U-Th-Pb isotopic systematics of lherzolitic shergottite Yamato-793605

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We have undertaken U-Th-Pb isotopic studies on lherzolitic shergottite, Yamato-793605. Four mineral separates (olivine, plagioclase, and two phases of pyroxene) and a whole-rock sample were leached with dilute acid in order to remove secondary Pb contamination. However, preferential leaching of U and Th over Pb occurred. The Pb isotopic data points of five residue fractions scattered, and a calculated Pb-Pb age does not clearly indicate either crystallization from a melt or a later disturbance to the U-Pb system, in contrast with other shergottites. The chord through U-Pb data points for pyroxene (PXl) leaches and residue intersects concordia at 4439±9 Ma and 212±62 Ma, suggesting a young disturbance event resulting in Pb loss. The later disturbance event partially reset the U-Pb system. The estimated ^<238>U/^<204>Pb (≡μ) value from the whole-rock residue for the source of Y-793605 is about 5,suggesting that Y-793605 was derived from a volatile-rich source environment and confirms a low-μ source for shergottites compared to volcanic rocks of the Earth.
国立極地研究所の論文