The chlorine isotope composition of iron meteorites: Evidence for the Cl isotope composition of the solar nebula and implications for extensive devolatilization during planet formation

Abstract

The bulk chlorine concentrations and isotopic compositions of a suite of non‐carbonaceous (NC) and carbonaceous (CC) iron meteorites were measured using gas source mass spectrometry. The δ37Cl values of magmatic irons range from −7.2 to 18.0‰ versus standard mean ocean chloride and are unrelated to their chlorine concentrations, which range from 0.3 to 161 ppm. Nonmagmatic IAB irons are comparatively Cl‐rich containing >161 ppm with δ37Cl values ranging from −6.1 to −3.2‰. The anomalously high and low δ37Cl values are inconsistent with a terrestrial source, and as Cl contents in magmatic irons are largely consistent with derivation from a chondrite‐like silicate complement, we suggest that Cl is indigenous to iron meteorites. Two NC irons, Cape York and Gibeon, have high cooling rates with anomalously high δ37Cl values of 13.4 and 18.0‰. We interpret these high isotopic compositions to result from Cl degassing during the disruption of their parent bodies, consistent with their low volatile contents (Ga, Ge, Ag). As no relevant mechanisms in iron meteorite parent bodies are expected to decrease δ37Cl values, whereas volatilization is known to increase δ37Cl values by the preferential loss of light isotopes, we interpret the low isotope values of <−5‰ and down to −7.2‰ to most closely represent the primordial isotopic composition of Cl in the solar nebula. Similar conclusions have been derived from low δ37Cl values down to −6, and −3.8‰ measured in Martian and Vestan meteorites, respectively. These low δ37Cl values are in contrast to those of chondrites which average around 0‰ previously explained by the incorporation of isotopically heavy HCl clathrate into chondrite parent bodies. The poor retention of low δ37Cl values in many differentiated planetary materials suggest that extensive devolatilization occurred during planet formation, which can explain Earth's high δ37Cl value by the loss of approximately 60% of the initial Cl content.

The Cl isotope composition and halogen contents of Apollo-return samples

“…The chlorine isotope system is of particular interest because the Cl isotope values of the Moon are uniquely high. The high values, and increases in the Cl isotope composition of a planetary body from the nebular baseline (around −5 to −7‰ compared to Earth), are related to the amount of Cl that was lost throughout its accretionary and differentiation history (17,18).…”

The Zn, S, and Cl isotope compositions of mare basalts: Implications for the effects of eruption style and pressure on volatile element stable isotope fractionation on the Moon

“…Green bar represents the estimated  37 Cl value of the silicate Earth (Sharp et al, 2013). Purple bar represents the estimated nebular value from Gargano & Sharp (2019) and Sharp et al, (2016). Day et al, (2020), Paniello et al, (2012) & Moynier et al, (2006.…”

“…The effect of differentiation on Cl isotope fractionation has not been studied experimentally, although is generally assumed to be minimal given the small isotopic fractionations at high temperatures (Gargano and Sharp, 2019;Schauble et al, 2003) and the absence of multiple Cl oxidation states in magmatic systems. Instead, the low  37 ClWSC (1.8 ± 2.5‰) relative to the high  37 ClSBC (7.3 ± 3.5) values is interpreted to reflect kinetic isotope fractionation of Cl via degassing, followed by subsequent deposition of isotopically light Clbearing vapor (Gargano et al, 2020;Sharp et al, 2010).…”

The biogeochemical cycling of chlorine

“…The only reservoir in which chlorine cycling is modeled accurately is the atmosphere (Wang et al, 2019). The chlorine content of the core and mantle is uncertain, which in turn creates uncertainty in the total mass of chlorine remaining on Earth (Eggenkamp, 2014; Gargano & Sharp, 2019; Guo & Korenaga, 2021). The cycling of organochlorine and oxidized chlorine species is not well constrained across space and time in the ocean or terrestrial environments (Rajagopalan et al, 2009; Svensson et al, 2021), perhaps because these compounds were only recently recognized to be naturally occurring.…”