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 partitioning and isotopic fractionation of boron into carbonates. At pH ≈ 8, both polymorphs have boron isotopic compositions I6.5%0 lower than the boron isotopic composition of the experimental fluid (δ¹¹B = 0 ± 0.7א.). The isotopic composition of the synthetic minerals is identical to the calculated boron isotopic composition of the B(OH)₄^- species in the parent solutions, consistent with only B(OH)₄^- interacting with the crystal surface. The fractionation factor calculated from this isotopic offset is within analytical error of that measured between seawater and modern marine carbonates. The boron concentrations of the experimentally grown aragonite are 13.4, 85.1, and 379 ppm for total fluid boron concentrations of 0.59, 5.1, and 37.0 ppm, respectively. High-Mg calcite (HMC) coprecipitating with the aragonite has bulk boron concentrations of 5.6, 56.2, 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, 44.l, and 499 ppm, respectively. Boron uptake by aragonite is greater than boron uptake by calcite grown under nearly identical conditions, while boron uptake by HMC is intermediate. Synthetic calcite crystals grown in solutions with greater than 4 ppm boron exhibit habit modification with the appearance of a subordinate form {1120} in addition to the dominant {1014}, the perfect cleavage rhombohedron, found in low-boron calcites. This is evidence of strong interaction of the borate with the crystal surface.