Origin of ureilites inferred from a SIMS oxygen isotopic and trace element study of clasts in the Dar al Gani 319 polymict ureilite

Abstract

Secondary ion mass spectrometer (SIMS) oxygen isotope analyses were performed on 24 clasts, representing 9 clast types, in the Dar al Gani (DaG) 319 polymict ureilite with precisions better than 1‰. Olivine-rich clasts with typical ureilitic textures and mineral compositions have oxygen isotopic compositions that are identical to those of the monomict ureilites and plot along the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line. Other igneous clasts, including plagioclase-bearing clasts, also plot along the CCAM line, indicating that they were derived from the ureilite parent body (UPB). Thus, we suggest that some of the plagioclase-bearing clasts in the polymict ureilites represent the “missing basaltic component” produced by partial melting on the UPB. Trace element concentrations (Mg, K, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Rb, Sr and Ba) in ureilitic plagioclase and glass from 13 clasts were obtained by using the SIMS high mass resolution method. The trace element contents of the plagioclase generally show monotonic variations with anorthite content (mol%) that are consistent with partial melting and fractional crystallization. Incompatible trace element concentrations (K, Ti, and Ba) are low and variable for plagioclase with An > 40, indicating that the parental magmas for some clasts were derived from a depleted source. We performed partial melt modeling for CI and CM precursor compositions and compared the results to the observed trace element (K, Ba, and Sr) abundances in the plagioclase. Our results indicate that (1) the UPB evolved from a alkali-rich carbonaceous chondritic precursor, (2) parent melts of porphyritic clasts could have formed by 5–20% equilibrium partial melting and subsequent fractional crystallization, and (3) parent melts of the incompatible trace element-depleted clasts could be derived from fractional melting, where low degree (<10%) partial melts were repeatedly extracted from their solid sources. Thus, both the oxygen isotopic and trace element compositions of the plagioclase bearing clasts in DaG-319 suggest that the UPB underwent localized low degree-partial melting events. The partial melts could have been repeatedly extracted from the precursor, resulting in the formation of the olivine-pigeonite monomict ureilites as the final residue.