Minor and trace element concentrations in adjacent kamacite and taenite in the Krymka chondrite

Abstract

International audienceWe report in situ NanoSIMS siderophile minor and trace element abundances in individual Fe-Ni metal grains in the unequilibrated chondrite Krymka (LL3.2). Associated kamacite and taenite of 10 metal grains in four chondrules and one matrix metal were analyzed for elemental concentrations of Fe, Ni, Co, Cu, Rh, Ir and Pt. The results show large elemental variations among the metal grains. However, complementary and correlative variations exist between adjacent kamacite-taenite. This is consistent with the unequilibrated character of the chondrite and corroborates an attainment of chemical equilibrium between the metal phases. The calculated equilibrium temperature is 446 ± 9 °C. This is concordant with the range given by Kimura et al. (2008) for the Krymka post-accretion thermal metamorphism. Based on Ni diffusivity in taenite, a slow cooling rate is estimated of the Krymka parent body that does not exceed ~1K/Myr, which is consistent with cooling rates inferred by other workers for unequilibrated ordinary chondrites. Elemental ionic radii might have played a role in controlling elemental partitioning between kamacite and taenite. The bulk compositions of the Krymka metal grains have non-solar (mostly subsolar) element/Ni ratios suggesting the Fe-Ni grains could have formed from distinct precursors of non-solar compositions or had their compositions modified subsequent to chondrule formation events

In situ analysis of platinum group elements in equilibrated ordinary chondrite kamacite and taenite

“…) and ordinary chondrites (Campbell and Humayun ; Meftah et al. ), which demonstrate that PGEs preferentially partition into taenite. This preferential partitioning is controlled by the size and electron configuration of the PGEs as well as the crystal structures of kamacite and taenite (body‐centered cubic versus face‐centered cubic; Hsu et al.…”

“…The ionic radius of the PGE's main oxidation state has also been related to partitioning between kamacite and taenite (Meftah et al. ). Our study shows consistently higher taenite PGE concentrations, but also significant variability (Table ; Fig.…”

“…Metal analysis of the Krymka unequilibrated LL3.2 chondrite has also revealed a lack of correlation between Rh, Ir, and Pt and Ni concentrations (Meftah et al. ). In order to test whether PGEs correlate with Ni in less‐equilibrated chondrite metal, the Benares (a) results from this study are plotted in Fig.…”

“…; Meftah et al. ). Given that PGE concentrations are relatively low on Earth, there is also interest in the economic development of asteroids to mine these elements (e.g., Kargel ; Elvis ).…”

“…; Campbell and Humayun 1999a, ; Humayun and Campbell , ; Meftah et al. ). These studies have provided insight into a range of questions regarding trace element concentrations, metal origins, and fractionation of PGEs.…”