T233 Apatite, from Magma to Medicine: In Honor of John M. Hughes

Wednesday, 7 November 2018
8:00 AM - 12:00 PM
Room 242

APATITE AS A TOOL FOR INVESTIGATING THE EVOLUTION OF HYDROGEN IN THE MARTIAN CRUST

Paper number: T233-14
Program: Technical Sessions
Time: 11:15 AM – 11:30 AM

Authors

Abstract

In the absence of a magnetosphere, the martian atmosphere has been stripped of the lighter 1H isotope over billions of years by the solar wind. This has resulted in a heavily fractionated atmosphere compared to the mantle. Northwest Africa (NWA) 7034 and its pairings represent a regolith breccia of a basaltic bulk composition, the fine-grained matrix of which bears a strong resemblance to the major and trace element composition estimated for the ancient southern highlands crust on Mars. Clasts in NWA 7034 have ages as old as 4.4Ga with phosphates recording a Pb-loss event at 1.5 Ga (McCubbin et al., 2016 JGR). On the other hand, Allan Hills (ALH) 84001 is a coarse-grained, orthopyroxenite and is the oldest sample we have from Mars, dated at ~4.1 Ga (Lapen et al. 2010, Science v328). A younger age for phosphates (~4.0 Ga, Terada et al., 2003 MaPS v38) and presence of carbonates indicate crustal alteration of ALH 84001. Together these samples provide an opportunity to investigate the evolution of hydrogen on Mars.

We analyzed the chemistry and isotopic composition of hydrogen in apatite [Ca5(PO4)3(F,Cl,OH)]. In NWA 7034 apatite occurs in a number of different lithic clasts as well as mineral clasts in the matrix. Electron microprobe analysis confirmed the presence of at least two generations of apatite in ALH 84001.

The equilibration of apatite with crustal fluids is supported by the chlorine-rich compositions exhibited by apatites in NWA 7034 and ALH 84001 in comparison to apatites from other martian meteorites. All of the H-isotopic compositions that we obtained fall within ~500‰ of the range of values reported for the intermediate H reservoir predicted for the martian crust by Usui et al. (2015 EPSL v410). These observations indicate that at least portions of the martian regolith have not exchanged completely with the martian atmosphere and could suggest that the crust and atmosphere have been decoupled since about 4Ga.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-320326

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