The silicon isotopic composition of inner Solar System materials

This study uses high precision silicon isotopic measurements to understand events that occurred during the earliest stages of formation of the terrestrial planets. The isotopic compositions of diverse materials such as chondrites, lunar rocks and asteroidal basalts can shed light on the homogeneity of the solar nebula, metal-silicate differentiation on planetary bodies, and terrestrial moon formation.Limited variation in the Si isotopic composition of meteorites is evidence for a relatively homogeneous inner solar system with respect to silicon isotopes. The Si isotopic composition of bulk silicate Earth (BSE) is, however, heavier than meteorites. This points to an event unique to Earth that fractionated Si isotopes, such as core formation at terrestrial conditions. The Δ30SiBSE-meteorite value from this study indicates that the Earth’s core contains 8.7 (+8.1/−6.2) wt% Si.No systematic δ30Si differences were found between any of the lunar lithologies analysed, implying a Si isotopic homogeneity of the sampled lunar source regions. The lunar average, δ30Si = −0.29±0.08permil (2σSD), is identical to the recent value of Savage et al. (2010) for BSE of δ30Si = −0.29 ± 0.08permil (2σSD). The best explanation of the data is that Si isotopes must have homogenised in the aftermath of the Moon-forming impact with no subsequent fractionation in the proto-lunar disk.The Si isotopic composition of olivine within lunar basalts was found to be the same or heavier than δ30Si(pyroxene). This is not consistent with terrestrial data where δ30Si(pyroxene) is always lighter than δ30Si(olivine). Crystallisation history cannot explain the data, and the slow diffusion rates of Si rule out cooling rates as a cause. Therefore, it appears that inter-mineral fractionation of Si isotopes occurs differently on the Moon.The δ30Si of chondrules picked from Allende spanned a range of ~0.6permil, a factor of two greater than the bulk meteorite range. There is no evidence for the variable δ30Si of the chondrules being the result of post-formation alteration and there is no convincing evidence for precursor heterogeneity being the primary cause. It is likely that Si isotopic composition of chondrules is the result of evaporation and reequilibration with the evaporated phase.