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Hf-W Chronometry for an Early Differentiation in the Parent Body of Ureilites

Ureilites are primitive achondrites that are composed mainly of olivine and pyroxenes with abundant interstitial and crosscutting carbon polymorphs, Fe-Ni metal, sulfides, and silicates. Studies of ureilites indicate that the parent body went through a series of igneous processes, while, at the same time, ureilites contain features that can only be explained by mixing of primitive nebular materials [1]. Several models have been proposed to explain the origin of ureilites, and these include multi-stage igneous cumulate processes [2], residues of partial melting [3], collision of primitive planetesimals [4], and smelting of an olivine-rich parent body [5]. Despite the fact that no single model can yet satisfy all the observed chemical and physical evidence, there is little doubt that ureilites represent the silicate portion of a differentiated body. In this study, we present preliminary Hf-W (t1/2 ~ 9 myrs) data for a suite of ureilites in order to further constrain the nature and the timing of this partial melting/smelting event. Four ureilites, including ALHA77257, Dhofar132, EET96042, and PCA82506, have been analyzed for their W isotope composition and Hf/W ratios at ETHZürich. About 1.5 grams of powder was prepared for each sample using an aluminum-oxide mortar and pestle under a laminar flow of HEPA-filtered air. The experimental procedure has been described in detail elsewhere [6]. All four ureilites show significant W deficits relative to that of the terrestrial W standard. The ew varies from -3.6 to -2.7, which is sub-chondritic [7,8] consistent with sub-chondritic Hf/W ratios (Hf/W varies from 0.07 to 0.3). In fact, these ureilites have W isotopic compositions comparable to that of early metals extracted from ordinary and enstatite chondrites and iron meteorites [9,10], in contrast to other basaltic achondrites all of which have positive to chondritic ew [11,12]. There exists a positive but poorly defined relationship between the measured W isotope and Hf/W ratios among these four ureilites, with an initial ew at ~ -3.6, suggesting a possible common origin among the four samples. The unradiogenic ew observed in all four ureilites provides evidence that the parent body differentiated early, consistent with recently reported Mn-Cr data [13] and comparable to the timescales for differentiation of the HED parent body [11,12,14]. The unradiogenic ew signature in ureilites predominantly reflects their sub-chondritic Hf/W ratios and this is thought to be the product of the partial melting / smelting event that was responsible for the formation of ureilites [15]. This reconnaissance Hf-W study has examined only four samples, and these do not cover the entire chemical and petrographic types of the ureilite families either. Therefore, it is premature to conclude much based on these data alone. Nonetheless, it seems clear that the ureilite parent body differentiated early. Future work should combine chemical and petrographic evidence to study ureilites of different chemical and petrographic types, such that a more complete picture to the evolution history of the ureilite parent body can be obtained.