Uranium isotope fractionation during coprecipitation with aragonite and calcite

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• 作者：Xinming Chen^a ; ^{Xinming.Chen@asu.edu" class="auth_mail" title="E-mail the corresponding author} ; Stephen J. Romaniello^a ; Achim D. Herrmann^b ; Laura E. Wasylenki^c ; Ariel D. Anbar^a ; ^d
• 关键词：U isotopes ; Paleoredox ; Calcium carbonate ; Speciation
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：1 September 2016
• 年：2016
• 卷：188
• 期：Complete
• 页码：189-207
• 全文大小：1610 K

文摘

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, P_CO2${\text{P}}_{{\text{CO}}_2}$, Ca²⁺, or Mg²⁺ concentrations. In general, these effects are likely to be small (<0.13‰), but are nevertheless potentially significant because of the small natural range of variation of ²³⁸U/²³⁵U.