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In-Situ (U-Th)/He Dating of Martian Apatites: Low-Temperature Thermal Processes in the Late Amazonian Martian Regolith Recorded by NWA 7034
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  • Christopher McDonald,
  • Matthijs van Soest,
  • Kip Hodges,
  • Jemma Davidson
Christopher McDonald
Arizona State University

Corresponding Author:[email protected]

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Matthijs van Soest
Arizona State University
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Kip Hodges
Arizona State University
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Jemma Davidson
Arizona State University
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Abstract

Apatite – Ca5(PO4)3(OH,F,Cl) – is a common accessory mineral in many terrestrial rocks and has a low (~75˚C) closure temperature for He retention[1]. As such the (U-Th-Sm)/He thermochronometer has become one of the most used dating techniques to constrain low temperature cooling histories of terrestrial samples. Apatite has been documented in many meteoritic, lunar, and martian samples, and is a particularly common accessory phase in many martian lithologies[2]. In meteoritic samples, (U-Th-Sm)/He dating of apatites provides some methodological challenges in that grains are frequently anhedral making it complicated to do a FT correction[2]. Results are commonly interpreted to represent the timing of shock metamorphism related to impacts onto the (planetary) surface which often correlates with cosmic-ray exposure ages[2]. NWA 7034, and its pairings, represents a piece of martian regolith that documents events on the martian surface that span approximately 4 Ga[3]. NWA 7034 is known to contain upwards of 4 wt.% apatite as detrital mineral fragments and as accessory phases in many lithic and impact-derived clasts[4]. These apatites have been previously analyzed geochronologically via U-Pb apatite and isotopically via δD, δ37Cl, and H2O using microbeam techniques[5,6]. At present, (U-Th)/He dating of NWA 7034 and its pairings has been restricted to bulk rock analyses[4,7,8,9]. This whole-rock approach relies on assumptions that the He, and its parent isotopes, are evenly distributed throughout the sample and that resetting of the WR-He thermochronometer is universal throughout the sample in response to the thermal pulse associated with the impact. These studies have yielded widely dispersed datasets that range from 50 Ma to 200 Ma, putting some of those assumptions into question[4,7,8,9]. In this contribution, we utilize the laser ablation double-dating (LADD) technique on individual apatites present within several polished slabs of NWA 7034. This targeted approach allows us to acquire (U-Th-Sm)/He ages, U-Pb ages, and trace element information within a petrological framework. The (U-Th-Sm)/He ages will provide additional constraints on the low temperature history of NWA7034 and help test the veracity of WR-He retention ages. References: [1] Zeitler et al. 1987 GCA, [2] Min 2005 Rev. Mineral Geochem; [3] Cassata et al. 2018, SciAdv; [4] Agee et al. 2013 Sci; [5] Hu et al. 2019 MAPS; [6] Davidson et al. 2020 EPSL; [7] Cartwright et al. 2014 EPSL; [8] Lindsay et al. 2021 MAPS; [9] Stephenson et al. 2017 MAPS