The effect of growth rate on uranium partitioning between individual calcite crystals and fluid

Abstract

Elemental to calcium ratios in calcium carbonate minerals are used to study environmental conditions. In particular, uranium to calcium ratio (U/Ca) has been proposed as a proxy for seawater carbonate ion concentration (CO32−) and seawater pH. This work is focused on the evaluation of growth rate and its effect on uranium partitioning between calcite and fluid. We grew inorganic calcite (by diffusion of CO2) isothermally from NH4Cl-CaCl2 doped with uranium. The growth rate of calcite (crystal extension rate, V) was monitored by sequentially spiking calcite-precipitating fluids with rare earth element (REE) dopants. The REE was analyzed with secondary ion mass spectrometry (SIMS) at spots matching those where U/Ca was determined. Partition coefficient KU=(U/Ca)calcite/(U/Ca)fluid increases with increasing growth rate (V). Specifically, KU increases from 0.02 to 0.06 when V increases from 0.01 to 0.14nm/s and remains nearly constant at faster rates. Numerical simulations using the growth entrapment model (GEM) and unified uptake kinetics model (UUKM) were undertaken to explain KU-V relationship in the recent data on calcite.