Lithium isotope compositions of Martian and lunar reservoirs

Abstract

Lithium isotope compositions and concentrations of 12 lunar samples (including two high-Ti, three low-Ti mare basalts, five highland breccias, one orange and one green glass) and 7 Martian meteorites (three basaltic and one lherzolitic shergottite, two nakhlites, and the orthopyroxenite ALHA 84001 were measured using MC-ICP–MS. Most of the Martian samples have a narrow range of δ 7Li (+ 3.6 to + 5.2‰). Only ALHA 84001 is isotopically lighter, with δ 7Li = − 0.6‰. The range in Li concentrations is limited and all shergottites have identical Li concentrations (1.8–2.1 μg/g) and isotope compositions within the error. Despite a larger variation in Li concentrations (5–49 μg/g), Li isotope variation of most lunar samples is also very limited (+ 3.5 to + 6.6‰) with an average of + 5.2‰ (± 1.2, 2 σ). The only exception is one KREEP-rich highland breccia (15445a), which has a δ 7Li value of + 18.6‰. Consequently, the majority of lunar and Martian samples have an isotopic signature similar to the Earth's mantle (MORB and OIB). These results imply a homogeneous Li isotope composition of the inner solar system with a δ 7Li ≈ + 4‰. The results further indicate that planetary silicate differentiation by partial melting on planets under either wet or dry conditions does not significantly fractionate Li isotope compositions. Lithium abundances of lunar basalts and glasses are similar to those of terrestrial basalts. In contrast, Martian basalts have generally lower Li concentrations, more similar to BSE, although the concentrations in shergottitic clinopyroxenes and nakhlitic pyroxenites do not indicate a lower Li abundance for bulk Mars. These systematics imply that the Martian basalts were depleted in Li by a process that did not fractionate the Li isotope composition.