Journal
METEORITICS & PLANETARY SCIENCE
Volume 51, Issue 11, Pages 2111-2126Publisher
WILEY
DOI: 10.1111/maps.12591
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Funding
- NASA [NNX14AG44G]
- Humboldt Fellowship
- NSF
- NASA [683946, NNX14AG44G] Funding Source: Federal RePORTER
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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.
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