Boron isotope fractionation during brucite deposition from artificial seawater

Abstract. Experiments involving boron incorporation into brucite (Mg(OH)2) from magnesium-free artificial seawater with pH values ranging from 9.5 to 13.0 were carried out to better understand the incorporation behavior of boron into brucite. The results show that both concentration of boron in deposited brucite ([B]d) and its boron partition coefficient (Kd) between deposited brucite and final seawater are controlled by pH of the solution. The incorporation capacity of boron into brucite is much stronger than that into oxides and clay minerals. The isotopic compositions of boron in deposited brucite (δ11Bd) are higher than those in the associated artificial seawater (δ11Bisw) with fractionation factors ranging between 1.0177 and 1.0569, resulting from the preferential incorporation of B(OH)3 into brucite. Both boron adsorptions onto brucite and precipitation reaction of H3BO3 with brucite exist during deposition of brucite from artificial seawater. The simultaneous occurrence of both processes determines the boron concentration and isotopic fractionation of brucite. The isotopic fractionation behaviors and mechanisms of boron incorporated into brucite are different from those into carbonates. Furthermore, the isotopic compositions of boron in modern corals might be affected by the existence of brucite in madrepore and the preferential incorporation of B(OH)3 into brucite. An exploratory study for the influence of brucite on the boron isotopic composition in modern corals is justifiable.