Lead-lead (Pb-Pb) dating of eucrites and mesosiderites: Implications for the formation and evolution of Vesta

Abstract

Asteroid 4 Vesta is the largest known differentiated body in the asteroid belt, and is thought to be the likely parent body of howardite, eucrite, and diogenite (HED) meteorites. Howardites likely represent the vestan surface, eucrites likely crystallized as lavas in the upper crust, and diogenites likely originated from a plutonic layer deep in the crust. HEDs are potentially linked to another group of meteorites: the stony-iron mesosiderite group, due to mineralogical, geochemical, and isotopic similarities. Collision and disruption processes in the asteroid belt are key to understanding the evolution of asteroids, with impact events generating significant volumes of melt, which, if dated, can provide information on the timing and nature of such events. We performed in situ lead-lead (Pb-Pb) dating using Secondary Ionization Mass Spectrometry (SIMS) on melt clasts (mainly comprised of pyroxene, plagioclase, ± iron-nickel metal and/or glass) in two eucrites (Serra Pelada and Northwest Africa (NWA) 2696) and phosphates targeted within three mesosiderites (Vaca Muerta, Hainholz, and Estherville), respectively. The eucrite melt clasts yielded ages of 4520 ± 11 Ma and 4528.6 ± 6.3 Ma, in Serra Pelada and NWA 2696, respectively, and are likely indicative of a major heating event, such as an impact, metamorphism due to burial, or prolonged magmatism on the parent body. Our results from targeted mesosiderite phosphate analysis yielded a younger age range of ∼3968 – 4112 Ma, similar to ages reported previously for phosphate analysis in eucrites, as well as the broad range observed previously for many HEDs, and towards the upper end of the age range observed in lunar materials. These data may suggest a period of increased impact flux or possibly several higher-magnitude impacts on the mesosiderite parent body within that timeframe. Our results add further support to the likelihood of the existence of (at least) two different parent bodies for the HEDs (Vesta) and mesosiderites (Mesosiderite Parent Body – MPB).