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

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 delta Cl-37 values range from -3.8 to + 8.6 parts per thousand. The olivinephyric shergottites are the isotopically lightest samples, with delta Cl-37 mostly ranging from -4 to -2 parts per thousand. Samples with evidence for a crustal component have positive delta Cl-37 values, with an extreme value of 8.6 parts per thousand. Most of the basaltic shergottites have intermediate delta Cl-37 values of -1 to 0 parts per thousand, 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 parts per thousand. This most likely represents the original bulk Martian Cl isotope value. The other endmember is a Cl-37-enriched crustal component. We speculate that preferential loss of (35) Cl to space has resulted in a high delta Cl-37 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 delta Cl-37 value of primitive Mars is different from Earth and most chondrites, both of which are close to 0 parts per thousand. We are not aware of any parent-body process that could lower the delta Cl-37 value of the Martian mantle to -4 to -3 parts per thousand. Instead, we propose that this low delta Cl-37 value represents the primordial bulk composition of Mars inherited during accretion. The higher delta Cl-37 values seen in many chondrites are explained by later incorporation of Cl-37-enriched HCl-hydrate.