A LASER-BASED MICROANALYTICAL METHOD FOR THE INSITU DETERMINATION OF OXYGEN ISOTOPE RATIOS OF SILICATES AND OXIDES

A new laser-based microanalytical method for oxygen isotope determinations of silicates and oxides has been developed whereby analyses are made on <100 μg samples or of in situ spot determinations of <300 μm in diameter. This new method differs from the conventional fluorination technique in that samples are heated directly by a laser, much lower blanks are achieved, and far smaller amounts of material can be analyzed. The intense and focussed heat source allows for in situ analyses to be made on even the most refractory minerals. Sample separates are placed on a nickel sample holder in a 304-stainless steel sample chamber and are heated with a 20W CO2 laser. The infrared laser radiation is admitted through a BaF2 window, a material that is transparent to both infrared and visible radiation, and does not react with fluorine. Either BrF5 or ClF3 is used as the fluorinating agent. The released oxygen is passed successively over a cold trap and through a hot mercury fluorine-getter, and is converted to CO2 by reaction with hot carbon in the presence of a Pt-catalyst. The CO2 is admitted on-line to the mass spectrometer. Up to 19 samples can be loaded and degassed simultaneously with the current design, and reaction times for each sample are generally less than 2 min. Quartz, feldspar, kyanite, olivine, diopside, garnet, muscovite, biotite, MnO2, and bulk-rock samples have all been analyzed successfully. The precision and accuracy (±0.1%) is equal to the conventional fluorination method. Replicate in situ spot-analyses were made of quartz and magnetite on a polished thick section and of olivine on a single large olivine crystal. Analyses yielding from <1-4 μmoles of CO2 gave the expected δ18O value within 0.5%. © 1990.