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

S. Mikhail, C. Guillermier, I. A. Franchi, A. D. Beard, K. Crispin, A. B. Verchovsky, A. P. Jones, H. J. Milledge

Research output: Contribution to journalArticlepeer-review

27 Scopus citations

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. Key Points In the mantle, C isotope fractionation may be larger in the hotter deep mantle Carbide precipitation can produce isotopically distinct mantle carbon reservoirs Core formation likely fractionated the C isotope value of the BSE

Original languageEnglish (US)
Pages (from-to)855-866
Number of pages12
JournalGeochemistry, Geophysics, Geosystems
Volume15
Issue number4
DOIs
StatePublished - Apr 2014

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology

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