Boron speciation in aqueous fluids at 22 to 600 degrees C and 0.1 MPa to 2 GPa

摘要

The speciation of boron in H2O + H3BO3 +/- NaCl and H2O + Na2B4O7 fluids was studied in situ at temperatures between 22 and 600degreesC and pressures from 0.1 MPa to similar to2 GPa using Raman spectroscopy and a hydrothermal diamond anvil cell. Additionally, we determined the frequency shifts of the 877 cm(-1) Raman line of [B(OH)(3)](0) in aqueous fluids with temperature (partial derivativev(877)/partial derivativeT)(p) = (0.1 MPa) = -0.02532 cm(-1) K-1 and pressure (partial derivativev(877)/partial derivativeP)(T) = (22degreesC) = 4.06 cm(-1) GPa(-1). The observed species in acidic fluids were [B(OH)(3)](0) and smaller amounts of a four-coordinated boron species which may be attributed to dissolved metaboric acid HBO2(aq). The ratio of this B-[4]-O species to [B(OH)(3)](0) increases with temperature and decreases slightly with addition of NaCl. In alkaline solutions, polyboric ions depolymerize rapidly with temperature. Thus, [B(OH)(3)](0) and [B(OH)(4)](-) were the only remaining detectable species at 500 and 600degreesC. The Raman spectra showed an increase of [B(OH)(3)](0) relative to [B(OH)(4)](-) with temperature and an increase of [B(OH)(4)](-) relative to [B(OH)(3)](0) with pressure.The general trend in the boron speciation is a higher stability of simpler complexes with temperature. The experimental observations strongly indicate that planar three-coordinated [B(OH)(3)](0) is the predominant boron species in the aqueous phase over a wide range of P-T-pH conditions. This supports the validity of previous assumptions on boron coordination in crustal and mantle wedge fluids. Copyright (C) 2005 Elsevier Ltd.