ALBERT

Description: 〈P〉The speciation of boron in H〈FONT SIZE="-1"〉2〈/FONT〉O+H〈FONT SIZE="-1"〉3〈/FONT〉BO〈FONT SIZE="-1"〉3〈/FONT〉±NaCl and H〈FONT SIZE="-1"〉2〈/FONT〉O+Na〈FONT SIZE="-1"〉2〈/FONT〉B〈FONT SIZE="-1"〉4〈/FONT〉O〈FONT SIZE="-1"〉7〈/FONT〉 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〈SUP〉-1〈/SUP〉 Raman line of [B(OH)〈FONT SIZE="-1"〉3〈/FONT〉]〈SUP〉0〈/SUP〉 in aqueous fluids with temperature (〈FONT FACE="Math1"〉d〈/FONT〉〈FONT FACE="Math5"〉v〈/FONT〉〈FONT SIZE="-1"〉877〈/FONT〉/〈FONT FACE="Math1"〉dT〈/FONT〉)〈FONT SIZE="-1"〉P=0.1 MPa〈/FONT〉 = -0.02532 cm〈SUP〉-1〈/SUP〉K〈SUP〉-1〈/SUP〉 and pressure (〈FONT FACE="Math1"〉d〈/FONT〉〈FONT FACE="Math5"〉v〈/FONT〉〈FONT SIZE="-1"〉877〈/FONT〉/〈FONT FACE="Math1"〉d〈/FONT〉P)〈FONT SIZE="-1"〉T=22°C〈/FONT〉 = 4.06 cm〈SUP〉-1〈/SUP〉GPa〈SUP〉-1〈/SUP〉. The observed species in acidic fluids were [B(OH)〈FONT SIZE="-1"〉3〈/FONT〉]〈SUP〉0〈/SUP〉 and smaller amounts of a four-coordinated boron species which may be attributed to dissolved metaboric acid HBO〈FONT SIZE="-1"〉2〈/FONT〉(aq). The ratio of this B〈SUP〉[4]〈/SUP〉-O species to [B(OH)〈FONT SIZE="-1"〉3〈/FONT〉]〈SUP〉0〈/SUP〉 increases with temperature and decreases slightly with addition of NaCl. In alkaline solutions, polyboric ions depolymerize rapidly with temperature. Thus, [B(OH)〈FONT SIZE="-1"〉3〈/FONT〉]〈SUP〉0〈/SUP〉 and [B(OH)〈FONT SIZE="-1"〉4〈/FONT〉]〈SUP〉-〈/SUP〉 were the only remaining detectable species at 500 and 600°C. The Raman spectra showed an increase of [B(OH)〈FONT SIZE="-1"〉3〈/FONT〉]〈SUP〉0〈/SUP〉 relative to [B(OH)〈FONT SIZE="-1"〉4〈/FONT〉]〈SUP〉-〈/SUP〉 with temperature and an increase of [B(OH)〈FONT SIZE="-1"〉4〈/FONT〉]〈SUP〉-〈/SUP〉 relative to [B(OH)〈FONT SIZE="-1"〉3〈/FONT〉]〈SUP〉0〈/SUP〉 with pressure. 〈BR〉 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)〈FONT SIZE="-1"〉3〈/FONT〉]〈SUP〉0〈/SUP〉 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.〈BR〉 〈/P〉