QUE 94204: A primitive enstatite achondrite produced by the partial melting of an E chondrite-like protolith

摘要

Queen Alexandra Range (QUE) 94204, an enstatite achondrite, is a coarse-grained, highly recrystallized, chondrule-free and unbrecciated rock dominated (about 70vol%) by anhedral, equigranular crystals of orthoenstatite of nearly endmember composition (Fs_(0.1-0.4), Wo_(0.3-0.4)) with interstitial plagioclase, kamacite, and troilite. Abundance of approximately 120° triple junctions and the close association of metal-sulfide and plagioclase-rich melts indicate that QUE 94204 has undergone limited partial melting with inefficient melt extraction. Mineral chemistry indicates a high degree of thermal metamorphism. Kamacite in QUE 94204 contains between 2.09 and 2.55wt% Si, similar to highly metamorphosed EL chondrites. Plagioclase has between 4.31 and 6.66wt% CaO, higher than other E chondrites but closer in composition to plagioclase from metamorphosed EL chondrites. QUE 94204 troilite contains up to 2.55wt% Ti, consistent with extensive thermal metamorphism of an E chondrite-like precursor. Results presented in this study indicate that QUE 94204 is the result of low degree, (about 5-20vol%, probably toward the lower end of this range) partial melting of an E chondrite protolith. Textural and chemical evidence suggests that during the metamorphism of QUE 94204, melts formed first at the Fe,Ni-FeS cotectic near approximately 900°C, followed by plagioclase-pyroxene silicate partial melts near approximately 1100°C. Neither the Fe,Ni-FeS nor the plagioclase-pyroxene melts were efficiently segregated or extracted. QUE 94204 belongs to a grouplet of similar "primitive enstatite achondrites" that are analogous to the acapulcoites-lodranites, but that have resulted from the partial melting of an E chondrite-like protolith.