Magnesium Isotopes in the Earth and Moon by Laser Ablation MC-ICPMS

Magnesium isotopes potentially offer new insights into a diverse range of high temperature processes ranging from evaporation and condensation in the solar nebula, to melting and metasomatism in the mantle, and hydrothermal circulation. For example, volatility-related Mg isotopic variations of up to 10 per mil/amu occur in early nebular phases that are interpreted as evaporation residues, and surprisingly large variations (up to 3 per mil/amu) have been reported recently for the terrestrial mantle. We have measured the Mg isotopic compositions of olivine phenocrysts from primitive ocean island and island arc picrites, and Apollo 12 mare basalts by laser ablation multi-collector ICPMS to investigate possible source effects in the mantle, and to search for evidence of temperature-dependent isotopic fractionation during formation of the Moon. Analyses were obtained with a Neptune MC-ICPMS operated in medium resolution mode, and an excimer laser (5 Hz, 50 micron spot diameter, 80 mJ/pulse). 26Mg was measured at an off-center peak position to avoid interference from 12C-14N. Replicate analyses of mantle olivines demonstrate an analytical precision of better than 0.2 per mil/amu (1 sigma stdev) using the standard-unknown bracketing method. Picritic olivines (Fo88-90) from Hawaii and Vanuatu show a range in Mg isotopic composition of about 0.5 per mil/amu relative to San Carlos olivine. No systematic differences that could be related to variations in source composition or extent of melting were found. In contrast, olivines from Apollo 12 picritic mare basalts show an apparently larger range of isotopic compositions of about 1.5 per mil/amu. The measured Mg isotope ratios of these phenocrysts vary systematically to heavier values with decreasing Fo content, from +0.2-0.4 per mil/amu (relative to San Carlos olivine) in the most primitive phenocrysts (Fo70-75) to +1.5 per mil/amu in the most evolved olivines (Fo45-50). To examine the possibility that the apparent isotopic variations in the lunar olivine actually reflect a composition-dependent analytical effect, we measured the Mg isotopic compositions of experimentally crystallized olivines with a similar range of major element compositions (Fo50-90) and prepared from a common starting material. These results demonstrate a matrix effect of approx. +0.3 per mil/amu per 10 Fo units, which reproduces the range of compositions observed in the lunar samples. The trend to heavier apparent Mg isotopic compositions with increasing iron content in the olivine is consistent with a space-charge effect in the MC-ICPMS, although an ablation-related mechanism is also possible. When corrected for matrix effects, the Mg isotopic composition of the Moon is identical to the Earth's mantle, placing strong constraints on any volatility-related fractionation that could have occurred during formation of the Moon.