Metal phases of L chondrites: Their formation and evolution in the nebula and in the parent body

Metal phases of six (three equilibrated and three unequilibrated) L chondrites were studied by INAA, SEM, and Mössbauer spectroscopy. Characteristics retained in the bulk metals of unequilibrated chondrites (abundant carbon, high contents of Cr, V, Mn, and low contents of W, Mo, and Ga compared to metals of equilibrated chondrites, less enrichment of W than Mo, and fractionation of Co from Ni) demonstrate that chondrite metals are not nebular condensates. All those characteristics can be well explained by melting. Chondrite metals are not melting remnants of previously condensed metals, rather, they were produced by reduction of CI-or CM-like material during the melting process. The complementarity in composition and the similarity in melting feature suggest that chondritic metals and chondrules are the complementary components of the same melting event. Distribution of trace siderophile elements between taenite and bulk metal indicates that kamacite and taenite can only be the lowtemperature diffusion products and must have been developed in the chondrite parent body. The difference in the taenite composition between equilibrated and unequilibrated chondrites reveals that the equilibrated chondrites were located near the surface while the unequilibrated chondrites were in the interior if they were derived from the common parent body. Thus, while the exsolution of chondrite metal into kamacite and taenite was due to the internal thermal activity, the crystallization of EOC silicates resulted from an external heating. The internal metamorphism was mild (400-600°C) and long whereas the external heating was intense (with a maximum temperature in range of 800-950°C) and short. Tetrataenite is present not only in UOCs but also in EOCs, suggesting that the external heating occurred during the internal metamorphism, i.e., within 100 myr of chondrite formation.