Uranium isotope fractionation during coprecipitation with aragonite and calcite

作者：Chen Xinming; Romaniello Stephen J; Herrmann Achim D; Wasylenki Laura E; Anbar Ariel D

来源：Geochimica et Cosmochimica Acta, 2016, 188: 189-207.

DOI：10.1016/j.gca.2016.05.022

• U isotopes
• Paleoredox
• Calcium carbonate
• Speciation

Natural variations in U-238/U-235 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 similar to 7.5 and similar to 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 similar to 8.5, with heavier U isotopes preferentially enriched in the solid phase. U-238/U-235 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 similar to 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, Ca2+, or Mg2+ concentrations. In general, these effects are likely to be small (<0.13 parts per thousand), but are nevertheless potentially significant because of the small natural range of variation of U-238/U-235.

• 出版日期2016-9-1