Vesta and extensively melted asteroids: Why HED meteorites are probably not from Vesta

摘要

Most researchers hold that the HED clan of differentiated meteorites originated on Vesta largely based on the assumption that nearly all V-type asteroids with basaltic reflection spectra are fragments spalled off Vesta. Although it is a reasonable working hypothesis that most of the V-type asteroids in the Vesta family originated on Vesta, the spectra are not unique enough to confirm this; a sizable fraction may have been produced during the destruction of a differentiated asteroid in the same large region of dynamic space. Observations of asteroids in the inner Asteroid Belt show that more than half of the V-type asteroids do not belong to the Vesta dynamic family.Iron-meteorite evidence shows that at least 26 asteroids experienced extensive melting and would have generated basalts and other differentiated stony meteorites. Most iron meteorites show high degrees of elemental fractionations that lead to the conclusion that they experienced fractional crystallization; it is probable that all these bodies generated basalts. There are 9 of these "magmatic" iron-meteorite groups and test criteria mainly based on extreme fractionations indicate that an additional 17 disrupted asteroids hosted fractionally crystallized cores and thus that ≥26 asteroids experienced extensive melting; this estimate is much lower than previous estimates that included nonmagmatic irons.Within expected planetary heterogeneities the O-isotopic composition of HEDs is the same as that in oxides from IIIAB irons, the largest magmatic group of iron meteorites. εCr54 values are also very similar in IIIABs and HEDs. The O- and Cr-isotopic ties are much stronger than the spectral tie thus the working hypothesis should be that HEDs are from the IIIAB parent asteroid.Remote elemental analysis could confirm that HEDs are not from Vesta. If future remote analysis measures K contents ≥0.6 mg/g this will indicate that HEDs did not originate on Vesta.