In Situ SIMS Carbon Isotopic Analysis Of Carbon-Bearing Minerals In Nantan And Aletai Iron Meteorites: Implications On Genesis

Abstract

: A better understanding of the fractionation properties of carbon isotope between different carbon-bearing minerals will provide insights into the core-forming process of planets. In this study, we carried out an integrated study of petrography and in situ secondary ion mass spectroscopy (SIMS) carbon isotopic analysis for Nantan and Aletai iron meteorites. Haxonite in Aletai IIIE-an iron meteorite has a δ 13 C value of −14.80 ± 2.31‰, which is different from that of haxonite in Colonia Obrera IIIE iron meteorite. This suggests that the parent body of Aletai has a different initial carbon isotopic composition or experienced different planetary processes ( e.g. , degassing of CO 2 /CH 4 ). A graphite-rich nodule in Nantan IAB iron meteorite, characterized by a core-mantle-rim texture, was systematically studied. Some graphite grains in the nodule mantle (GNM; δ 13 C value as low as −14.65‰) and all graphite grains in the nodule rim (GNR; δ 13 C = −12.65 ± 2.90‰) are more depleted in 13 C than those in the nodule core (GNC; δ 13 C = −7.17 ± 2.42‰). This could be due to the preferential incorporation of 13 C into the early-crystallized GNC. The carbon isotopic fractionation (Δ 13 C = 6.9 ± 2.7 ‰) between coexisting GNR (δ 13 C = −12.65 ± 2.90‰) and cohenite (δ 13 C = −19.60 ± 2.59‰) yielded an equilibrium temperature of ~950–1310 °C, supporting the melt-crystallization genesis for nodule. We concur with previous studies that the early differentiation of Earth could have led to positive carbon isotopic fractionation between graphite/diamond in the mantle and metallic melt sinking to the core.