SIMS Pb-Pb and U-Pb age determination of eucrite zircons at < 5 μm scale and the first 50 Ma of the thermal history of Vesta

Eucrites, a subgroup of basaltic achondrites, represent the remnants of the earliest magmatic stage of the Solar System formation, possibly on the asteroid 4 Vesta. The precise determination of their crystallization age is therefore important for constraining the evolutionary history of the asteroid, including melting and differentiation processes of the parent body. Zircons would be the best mineral for such purpose. However, extra-terrestrial zircons are rare, and those found in eucrites are typically small (< 5 mu m). Using a Cameca IMS-1280 high-resolution ion microprobe, precise uranium-lead (U-Pb) and Pb-Pb ages of the non-cumulate basaltic eucrite zircons in Bereba, Cachari, Caldera, Camel Donga, and Juvinas were determined successfully at scale of < 5 mu m. We obtained a weighted average Pb-207/Pb-206 age of 4541 +/- 11 Ma and U-Pb concordia age of 4525 +/- 24 Ma for the five eucrite samples studied. A synthesis of available age data of howardites, eucrites and diogenites indicate a protracted magmatic history on Vesta, with a most pronounced core-mantle differentiation event marked at 4564 +/- 2 Ma, followed by a peak of basaltic magmatism at 4552 +/- 7 Ma, which gradually diminished over the next 50 Ma. Thermal modeling of Vesta suggests that partial melt fraction of greater than 10%, with temperature well exceeding 1500 K could be maintained over this period throughout its interior. This is consistent with the chronological records of eucrites suggesting persistent magmatism on Vesta during the first 50 Ma in the early Solar System. (C) 2013 Elsevier Ltd. All rights reserved.