Silicon and Oxygen Isotope Evolution of the Inner Solar System

Enstatite chondrites have been regarded as major building blocks of the Earth and other differentiated inner planetary bodies due to the similarity of Delta O-17 (deviation of the delta O-17 value from the terrestrial silicate fractionation line) and nucleosynthetic isotope anomalies. However, this hypothesis has been rebutted by the fact that the Earth and enstatite chondrites show distinct Si isotopic compositions. It has been debated whether the origin of this Si isotope difference is the result of nebular or planetary processes. Here we show that the delta Si-30 (deviation of Si-30/Si-28 relative to NBS 28 standard) and the Delta O-17 values of chondrules in unequilibrated enstatite chondrites are between -0.20 parts per thousand and -0.54 parts per thousand and -0.36 parts per thousand and +0.26 parts per thousand, respectively. Furthermore, the chondrules with higher Delta O-17 values tend to have lower delta Si-30. The data exhibit values consistent with most of the noncarbonaceous group differentiated planetary bodies. This consistency suggests that the Si and O isotopic compositions of enstatite chondrules record those of the major precursors that formed the differentiated planetary bodies in the inner solar system. Model calculations based on the results reveal that the Si and O isotope variations of the enstatite chondrite chondrules were generated by an interaction between the evaporation-driven SiO-rich gas and partially or fully melted forsterite-rich precursor chondrules. The Mg/Si of the evaporated dust-gas mixtures increased with increasing silicate/metal ratio in the evaporated dust, which may have increased the bulk Mg/Si and delta Si-30 value of the inner planetary bodies.