Lead and Mg isotopic age constraints on the evolution of the HED parent body

Martin SCHILLER, James N. CONNELLY, and Martin BIZZARRO
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12848]
Centre for Star and Planet Formation, Natural History Museum of Denmark, Copenhagen, Denmark
Published by Arrangement with John Wiley & Sons

The large collection of howardite-eucrite-diogenite (HED) meteorites allows us to study the initial magmatic differentiation of a planetesimal. We report Pb-Pb ages of the unequilibrated North West Africa (NWA) 4215 and Dhofar 700 diogenite meteorites and their mass-independent 26Mg isotope compositions (26Mg*) to better understand the timing of differentiation and crystallization of their source reservoir(s). NWA 4215 defines a Pb-Pb age of 4484.5 ± 7.9 Myr and has a 26Mg* excess of +2.3 ± 1.6 ppm whereas Dhofar 700 has a Pb-Pb age of 4546.4 ± 4.7 Myr and a 26Mg* excess of +25.5 ± 1.9 ppm. We interpret the young age of NWA 4215 as a thermal overprint, but the age of Dhofar 700 is interpreted to represent a primary crystallization age. Combining our new data with published Mg isotope and trace element data suggests that approximately half of the diogenites for which such data are available crystallized within the first 1–2 Myr of our solar system, consistent with a short-lived, early-formed magma ocean undergoing convective cooling. The other half of the diogenites, including both NWA 4215 and Dhofar 700, are best explained by their crystallization in slowly cooled isolated magma chambers lasting over at least ~20 Myr.

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