Ruthenium isotopic fractionation in primitive achondrites: Clues to the early stages of planetesimal melting

Abstract

Primitive achondrites derive from the residual mantle of incompletely differentiated planetesimals, from which partial silicate and metallic melts were extracted. As such, primitive achondrites are uniquely useful to examine the early stages of planetesimal melting and differentiation. To better understand this early melting and melt segregation as well as the nature of the melts involved, we obtained mass-dependent Ru isotopic compositions of 17 primitive achondrites, including winonaites, acapulcoite-lodranites, ureilites, brachinites, and two ungrouped samples. Most primitive achondrites with subchondritic Ru concentrations are characterized by heavy Ru isotopic compositions relative to chondrites, likely reflecting the extraction of isotopically light partial metallic melts. While the segregation of early-formed partial Fe-Ni-S melts likely had no effect on the Ru isotope compositions, the subsequent extraction of S-free partial metallic melts at higher temperatures provides a viable mechanism for producing the observed Ru isotopic fractionation and fractionated highly siderophile element ratios among primitive achondrites. Together, these observations indicate that differentiation of primitive achondrite parent bodies involved the segregation of distinct partial metallic melts over a range of temperatures, and that these melts ultimately formed a partial core with fractionated and light Ru isotopic composition. This contrasts with the unfractionated Ru isotope signatures previously estimated for bulk iron meteorite cores, which therefore indicates quantitative metal segregation during core formation in the iron meteorite parent bodies. The less efficient metal segregation in primitive achondrite parent bodies most likely reflects lower initial amounts of heat-producing 26Al due to later accretion or impact disruption of the parent bodies during differentiation.