Journal Article

The oxidation state of the mantle and the extraction of carbon from Earth’s interior

Ojwang, Dickson O., McCammon, Catherine A., Frost, Daniel J., Stagno, Vincenzo

2013

Overview

The oxygen fugacity of the deepest rock samples from Earth’s mantle is found to be more oxidized than previously thought, with the result that carbon in the asthenospheric mantle will be hosted as graphite or diamond but will be oxidized to produce carbonate melt through the reduction of Fe 3+ in silicate minerals during upwelling. Graphite and diamond in the upper mantle Vincenzo Stagno and colleagues report experiments on mantle xenoliths, and find that the oxygen fugacity of the deepest rocks they analyse is at least one order of magnitude more oxidized than previous estimates. They conclude from this that carbon in the asthenospheric mantle will be hosted as graphite or diamond, but it will be oxidized to produce carbonate melt during upwelling. This 'redox melting' relationship has important implications for the extraction of CO 2 from the mantle through decompressive melting. Determining the oxygen fugacity of Earth’s silicate mantle is of prime importance because it affects the speciation and mobility of volatile elements in the interior and has controlled the character of degassing species from the Earth since the planet’s formation 1 . Oxygen fugacities recorded by garnet-bearing peridotite xenoliths from Archaean lithosphere are of particular interest, because they provide constraints on the nature of volatile-bearing metasomatic fluids and melts active in the oldest mantle samples, including those in which diamonds are found 2 , 3 . Here we report the results of experiments to test garnet oxythermobarometry equilibria 4 , 5 under high-pressure conditions relevant to the deepest mantle xenoliths. We present a formulation for the most successful equilibrium and use it to determine an accurate picture of the oxygen fugacity through cratonic lithosphere. The oxygen fugacity of the deepest rocks is found to be at least one order of magnitude more oxidized than previously estimated. At depths where diamonds can form, the oxygen fugacity is not compatible with the stability of either carbonate- or methane-rich liquid but is instead compatible with a metasomatic liquid poor in carbonate and dominated by either water or silicate melt. The equilibrium also indicates that the relative oxygen fugacity of garnet-bearing rocks will increase with decreasing depth during adiabatic decompression. This implies that carbon in the asthenospheric mantle will be hosted as graphite or diamond but will be oxidized to produce carbonate melt through the reduction of Fe 3+ in silicate minerals during upwelling. The depth of carbonate melt formation will depend on the ratio of Fe 3+ to total iron in the bulk rock. This ‘redox melting’ relationship has important implications for the onset of geophysically detectable incipient melting and for the extraction of carbon dioxide from the mantle through decompressive melting.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 704/2151/209 / 704/2151/431 / Alloys / Carbon / Carbon dioxide / Crystalline rocks / Degassing / Earth / Earth sciences / Earth, ocean, space / Exact sciences and technology / Fluids / Fugacity (Thermodynamics) / Humanities and Social Sciences / Igneous and metamorphic rocks petrology, volcanic processes, magmas / Internal geophysics / letter / Lithosphere / Magma / Mantle / Melting / Mineralogy / Minerals / multidisciplinary / Oxygen / Rocks / Science / Solid-earth geophysics, tectonophysics, gravimetry / Speciation / Spectrum analysis / Upwelling