Bombardment history of Asteroid 4 Vesta recorded by brecciated eucrites: Large impact event clusters at 4.50 Ga and discreet bombardment until 3.47 Ga

1Trudi Kennedy,1Fred Jourdan,2Ela Eroglu,1Celia Mayers
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.06.027]
1Western Australian Argon Isotope Facility, JdL Centre & Applied Geology, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia
2Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia
Copyright Elsevier

The thermal/impact histories of sixteen eucrite meteorites were investigated: three monomict eucrites (NWA 999, A-87272,87, and Stannern), five polymict eucrites (NWA 1000, NWA 1666, NWA 5601, Y-980066,100, and Y-980255,100), three quench-textured, eucrite melt rocks (Y-981646,21, Y-981651,105, and MIL 0766214), one eucrite dominantly comprised of quench-textured clasts (QUE 99005,11), three unclassified eucrite breccias (LAP 031316,9, LAR 06870,5, QUE 99799,4) and one unbrecciated eucrite (EET 92004,17), included here due to its shock features. We have measured fifteen high-precision new 40Ar/39Ar plateau ages on plagioclase and matrix for ten of these meteorites with a tight cluster of nine ages obtained from three different polymict breccias. These ages range from 4534 ± 56 Ma to 4491 ± 16 Ma resulting in a concordant age population (P = 0.16). The fact that such a cluster of ages is recorded in unrelated breccias which are made of a priori unrelated components, leads us to propose that those ages recorded a single heating event on a large scale, and is interpreted here as a high-energy impact event, early in the history of Vesta at 4500 ± 4 Ma. We propose that the debris was ejected and isolated from subsequent large impacts in a secondary rubble pile asteroid where the energy of the outgoing shock wave from an impact is significantly reduced as it compacts the target material (Holsapple et al., 2002, and references therein) .

The other analyses define a spread of five plateau ages ranging from 3851± 21 Ma to 3469 ± 35 Ma, over ∼380 Ma. An additional apparent plateau age of 4288 ± 38 Ma, but with a diffusion profile of cumulative 39Ar release, along with published U-Pb apatite age of ∼4.14 Ga, suggests that the data might either define a true continuum (normal background bombardment) from 4.5 Ga to 3.47 Ga or cluster between ∼3.85 Ga and ∼3.47 Ga (excavation of a fresh surface at 3.85 Ga continuously bombarded until 3.47 Ga). Both scenarios are compatible with a final ejection age of 3.47 Ga when a major impact liberates the bulk of the brecciated meteorites into another secondary rubble pile asteroid, where the brecciated eucrites stayed relatively well protected from subsequent major impacts. Based on these results and recent crater counting measurements, we propose that the excavation and bulk ejection were caused by the Rheasilvia (ca. 3.47 Ga) basin-forming impact.

Diffusion models on plagioclase crystals with different Ar age spectrum signatures, from a single breccia (NWA1666; 4501 ± 7 Ma), suggest that either: (1) the formation of the breccia is very young, or (2) different plagioclase crystals have different diffusion characteristics, and/or (3) the porosity caused heterogeneous temperatures during an impact heating event, particularly likely if the 4.5 Ga brecciated eucrites were stored in a rubble pile asteroid. Many or possibly most large asteroids being re-accumulated rubble piles with potential large porosity (Holsapple et al., 2002). Scenarios (2) and (3) preclude the usage of multi-grain aliquots to decipher the time-temperature history of most impact breccia using 40Ar/39Ar thermochronology.

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