Highly Siderophile Elements, 187Re-187Os and 182Hf-182W Isotopic Systematics of Early Solar System Materials: Constraining the Early Evolution of Chondritic and Achondritic Parent Bodies

Abstract

Highly siderophile element (HSE) abundances and 187Re-187Os isotopic systematics for H chondrites and ungrouped achondrites, as well as 182Hf-182W isotopic systematics of H and CR chondrites are reported.

Achondrite fractions with higher HSE abundances show little disturbance of 187Re-187Os isotopic systematics. By contrast, isotopic systematics for lower abundance fractions are consistent with minor Re mobilization. For magnetically separated H chondrite fractions, the magnitudes of disturbance for the 187Re-187Os isotopic system follow the trend coarse-metal<fine-metal<silicate.

Ungrouped achondrite NWA 6704 has calculated whole-rock HSE abundances that are nearly chondritic and consistent with limited large-scale differentiation of its parent body. Most likely, this rock formed on a chondritic parent body, and crystallized from a melt from which little or no metal was removed. Modest variations in the relative HSE abundances among bulk pieces may have been the result of sulfide loss or evolving oxidation state during crystallization. The HSE abundances of ungrouped achondrite NWA 7325 are highly fractionated and depleted, relative to bulk chondrites. Therefore, its parent body must have undergone complex processing, including core formation, late accretion, and igneous processes.

A negative thermal ionization mass spectrometry (N-TIMS) technique was developed to measure the W isotopic compositions of H chondrite metal fractions. This method is capable of measuring 182W/184W to an external precision of 5.7 ppm and 183W/184W to 6.7 ppm, which is ~2-3x more precise than the most recently published technique capable of measuring 182W/184W and 183W/184W.

The HSE abundances of H chondrite nonmagnetic fractions are too high to reflect equilibration between metals and silicates. There is also no correlation between metamorphic grade and apparent degree of equilibration. The 182Hf-182W isotopic systematics of H chondrite magnetic separates do not form precise isochrons for most H chondrites, consistent with disequilibrium among the separates. However, the slightly magnetic and nonmagnetic separates from most H chondrites do form precise isochrons. The calculated slope ages are inconsistent with the onion shell thermal model, in which H4 chondrites cooled fastest and H6 chondrites cooled slowest. Instead, the data indicate that the H chondrite parent body must have been disrupted, possibly by impact, early in Solar System history.