Magnesium isotope systematics in Martian meteorites

Tomáš Magna, Yan Hu, Fang-Zhen Teng, Klaus Mezger

Earth and Planetary Science Letters
In Press, Corrected Proof, Available online 26 July 2017


• The Martian mantle has an estimated δ26Mg of −0.27 ± 0.04‰.
• Inner Solar System planetary bodies have identical bulk Mg isotope compositions.
• Clinopyroxene accumulation responsible for slight δ26Mg variations in Martian samples.
• Slightly higher δ26Mg found for crustal breccia NWA 7034.
• Narrow δ26Mg range precludes large-scale recycling in Martian history.”

“Magnesium isotope compositions are reported for a suite of Martian meteorites that span the range of petrological and geochemical types recognized to date for Mars, including crustal breccia Northwest Africa (NWA) 7034. The δ26Mg values (per mil units relative to DSM-3 reference material) range from −0.32 to −0.11‰; basaltic shergottites and nakhlites lie to the heavier end of the Mg isotope range whereas olivine-phyric, olivine–orthopyroxene-phyric and lherzolitic shergottites, and chassignites have slightly lighter Mg isotope compositions, attesting to modest correlation of Mg isotopes and petrology of the samples. Slightly heavier Mg isotope compositions found for surface-related materials (NWA 7034, black glass fraction of the Tissint shergottite fall; δ26Mg > −0.17‰) indicate measurable Mg isotope difference between the Martian mantle and crust but the true extent of Mg isotope fractionation for Martian surface materials remains unconstrained. The range of δ26Mg values from −0.19 to −0.11‰ in nakhlites is most likely due to accumulation of clinopyroxene during petrogenesis rather than garnet fractionation in the source or assimilation of surface material modified at low temperatures. The rather restricted range in Mg isotope compositions between spatially and temporally distinct mantle-derived samples supports the idea of inefficient/absent major tectonic cycles on Mars, which would include plate tectonics and large-scale recycling of isotopically fractionated surface materials back into the Martian mantle. The cumulative δ26Mg value of Martian samples, which are not influenced by late-stage alteration processes and/or crust–mantle interactions, is −0.271±0.040‰−0.271±0.040‰ (2SD) and is considered to reflect δ26Mg value of the Bulk Silicate Mars. This value is robust taking into account the range of lithologies involved in this estimate. It also attests to the lack of the Mg isotope variability reported for the inner Solar System bodies at current analytical precision, also noted for several other major elements.”