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Origin of the isotopic composition of natural perchlorate: Experimental results for the impact of reaction pathway and initial ClOx reactant

APTIM Authors/Contributors


Natural perchlorate (ClO4−) exists in many places on Earth, in lunar regolith, meteorites, and on the surface of Mars. Terrestrial natural ClO4− has widely variable Cl and O stable isotopic compositions (δ37Cl, δ18O, Δ17O). The δ18O and Δ17O values of ClO4− from the most hyper-arid locations co-vary. ClO4− from less arid areas has relatively little 17O excess and poor Δ17O-δ18O correlation. ClO4− from the Atacama Desert has unusually low δ37Cl (<−10‰) and exhibits a positive correlation between δ37Cl and δ18O, while the δ37Cl of ClO4− from all other locations varies between −5 and +7‰ with no δ37Cl-δ18O covariation. To evaluate the impact of different precursors (ClOx) and reaction pathways on the isotopic composition of ClO4−, we measured the isotopic composition of ClO4− produced in the laboratory by UV or O3 mediated aqueous oxidation of Cl−, OCl−, ClO2−, and ClO2° as well as O3 mediated oxidation of dry NaCl. ClOx oxidation in aqueous or dry systems enriched in O3 produced ClO4− with Δ17O values that generally increased with the number of O atoms required and included evidence that the site-specific 17O anomaly in O3 was preferentially transferred to ClO4−. Based on the inferred number of O atoms sourced from O3, and known Cl and O reaction pathways, it appears that ClO2° and ClO3* were required intermediates in the production of ClO4− in the O3 experiments. ClOx aqueous oxidation by UV irradiation produced ClO4− with a large range of δ18O values and little or no 17O anomaly. ClO3− was produced to a much greater extent than ClO4− in all experiments except dry oxidation of NaCl by O3. The isotopic composition of ClO3− was distinct from that of ClO4− produced from the same initial reactants. Combined results of O3 and UV mediated reactions largely bracketed the range of natural ClO4− δ18O and Δ17O values as well as δ37Cl values of non-Atacama natural samples, but no conditions produced the low δ37Cl values of Atacama ClO4−. Our results indicate that variation in production mechanisms, possibly combined with isotopically variable precursors, could be responsible for much of the observed isotopic variation in natural ClO4− and ClO3−.

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