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

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 delta Cl-37 values of magmatic irons range from -7.2 to 18.0 parts per thousand versus standard mean ocean chloride and are unrelated to their chlorine concentrations, which range from 0.3 to 161ppm. Nonmagmatic IAB irons are comparatively Cl-rich containing >161ppm with delta Cl-37 values ranging from -6.1 to -3.2 parts per thousand. The anomalously high and low delta Cl-37 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 delta Cl-37 values of 13.4 and 18.0 parts per thousand. 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 delta Cl-37 values, whereas volatilization is known to increase delta Cl-37 values by the preferential loss of light isotopes, we interpret the low isotope values of <-5 parts per thousand and down to -7.2 parts per thousand to most closely represent the primordial isotopic composition of Cl in the solar nebula. Similar conclusions have been derived from low delta Cl-37 values down to -6, and -3.8 parts per thousand measured in Martian and Vestan meteorites, respectively. These low delta Cl-37 values are in contrast to those of chondrites which average around 0 parts per thousand previously explained by the incorporation of isotopically heavy HCl clathrate into chondrite parent bodies. The poor retention of low delta Cl-37 values in many differentiated planetary materials suggest that extensive devolatilization occurred during planet formation, which can explain Earth's high delta Cl-37 value by the loss of approximately 60% of the initial Cl content.