The use and usefulness of boron isotopes in natural silicate-water systems

Boron and its two stable isotopes 10B and 11B are considered powerful tracers in low temperature regimes like ocean waters and subduction zones, while very little is known about the isotopic fractionation in high pressure-temperature (PT) regimes. However, recent studies indicate that boron fractionation in silicate-water systems may follow a systematic relationship with temperature, regardless of the geological regime. In this paper we review previous B isotope studies and test the empirical relationship proposed by Williams et al. [Boron isotope geochemistry during diagenesis. Part I. Experimental determination of fractionation during illitization of smectite, Geochim. Cosmochim. Acta 65 (2001a) 1769-1782; Boron isotope geochemistry during diagenesis. Part II. Applications to organic sediments, Geochim. Cosmochim. Acta 65 (2001b) 1783-1794; Application of boron isotopes to the understanding of fluid-rock interactions in a hydrothermally stimulated oil reservoir in the Alberta Basin, Canada, Geofluids 1 (2001c) 229-240] by comparing it with the wealth of earlier data from studies of different geological scenarios. Our main conclusion is that, given the large variations of B isotope fractionation patterns in natural silicate-water systems, the majority of these systems are not represented by the proposed relationship. Factors more complex than temperature alone appear to control B isotope geochemistry, which include species of the silicate mineral, starting fluid, pH, geological setting, fluid/rock ratio (i.e. porosity), or time for equilibration between the solid and fluid phase.