182Hf-182W chronometry and early differentiation of the ureilite parent body

Eight samples spanning the bulk chemical range of monomict ureilites, have been analyzed for their W isotopic composition and Hf/W in order to elucidate the differentiation history of the ureilite parent body. The ureilites studied here show sub-chondritic Hf and W concentrations and low Hf/W ratio, and all show clear W-182 deficits relative to bulk chondrites. The W isotope data provide evidence that the ureilite parent body differentiated within 1 to 2 million years after the start of the solar system. Such early accretion and differentiation is comparable to the parent bodies of various magmatic iron meteorites, as well as the HED parent body. However, the trace element fractionations and by inference melting processes are very different. An early smelting/partial melting event coupled with metal-silicate segregation, however, can reproduce the observed Hf-W data in ureilites, and is consistent with petrological and other chemical evidence for a missing basaltic component and with primitive oxygen and noble gases compositions. (C) 2009 Elsevier B.V. All rights reserved.