Magnesium isotopic composition of achondrites

Magnesium isotopic compositions of 22 well-characterized differentiated meteorites including 7 types of achondrites and pallasite meteorites were measured to estimate the average Mg isotopic composition of their parent bodies and evaluate Mg isotopic heterogeneity of the solar system. The δ26Mg values are −0.236‰ and −0.190‰ for acapulcoite-lodranite and angrite meteorites, respectively and vary from −0.267‰ to −0.222‰ in the winonaite-IAB-iron silicate group, −0.369‰ to −0.292‰ in aubrites, −0.269‰ to −0.158‰ in HEDs, −0.299‰ to −0.209‰ in ureilites, −0.307‰ to −0.237‰ in mesosiderites, and −0.303‰ to −0.238‰ in pallasites. Magnesium isotopic compositions of most achondrites and pallasite meteorites analyzed here are similar and reveal no significant isotopic fractionation. However, Mg isotopic compositions of D′Orbigny (angrite) and some HEDs are slightly heavier than chondrites and the other achondrites studied here. The slightly heavier Mg isotopic compositions of angrites and some HEDs most likely resulted from either impact-induced evaporation or higher abundance of clinopyroxene with the Mg isotopic composition slightly heavier than olivine and orthopyroxene. The average Mg isotopic composition of achondrites (δ26Mg = −0.246 ± 0.082‰, 2SD, n = 22) estimated here is indistinguishable from those of the Earth (δ26Mg = −0.25 ± 0.07‰; 2SD, n = 139), chondrites (δ26Mg = −0.28 ± 0.06‰; 2SD, n = 38), and the Moon (δ26Mg = −0.26 ± 0.16‰; 2SD, n = 47) reported from the same laboratory. The chondritic Mg isotopic composition of achondrites, the Moon, and the Earth further reflects homogeneity of Mg isotopes in the solar system and the lack of Mg isotope fractionation during the planetary accretion process and impact events.