Substantial Variations of Refractory Siderophile Element Ratios in Components of Unequilibrated Chondrites - Implications for Terrestrial Planet Compositions

Abstract:

Abundance ratios of refractory elements in bulk compositions of chondritic meteorites show limited variability relative to CI chondrites, typically < 10%. Whether or not this was also the case for the building materials of the terrestrial planets remains an important open question with implications for planetary chemistry and chronology (e.g. the Sm/Nd ratio of the terrestrial planets). The limited variation of these elements in chondrites has been explained by complete condensation of these elements from solar gas into oxides, silicates and metal. However, small systematic differences in abundances and ratios of specific refractory elements in different groups of chondrites suggest variable mixing ratios of refractory components in different compartments of the solar nebula. The systematic fractionations of Re/Os, Y/Ho (and other REE) between different classes of chondrites, the presence of strongly fractionated refractory metal nuggets in some carbonaceous chondrites and stable isotope variations of nucleosynthetic origin (Ru, Mo, Ti, Cr) in the bulk rocks and in leachates of chondrites suggest that the origin of refractory element fractionations in chondrites is more complicated than assumed in early models. Abundance ratios of refractory siderophile elements in physically separated components of unequilibrated chondrites show that components of the least equilibrated chondrites (petrologic type 2 to 3.4) display larger variations than bulk rocks (e.g., Pt/Ir, Rh/Ir, Ru/Ir, Re/Os vary by 20-30% or more). These data and the isotopic variations are difficult to ascribe to parent body processes or metal-sulfide-silicate partitioning during chondrule formation. Rather the different end members appear to preserve differences inherited from fractional condensation or evaporation of dust precursors of chondrules, matrix and Fe-Ni metal that were later efficiently mixed in nebular compartments to yield approximately CI chondrite like ratios of refractory elements. Larger fractions of dust processed at higher temperatures may have existed in the terrestrial planet zone in comparison to the early main belt. Thus, modest differences in the ratios of refractory elements of the building materials of the terrestrial planets vs. main belt objects such as chondrite parent bodies can be expected.