New constraints on the lithium isotope compositions of the Moon and terrestrial planets

Abstract

High-precision lithium (Li) isotope data are reported for samples from the Earth, Moon, Mars and Vesta and provide evidence of broadly similar compositions that are slightly heavy relative to those of chondrites. Mare basalts exhibit a large range of Li isotope compositions ( δ 7Li = + 3.4‰ to + 6.4‰) that correlate with indices of magmatic differentiation. Three samples (quartz-normative basalts and picritic orange glass) that are thought to have formed by melting of relatively primitive source regions yield a mean δ 7Li = + 3.8 ± 0.4‰ taken as the best estimate for the average composition of the Moon. Other samples are isotopically heavier correlating with increases in Rb and Hf and probably reflecting transport of isotopically heavy Li that formed in specific high-Ti cumulate–melt layers during crystallisation of the magma ocean. The most extreme lunar δ 7Li is found in a ferroan anorthosite (+ 8.9‰). Terrestrial mantle olivines fall into a tight range between + 3.6‰ and + 3.8‰. If these olivines reflect the composition of the bulk Earth, the Li isotope compositions of Earth and Moon are identical. The Li isotope compositions of samples from the Moon, Earth, Mars and Vesta provide no evidence for differences between large inner solar system mantle reservoirs. This in turn provides evidence that core formation, volatile loss and the presence of a crust and hydrosphere have not significantly influenced the bulk Li isotope composition of the mantles of these objects. The fact that chondrites are isotopically light compared with differentiated planetary bodies of the inner solar system is consistent with a small but significant Li isotope fractionation within the accretionary disc or chondrite parent bodies, the origin of which is presently unclear.