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キーワード:
martian meteorites; Hf-W chronometry; Sm-Nd chronometry; planetary
differentiation; magma ocean; crust formation; Earth Science
要旨:
Mars probably accreted within the first 10 million years of Solar System formation and likely underwent magma ocean crystallization and crust formation soon thereafter. To assess the nature and timescales of these large-scale mantle differentiation processes we applied the short-lived 182Hf-182W and 146Sm-142Nd chronometers to a comprehensive suite of martian meteorites, including several shergottites, augite basalt NWA 8159, orthopyroxenite ALH 84001 and polymict breccia NWA 7034. Compared to previous studies the 182W data are significantly more precise and have been obtained for a more diverse suite of martian meteorites, ranging from samples from highly depleted to highly enriched mantle and crustal sources. Our results show that martian meteorites exhibit widespread 182W/184W variations that are broadly correlated with 142Nd/144Nd, implying that silicate differentiation (and not core formation) is the main cause of the observed 182W/184W differences. The combined 182W-142Nd systematics are best explained by magma ocean crystallization on Mars within ∼20-25 million years after Solar System formation, followed by crust formation ∼15 million years later. These ages are indistinguishable from the I-Pu-Xe age for the formation of Mars' atmosphere, indicating that the major differentiation of Mars into mantle, crust, and atmosphere occurred between 20 and 40 million years after Solar System formation and, hence, earlier than previously inferred based on Sm-Nd chronometry alone.
