Origin of opaque assemblages in C3V meteorites: Implications for nebular and planetary processes

Abstract

Mineral phases from opaque assemblages (OAs) in Ca, Al-rich refractory inclusions (CAIs), chondrules and matrix in C3V meteoites were chemically analyzed and compared with experimentally determined phase equilibria and partitioning data in the Ni-Fe-S, Ni-Fe-S and Ni-Fe-O systems to estimate the temperature, sulfur fugacity (f_(S2)) and oxygen (f_(O2)) of OA formation. The kinetics of dissolution and exsolution of metallic phases in the Ni-Fe-Ru system were used to constrain the thermal history of OAss that occur in CAIs. Based on this work, we suggest that OAs formed after the crystallization of host CAIs by exsolution, sulfidation and oxidation of precursor alloys at low temperatures (~ 770 K) and higher than solar gas f_(S2) and f_(S2). Our model contrasts with previous models that call upon the formation of CAI OAs by aggregation of previously formed phases in the solar nebula prior to the crystallization of CAIs. Opaque assemblages in CAIs and chondrules probably originated as homogeneous alloys during melting of the silicate protions of CAIs and chondrules. The compositions of these precursor alloys reflect high-temperature and low-f_(O2) conditions in the early solar nebula. The similarities in the temperature, f_(S2) and f_(O2) of equilibrium for OAs that occur in CAIs, chondrules and matrix suggest that these three components of C3V meteorites share a common, late low-temperature history. The mineral phases in OAs do not preserve an independent history prior to CAI and chondrule melting and crystallization, but instead provide important information on the post-accretionary history of C3V meteorites and allow us to quantify the temperature, f_(S2) and f_(O2) of cooling planetary environments.

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