Uranium Isotope Fractionation During Coprecipitation with Aragonite and Calcite

Abstract:

Natural variations in the ²³⁸U/²³⁵U ratio of marine carbonates may provide a useful way of constraining past variations in ocean redox conditions. However, before applying this novel redox proxy, it is essential to explore possible isotopic fractionation during U coprecipitation with aragonite and calcite. We investigated these effects in laboratory experiments.

Aragonite and calcite coprecipitation experiments were conducted at pH 8.5±0.1 using a constant addition method [1]. More than 90% of the U was incorporated into the solid phase at the end of each experiment. Samples were purified using UTEVA chemistry and δ^238/235U was measured using ²³³U-²³⁶U double-spike MC-ICP-MS with a precision of ±0.10‰ [2]. The aragonite experiment demonstrated a ²³⁸U/²³⁵U Rayleigh fractionation factor of α=1.00008±0.00002 with the ²³⁸U preferentially incorporated. In contrast, the calcite experiment demonstrated no resolvable U isotope fractionation (α=1.00001±0.00003).

To determine if U isotopes are affected during the early diagenetic conversion of aragonite to calcite, natural carbonate samples were collected along an aragonite-calcite transition across a single coral head in the Key Largo limestone, and characterized for U concentration and δ^238/235U [3]. We found that the mean δ^238/235U in aragonite (-0.33±0.07‰ 2se) was slightly heavier than that in calcite (-0.37±0.02‰ 2se).

Further work is needed to address the mechanisms leading to differential isotopic fractionation of U(VI) during incorporation into aragonite and calcite. Possible drivers include differences in coordination in the crystal structure or equilibrium isotopic fractionation between various aqueous U(VI) species prior to incorporation.

[1] Reeder et al. (2001) GCA 65, 3491-3503.

[2] Weyer et al., (2008) GCA 72, 345-359.

[3] Gill et al., (2008) GCA 72, 4699-4722.