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Micrometer-scale U-Pb age domains in eucrite zircons, impact re-setting, and the thermal history of the HED parent body

Abstract : Meteoritic zircons are rare, but some are documented to occur in asteroidal meteorites, including those of the howardite-eucrite-diogenite (HED) achondrite clan (Rubin, A. [1997]. Meteorit. Planet. Sci. 32, 231-247). The HEDs are widely considered to originate from the Asteroid 4 Vesta. Vesta and the other large main belt asteroids record an early bombardment history. To explore this record, we describe submicrometer distributions of trace elements (U, Th) and U-235,U-238-Pb-207,Pb-206 ages from four zircons (7-40 mu m empty set) separated from bulk samples of the brecciated eucrite Millbillillie. Ultra-high resolution (similar to 100 nm) ion microprobe depth profiles reveal different zircon age domains correlative to mineral chemistry and to possible impact scenarios. Our new U-Pb zircon geochronology shows that Vesta's crust solidified within a few million years of Solar System formation (4561 +/- 13 Ma), in good agreement with previous work (e.g. Carlson, R.W., Lugmair, G.W. [2000]. Timescales of planetesimal formation and differentiation based on extinct and extant radioisotopes. In: Canup, R., Righter, K. (Eds.), Origin of the Earth and Moon. University of Arizona Press, Tucson, pp. 25-44). Younger zircon age domains (ca. 4530 Ma) also record crustal processes, but these are interpreted to be exogenous because they are well after the effective extinction of Al-26 (t(1/2) = 0.72 Myr). An origin via impact-resetting was evaluated with a suite of analytical impact models. Output shows that if a single impactor was responsible for the ca. 4530 Ma zircon ages, it had to have been = 10 km in diameter and at high enough velocity (5 km s(-1)) to account for the thermal field required to re-set U-Pb ages. Such an impact would have penetrated at least 10 km into Vesta's crust. Later events at ca. 4200 Ma are documented in HED apatite U-235,U-238-Pb-207,Pb-206 ages (Zhou, Q. et al. [2011]. Early basaltic volcanism and Late Heavy Bombardment on Vesta: U-Pb ages of small zircons and phosphates in eucrites. Lunar Planet. Sci. 42. Abstract #2575) and Ar40-39 age spectra (Bogard, D.D. [2011]. Chem. Erde 71, 207-226). Yet younger ages, including those coincident with the Late Heavy Bombardment (LHB; ca. 3900 Ma), are absent from Millbillillie zircon. This is attributable to primordial changes to the velocity distributions of impactors in the asteroid belt, and differences in mineral closure temperatures (T-c zircon apatite). (C) 2014 Elsevier Inc. All rights reserved.
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M. D. Hopkins, S. J. Mojzsis, W. F. Bottke, O. Abramov. Micrometer-scale U-Pb age domains in eucrite zircons, impact re-setting, and the thermal history of the HED parent body. Icarus, Elsevier, 2015, 245, pp.367-378. ⟨10.1016/j.icarus.2014.08.025⟩. ⟨hal-02341069⟩



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