Crystallization Ages of Zircons on Eucrite Parent Body from Hf-W systematics

The accretion of planetary bodies like Earth, Moon, Mars, Vesta and other meteorite parent bodies (diameter > 50 kilometers) was quickly followed by melting and metal-silicate differentiation resulting in core formation. The decay of 182 Hf [half-life (T 1/2) ~ 9 million years (Ma)] to 182 W has proved to be a very useful relative chronometer to determine the core formation and silicate differentiation time scales. The refractory lithophile Hf and siderophile W present in near chondritic proportions in planetary bodies undergo significant fractionation during metal-silicate fractionaton prior to core formation. Evolution of the mantle through silicate differentiation causes further fractionation because W is more incompatible compared to Hf [1]. Eucrites, howardites and diogenites (HEDs) are a group of differentiated meteorites that possibly originated on asteroid 4 Vesta. The eucrites have 182 W excess [2]] indicating a very early differentiation process on Vesta. The early differentiation of Vesta is consistent with the inferences derived from other short-lived radionuclides 26 Al [half-life ~ 0.7 ma], 53 Mn [half-life ~ 3.5Ma] [3]. Zircon is an ideal mineral for determining the 182 Hf abundance at the time of its formation. It has a high concentration of Hf (1-2%) and significantly low abundance of W. Previous measurements on Hf-W systematics carried out using SHRIMP, SHRIMP-RG and Cameca ims 4f [4] were limited in measurement quality and precision. These measurements were carried out using either single detector system, high mass resolution ranging from ~7000 to 11,000 or energy filtering which resulted in severe loss of signal and increased measurement time. The signal strengths constrained by meteorite zircon size (~ 10-20µm) limit the counting statistics and the measurement precision. In this abstract we report the new 182 Hf-182 W evolution systematics and δ 183 W composition of several zircons from A881388 eucrite. We also report the data the 182 Hf-182 W evolution systematics for A881467 zircons which was reported previously [5] but have been corrected for their ionization efficiency following the prescription for the same as followed by [6]. These measurements were carried out using the multi-collector Cameca ims1270 large geometry ion microprobe (NORDSIMS) at the Swedish Museum of Natural History. These measurements have attempted to overcome the difficulties of the earlier work [4]. The Hf-W measurements were carried out using the multi-detector assembly at a nominal mass resolution of ~ 7500. The W isotopes were measured as metal [14]. A881388 and A881467 [previous abstract] have …