11,10B Isotopic Fractionation Between B(OH)3 and B(OH)4- and an Inorganic Mechanism for their Incorporation Into Calcite

It has recently been established experimentally by Byrne, et al. (2006) and Klochko, et al. (2006) that the equilbrium constant for the isotopic exchange reaction: 10B(OH)3 + 11B(OH)4- = 11B(OH)3 + 10B(OH)4- (1) has a value around 1.027 for seawater at 25^{circ}C, for total B concentrations from 0.01 to 0.05 molal. These experimental studies involved essentially the accurate determination of the small pKa difference between the 11B and 10B isotopomers of boric acid. This equilibrium constant value is significantly higher than the traditional value of 1.0194 from Kakihana, et al. (1977). It agrees well with calculated values from Liu and Tossell (2005) but disagrees with a considerably smaller value obtained from spectral studies by Sanchez- Valle, et al. (2005). We will present additional calculations supporting and extending the study of Liu and Tossell (2005) and we will explain the discrepancy with the study of Sanchez-Valle, et al. (2005) and discuss the general unsuitability of methods employing experimental spectral data. We will decompose the free energy change for reaction (1) into its enthalpic and entropic components and will discuss isotopic fractionation between B(OH)3 (aq) and various vapor species. We will also present a model for the incorporation of B(OH)3 or B(OH)4- into calcite by a chemical reaction with HCO3- on the surface of hydrated calcite which produces a B(OH)2CO3- complex, in which the B may be either 3- or 4-coordinate. References Byrne, et al. Deep-Sea Research I (2006) 53, 684-688. Kakihana, et al. Bull. Chem. Soc. Jpn. (1977) 50, 158-163. Klochko, et al. Earth Planet. Sci. Lett. (2006) 248, 276-285. Liu and Tossell Geochim. Cosmochim. Acta. (2005) 69, 3995- 4006. Sanchez-Valle, et al. Geochim. Cosmochim. Acta (2005) 69, 4301-4313.