Refractory inclusions from the Leoville, Efremovka, and Vigarano c3v chondrites: Major element differences between types A and B, and extraordinary refractory siderophile element compositions

Abstract

We have measured the bulk major and trace element compositions often CAIs from Leoville, Efremovka, and Vigarano, members of the reduced subgroup of C3V chondrites, by INAA. Like CAIs from the reduced subgroup studied previously and unlike CAIs from Allende, a member of the oxidized subgroup, nine of the ten CAIs studied here possess only small amounts of secondary phases and, hence, preserve their pre-alteration bulk major element compositions. Using those compositions and model pre-alteration compositions of Allende CAIs, we cannot distinguish Type B1s clearly from B2s, nor some CTAs from FTAs. All Type As and most Type Bs have superchondritic CaO Al 2O 3 ratios, suggesting that their precursors condensed from a solar gas after ~20% of the Al was removed by high-temperature, Al-rich phases. Type Bs possess higher concentrations of MgO and SiO 2 than do Type As because the precursor phases of Type Bs reacted with the cooling nebular gas to form fassaite, while those of Type As did not. Eight of the ten CAIs have unusual or unique refractory trace element characteristics compared to those seen in Allende inclusions. Vig1, L2, Ef1, and Ef2 have remarkably high concentrations of Re and Os (~57–145 × Cl) because they sampled 4–9 times more hcp condensate metal alloy than did most Allende inclusions. Vig1, L2, Ef1, Ef2, L1, and Vig2 have unusually large Os Ru ratios (up to 6.9 × Cl) because the removal temperatures of their hcp alloy precursors were unusually high (up to ~ 1655 K at 10 −3atm). L2 and L4 have low Zr Hf ratios (~0.4 × Cl); L1 and Ef2, low V/Yb ratios (~0.25 × Cl); Ef2, high Th La (~1.6 × Cl), and Sr Yb (~-2.4 × Cl) ratios; and ED, high Hf Lu (~2.6 × Cl), Sc Lu (~3.1 × Cl), and Zr Lu (~2.8 × Cl) ratios. Most of these fractionations are explained by non-representative nebular sampling of one or more of hibonite, zirconium oxide, perovskite, and melilite, which may be condensates or evaporation residues. Of twenty-five inclusions from the reduced subgroup now studied in this laboratory, seventeen have at least one pair of refractory trace elements which are more fractionated relative to one another compared to Cl chondrites than are the same elements in Allende CAIs, indicating that the host phases of these elements were not as thoroughly mixed together in the nebular region where CAIs of the reduced subgroup accreted as where those in Allende did. Parts of the nebula sampled by C3V chondrites of the reduced subgroup were not well sampled by Allende, and vice versa. Like most coarse-grained inclusions from the reduced subgroup studied previously, most of those analyzed here have lower concentrations of Na, Au, Fe, Zn, and Mn than their Allende counterparts, consistent with the idea that they experienced secondary alteration at a higher temperature or for a shorter time than did Allende inclusions.