Lithium isotope composition of Mars - corollary of radial heterogeneity in the early Solar System?

Magna, T.; Mezger, K. and Fehr, M. (2010). Lithium isotope composition of Mars - corollary of radial heterogeneity in the early Solar System? In: 20th V.M. Goldschmidt Conference, 13-18 Jun 2010, Knoxville, Tennessee, USA.

DOI: https://doi.org/10.1016/j.gca.2010.04.039

Abstract

Lithium concentrations and isotope ratios of a suite of Martian meteorites comprising all available lithologies show moderate variation in Li abundances (1.9–8.0 ppm), but a surprisingly large spread of δ7Li (-0.9 to 6.2‰) exceeding that of common terrestrial basalts. There is no systematic difference in δ7Li between shergottites and nakhlites, whereas Li contents are higher in the latter. Proportionally important mesostasis in nakhlites carries significant amounts of Li, yet without a detectable impact on δ7Li. Strong linear and positive correlation of δ7Li and mg# in nakhlites (r2=0.99) suggests that the magmatic differentiation processes are mainly responsible for the Li isotope variability and that nakhlites may be co-genetic (or magmatic differentiation follows single common path for nakhlites Mars-wide). Chemically diverse ultramafic lithologies (ALH 77005, Chassigny, NWA 2737) have identical δ7Li =4.0‰, adopted as the reference value for the Martian mantle. Leachates (H2O, HCl) of nakhlite NWA 817 (~15% mesostasis) have δ7Li that are indistinguishable from the whole-rock. This further constrains the assumed presence of water on Mars and allows for only ephemeral existence of surface liquid water which would have otherwise developed high δ7Li in these samples. The uniform and slightly heavier δ7Li of the Martian mantle compared to that of the Earth may imply existence of resolvable radial heterogeneity of δ7Li with increasing heliocentric distance in the early Solar System. Whether this difference originates from the initial heterogeneous distribution of Li isotopes in the early Solar nebula or is related to particular geochemical properties of Li is unclear. However, resolvable δ7Li differences between the Earth and Moon could be explained by easier 6Li loss from vaporized Earth’s mantle immediately following the Moonforming Giant Impact. An alternative scenario implies preferential destruction of less stable 6Li in the early Solar System.

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