Abstract (EDOC: 5938)
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 about 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 (dv877/dT)P=0.1 MPa = -0.02532 cm-1K-1
and pressure (dv877/dP)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-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.
(2005): Boron speciation in aqueous fluids at 22 to 600°C and 0.1 MPa to 2 GPa. Geochimica et Cosmochimica Acta, 69, 2, 275-281.