Mn‐Cr isotope systematics of the D'Orbigny angrite

Abstract— We have conducted a detailed study of the Mn‐Cr systematics of the angrite D'Orbigny. Here, we report Cr isotopic abundances and Mn/Cr ratios in olivine, pyroxene, glass, chromite, and bulk rock samples from D'Orbigny. ⁵³Cr excesses in these samples correlate well with their respective Mn/Cr ratios and define an isochron with a slope that corresponds to an initial ⁵³Mn/⁵⁵Mn ratio = (3.24 ± 0.04) × 10^?6 and initial ⁵³Cr/⁵²Cr ratio of ?(53) = 0.30 ± 0.03 at the time of isotopic closure. The ⁵³Mn/⁵⁵Mn ratio of the D'Orbigny bulk rock is more than two‐fold the ⁵³Mn/⁵⁵Mn ratio of the angrites Lewis Cliff 86010 (LEW) and Angra dos Reis (ADOR) and implies an older Mn‐Cr age of 4562.9 ± 0.6 Ma for D'Orbigny relative to a Pb‐Pb age of 4557.8 ± 0.5 Ma for LEW and ADOR. One of the most unusual aspects of D'Orbigny is the presence of glass, a phase that has not been identified in any of the other angrites. The Mn‐Cr data for glass and a pyroxene fraction found in druses indicate that they formed contemporaneously with the main phases of the meteorite. Since the Mn‐Cr age of D'Orbigny is ?5 Ma years older than the angrites LEW and ADOR, D'Orbigny likely represents an earlier stage in the evolution of the angrite parent body.