Partial melting and melt migration in the acapulcoite-lodranite parent body

Abstract

We review petrologic evidence that the acapulcoites and lodranites formed by < 1 vol% to probably 20 vol% whole rock partial melting of a chondritic precursor material. At low degrees of partial melting, only Fe,NiFeS cotectic melting occurred. Migration distances for partial melts were short, resulting in the formation of acapulcoites with essentially chondritic troilite and plagioclase contents, but achondritic textures. At high degrees of partial melting, both Fe,NiFeS and basaltic (plagioclase-pyroxene) partial melts formed, and the melts may have migrated out of the source rock. The partial melt residues, which are more or less depleted in Fe,NiFeS and plagioclase, are the lodranites. Melt migration was complex: most acapulcoites, which experienced relatively low degrees of partial melting, lost little if any of the partial melt. One acapulcoite, LEW 86220, represents a unique case in which Fe,NiFeS and basaltic partial melts appear to have migrated from a lodranite source region into a cooler acapulcoite region, where they were trapped. In cases of the relatively high degrees of partial melting experienced by lodranites, melts may have been partly, selectively, or totally removed from the rocks, and Fe,NiFeS and/or basaltic partial melts may have been removed to different degrees and may, in fact, have been trapped on occasion in greater than chondritic proportions. We model vein and dike formation and melt migration by calculating the excess pressures and vein and dike sizes for varying degrees of partial melting. Our calculations are broadly consistent with observations, indicating that melt migration is inefficient at low degrees of partial melting and extremely efficient at high degrees of partial melting. Although the size of the acapulcoite-lodranite parent body and the volatile contents of the chondritic precursor rocks are poorly constrained, the lack of basaltic rocks in the world's meteorite collections complementary to the lodranites suggests that basaltic partial melts may have been accelerated off the body by explosive volcanism of the type envisioned by Wilson and Keil (1991) and ejected into space. The diversity of rocks from the acapulcoite-lodranite parent body may provide a basis for better understanding the diverse range of spectral subtypes recognized among the S-type asteroids.