Hf-w Chronology of the Angrite Parent Body: Timing of Accretion, Core Formation and Magmatism

Introduction: Key issues regarding the early evolution of planetesimals include the timescales of accretion, core formation, and magmatism. The short-lived Hf-W system has proven particularly useful as a chronometer for these early planetary processes. For instance, W model ages for magmatic iron meteorites reveal that core formation in their parent bodies occurred within ~1 Myr after formation of Ca,Al-rich inclusions (CAIs) [e.g., 1,3]. The application of the Hf-W system to date core formation in the parent bodies of basaltic achondrites is less straightforward because the Hf-W systematics of these rocks may not only reflect Hf-W fractionation due to core formation but additional Hf-W fractionation during later events. This makes it difficult to estimate the Hf/W and W/W ratios of the bulk mantle, which need to be known for calculating core formation ages. An alternative approach is to determine the W isotope evolution of the mantle of a differentiated planetesimal using the initial W/W and Hf/Hf ratios obtained from internal isochrons. Angrites are ideally suited for this approach because (i) they formed over a period that is sufficiently long for a resolvable evolution of the W/W ratio, (ii) their isotope systematics do not seem to be significantly disturbed by thermal and impact metamorphism, and (iii) precise internal Hf-W isochrons can be obtained due to the high Hf/W ratios of angrite pyroxenes. We present Hf-W mineral isochrons for a suite of angrites including D’Orbigny, Sahara 99555, NWA 4590, NWA 4801 and LEW 86010 and use these data to constrain the chronology of the accretion and earliest differentiation of their parent body. Methods and results: All angrites were gently crushed in an agate mortar and pyroxene, olivine, and plagioclase separates were obtained using heavy liquids and hand-picking, and were washed in ethanol. Sample dissolution, W purification and Hf-W concentration measurements followed our previously established techniques [e.g., 3]. All measurements were performed using a Nu Plasma MC-ICPMS at ETH Zurich. Tungsten isotope ratios of the samples were determined relative to two bracketing measurements of the W standard and are expressed in W, which is the 0.01% deviation from the terrestrial W/W. Olivine and plagioclase separates from all angrites have low Hf/W and W/W ratios, whereas the pyroxenes exhibit high Hf/W and radiogenic W/W ratios up to ~15 W. For all angrites, the Hf-W data for the mineral separates define precise isochron that yield the initial W/W and Hf/Hf ratios at the time of Hf-W closure. Discussion: Comparison of Hf-W to Pb-Pb, Al-Mg, and Mn-Cr ages. The Hf-W isochron ages for angrites are not only important for determining the chronology of the angrite parent body but also allow a comparison to results from the Mn-Cr, Al-Mg, and PbPb systems, all of which have also been applied to several of the angrites. In Fig. 1, the initial Hf/Hf ratios of angrites are plotted against their Pb-Pb ages. All angrites plot on a straight line, whose slope is identical to the one predicted from the Hf half-life. The excellent agreement between the Hf-W and Pb-Pb ages provide evidence that both Hf-W and Pb-Pb systematics provide reliable ages for angrite formation.