A Comparison of Re-Os Isotopes and Abundances of Highly Siderophile Elements in Chondritic Meteorites: Constraints on Terrestrial Late Accretion

Understanding the distributions of refractory, highly-siderophile elements (HSE) in chondritic meteorites is necessary to better constrain the history and distributions of those elements in terrestrial and planetary mantles. It is now widely, although not unanimously, agreed that the bulk of the highly siderophile elements in the Earth's mantle were added subsequent to terrestrial core segregation, by late accretion of broadly chondritic materials. New, precise abundance data for the HSE in chondrites, coupled with Re-Os isotopic data for the same meteorites, indicate that each of the three major chondrite groups is uniquely defined. Re-Os isotopic data show isotopic evidence for a major difference in long-term Re/Os (as shown in their modern ¹⁸⁷Os/¹⁸⁸Os) between carbonaceous chondrites relative to and ordinary and enstatite chondrites. Ordinary chondrites have ¹⁸⁷Os/¹⁸⁸Os=0.1283+/-0.0031. Enstatite chondrites have ¹⁸⁷Os/¹⁸⁸Os=0.1281+/-0.0009; and carbonaceous chondrites have ¹⁸⁷Os/¹⁸⁸Os=0.1260+/-0.0021. These differences reflect 7-8% lower Re/Os ratio in carbonaceous chondrites, compared to enstatite and ordinary chondrites. This fractionation process occurred very early in the history of the solar system, possibly during condensation. It has been long recognized that enstatite chondrites have relatively high Pd concentrations, compared to the other highly siderophile elements. Our new data show that enstatite chondrites have Pd/Ru and Pd/Ir ratios of 0.987+/-0.102 and 1.48+/-0.18. Carbonaceous chondrites and ordinary chondrites have distinctly lower Pd/Ru and Pd/Ir ratios. Pd/Ru and Pd/Ir in carbonaceous chondrites average 0.754+/-0.104 and 1.08+/-0.17, respectively. Similarly, ordinary chondrites have average Pd/Ru and Pd/Ir ratios of 0.759+/-0.088 and 1.153+/-0.153. The cause of high relative Pd concentrations in enstatite chondrites is not well understood. Its ubiquitous presence, regardless of the presence of brecciation, degree of metamorphism and terrestrial weathering, suggests that it also occurred early in the history of the solar system. The distinguishing HSE characteristics of chondrites may permit better characterization of the source of the terrestrial and lunar late influx materials. The Os isotopic composition of the terrestrial primitive upper mantle indicates late accretion dominated by materials with Re/Os most similar to enstatite and ordinary chondrites than to carbonaceous chondrites. Published data for samples from fertile, terrestrial mantle yield broadly chondritic ratios of Pd/Ir and Pd/Ru, but are suggestive of some variation in those ratios among upper mantle reservoirs. Better constraints on the composition of terrestrial late influx may come from new analyses of upper mantle material and in lunar impact breccias.