Premium
The chlorine isotope composition of Martian meteorites 2. Implications for the early solar system and the formation of Mars
Author(s) -
Sharp Zachary,
Williams Jeffrey,
Shearer Charles,
Agee Carl,
McKeegan Kevin
Publication year - 2016
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.12591
Subject(s) - meteorite , martian , mars exploration program , geology , mantle (geology) , chondrite , geochemistry , basalt , astrobiology , martian surface , carbonaceous chondrite , parent body , isotope , formation and evolution of the solar system , mineralogy , physics , quantum mechanics
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 δ 37 Cl values range from −3.8 to +8.6‰. The olivine‐phyric shergottites are the isotopically lightest samples, with δ 37 Cl mostly ranging from −4 to −2‰. Samples with evidence for a crustal component have positive δ 37 Cl values, with an extreme value of 8.6‰. Most of the basaltic shergottites have intermediate δ 37 Cl 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 a 37 Cl‐enriched crustal component. We speculate that preferential loss of 35 Cl to space has resulted in a high δ 37 Cl 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 δ 37 Cl 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 δ 37 Cl value of the Martian mantle to −4 to −3‰. Instead, we propose that this low δ 37 Cl value represents the primordial bulk composition of Mars inherited during accretion. The higher δ 37 Cl values seen in many chondrites are explained by later incorporation of 37 Cl‐enriched HC l‐hydrate.