Tracking the weathering of basalts on Mars using lithium isotope fractionation models

An edited version of this paper was published by AGU. Copyright (2015) American Geophysical UnionLithium (Li), the lightest of the alkali elements, has geochemical properties that include high aqueous solubility (Li is the most fluid mobile element) and high relative abundance in basalt-forming minerals (values ranking between 0.2 and 12 ppm). Li isotopes are particularly subject to fractionation because the two stable isotopes of lithium - 7Li and 6Li - have a large relative mass difference (∼15%) that results in significant fractionation between water and solid phases. The extent of Li isotope fractionation during aqueous alteration of basalt depends on the dissolution rate of primary minerals - the source of Li - and on the precipitation kinetics, leading to formation of secondary phases. Consequently, a detailed analysis of Li isotopic ratios in both solution and secondary mineral lattices could provide clues about past Martian weathering conditions, including weathering extent, temperature, pH, supersaturation, and evaporation rate of the initial solutions in contact with basalt rocks. In this paper, we discuss ways in which Martian aqueous processes could have lead to Li isotope fractionation. We show that Li isotopic data obtained by future exploration of Mars could be relevant to highlighting different processes of Li isotopic fractionation in the past, and therefore to understanding basalt weathering and environmental conditions early in the planet's historyData supporting our models and calculations are available as supporting information. The research leading to these results is a contribution from the Project ‘icyMARS’’, funded by the European Research Council, Starting Grant no 307496. This work was also partially supported by the European FEDER program and the Spanish Ministry of Science (MICINN) through the project CGL2011–30079. Comments by R. James and four anonymous reviewers helped us to clarify and strengthen our wor