Boron isotope exchange between seawater and the oceanic crust

Abstract

Dissolved boron in seawater from the Atlantic and Pacific is isotopically homogeneous at 39.5 per mil (11B10B ratios are expressed as per mil deviations from NBS SRM 951). Unaltered mid-ocean ridge basalts (MORE) from the crest of the East Pacific Rise (EPR) at 21° and 13°N have B contents of 0.39 ± 0.03 and 0.46 ± 0.03 ppm (about one order of magnitude lower than previous estimates) and δ11B of −3.6 ± 0.4 and −2.2 ± 0.6 per mil respectively. Large scale B exchange between seawater and the oceanic crust has been demonstrated at both high and low temperature. Hydrothermal solutions from nine separate vent fields at 21° and 13°N (EPR) have variable B enrichments, relative to seawater (416 μmoles/kg), of between 8 and 32% and have δ11B values between 30.0 ± 0.4 and 36.8 ± 0.4 per mil. Boron has been extracted from the basalts with no resolvable isotopic fractionation. High temperature waterrock ratios, based on the B concentrations, are between 0.28 and 3.0. The hydrothermal flux of B into the oceans is between 0.4 and 0.8 × 109 moles/yr, assuming that only pristine basalts are present in the reaction zone. Basalts altered at low temperature and serpentinites are variably enriched in B. The B content of altered whole rocks correlates strongly with δ18O, and increases with degree of alteration. Altered basalts (n = 7) containing between 8.9 and 69 ppm B have δ11B between 0.1 and 9.2 ± 0.4 per mil. Model calculations give waterrock ratios greater than 100 for rocks recovered from DSDP Hole 418A. Serpentinized peridotites (n = 4) with between 50 and 81 ppm B have δ11B between 8.3 ± 0.4 and 12.6 ± 0.4 per mil. The flux of B into the crust during low temperature alteration is about 13 × 109 moles/yr. The maximum diffusive flux of B into the crust from sediment pore waters is 0.8 × 109 moles/yr with a δ11B less than 43 per mil.