Rare Earth Element abundances in presolar SiC

T.R. Irelanda,b, J.N. Ávilaa,b, M. Lugaroc, S. Cristallod, P. Holdena, P. Lanca, L. Nittlere, C.M.O’D. Alexandere, F. Gyngardf, S. Amarif
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.027]
aResearch School of Earth Sciences, The Australian National University, Canberra ACT 2601, Australia
bPlanetary Science Institute, The Australian National University, Canberra ACT 2601, Australia
cKonkoly Observatory, Hungarian Academy of Sciences, Budapest, Hungary
dINAF – Osservatorio Astronomico di Teramo, 64100, Italy
eDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington DC 20015, USA
fLaboratory for Space Sciences, Physics Dept., Washington University, St. Louis, MO 63130, USA
Copyright Elsevier

Individual isotope abundances of Ba, lanthanides of the rare earth element (REE) group, and Hf have been determined in bulk samples of fine-grained silicon carbide (SiC) from the Murchison CM2 chondrite. The analytical protocol involved secondary ion mass spectrometry with combined high mass resolution and energy filtering to exclude REE oxide isobars and Si-C-O clusters from the peaks of interest. Relative sensitivity factors were determined through analysis of NIST SRM reference glasses (610 and 612) as well as a trace-element enriched SiC ceramic. When normalised to chondrite abundances, the presolar SiC REE pattern shows significant deficits at Eu and Yb, which are the most volatile of the REE. The pattern is very similar to that observed for Group III refractory inclusions. The SiC abundances were also normalised to s-process model predictions for the envelope compositions of low-mass (1.5 to 3 fx1) AGB stars with close-to-solar metallicities (Z=0.014 and 0.02). The overall trace element abundances (excluding Eu and Yb) appear consistent with the predicted s-process patterns. The depletions of Eu and Yb suggest that these elements remained in the gas phase during the condensation of SiC. The lack of depletion in some other moderately refractory elements (like Ba), and the presence of volatile elements (e.g. Xe) indicates that these elements were incorporated in to SiC by other mechanisms, most likely ion implantation.

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