Dating the Giant Moon--Forming Impact and the End of Earth's Accretion

Details Dating the Giant Moon--Forming Impact and the End of Earth's Accretion Dating the Giant Moon--Forming Impact and the End of Earth's Accretion

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p41e..04k&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #P41E-04

Mathematics

Logic

6200 Planetary Sciences: Solar System Objects

Scientific paper

The most widely--accepted model for the formation of the Moon involves a catastrophic collision between a Mars--sized body and the proto--Earth. Simulations of this Giant Impact provide evidence that it most likely occurred during the final stages of accretion contributing roughly the last 10% of Earth's mass. As such, the age of the Moon provides the best time-marker for the completion of Earth's accretion. Determining the age of the Moon has proven difficult however. Early attempts provided only minimum age constraints from long-lived chronometers that dated the most ancient lunar rocks but thereby left the Moon's exact age with considerable uncertainty. The age of the Moon can in principle be determined by dating the Moon's early large--scale (chemical) differentiation. Owing to the high temperatures produced during the Giant Impact, the origin of the Moon is associated with the generation of a global magma ocean. Although the time of formation of this magma ocean cannot be determined directly, dating its crystallization provides a minimum formation age of the Moon. The 182Hf-182W chronometer potentially provides the most powerful constraints on the age and early differentiation of the Moon but cosmogenic 182W--production mainly via neutron--capture of 181Ta at the lunar surface precludes chronological interpretations. We report the first W isotope data for lunar metals, which have low Ta/W and, hence, contain no cosmogenic 182W. The data reveal differences in indigenous 182W/184W of lunar mantle reservoirs, indicating crystallization of the lunar magma ocean less than ~50 Myr after formation of the solar system. The Hf--W age of the lunar magma ocean of <50 Myr combined with the Hf--W model age of >30 Myr for Earth's accretion tightly constrains the age of the Moon and the final stage of Earth's accretion to 30--50 Myr after the formation of the solar system. The relatively late formation of the Moon more than ~30 Myr after that of asteroids and Mars underpins the Moon's origin by a unique event, as required in the Giant Impact hypothesis.

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