Empirical evidence for the fractionation of carbon isotopes between diamond and iron carbide from the Earth's mantle. (2nd April 2014)

• Record Type: Journal Article
• Authors: Mikhail, S.; Guillermier, C.; Franchi, I. A.; Beard, A. D.; Crispin, K.; Verchovsky, A. B.; Jones, A. P.; Milledge, H. J.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 15:Number 4(2014:Apr.)
• Page Start: 855
• Page End: 866
• Publication Date: 2014-04-02
• Subjects: Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1002/2013GC005138
• Languages: English
• ISSNs: 1525-2027

Abstract

<abstract abstract-type="main"> <title>Abstract</title> <p>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 <sup>13</sup>C in a terrestrial natural high‐pressure and high‐temperature (HPHT) system. The difference for the δ<sup>13</sup>C 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 <sup>13</sup>C‐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 <sup>13</sup>C between these two phases. These data suggest that significant isotopic fractionation of <sup>13</sup>C/<sup>12</sup>C (Δ<sup>13</sup>C 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 <sup>13</sup>C‐depleted carbon that have conventionally been interpreted as crustal in origin. Finally, the large Δ<sup>13</sup>C for diamond‐iron carbide shown here demonstrates Δ<sup>13</sup>C 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.</p> </abstract>