Accessory mineral microstructure and chronology reveals no evidence for late heavy bombardment on the asteroid 4-VestaOPEN ACCESS 

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L.F. White, D.E. Moser, J.R. Darling, B.G. Rider-Stokes, B. Hyde, K.T. Tait, K. Chamberlain, A.K. Schmitt, J. Dunlop, M. Anand

Earth and Planetary Science Letters
Volume 636, 15 June 2024, 118694

LINK (OPEN ACCESS)

“Highlights

  • We conduct correlated microstructural and isotopic analysis of six eucrites.
  • U-bearing accessory minerals baddeleyite, zircon, and apatite were targeted.
  • No sample records ages around 3.9 Ga, the hypothesised late heavy bombardment.
  • Older (> 4.2 Ga) grains record no structural evidence of later shock deformation.
  • We find no evidence for a increased impact flux (LHB) in the studied eucrites.”

“A long-standing paradigm in planetary science is that the inner Solar System experienced a period of intense and sustained bombardment between 4.2 and 3.9 Ga. Evidence of this period, termed the Late Heavy Bombardment is provided by the 40Ar/39Ar isotope systematics of returned Apollo samples, lunar meteorites, and asteroidal meteorites. However, it has been largely unsupported by more recent and robust isotopic age data, such as isotopic age data obtained using the U-Pb system. Here we conduct careful microstructural characterisation of baddeleyite, zircon, and apatite in six different eucrites prior to conducting SIMS and LA-ICP-MS measurement of U, Th, and Pb isotopic ratios and radiometric dating. Baddeleyite, displaying complex internal twinning linked to reversion from a high symmetry polymorph in two samples, records the formation of the parent body (4554 ± 3 Ma 2σ; n = 8), while structurally simple zircon records a tight spread of ages representing metamorphism between 4574 ± 14 Ma and 4487 ± 31 Ma (n = 6). Apatite, a more readily reset shock chronometer, records crystallisation ages of ∼4509 Ma (n = 6), with structurally deformed grains (attributed to impact events) yielding U-Pb ages of 4228 Ma (n = 12). In concert, there is no evidence within the measured U-Pb systematics or microstructural record of the eucrites examined in this study to support a period of late heavy bombardment between 4.2 and 3.9 Ga.”

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