Fractional crystallization of olivine melt inclusion in shock-induced chondritic melt vein

The formation of ringwoodite, wadsleyite and majorite from their parental low-pressure polymorphs in melt veins in chondritic meteorites is usually interpreted to be the result of shock-induced solid-state phase transformation. Formation and survival of individual mineral melt enclaves in the chondritic high-pressure melt was not considered a viable possibility. We report evidence for melting of individual large olivine fragments entrained in melt veins, their survival as melt enclaves in the chondritic melts and their subsequent fractional crystallization at high-pressures and temperatures. The fractionally crystallized olivine melt enclaves appear to be ubiquitous in chondrites. In contrast, Ca-poor pyroxene fragments in the same veins and Ca-poor pyroxene in chondrules entrained do not show any sign of melting. Texture and compositions of olivine fragments are indicative of fractional crystallization from individual olivine melts alone. Fragments of original unzoned olivine (Fa(24-26)) melted, and melts subsequently fractionally crystallized to Mg-rich wadsleyite (Fa(6-10)) and Mg-poor ringwoodite (Fa(28-33)) with a compositional gap of <= 26 mol% fayalite. In contrast, compositions of ringwoodite and wadsleyite that emerged from solid-sate phase transformations are identical to that of parental olivine thus erasing any source of enigma. The olivine monomineralic melts barely show any signs of mixing with the chondritic liquid prior to or during their individual fractional crystallization. Our findings demonstrate that the formation of high-pressure minerals during shock events in asteroids also results from melting and fractional crystallization from some individual mineral melts that barely mixed with the chondritic melt host, a mechanism previously not recognized or accepted. (C) 2009 Elsevier B.V. All rights reserved.