Petrology of the basaltic achondrite meteorites

Abstract

Mineralogical and textural evidence indicates that the basaltic achondrites originated in one or more magmatic episodes in a variety of cooling environments that resulted in textures ranging from gabbroic to diabasic. Chemical compositional and mineralogical variations are consistent with a common origin for the basaltic achondrites by magmatic differentiation. The characteristics of the mineralogical, major element and trace element variations are similar to those of the Skaergaard intrusion, but the basaltic achondrite magmas started from different compositions and crystallized under much lower partial pressures of oxygen(in the stability field of metallic iron) than did the magmas of the Skaergaard intrusion. The differentiation trends shown by the basaltic achondrites indicate that the starting material had calcic plagioclase and was depleted in alkalis with respect to chondritic meteorites, in which the plagioclase is sodic. Brecciation is a conspicuous feature of most basaltic achondrites, which can be most satisfactorily subgrouped on the basis of breccia type as brecciated eucrites (monomict breccias), eucrites (unbrecciated), and howardites (polymict breccias). Petrographic evidence suggests that some textural metamorphism and brecciation were produced by shock effects that accompanied impact events on the surface of the parent body. The abundances of basaltic achondrite falls are consistent with a surface-sampling mechanism such as meteorite impact ejection. A preponderance of near-surface samples and the distinct differences between the basaltic achondrites and chondrites suggest that the moon is the probable parent body of the basaltic achondrites.