Trace element chemistry of Cumulus Ridge 04071 pallasite with implications for main group pallasites

Abstract

Pallasites have long been thought to represent samples from the metallic core–silicate mantle boundary of a small asteroid-sized body, with as many as ten different parent bodies recognized recently. This report focuses on the description, classification, and petrogenetic history of pallasite Cumulus Ridge (CMS) 04071 using electron microscopy and laser ablation ICP-MS. Most olivines are angular in CMS 04071, but there are some minor occurrences of small rounded olivines, such as in the Eagle Station pallasite. Olivine, chromite, and metal compositions indicate that CMS 04071 can be classified as a Main Group pallasite. The kamacite/taenite partition coefficients (D) for highly siderophile elements (HSE) are all close to 1, but comparison with previous studies on iron meteorites and pallasites shows that variation of some D values is controlled by the Ni content oftaenite. D(HSE)metal/sulfide for Re, Cu, and Cr all are <1, indicating chalcophile behavior for these three elements, in agreement with experimental Dmetal/sulfide. D(HSE)metal/olivine are variable, which isperhaps due to small metallic inclusions in the olivine that are present to variable extents in different pallasites. All of these data, together with results from previous studies, indicate that the CMS pallasites were likely formed at the core-mantle boundary of a small asteroid, but not necessarily related to the core that produced the IIIAB irons. In addition, they share a similar volatile element depletion to HEDs that is distinct from other bodies such as Earth, Mars, Angrite Parent Body, and the parent body of the brachinites.