Effects of Desert Weathering on Meteoritic Hydrogen Isotope Systematics: Insights from Tissint

Introduction: The hydrogen isotopic composition of meteorites and their components can provide insights into the source of water on their parent bodies and secondary processes that may have occurred on these bodies. As such, determining the indigenous D/H ratio of meteorites and their constituent phases is crucial. Nonetheless, terrestrial weathering can affect many of the properties of meteorites, such as their chemistry, mineralogy, composition of organic matter, as well as the isotopic compositions of a range of elements including H, C, O and noble gases [1]. For example, the Holbrook meteorite shows a significant δO shift over the 99 years of sample recovery from its fall location in Arizona [2]. Approximately 98% of meteorites in collections are finds and have been subjected to varying degrees of terrestrial weathering. Even if the effects of weathering on meteorites are broadly similar for a variety of terrestrial environments (e.g., oxidation of metal, formation of carbonates), the modification of a meteorite’s specific physicochemical characteristics is a function of the terrestrial residence time, climate, and soil composition at the recovery site [1,3-4]. Therefore, determining the effects of terrestrial exposure on meteorites recovered from a variety of terrestrial environments is important since this information would allow us to deconvolve such effects from their indigenous characteristics [5]. Water in the liquid and vapor state in the terrestrial environment can be added to, and exchanged with, the indigenous water in a meteorite and can overprint its indigenous D/H ratio. One approach used to evaluate such effects is to conduct laboratory experiments involving isotopically spiked water [6,7]. However, such experiments are of short duration compared to the much longer exposure typically experienced by the majority of meteoritic finds and did not result in significant changes in the D/H ratio (i.e., ≤15%). An alternative approach is to assess the effects of terrestrial contamination based on the hydrogen isotope analyses of natural meteoritic samples that have experienced varying degrees of terrestrial exposure. In particular, the D/H ratio has been used to distinguish between secondary aqueous alteration occurring under martian versus terrestrial conditions, because the martian surface water reservoir that may have interacted with the shergottites and nakhlites is thought to be significantly D-enriched relative to terrestrial reservoirs (e.g., [8,9]). Thus, measuring the D/H ratios of individual phases in relatively fresh martian meteorite falls could provide a measure of the rate of hydrogen isotopic exchange in the terrestrial environment. In this study, we report the results of experiments that we conducted using the most recent martian meteorite fall, Tissint, to assess the effects of weathering on hydrogen isotope systematics resulting from different residence times in a desert environment.