Uranium isotope fractionation during coprecipitation with aragonite and calcite

Xinming Chen, Stephen J. Romaniello, Achim D. Herrmann, Laura E. Wasylenki, Ariel Anbar

Research output: Contribution to journalArticle

42 Scopus citations

Abstract

Natural variations in 238U/235U of marine calcium carbonates might provide a useful way of constraining redox conditions of ancient environments. In order to evaluate the reliability of this proxy, we conducted aragonite and calcite coprecipitation experiments at pH ~7.5 and ~8.5 to study possible U isotope fractionation during incorporation into these minerals.Small but significant U isotope fractionation was observed in aragonite experiments at pH ~8.5, with heavier U isotopes preferentially enriched in the solid phase. 238U/235U of dissolved U in these experiments can be fit by Rayleigh fractionation curves with fractionation factors of 1.00007 + 0.00002/-0.00003, 1.00005 ± 0.00001, and 1.00003 ± 0.00001. In contrast, no resolvable U isotope fractionation was observed in an aragonite experiment at pH ~7.5 or in calcite experiments at either pH. Equilibrium isotope fractionation among different aqueous U species is the most likely explanation for these findings. Certain charged U species are preferentially incorporated into calcium carbonate relative to the uncharged U species Ca2UO2(CO3)3(aq), which we hypothesize has a lighter equilibrium U isotope composition than most of the charged species. According to this hypothesis, the magnitude of U isotope fractionation should scale with the fraction of dissolved U that is present as Ca2UO2(CO3)3(aq). This expectation is confirmed by equilibrium speciation modeling of our experiments. Theoretical calculation of the U isotope fractionation factors between different U species could further test this hypothesis and our proposed fractionation mechanism.These findings suggest that U isotope variations in ancient carbonates could be controlled by changes in the aqueous speciation of seawater U, particularly changes in seawater pH, PCO2, Ca2+, or Mg2+ concentrations. In general, these effects are likely to be small (238U/235U.

Original languageEnglish (US)
Pages (from-to)189-207
Number of pages19
JournalGeochimica et Cosmochimica Acta
Volume188
DOIs
StatePublished - Sep 1 2016

Keywords

  • Calcium carbonate
  • Paleoredox
  • Speciation
  • U isotopes

ASJC Scopus subject areas

  • Geochemistry and Petrology

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