Impact mixing among rocky planetesimals in the early Solar System from angrite oxygen isotopes

Rider-Stokes, B. G.; Greenwood, R. C.; Anand, M.; White, L. F.; Franchi, I. A.; Debaille, V.; Goderis, S.; Pittarello, L.; Yamaguchi, A.; Mikouchi, T. and Claeys, P. (2023). Impact mixing among rocky planetesimals in the early Solar System from angrite oxygen isotopes. Nature Astronomy, 7 pp. 836–842.



Angrite meteorites are thought to represent ancient basaltic igneous rocks that formed inward of Jupiter’s orbit on the basis of their isotopic parameters such as ε50Ti, ε54Cr and Δ17O in addition to Fe/Mn ratios of pyroxene. New bulk oxygen isotope measurements of nine angrites, and of olivine ‘xenocrysts’ and groundmass fractions from three quenched angrites, however, reveal clear isotopic disequilibrium, implying an impact melt origin. Groundmass fractions from Asuka 12209, Asuka 881371 and Northwest Africa 12320 quenched angrites demonstrate an average Δ17O value of −0.003 ± 0.020‰. Here, excluding the bulk value and all groundmass fractions of Northwest Africa 12320, which is contaminated by an impactor, we determine a new well constrained average Δ17O value for the angrite parent body (−0.066 ± 0.016‰). Microstructural investigations of Northwest Africa 12320 reveal the presence of both fully recrystallized and undeformed olivine xenocrysts, indicating that some xenocrysts underwent high-temperature processes. These results suggest that angrites bear signatures of impact-driven isotopic mixing, possibly in response to early giant planet migration. The evidence for impact mixing raises doubts about the utility of quenched angrites as a suitable Pb–Pb isotopic anchor, which in turn has consequences for accurately defining the timeline of other Solar System events.

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