Modelling the geochemical cycle of boron:: Implications for the long-term δ11B evolution of seawater and oceanic crust

The boron geochemical cycle has been simulated using a time-dependent geochemical box model that was coupled to a one-dimension model of seawater-oceanic crust interactions. Boron elemental and isotopic compositions of oceanic rocks as a function of depth were calculated by mass balance, using the temperature and porosity profiles of the crust as well as the available experimental and empirical distribution coefficients and fractionation factors between mineral and water. Ranges of boron elemental and isotopic variations of seawater were calculated for crust-seawater interactions that take place from the ridge-axis to the off-axis closure of the hydrothermal system. The present-day delta B-11 of seawater (40%o) could represent a steady-state value. However, depending on crustal permeability, lifetime of water-rock interactions, and expansion rate of the oceanic ridge, the delta B-11 of seawater may vary from 30 parts per thousand to 50 parts per thousand at the 10 million year scale. Some boron isotope compositions of Cretaceous biogenic carbonates and ophiolitic serpentinites from Oman are comparable to modem rock samples, suggesting that the delta B-11 of Cretaceous seawater was close to the present-day value. Low delta B-11 values of some biogenic carbonates cannot be attributed to low pH values of past seawater, but more probably to delta B-11 variations of seawater or diagenetic alteration by crustal aqueous fluids. Boron isotope composition of hydrothermally altered serpentines could be considered as a promising proxy of the seawater composition. (c) 2005 Elsevier B.V. All rights reserved.