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

Nubia L. Estrada, Todd A. Anderson, J. K. Böhlke, Baohua Gu, Paul B. Hatzinger, Stanley J. Mroczkowski, Balaji Rao, Neil C. Sturchio, W. Andrew Jackson

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Abstract

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.

Original languageEnglish
Pages (from-to)292-315
Number of pages24
JournalGeochimica et Cosmochimica Acta
Volume311
DOIs
StatePublished - Oct 15 2021

Keywords

  • Chlorate
  • Oxyanions
  • Ozone
  • Perchlorate
  • Photolysis
  • Stable isotopic composition
  • chlorine isotopes
  • oxygen isotopes

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