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

1Anthony Gargano,2Zachary Sharp
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13303]
1Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, 87131‐0001 USA
2Center for Stable Isotopes, University of New Mexico, Albuquerque, New Mexico, 87131‐0001 USA
Published by arrangement with John Wiley & Sons

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.

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