The petrogenesis of L-6 chondrites: insights from the chemistry of minerals

Major, minor and trace element concentrations have been measured in all the volumetrically significant minerals from the L-6 chondrites Alfianello, Colby (WI) and Leedey. Most of the elements show the compositional uniformity observed in other studies. However, REE, Co and Ni are distinctly different in their abundances and their phase distributions in each of the three meteorites. Such differences must manifest differing physical and chemical conditions involved in the formation of each of the three rocks. The distributions of REE and Ni between various phases are compatible with models which assume equilibrium between presently coexisting minerals. Such a model demonstrates how plausible differences in temperature and Ni content of metallic minerals could produce the observed differences in Ni abundances in olivine and clinopyroxene from each of the three chondrites. REE are too abundant in the minerals to have been produced by crystal-liquid fractionation from a magma of whole rock composition. Alkali element abundances are incompatible with a model of closed system equilibration of the meteorites. Rb is underabundant in feldspar relative to Na when compared to the equilibrium model predictions. A portion of the Cs seems to be included in feldspar by diadochic substitution, however, significant amounts are randomly associated with other minerals, perhaps by incorporation into lattice defects and at interstices. Partial loss of this nonfeldspathic Cs may be responsible for the large range of Cs abundances observed in chondrites.