PARTIAL MELTING AND MELT MIGRATION IN THE ACAPULCOITE-LODRANITE PARENT BODY

We review petrologic evidence that the acapulcoites and lodranites for med by <1 vol% to probably >20 vol% whole rock partial melting of a ch ondritic precursor material. At low degrees of partial melting, only F e,Ni-FeS cotectic melting occurred. Migration distances for partial me lts were short, resulting in the formation of acapulcoites with essent ially chondritic troilite and plagioclase contents, but achondritic te xtures. At high degrees of partial melting, both Fe,Ni-FeS and basalti c (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 rel atively low degrees of partial melting, lost little if any of the part ial melt. One acapulcoite, LEW 86220, represents a unique case in whic h Fe,Ni-FeS and basaltic partial melts appear to have migrated from a lodranite source region into a cooler acapulcoite region, where they w ere trapped. In cases of the relatively high degrees of partial meltin g experienced by lodranites, melts may have been partly, selectively, or totally removed from the rocks, and Fe,Ni-FeS and/or basaltic parti al 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 parti al melting. Our calculations are broadly consistent with observations, indicating that melt migration is inefficient at low degrees of parti al melting and extremely efficient at high degrees of partial melting. Although the size of the acapulcoite-lodranite parent body and the vo latile contents of the chondritic precursor rocks are poorly constrain ed, the lack of basaltic rocks in the world's meteorite collections co mplementary 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 dive rsity of rocks from the acapulcoite-lodranite parent body may provide a basis for better understanding the diverse range of spectral subtype s recognized among the S-type asteroids. Copyright (C) 1997 Elsevier S cience Ltd.