Refractory inclusions with unusual chemical compositions from the Vigarano carbonaceous chondrite

Abstract

Ten inclusions, nine from Vigarano and one from Leoville, both members of the reduced subgroup of C3V chondrites, were analyzed for major and trace elements by neutron activation. Most have some refractory element characteristics that are common in refractory inclusions from Allende, a member of the oxidized subgroup of C3V chondrites. For instance, three (Leo 3537-2, Vig 1623-8, Vig 1623-11) are Type B inclusions with Group I or modified Group I REE patterns, three (Vig 1623-5, Vig 1623-10, Vig 1623-13) are olivine-rich inclusions, each of which is rather uniformly diluted in most refractory elements compared to Group I inclusions, and two (Vig 1623-14, Vig 1623-16) are fine-grained inclusions with Group II REE patterns. Six of the Vigarano inclusions, however, have refractory element fractionations that are unusual in Allende inclusions. Vig 1623-11 has Cl chondrite-normalized enrichment factors for Th and refractory siderophiles that are lower than those for REE by 60% or more, a positive Mo anomaly ( Mo Ir = 1.92 ± 0.02 relative to Cl chondrites) and an unusually low Zr Hf ratio (0.67 ± 0.06 relative to Cl). Relative to Cl chondrites, the Ir enrichment factor is 1.7 times greater than the mean enrichment factor for Re, Os, and Ru in Vig 1623-10. Vig 1623-13 has one of the lowest Ir Sm ratios (0.00044 ± 0.00007 relative to Cl) ever measured in an olivine chondrule. Vig 1623-16 has a modified Group II REE pattern which reflects removal of refractory REEs from the nebular gas at unusually low temperatures. Vig 477-5 has a REE pattern with a small negative Ce anomaly ( Ce La = 0.79 ± 0.01 relative to Cl), a large negative Yb anomaly ( Yb Lu = 0.57 ± 0.02 relative to Cl) and no Eu anomaly. This inclusion also has a positive Mo anomaly ( Mo Ir = 2.25 ± 0.03 relative to Cl) and enrichment factors for Zr and Th that are lower than those for Lu by about 65%. Vig 477-B has a REE pattern that may be a mixture of five parts of a component with an ultrarefractory REE pattern and ninety-five parts of another with a modified Group II REE pattern. Vigarano apparently sampled a different population of refractory inclusions from Allende, presumably because refractory nebular materials were not well mixed where and when C3V chondrites accreted. The Vigarano and Leoville coarse-grained inclusions studied here, like others analyzed previously, tend to have lower concentrations of Na and Au than Allende coarse-grained inclusions, but the two Vigarano fine-grained inclusions of this study have concentrations of these elements that are comparable to those of Allende fine-grained inclusions. Thus, although coarse-grained inclusions in the reduced subgroup of C3V chondrites were altered under different conditions from those in the oxidized subgroup, some fine-grained inclusions in the reduced subgroup were altered under similar conditions as some in the oxidized subgroup.