Avice, G.; Meier, M.M.M.; Marrocchi, Y.
Geochemical Perspectives Letters, 2022, 23, 1-4
Voir en ligne : https://doi.org/10.7185/geochemlet.2228
Abstract :
Noble gases trapped in primitive meteorites (chondrites) allow quantification of the physical processes that operated during the evolution of the protoplanetary disk (e.g., Kuga et al., 2015). Although these elements are present in different carriers contained in meteorites (including presolar SiC, diamonds, graphite; Ott, 2014), they are mainly hosted in a phase − referred to as phase Q− whose nature is still poorly characterised (Busemann et al., 2000). Notwithstanding this uncertainty, it has been shown that phase Q dominates the heavy noble gas budget of chondrites and is closely associated with carbonaceous material that survives HF/HCl attack of bulk meteorites (Lewis et al., 1975). Thanks to its extreme sensitivity to oxidation, the xenon isotopic composition of phaseQhas been precisely determined, revealing a mass dependent isotopic fractionation relative to solar wind (SW-Xe) in favour of the heavy isotopes relative to the light ones (Wieler et al., 1991; Busemann et al., 2000; Gilmour, 2010). However, the commonly used Xe-Q isotopic composition hinges on the average of measurements of several carbonaceous chondrites (CCs) showing distinct Xe isotopic compositions between and within each group, especially for 129Xe (Busemann et al., 2000). Such 129Xe excesses result from the decay of extinct 129I (t1/2 = 16 Myr), which was producing radiogenic 129Xe* during the first ∼100 million years of the solar system (Jeffery and Reynolds, 1961). The measurement of xenon isotopes in the coma of comet 67P/Churyumov-Gerasimenko revealed extreme 129Xe enrichment relative to 132Xe and the solar composition (Marty et al., 2017). As this large monoisotopic excess would require unlikely 129I enrichment, it has been interpreted as originating from a specific nucleosynthetic process producing 129Xe that was sampled by icy bodies formed in the outer solar system (Marty et al., 2017). Interestingly, the carbon-rich primitive chondrites Tagish Lake and Tarda are thought to originate from D-type asteroids (Hiroi et al., 2001; Marrocchi et al., 2021) considered to have formed at large heliocentric distances beyond the current orbit of Saturn, and potentially as far as the Kuiper Belt (i.e. 30–50 astronomical units = au; Levison et al., 2009). Here we report the results of a comprehensive study of the isotopic compositions of noble gases contained in Tagish Lake and Tarda to evaluate if material accreted in the outer solar system presents specific signatures. We compare our data to other CCs and discuss the origin of the variable radiogenic 129Xe excesses between and within each CC groups.