In-Situ (U-Th)/He Dating of Martian Apatites: Low-Temperature Thermal
Processes in the Late Amazonian Martian Regolith Recorded by NWA 7034
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