Oxygen isotope evidence for rapid mixing of the HED meteorite parent body

The 16O, 17O and 18O abundances of howardites, eucrites, and diogenites have been used to assign them to a single ‘HED’ parent body, thought to be asteroid 4 Vesta. We report the first evidence of oxygen isotopic heterogeneity among HED meteorites indicating incompletely mixed sources. New high-precision oxygen isotope measurements of 34 HED meteorites reveal that most have the same v17OP, consistent with a very rapid early history of large-scale mixing on Vesta. However, howardites are on average very slightly enriched in 16O, whereas Ibitira, Caldera, Pasamonte, and ALHA78132 are 16O-depleted compared to other investigated eucrites. The v17OP of Ibitira is completely different from all other HEDs measured. Some of the results for eucrites and diogenites can be explained by partial melting and rapid mixing of the interior of Vesta. Others require a separate parent body or indicate that parts of the outer layer of Vesta retained some primary isotopic heterogeneity. The oxygen isotopic composition of howardites provides an upper limit for the amount of admixed carbonaceous chondritic material into the HED parent body regolith. 8 2004 Elsevier B.V. All rights reserved.