Modelling the geochemical cycle of boron: Implications for the long-term d11B evolution of seawater and oceanic crust
Résumé
The boron geochemical cycle has been simulated using a time-dependent geochemical box model that was coupled to a onedimension 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 d11B of seawater (40‰) could represent a steady-state value. However, depending on crustal permeability, lifetime of water–rock interactions, and expansion rate of the oceanic ridge, the d11B of seawater may vary from 30‰ to 50‰ at the 10 million year scale. Some boron isotope compositions of Cretaceous biogenic carbonates and ophiolitic serpentinites from Oman are comparable to modern rock samples, suggesting that the d11B of Cretaceous seawater was close to the present-day value. Low d11B values of some biogenic carbonates cannot be attributed to low pH values of past seawater, but more probably to d11B variations of seawate 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.