Solar system planetary bodies are generally volatile depleted compared to chondritic meteorites (e.g., O’Neill & Palme 2008; Braukmüller et al. 2019). For example, Rb–Sr and K–U are elemental pairs with similar geochemical behaviors, but Rb and K are much more volatile than Sr and U, and Earth has Rb/Sr and K/U ratios that are ∼10 times lower than that of Ivuna-like carbonaceous (CI) chondrites (Allègre et al. 2001; Halliday & Porcelli 2001). However, the mechanisms and conditions that led to this relative volatile-element depletion remain ambiguous. The stable isotopes of elements can be fractionated by volatilization processes and the shifts in their isotopic compositions have been applied to understand volatile-element depletion processes (Humayun & Clayton 1995; Luck et al. 2005; Hin et al. 2017; Kato & Moynier 2017b; Pringle & Moynier 2017; Young et al. 2019; Bloom et al. 2020; Hellmann et al. 2021). Cr is a moderately volatile element (Lodders 2003; Wood et al. 2019) that may show a special behavior during volatilization at higher temperatures. Under moderately reducing to oxidizing conditions (fO2 > IW+1), relevant during evaporation processes from magma oceans on planets (Visscher & Fegley 2013), Cr may be present in the gas as oxide species CrO(g), CrO2(g) and CrO3(g) (Chase 1998). Equilibrium isotopic fractionation between these gas phase species and melt will cause enrichment of the lighter Cr isotopes in the condensed phases, in which Cr is either di- or trivalent, relative to the vapor (Sossi et al. 2018). This contrasts with its speciation under more reduced nebular conditions in which Cr may be present as Cr2+ (major; in silicates) or Cr0 (minor; in Fe–Ni metal) in condensed phases and Cr0 in the gas (Grossman et al. 2008), in which case, through either equilibrium or kinetic fractionation, the condensed phase will be enriched in the heavy Cr isotopes. Solids or magmas that evaporated significant quantities of moderately volatile elements such as Zn, K, Rb, or Ga normally show enrichments of heavy isotopes of these elements, both for kinetic and equilibrium isotope fractionation processes (Paniello et al. 2012; Wang & Jacobsen 2016; Kato & Moynier 2017a; Pringle & Moynier 2017). The behavior of Cr during evaporation has been a motivation to apply this element to study the origin of volatile depletion in the Moon and of the eucrite parent body (likely asteroid 4-Vesta) (Sossi et al. 2018; Zhu et al. 2019b). Both lunar samples and HED meteorites from Vesta are enriched in the lighter isotopes of Cr compared to Earth and chondrites. These isotopic enrichments have been interpreted to reflect equilibrium Cr isotope fractionation between melt (reduced, and enriched in CrO) and gas (oxidized, and enriched in CrO2) during evaporation from a magma ocean. Given that at equilibrium, isotopic fractionation scales with 1/T
2, the Cr evaporation from the Moon and Vesta was estimated to have occurred under relatively low temperatures (∼2000 K), which in the case of the Moon is comparable to the surface temperature of a lunar magma ocean (Sossi et al. 2018).
Citation: Sossi et al. 2018
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Citation: Sossi et al. 2018
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Citation: Sossi et al. 2018
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