In situ study of boron partitioning between calcite and fluid at different crystal growth rates Rinat I. Gabitov a,⇑ , Claire Rollion-Bard b,1 , Aradhna Tripati c , Aleksey Sadekov d a Mississippi State University, Mississippi State, MS 39762, USA b Centre de Recherches Pe ´trographiques et Ge ´ochimiques, CRPG-CNRS, Universite ´ de Lorraine, UMR 7358, BP 20, F-54501 Vandoeuvre-Les-Nancy Cedex, France c University of California, Los Angeles, CA 90095, USA d University of Cambridge, Cambridge CB23EQ, UK Received 30 October 2012; accepted in revised form 4 April 2014; Available online 19 April 2014 Abstract The boron isotopic and chemical content of carbonates (expressed as d 11 B and B/Ca ratios) have been proposed as proxies for seawater carbonate system parameters. Thermodynamic and kinetic effects on d 11 B 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 d 11 B were performed on single crystal of calcite synthesized by diffusion of CO 2 under controlled conditions from a solution of NH 4 Cl–CaCl 2 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 d 11 B ratios were measured. Values for the boron Nernst partition coefficient, defined as D B =B (calcite) /B (fluid) , increase from 0.5 (pH NBS = 8.15) to 0.9 (pH = 8.00) with values of V increasing from 0.04 to 0.13 nm/s. For d 11 B behavior, it was not possible to draw conclusive results due to the analytical error (2.2&;2r). pH effects on D B 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. Ó 2014 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Dissolved inorganic boron in fluids is present mainly as the two chemical species B(OH) 3 and B(OH) 4  , with the fraction of each species present dependent on solution pH at equilibrium (Hershey et al., 1986; Hemming and Hanson, 1992). There is an isotopic fractionation between these two species of about 27.2& (Klochko et al., 2006), with B(OH) 3 being enriched in 11 B. It is generally assumed that only B(OH) 4  is incorporated into calcite (Hemming et al., 1995). Thus the partition coefficient of boron, as defined in Hemming and Hanson (1992), has been proposed as a proxy for solution [B(OH) 4  ]/[HCO 3  ] ratios (Yu and Elderfield, 2007; Yu et al., 2007a,b; Tripati et al., 2009, 2011; Allen et al., 2012), as well as the [CO 3 2 ] of seawater (Yu and Elderfield, 2007; Rae et al., 2011). Furthermore, the boron isotopic composition of carbonates has been used as a proxy for the pH of solution from which carbonate http://dx.doi.org/10.1016/j.gca.2014.04.014 0016-7037/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +1 662 268 1032x218. E-mail address: rinat.gabitov@gmail.com (R.I. Gabitov). 1 Present address: Institut de Physique du Globe de Paris (IPGP), Universite ´ Paris Diderot, UMR CNRS 7154, 1 rue Jussieu, F-75238 Paris Cedex 05, France. www.elsevier.com/locate/gca Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 137 (2014) 81–92