Timing of Formation of the Earth-Moon System

Details Timing of Formation of the Earth-Moon System Timing of Formation of the Earth-Moon System

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v11e..02h&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #V11E-02

Physics

1026 Composition Of The Moon, 1040 Radiogenic Isotope Geochemistry, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1115 Radioisotope Geochronology

Scientific paper

It is generally agreed that proto-Earth had accreted from planetesimals of broadly chondritic composition by 4.55 Ga. It is also widely believed that the Earth-Moon system formed following the subsequent collision with a Mars-sized-object. Such an event would surely have vaporized a large portion of both impactor and target and melted the rest of the combined system. However, the timing of this hypothesized event and the duration of crystallization of the molten outer layers of both bodies are controversial. The timing of terrestrial core formation (which would place an upper bound on the age of formation of the metal-poor Moon and thus collision) is not well known. Hf-W model ages range from 20 to >100 m.y., depending on assumed value of Hf/W in the silicate Earth or Moon, but recent reports favor the higher end of this range. 142,143Nd/144Nd data for lunar basalts have been interpreted to indicate that the lunar magma ocean (LMO) was still active at 4.35-4.31 Ga, although such relatively youthful ages could instead reflect isotopic resetting or source heterogeneity. Interpretation of zircon Lu-Hf data, however, is less sensitive to underlying assumptions that limit the aforementioned estimates. We have recovered exotic zircons in the age range 4.35 to 3.91 Ga from both terrestrial (Jack Hills quartzites) and lunar (Apollo 14 breccias) samples and undertaken laser ablation Lu-Hf analyses to obtain initial 176Hf/177Hf ratios (Hfo). Terrestrial zircons (n=221) show dominantly negative ɛHf(T) throughout the Hadean and yield an essentially uniform spectrum of mantle extraction ages between 4.5-4.2 Ga. Four robust analyses yield Hfo close to the solar system initial indicate source sequestration in an ultra-low 176Lu/177Hf environment by 4.52±0.02 Ga (using the most recent chondritic Lu-Hf parameters). The lunar zircons crystallized from highly incompatible-trace-element enriched (KREEP) sources that evolved during differentiation of the LMO. Nearly half of the model ages calculated assuming a source 176Lu/177Hf of 0.014 (derived from the overall data trend but consistent with independent estimates) cluster at 4.48±0.03 Ga which we interpret to be a lower bound on the last stage of LMO crystallization. We thus suggest that collisional events leading to the formation of the Earth-Moon system and the solidification of the molten outer layers of both bodies may have been complete within 70 m.y. of CAI formation.

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