IN-SITU OXYGEN-ISOTOPE ANALYSIS WITH AN EXCIMER-LASER USING F-2 AND BRF5 REAGENTS AND O-2 GAS AS ANALYTE

In situ oxygen isotope analysis of silicate minerals has been validate d by interlaboratory calibration using an excimer laser (KrF fill gas, 248 nm), F-2 gas fluorinating reagent, and O-2 gas as mass spectromet er analyte. A value of delta(18)O(SMOW) = 5.82 parts per thousand (+/- 0.03, 4 analyses) was obtained for almandine UWG-2 compared to a recom mended value of 5.8 parts per thousand (Valley et al., 1995). The inte rhalogen BrF5 was tested by comparing in situ analyses made alternativ ely with either BrFS or Fa gas on crystals of almandine, epidote, fors terite, tourmaline, and zircon. Results for different pressures of BrF 5 (20 to 40 ton) in the reaction chamber show that delta(18)O values d ecrease by 1 permil for an increase of 10 ton in BrF5 pressure. In str ong contrast, delta(18)O values measured with purified F-2 gas are pre cise and accurate over a range of from 15 to 150 ton F-2 gas pressure. Use of F-2 gives higher yields of O-2 than with BrF5. Values of delta (17)O and delta(18)O measured in situ on a variety of silicates all pl ot on the terrestrial mass fractionation line, provided that NF3 conta mination is eliminated from the O-2 gas analyte. Copyright (C) 1997 El sevier Science Ltd.