Refractory element fractionation in the Allende meteorite: Implications for solar nebula condensation and the chondritic composition of planetary bodies
Résumé
Chondritic meteorites represent primitive undifferentiated solar system material that is compositionally similar to the nonvolatile fraction of the Sun. The mineralogy and texture of chondritic meteorites is complex, however, because they are mixtures of several components that formed under different conditions in the solar nebula and were further processed on their parent bodies: chondrules, a volatile rich, fine-grained matrix, including a variety of mineral and lithic clasts, metal, sulfides, and Ca, Al-rich inclusions (CAI). The bulk chemistry of a single aliquot of a chondritic meteorite consequently depends on the size and distribution of its constituents. Here, we investigate the effect of sample heterogeneity on the major and trace element composition of the CV chondrite Allende using a single 30 g slice, which is 22.5 cm(2) in dimension and 4 mm thick. Thirty-nine equally sized pieces with an average sample weight of ca. 0.6 g (corresponding to a cube with an edge length of 5 to 6 mm) were powdered and aliquots of 0.12 g and 0.02-0.03 g were analyzed by XRF for major and ICP-MS for trace elements. One sample contained a large CAI, another sample was dominated by a dark inclusion (DI). Excluding these two samples, the concentrations of the major elements Mg, Si and Fe are constant within analytical uncertainty at the millimeter-centimeter scale (S.D. 0.9, 1.3 and 2.6%, respectively). Non-refractory minor and trace elements are similarly constant, including geochemically very different elements such as Mn, Cr, Ni, Co, P, Zn and Pb. This reflects a uniform mixture of the various host phases of these elements during accretion, and excludes elemental redistribution above a millimeter-scale by aqueous alteration and/or thermal metamorphism on the parent body. The refractory elements Al, Ca, Ti etc. are more variable (S. D. 17, 10 and 9%, respectively), which is mainly the result of different proportions of millimeter-size CAI, many of them with strongly fractionated group II rare earth element patterns, i.e., variable enrichment of the more volatile refractory elements (Ta, U, Nb, Sr, Tm, Nd) over the strongly refractory elements (Lu, Zr, Hf). Admixture of group II CAI can also account for the sub-chondritic Nb/Ta and Zr/Nb ratios in CV chondrites. The total average of all 37 samples has a clear group II-type rare earth element pattern. If this fractionated refractory element pattern is representative of the Allende parent body, this observation suggests that bulk planetary bodies, possibly including the Earth-forming planetary embryos, may have refractory element patterns that are fractionated relative to those of CI chondrites.