MINERAL-FLUID PARTITIONING AND ISOTOPIC FRACTIONATION OF BORON IN SYNTHETIC CALCIUM-CARBONATE

Aragonite, high-Mg calcite, and Mg-free calcite have been grown under monitored laboratory conditions to investigate mineral-fluid partition ing and isotopic fractionation of boron into carbonates. At pH almost- equal-to 8, both polymorphs have boron isotopic compositions 16.5parts per thousand lower than the boron isotopic composition of the experim ental fluid (delta B-11 = 0 +/- 0.7parts per thousand). The isotopic c omposition of the synthetic minerals is identical to the calculated bo ron isotopic composition of the B(OH)4- species in the parent solution s, consistent with only B (OH)4- interacting with the crystal surface. The fractionation factor calculated from this isotopic offset is with in analytical error of that measured between seawater and modern marin e carbonates. The boron concentrations of the experimentally grown ara gonite are 13.4, 85.1, and 379 ppm for total fluid boron concentration s of 0.59, 5.1, and 37.0 ppm, respectively. High-Mg calcite (HMC) copr ecipitating with the aragonite has bulk boron concentrations of 5.6, 5 6.2, 260, and 1771 ppm for fluid boron concentrations of 0.59, 5.1, 37 , and 272 ppm, respectively. Calcite boron concentrations are 1.78, 14 .0, 137.7, and 1228 ppm for fluid boron concentrations of 0.45, 4.0, 4 4.1, and 499 ppm, respectively. Boron uptake by aragonite is greater t han boron uptake by calcite grown under nearly identical conditions, w hile boron uptake by HMC is intermediate. Synthetic calcite crystals g rown in solutions with greater than 4 ppm boron exhibit habit modifica tion with the appearance of a subordinate form {112BAR0} in addition t o the dominant {101BAR4}, the perfect cleavage rhombohedron, found in low-boron calcites. This is evidence of strong interaction of the bora te with the crystal surface.