Aqueous geochemistry at gigapascal pressures: NMR spectroscopy of fluoroborate solutions

Abstract

Aqueous geochemistry could be extended considerably if nuclear-magnetic resonance (NMR) methods could be adapted to study solutions at elevated temperatures and pressures. We therefore designed an NMR probe that can be used to study aqueous solutions at gigapascal pressures. Fluoride solutions were chosen for study because 19F couples to other nuclei in the solutions (31P and 11B) in ways that make peak assignments unequivocal. Correspondingly, NMR spectra of 19F- and 11B were collected on aqueous HBF4-NH4PF6 solutions to pressures up to 2.0 GPa. At pressure, peaks in the 19F spectra were clear and assignable to the BF4−(aq), F−(aq) and BF3OH− (aq) ions, and these aqueous complexes varied in signal intensity with pressure and time, for each solution. Peaks in the 11B spectra at pressure could be assigned to the BF4−(aq) and BF3OH−(aq) species. Additionally, there is a single peak that is assignable to H3BO3o(aq) and B(OH)4−(aq) in rapid-exchange equilibria. These peaks broaden and move with pressure in ways that suggest reversible interconversion of borate and fluoroborate species. The PF6− ion was found to provide a suitable 19F shift and intensity standard for high-pressure spectra because it was chemically inert. The positions and intensities of the doublet peak also remains constant as a function of pressure and pH. Addition of electrolytes considerably distorts the phase diagram of water such that the stability region of the aqueous solution expands to well beyond the 0.8 GPa freezing pressure of pure water; some fluoroborate solutions remain liquid until almost 2.0 GPa.