equilibrium involving dolomite solid solution. The presence of Fe and silica in the liquid carbonate may account for the lower fo2 than predicted for the solid carbonate buffer. In contrast to previous arguments, the carbonate melt stability field extends to lower oxygen fugacity in eclogitic rocks compared to peridotite rocks. The same set of carbonated eclogite experiments also contained monomineralic layers of clinopyroxene and garnet that allowed the ferric Fe contents of these minerals to be measured using Mössbauer spectroscopy. Using the equilibrium, 5CaFeSi2O6 + 1/3Ca3Al2Si3O12 + O2 = 2Ca3Fe2Si3O12 + 1/3Fe3Fe2Si3O12 + 4SiO2 a relatively simple model was derived for determining the oxygen fugacity of eclogitic rocks from the compositions of garnet and clinopyroxene coupled with Mössbauer spectroscopy determinations of the garnet ferric Fe content. The model was able to reproduce the experimental data to within 0.5 log units. The equilibrium was also used to determine the fo2 of eclogitic xenoliths from the Siberian and Kaapvaal cratons. The Siberian eclogites displayed oxygen fugacities in the range between FMQ -0.5 and FMQ -1 log unit, which would be incompatible with the equilibrium existence of diamond or graphite. The Kaapvaal eclogites were more reduced and in the range -1.9 to -3.4 log units below FMQ. The oxygen fugacity at which magnesite (MgCO3) is reduced to diamond in a typical mantle assemblage was also determined between 16 and 45 GPa and 1500-1700 °C in experiments employing a multianvil device. This oxygen fugacity, measured using a sliding redox sensor that employs Ir-Fe alloy, was found to be greater than 2 log units above the iron-wüstite oxygen buffer (ΔIW+2). Reversal experiments employing Fe-Ni alloy confirmed complete oxidation of Ni in the presence of magnesite and diamond even at 45 GPa. As the oxygen fugacity of the transition zone and lower mantle is most likely at or below the IW buffer this confines the stability of solid carbonate to the upper mantle or to unusually oxidized regions of the deeper mantle. Within the uncertainty of our measurements it is possible that the oxygen fugacity at which magnesite and diamond coexist decreases with pressure, with a gradient that would allow magnesite to become the stable host for carbon at the very base of the lower mantle or coexisting with minor amounts of carbon dissolved in Fe-Ni metal.
Publications
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(2011) The stability of magnesite in the transition zone and the lower mantle as a function of oxygen fugacity. Geophysical research Letters 38: Art. No. L19309
Stagno V, Tange Y, Miyajima N, McCammon CA, Irifune T, Frost DJ
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(2013) The oxidation state of the mantle and the extraction of carbon from Earth’s interior. Nature 493, 84-88
Stagno V, Ojwang DO, McCammon CA, Frost DJ
