The chlorine isotope composition of Martian meteorites 2. Implications for the early solar system and the formation of Mars

Abstract We determined the chlorine isotope composition of 16 Martian meteorites using gas source mass spectrometry on bulk samples and in situ secondary ion microprobe analysis on apatite grains. Measured δ37Cl values range from −3.8 to +8.6‰. The olivine‐phyric shergottites are the isotopically lightest samples, with δ37Cl mostly ranging from −4 to −2‰. Samples with evidence for a crustal component have positive δ37Cl values, with an extreme value of 8.6‰. Most of the basaltic shergottites have intermediate δ37Cl values of −1 to 0‰, except for Shergotty, which is similar to the olivine‐phyric shergottites. We interpret these data as due to mixing of a two‐component system. The first component is the mantle value of −4 to −3‰. This most likely represents the original bulk Martian Cl isotope value. The other endmember is a37Cl‐enriched crustal component. We speculate that preferential loss of35Cl to space has resulted in a high δ37Cl value for the Martian surface, similar to what is seen in other volatile systems. The basaltic shergottites are a mixture of the other two endmembers. The low δ37Cl value of primitive Mars is different from Earth and most chondrites, both of which are close to 0‰. We are not aware of any parent‐body process that could lower the δ37Cl value of the Martian mantle to −4 to −3‰. Instead, we propose that this low δ37Cl value represents the primordial bulk composition of Mars inherited during accretion. The higher δ37Cl values seen in many chondrites are explained by later incorporation of37Cl‐enrichedHCl‐hydrate.