Boron speciation in aqueous fluids at 22 to 600°C and 0.1 MPa to 2 GPa |
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Authors: | C. Schmidt R. Thomas |
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Affiliation: | a GeoForschungZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany |
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Abstract: | 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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