^1,2Tomáš Magna, ³James M.D. Day, ^1,4Klaus Mezger, ^5,6Manuela A. Fehr, ⁷Ralf Dohmen, ⁸Hasnaa Chennaoui Aoudjehane, ⁹Carl B. Agee
¹Institut für Mineralogie, Universität Münster, Corrensstr. 24, D-48149 Münster, Germany
²Czech Geological Survey, Klárov 3, CZ-118 21 Prague 1, Czech Republic
³Geosciences Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093-0244, USA
⁴Institut für Geologie, Universität Bern, Baltzerstr. 1+3, CH-3012 Bern, Switzerland
⁵CEPSAR, Department of Environment, Earth & Ecosystems, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom
⁶Institut für Geochemie und Petrologie, ETH Zürich, Clausiusstr. 25, CH-8092 Zürich, Switzerland
⁷Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany
⁸Faculty of Sciences, Hassan II University, BP 5366 Maârif, Casablanca, Morocco
⁹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA

Lithium abundances and isotope compositions are reported for a suite of martian meteorites that span the range of petrological and geochemical types recognized to date for Mars. Samples include twenty-one bulk-rock enriched, intermediate and depleted shergottites, six nakhlites, two chassignites, the orthopyroxenite Allan Hills (ALH) 84001 and the polymict breccia Northwest Africa (NWA) 7034. Shergottites unaffected by terrestrial weathering exhibit a range in δ7Li from 2.1 to 6.2‰, similar to that reported for pristine terrestrial peridotites and unaltered mid-ocean ridge and ocean island basalts. Two chassignites have δ7Li values (4.0‰) intermediate to the shergottite range, and combined, these meteorites provide the most robust current constraints on δ7Li of the martian mantle. The polymict breccia NWA 7034 has the lowest δ7Li (−0.2‰) of all terrestrially unaltered martian meteorites measured to date and may represent an isotopically light surface end-member.
The new data for NWA 7034 imply that martian crustal surface materials had both a lighter Li isotope composition and elevated Li abundance compared with their associated mantle. These findings are supported by Li data for olivine-phyric shergotitte NWA 1068, a black glass phase isolated from the Tissint meteorite fall, and some nakhlites, which all show evidence for assimilation of a low-δ7Li crustal component. The range in δ7Li for nakhlites (1.8 to 5.2‰), and co-variations with chlorine abundance, suggests crustal contamination by Cl-rich brines. The differences in Li isotope composition and abundance between the martian mantle and estimated crust are not as large as the fractionations observed for terrestrial continental crust and mantle, suggesting a difference in the styles of alteration and weathering between water-dominated processes on Earth versus possibly Cl–S-rich brines on Mars. Using high-MgO shergottites (>14 wt.% MgO) it is possible to estimate the δ7Li of Bulk Silicate Mars (BSM) to be 4.2 ± 0.9‰ (2σ). This value is at the higher end of estimates for the Bulk Silicate Earth (BSE; 3.5 ± 1.0‰, 2σ), but overlaps within uncertainty.

Reference
Magna T, Day JMD, Mezger K, Fehr MA, Dohmen R, Aoudjehane CH, Agee CB (2015) Lithium isotope constraints on crust–mantle interactions and surface processes on Mars. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.029]

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