Precise and accurate determination of boron isotope ratios by multiple collector ICP-MS: origin of boron in the Ngawha geothermal system, New Zealand

Abstract

A new technique has been developed for the measurement of boron isotope ratios in fluids using a double focusing multiple collector inductively coupled plasma (ICP) mass spectrometer. Sample introduction using a direct injection nebuliser was found to eliminate memory problems that are common where spray chambers are involved. The 4–9% mass bias can be corrected for by bracketing sample measurements with standards. As matrices were found to affect the mass bias all samples must be purified and that sample and standard solutions must be similar. A new purification technique was developed that yields adequately purified samples. Using this technique, it is possible to make rapid measurements (4 min) from samples containing 250 ng B to a precision of ±0.2‰. This analytical technique has been applied to the Ngawha geothermal system in New Zealand in an attempt to determine the source of B in geothermal fluids where the B concentrations can exceed 100 mmol/l. The δ11B values range between −3.1‰ and −3.9‰ indicating that no seawater sources are involved and that the elevated B concentrations can only be accounted for by low water/rock ratios with the B being derived from basement greywacke/argillite. The similarity of B isotope ratios for hydrothermal fluids that have been diluted and cooled with groundwaters indicates that isotope fractionation due to adsorption is unlikely to occur in cool geothermal fluids (<40 °C).