In situ study of boron partitioning between calcite and fluid at different crystal growth rates

Abstract

The boron isotopic and chemical content of carbonates (expressed as δ11B and B/Ca ratios) have been proposed as proxies for seawater carbonate system parameters. Thermodynamic and kinetic effects on δ11B and B partitioning are not yet fully constrained, underscoring the importance of exploring possible effects of growth rate on boron incorporation in synthetic calcium carbonate minerals, which is the focus of this study. Secondary Ion Mass Spectrometry (SIMS) measurements of B/Ca and δ11B were performed on single crystal of calcite synthesized by diffusion of CO2 under controlled conditions from a solution of NH4Cl–CaCl2 doped with boron. Growth rates of calcite (V, nm/s) within crystals grown isothermally at 24.6°C were monitored by sequentially spiking calcite-precipitating fluids with rare earth element (REE) dopants. The REE were analyzed with SIMS at spots that match those locations where B/Ca and δ11B ratios were measured. Values for the boron Nernst partition coefficient, defined as DB=B(calcite)/B(fluid), increase from 0.5 (pHNBS=8.15) to 0.9 (pH=8.00) with values of V increasing from 0.04 to 0.13nm/s. For δ11B behavior, it was not possible to draw conclusive results due to the analytical error (2.2‰; 2σ). pH effects on DB during calcite precipitation, associated with an increase in pH from 8.00 to 8.15, appear to be masked by the competing effects of changing V. We conclude that over the range of growth rates and other conditions investigated in this study, growth rate effects on B partitioning need to be accounted for when using B/Ca ratios in biogenic calcite as a proxy for seawater carbonate system parameters.