Compositional and genetic relationship between chondrules, chondrule rims,metal, and matrix in the Renazzo chondrite

Individual chondrules, coarse and fine-grained metal, chondrule rims, and m atrix samples were separated from the Renazzo chondrite (CR2) and studied b y instrumental neutron activation analysis. Both Renazzo chondrule and matr ix fractions have CI Cr/Mg concentration ratios, unlike chondrules from oth er classes of chondrites, which have fractionated Cr/Mg ratios. This and ot her arguments suggest that Renazzo chondrules and matrix formed in the same nebular region. In Renazzo, fine-grained metal-the dominant metal componen t, is mostly located inside chondrules or in chondrule rims. Matrix contain s few coarse metal grains. Both types of metal, fine and coarse, exhibit si milar chemical signatures, comparatively high in Cr and low in Ni, suggesti ng a genetic relationship. Although metal is mostly contained in chondrules, chondrules and matrix of Renazzo have similar common siderophile/lithophile abundance ratios. This m ay imply that Renazzo chondrules and metal were formed by reduction of oxid ized precursors compositionally similar to CI chondrites. The CI-like conce ntrations of Sc, Mg, Se, and Zn in Renazzo matrix are consistent with this inference. The differences in siderophile element pattern between chondrule s, chondrule metal, and matrix indicate evaporation and recondensation of v olatile elements during chondrule formation. Recondensation of evaporated e lements was incomplete and the degree of recondensation correlates with ele ment volatility. The higher-than-CI K/Mg, Na/Mg, and Ga/Fe ratios of Renazz o matrix may reflect recondensation of K, Na, and Ga into the matrix. The C I Se/Sc and Zn/Sc concentration ratios of the matrix, however, reflect very limited degrees of recondensation of Se and Zn into matrix. The presence o f abundant phyllosilicates and organic material in Renazzo matrix requires a low background temperature (<200 K) during formation of the Renazzo meteo rite. Partial recondensation of volatile elements in a cold nebular environ ment implies instantaneous agglomeration of the Renazzo chondrite following chondrule formation. Copyright (C) 199 Elsevier Science Ltd.