Carbon Isotope Fractionation in Earth’s Mantle

A recent study by Sami Mikhail and colleagues has provided empirical data for the fractionation of 13C/12C below the IW buffer.

Stable isotopes of carbon are powerful tools used to quantify the abundance of carbon within Earth’s deep interior (reservoirs), and to trace the exchange of carbon between Earth’s crust and mantle (fluxes). Relevant examples include the degassing history of subducted slabs using volcanic gas compositions at convergent plate boundaries (reviewed in [1]), the flux of carbon emitted at mid-ocean ridge systems [2], and the origin of diamond-forming carbon (reviewed in [3]). Regarding the use of carbon isotopes as source indicators, three main parameters need to be constrained before data can be interpreted with confidence. These are [i] the initial 13C/12C ratio in the system, [ii] a database to relatively evaluate the data, and [iii] the magnitude and directions of isotopic fractionation under the P-T-fO2 conditions in question. These three criteria are mostly well satisfied with regard to stable isotopes of carbon in Earth under redox conditions around the quartz-fayalite-magnetite (QFM) buffer. However, carbon isotope fractionation in the mantle around the Fe-FeO (IW) buffer (≈ QFM -4 log units) is considerably less well constrained.

A recent study by DCO’s Sami Mikhail and colleagues [4] has provided empirical data for the fractionation of 13C/12C below the IW buffer (diamond - iron carbide). Two unique samples were sourced from the Jagersfontein kimberlite, South Africa, that is well known for producing samples from sublithospheric depths, and for samples containing metallic inclusions and carbides [5]. The new study shows isotopic fractionation of 13C/12C up to ~>±7 ‰ can occur in the mantle at minimum temperatures and pressures, consistent with the top of the diamond stability field when iron carbides are involved.

Co-genesis of carbide and diamond can therefore produce abiogenic reservoirs of 13C-depleted carbon that overlap isotopically “light” carbon conventionally attributed to subducted organic carbon. For perspective, the magnitudes of isotopic fractionation during diamond-formation in the mantle, buffered just above or below QFM at 1200°C, are only +1 ‰ for diamond-CH4 [6] and -3.5 ‰ for diamond-CO2 [7]. Ergo, carbon isotope fractionation in the deeper, hotter, and more reducing lower mantle may be larger than the shallower, colder, less reducing upper mantle; a previously counter-intuitive notion.

 

This work was funded by the Engineering and Physical Sciences Research Council, the Scientific and Technology Facilities Council, the Diamond Trading Company, the Department of Physical Sciences, The Open University (all UK), and the Carnegie Institution of Washington (USA). 

Image: Modified from (Rohrbach and Schmidt, 2011)

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