Bencubbin is a breccia containing metal and silicate clasts, along with occasional chondritic fragments. The breccia is cemented together by a small amount of shock-melted metal-silicate matrix. There is no evidence, however, for complete melting of either metal or silicate clasts after their incorporation within the breccia. The main metal phase occurs as rounded and angular clasts of Fe-Ni. Each clast is chemically homogeneous, but systematic chemical variations between clasts are observed with Ni concentrations varying from ≅ 5.3 wt% in some clasts up to 7.5 wt% in others. Cobalt concentrations vary between clasts from 0.25 to 0.35 wt% and are positively correlated with Ni. The Co and Ni concentrations are consistent with the metal condensation path (pressure ≅ 10-3 atm) predicted by Grossman and Olsen (1974, Geochim. Cosmochim. Acta 38, 173-187). The P vs Ni concentrations are consistent with the metal condensation path (pressure ≅ 10-4 atm) predicted by Wai et al. (1978, Lunar and Planetary Science IX, pp. 1193-1195). Thus we believe that Bencubbin metal clasts may record chemical information imparted during condensation of the metal from the nebula. Chromium concentrations in Bencubbin metal (0.05-0.30 wt%) greatly exceed concentrations observed in iron meteorites as predicted by Grossman and Olsen (1974, Geochim. Cosmochim. Acta 38, 173-187) for metal condensates from the solar nebula. The Cr-Ni trend in Bencubbin, however, is positively correlated with Ni, in contrast to the predicted condensation trend. This unexpected correlation may be the result of subsequent redistribution of Cr between metal and micron-sized troilite blebs. The incorporation of these troilite blebs within the metal clasts is difficult to explain in terms of low temperature (≅ 700 K) condensation of troilite. The possible explanations for the presence of the troilite may or may not be consistent with an unaltered primitive composition for the metal clasts. High temperature equilibrium condensation of Bencubbin metal, however, is also supported by the low Ga and Ge contents reported by Kallemeyn et al. (1978, Geochim. Cosmochim. Acta 42, 507-515). Three of the metal clasts were found to contain ≅2.3wt% Si in alloy with the metal. The compositions of these clasts are consistent with equilibrium condensation at a pressure of ≅ 1 atm from a gas of cosmic composition. The Si-rich clasts could also have condensed at lower pressures from a gas with a fractionated C O ratio relative to cosmic abundances. © 1979.
