The Effect of Growth Rate on Uranium Partitioning Between Calcite and Fluid

English Abstract:

Elemental to calcium ratios in calcium carbonate minerals are being used to study environmental conditions at which crystallization occurred. In particular, foraminiferal U/Ca has been proposed as proxy for determining seawater carbonate ion concentration (CO₃^2-) (e.g. Russel et al., 2004). However the kinetic effect of U/Ca incorporation into calcite has been the subject of investigations but is not well understood (e.g. Ni et al., 2007). Therefore, this work is focused on the evaluation of growth rate and its effect on uranium partitioning between calcite and fluid.

The calcites produced during this study were crystallized isothermally from NH₄Cl-CaCl₂ doped with uranium by diffusion of CO₂ from ammonium carbonate source. This method yielded growth of large crystals (>1 mm in size) without stirring of the fluid. Growth rate of calcite (crystal extension rate) was monitored by sequentially spiking calcite-precipitating fluids with rare earth element (REE) dopants. The U/Ca was analyzed with SIMS at spots matching those where REE were determined using CAMECA ims 1270 ion microprobe at UCLA (USA). Elemental analyses of the fluids were performed using Thermo Element XR, ICP-MS at the University of Cambridge (UK).

Partition coefficients K^U=(U/Ca)_calcite/(U/Ca)_fluid increases with increasing of growth rate (V). K^U increases by a factor of two when V increases from 0.01 to 0.14 nm/s and remained near constant at faster rates. Numerical simulations using the growth entrapment model (GEM) (Watson, 2004) and unified uptake kinetics model (UUKM) (Thien et al., 2014) explain the observed K^U-V trend.

References

Ni et al. (2007) Paleoceanogr., 22, PA3212, doi:10.1029/2006PA001337.

Russell et al. (2004) Geochim. Cosmochim. Acta, 68, 4347-4361.

Thien et al. (2014) Applied Geochemistry, 41, 135-150.

Watson (2004) Geochim. Cosmochim. Acta, 68, 1473-1488.