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Lithium isotope constraints on crust-mantle interactions and surface processes on Mars

TitleLithium isotope constraints on crust-mantle interactions and surface processes on Mars
Publication TypeJournal Article
Year of Publication2015
AuthorsMagna T., Day JMD, Mezger K., Fehr M.A, Dohmen R., Aoudjehane H.C, Agee C.B
JournalGeochimica Et Cosmochimica Acta
Date Published2015/08
Type of ArticleArticle
ISBN Number0016-7037
Accession NumberWOS:000355138000003
Keywords06319; aqueous alteration; carbonaceous chondrites; early differentiation; larkman nunatak; light lithophile elements; martian meteorite alh84001; nakhlite meteorites; sector icp-ms; solar-system; taupo volcanic zone

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 delta Li-7 from 2.1 to 6.2 parts per thousand, similar to that reported for pristine terrestrial peridotites and unaltered mid-ocean ridge and ocean island basalts. Two chassignites have delta Li-7 values (4.0 parts per thousand) intermediate to the shergottite range, and combined, these meteorites provide the most robust current constraints on delta Li-7 of the martian mantle. The polymict breccia NWA 7034 has the lowest delta Li-7 (-0.2 parts per thousand) 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-delta L-7 crustal component. The range in delta L-7 for nakhlites (1.8 to 5.2 parts per thousand), 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 (>15 wt.% MgO) it is possible to estimate the delta L-7 of Bulk Silicate Mars (BSM) to be 4.2 +/- 0.9 parts per thousand (2 sigma). This value is at the higher end of estimates for the Bulk Silicate Earth (BSE; 3.5 +/- 1.0 parts per thousand, 2 sigma), but overlaps within uncertainty. (C) 2015 Elsevier Ltd. All rights reserved.

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