AmericanChemicalSociety.com

Recent experiments have shown that the Earth’s upper mantle can dissolve trace amounts of carbon into the silicate minerals of the mantle, with concentrations increasing with increasing pressure.  These results have led to growing interest in the planet’s bulk carbon budget and implications for the mantle’s control of our climate.  Although it is well established that the vast majority of the Earth’s oil and natural gas resources form from surface-derived organic carbon, it has long been recognized that the carbon in the Earth’s interior has direct control on surface carbon budgets. 

In the first year of the project, we reported results on the potential for carbon sequestration in deep mantle materials, such as those found in the Earth’s lower mantle, highlighting the participation of the deep mantle in the global carbon cycle [Panero and Kabbes, 2008].  The results of this work highlighted the importance of the oxidation state of carbon in the deep mantle, paired with ongoing debate as to the oxidation state of iron in the deep mantle.  Recent experimental results indicating that iron in perovskite may undergo an “autoredox” reaction of 3Fe²⁺=2Fe³⁺+Fe⁰ and therefore that the lower mantle contains as much as 1% metallic iron.  Alternatively, because this autoredox reaction is independent of oxidation potential of the system, this reaction may lead to unexpected behaviors of those minor and trace elements in the mantle with multiple oxidation states.  As such, we have initiated a series of experiments to probe the relative oxygen buffers:  Siderite-Diamond-Iron (SDI) compared to Iron-Wüstite (IW) coupled with modeling these buffers under appropriate conditions, defining a first-order constraint of carbon relative to the dominant iron. 

Thermodynamic modeling suggests the IW buffer lies below the SDI buffer across the pressure and temperature range of Earth’s mantle, suggesting that FeCO₃ (siderite) will reduce to diamond.  Complimentary experiments at 10-70 GPa and and 1500-2500 K show the formation of diamond in all experiments.  These results indicate that the volume collapse of siderite is insufficient to stabilize carbonate under reducing lower mantle conditions.  Combined x-ray diffraction, micro Raman spectroscopy, and TEM of cross-sections of the samples show significant spatial variations, with those portions of the sample at the highest temperatures showing carbonate in equilibrium with iron oxides and diamond, and no remaining free iron [Kabbes et al., 2009; Kabbes 2010].  This indicates that at temperatures above ~2200 K (representative of mantle temperatures at that depth) that SDI is above IW.  As such, the stability of carbonate in the mantle is then a function of how high above the IW buffer the mantle at those depths may be.  These findings suggest that in the more reducing regions of a laterally and axially heterogeneous mantle, carbonates will be reduced to diamond and/or iron carbide(s), with the greatest reduction potential occurring just before the siderite spin transition.  In the more oxidizing regions, such as those near subduction zones and below D”, carbonate will be the stable host of carbon.  If carbon is a major light element of the core, it is likely that it would have to have been sequestered prior to the formation of the post-perovskite phase and the D” region.