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Empirical evidence for the fractionation of carbon isotopes between diamond and iron carbide from the Earth’s mantle

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Date
22/05/2014
Author
Mikhail, Sami
Guillermier, Christalle
Franchi, Ian
Beard, Andrew
Krispin, Catherine
Verchovsky, Alexander
Jones, Adrian
Milledge, H Judith
Keywords
Carbon isotope fractionation
Deep carbon cycle
Diamond
Iron carbide
QE Geology
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Abstract
We have studied two samples of mantle diamond containing iron carbide inclusions from Jagersfontein kimberlite, South Africa. Syngenetic crystal growth is inferred using morphological characteristics. These samples provide an opportunity to investigate the isotopic partitioning of 13C in a terrestrial natural high-pressure and high-temperature (HPHT) system. The difference for the δ13C values between the diamond and coexisting iron carbide averaged 7.2 ± 1.3‰. These data are consistent with available data from the literature showing iron carbide to be 13C-depleted relative to elemental carbon (i.e., diamond). We infer that the minerals formed by crystallization of diamond and iron carbide at HPHT in the mantle beneath the Kaapvaal Craton. It is unclear whether crystallization occurred in subcratonic or sublithospheric mantle; in addition, the source of the iron is also enigmatic. Nonetheless, textural coherence between diamond and iron carbide resulted in isotopic partitioning of 13C between these two phases. These data suggest that significant isotopic fractionation of 13C/12C (Δ13C up to >7‰) can occur at HPHT in the terrestrial diamond stability field. We note that under reducing conditions at or below the iron-iron wustite redox buffer in a cratonic or deep mantle environment in Earth, the cogenesis of carbide and diamond may produce reservoirs of 13C-depleted carbon that have conventionally been interpreted as crustal in origin. Finally, the large Δ13C for diamond-iron carbide shown here demonstrates Δ13C for silicate-metallic melts is a parameter that needs to be constrained to better determine the abundance of carbon within the Earth's metallic core.
Citation
Mikhail , S , Guillermier , C , Franchi , I , Beard , A , Krispin , C , Verchovsky , A , Jones , A & Milledge , H J 2014 , ' Empirical evidence for the fractionation of carbon isotopes between diamond and iron carbide from the Earth’s mantle ' , Geochemistry, Geophysics, Geosystems , vol. 15 , no. 4 , pp. 855–866 . https://doi.org/10.1002/2013GC005138
Publication
Geochemistry, Geophysics, Geosystems
Status
Peer reviewed
DOI
https://doi.org/10.1002/2013GC005138
ISSN
1525-2027
Type
Journal article
Rights
Copyright 2014. American Geophysical Union. All Rights Reserved. This work is made available online in accordance with the publisher’s policies. This is the final published version of the work, which was originally published at: https://dx.doi.org/10.1002/2013GC005138
Description
S.M. would like to thank the Engineering and Physical Sciences Research Council, the Diamond Trading Company, and the Department of Physical Sciences, The Open University (UK) for financial support during his time as a PhD student at University College London (UK). Date of Acceptance: 01/01/2014
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  • University of St Andrews Research
URL
http://onlinelibrary.wiley.com/doi/10.1002/2013GC005138/full#footer-support-info
URI
http://hdl.handle.net/10023/7813

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