Isotopic composition of natural and synthetic chlorate (δO, ΔO, δCl, Cl/Cl): Methods and initial results.

Natural chlorate (ClO) is widely distributed in terrestrial and extraterrestrial environments. To improve understanding of the origins and distribution of ClO, we developed and tested methods to determine the multi-dimensional isotopic compositions (δO, ΔO, δCl, Cl/Cl) of ClO and then applied the methods to samples of natural nitrate-rich caliche-type salt deposits in the Atacama Desert, Chile, and Death Valley, USA. Tests with reagents and artificial mixed samples indicate stable-isotope ratios were minimally affected by the purification processes. Chlorate extracted from Atacama samples had δO = +7.0 to +11.1‰, ΔO = +5.7 to +6.4‰, δCl = -1.4 to +1.3‰, and Cl/Cl = 48 × 10 to 104 × 10. Chlorate from Death Valley samples had δO = -6.9 to +1.6‰, ΔO = +0.4 to +2.6‰, δCl = +0.8 to +1.0‰, and Cl/Cl = 14 × 10 to 44 × 10. Positive ΔO values of natural ClO indicate that its production involved reaction with O, while its Cl isotopic composition is consistent with a tropospheric or near-surface source of Cl. The ΔO and δO values of natural ClO are positively correlated, as are those of ClO and NO from the same localities, possibly indicating variation in the relative contributions of O as a source of O in the formation of the oxyanions. Additional isotopic analyses of ClO could provide stronger constraints on its production mechanisms and/or post-formational alterations, with applications for environmental forensics, global biogeochemical cycling of Cl, and the origins of oxyanions detected on Mars.