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

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 13 C in a terrestrial natural high-pressure and high-temperature (HPHT) system. The difference for the d 13 C values between the diamond and coexisting iron carbide averaged 7.2 6 1.3&. These data are consistent with available data from the literature showing iron carbide to be 13 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 13 C between these two phases. These data suggest that significant isotopic fractionation of 13 C/ 12 C (D 13 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 13 C-depleted carbon that have conventionally been interpreted as crustal in origin. Finally, the large D 13 C for diamond-iron carbide shown here demonstrates D 13 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.

Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds

“…This relationship is firm evidence for a subducted protolith and makes it extremely unlikely that the associated negative δ 13 C values are related to deep mantle isotopic fractionation (Mikhail et al, 2014). In a subduction scenario, the low δ 13 C values (Fig.…”

“…Nevertheless, this interpretation is not unique. For example, Mikhail et al (2014) suggested that iron carbide inclusions in diamonds from Jagersfontein document isotopic fractionation (Δ 13 C carbide-diamond >7 ‰) sufficiently strong to cause the observed 13 C depleted signature. Similarly, evidence based on the rare earth element signatures (observation of negative Eu anomalies) of majoritic garnet inclusions from Jagersfontein, interpreted to reflect feldspar fractionation in crustal protoliths (Tappert et al, 2005b), was disputed by Griffin and O'Reilly (2007) and Corgne et al (2012) who considered Eu anomalies to reflect redoxrelated metasomatic signatures, or crystallisation from a melt at high pressures.…”

Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): constraints from carbon isotopes and inclusion compositions

“…However, experimental studies of isotopic fractionations are limited to ~4 ‰ in between carbon species in closed systems at 1,000 °C (Bottinga 1969;Deines 1980). A recent study of isotopic fraction between iron carbide and diamond in natural samples observed a fractionation of 7.2 ‰ (Mikhail et al 2014a), however, in the wrong sense to explain light diamond formation from mantle material. Increasing temperature would reduce fractionation, and the effects of pressure remain mostly unconstrained.…”

Carbon-bearing iron phases and the carbon isotope composition of the deep Earth

“…Very low δ 13 C values (−18‰ to −35‰) have been reported for SiC from several localities (25). Recently, Mikhail et al (8) C values, points to a genetic relationship between these diamonds with low δ…”

“…In the presence of Fe-rich metals, diamond can be converted to iron carbides (Fe 3 C and Fe 7 C 3 ) and dissolved C in Fe−Ni metals (5-7). The occurrence of cohenite, (Fe, Ni) 3 C, has been well documented from iron meteorites, but only recently, several investigators reported rare terrestrial occurrences of Fe carbides (Fe 3 C, Fe 2 C, and Fe 23 C 6 ) from kimberlite (8), from minerals of subcratonic lithosphere or possible lower-mantle origins (9, 10) and others (11). Thus, Fe carbides and Fe−Ni metals are increasingly recognized as potential C-bearing phases in the lower mantle and core.…”