INITIAL 182HF/180HF IN METEORITIC ZIRCONS

Abstract

Zircons from the Simmern H5 chondrite and Pomozdino eucrite have been analyzed for their Hf-W isotope systematics. Zircons have high intrinsic Hf contents and coupled with low W, make them ideal Hf-W chronometers. However, measurements of (182Hf/180Hf)0 are far from straightforward with low signals of radiogenic 182W and difficulties in calibrating Hf/W ratios. Zircons were analyzed from the Simmern chondrite, and the Pomozdino eucrite. The Simmern zircon has an ultrarefractory-enriched trace element pattern, a feature commonly associated with refractory inclusions. Analyses of Simmern zircon show variable Hf/W that is likely due to surface contamination. Simmern chondrite zircon yields a (182Hf/180Hf)0 of 7.2 (+/- 4.5) x 10-5. Pomozdino eucrite zircon analyses show very high Hf/W values indicating (182Hf/180Hf)0 of 1.7 (+/- 1.1) x 10-5. The Simmern value is in good agreement with the initial value of the solar system indicated from Hf-W systematics of chondrites. The Pomozdino value is lower than expected if eucrites formed within several million years of refractory inclusions as suggested from Al-Mg systematics of eucrites.