Metallic iron in basaltic achondrites

Abstract

The texture of metallic iron in many basaltic achondrites suggests that the metal formed during magmatic crystallization. In other basaltic achondrites the metal occurs in recrystallization textures. The nickel content of basaltic achondrite metal is in most cases lower than the nickel content of metal in chondritic meteorites. For meteorites where metal shows evidence of magmatic crystallization, nickel contents range from less than 0.1% to 0.6%. The distribution of nickel between silicate and metal phases in several meteorites is consistent with the fractionation obtained in experiments at low pressure and magmatic temperatures, showing that nickel is very strongly concentrated in the coexisting metal phase. The low total nickel content of most basaltic achondrites (≈4 ppm ) is best explained by separation of a nickel-rich metal phase during magmatic crystallization. The data suggest an inconsistency in upper-mantle models for an earth of chondritic composition, as it is difficult to reconcile the relatively high nickel content of ultrabasic rocks with the separation of a metallic core which should efficiently remove nickel.