ISOTOPE ANALYSIS OF OXYGEN IN MINERALS USING ND-YAG LASER-FLUORINATION - THE USE OF STAINLESS-STEEL WOOL TRAP AS A FLUORINE REMOVER

Oxygen from a range of minerals has been liberated by fluorination under the radiation from a Nd/YAG laser, and the isotope ratio of oxygen measured with a conventional ratio mass-spectrometer. During the fluorination, generation of fluorine gas often takes place through the dissociation of chlorine trifluoride (CIF3) and interferes with the measurement of the isotope ratio later on. Using mercury on gold to remove the F2 also led to a loss of oxygen. A simple trap made of stainless steel wool has proved to be very effective at removing of fluorine without any measurable loss of O2. The minerals analysed included quartz, feldspar, hornblende, mica, garnet, pyroxene, olivine, and spinel. The accuracy and precision of the method was evaluated. There is good agreement with data obtained by conventional fluorination for the analyses of hornblende, garnet and spinel. In the case of quartz, feldspar and pyroxene, the laser method gave isotope ratios (O-18/O-16) lighter than the ratios obtained by the conventional method by less than 1 per part per thousand, but its application to olivine and mica gave lighter isotope ratios by as much as 1 approximately 2 parts per thousand. The precision of this method is better than 0.3 parts per thousand for all the minerals investigated.