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

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

Jan 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005gecoa..69..275s&link_type=abstract

Geochimica et Cosmochimica Acta, Volume 69, Issue 2, p. 275-281.

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Scientific paper

The speciation of boron in H2O+H3BO3±NaCl and H2O+Na2B4O7 fluids was studied in situ at temperatures between 22 and 600°C and pressures from 0.1 MPa to ˜2 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 (∂ν877/∂T)p = 0.1 MPa = -0.02532 cm-1K-1 and pressure (∂ν877/∂P)T = 22°C = 4.06 cm-1GPa-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 600°C. 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.

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