A petrological and chemical reexamination of Main Group pallasite formation

The large number of pallasite parent bodies suggests that their formation was a common event in asteroid evolution. New microprobe data has been generated for pallasite olivines and chromites that indicate subsolidus redox processes, while literature data has been collated and correlated to form new results and interpretations. The new data include information on metal compositions; the metal cooling rates; the formation of round and fragmental olivine; the close-packing of olivine; the crystallization of the phosphates, phosphoran olivine and orthopyroxene; and the bulk pallasite P and S contents. A model for Main Group pallasites has been developed using fractional melting of a chondritic precursor to make a multi-layered parent body. One portion of this contains a gradational metal–olivine (pallasite) texture. Concentric downward crystallization of the molten metal is indicated, based on cooling rates and metal compositions. Fractional crystallization of the residual silicate and metallic melts produces the minor phases on cooling. Comparisons to and a brief review of past models, particularly those utilizing mixing, have been made.