Noble gas fractionation during low temperature alteration: an experimental approach.

bullock, M. A.; Schwenzer, S. P.; Bridges, J. C.; Chavez, C.; Filiberto, J.; Kelley, S. P.; Miller, M.; Moore, J. M.; Smith, H.; Swindle, T. D. and Treiman, A. H. (2015). Noble gas fractionation during low temperature alteration: an experimental approach. In: 46th Lunar and Planetary Science Conference, 16-20 Mar 2015, The Woodlands, TX, USA.

URL: http://www.hou.usra.edu/meetings/lpsc2015/pdf/1235...

Abstract

The heavy noble gases (Ar, Kr, Xe) are key tracers of planetary outgassing, atmospheric formation, and atmospheric evolution. On Mars, at least 3 distinct noble gas signatures have been observed: (1) Noble gas ratios different from the curent Mars atmosphere, but derived from mantle sources, was first found in Chassigny. (2) Shergottites, especially their shock glasses, contain unfractionated Martian atmosphere. (3) The nakhlite Martian meteorites and the ortho-pyroxenite ALH84001 contain fractionated Martian atmosphere that has been explained through several possible mechanisms: (1) The fractionation could represent ancient Martian atmosphere with a noble gas signature different from today. (2) The elementally fractionated noble gases could have entered the rocks via magma, either as a crustal component or via magmatic assimilation of aqueously altered crustal rock. (3) The elementally fractionated noble gases could have entered the rock during the formation of aqueous alteration minerals. Using data from Lafayette ‘iddingsite’, pure pyroxene mineral separate and whole rock samples, we have previously calculated that such an aquously fractionated component could have a ⁸⁴Kr/¹³²Xe of ~8.

All meteorites that show a fractionated Martian atmospheric component contain alteration products. Carbonates are the main alteration product in ALH 84001, accompanied by magnetite. In the nakhlites several alteration phases have been found; Lafayette contain carbonates, clay (ferric saponite, serpentine), sulfides, and an amorphous component.

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