The importance of oxygen isotope analysis in the field of meteoritics has been firmly established over the past 15 to 20 years, almost exclusively by Clayton and co-workers at the University of Chicago. The ability of the now classical oxygen three isotope plot to distinguish between fractionation and mixing processes has meant that such analyses are particularly powerful in helping to establish the generic relationships between different classes of meteorites. Conventionally, oxygen gas is usually extracted from the samples in nickel, or similar, reaction bombs at temperatures up to 700 degrees C in the presence of a powerful oxidizing reagent. Due to the nature of these experiments relatively large samples are required (35 mg or more) and the maximum temperatures used make analyses of more refractory minerals difficult if not dangerous. In the past two years a number of laboratories have been developing laser fluorination techniques. Such a technique greatly helps to reduce the blank, allow higher reaction temperatures, and speed up the reaction times, resulting in smaller sample size requirements, and potentially in situ analysis. However, to date this has only been applied to delta^18O analyses where the oxygen is analysed as CO2. We report here the development of a laser fluorination technique capable of running small samples for delta^17O and delta^18O using oxygen gas. The laser used is a 25W CO2 laser (10.6 micrometer radiation) and the reagent is ClF3. The sample size requirements are currently 0.5 to 1.0 mg. Replicate analyses of NBS-28 quartz has yielded precision on delta^18O and delta^17O of +-0.15 and +-0.17o/oo respectively and with other terrestrial samples define the expected fractionation line. The results of the first application of this technique to meteorite samples are shown in the figures. The LL6 Appley Bridge has been analyzed four times (Fig. 1a) giving mean values for delta^17O and delta^18O of +3.94o/oo (+-0.14) and +4.99o/oo (+-0.31) respectively, which compare favorably with previous results, delta^17O = +3.90 and +3.96, delta^18O = +4.97 and +5.17 (Clayton et al., 1991). To demonstrate its efficacy, two 1-mg aliquots of a putative Acapulcoite, FRO90011 weighing 1.8 g, have been analyzed yielding delta^17O values of +0.83 and +1.12o/oo and delta^18O of +3.81 and +3.64o/oo. As can be seen in Fig. 1b, these results are in good agreement with the results obtained from known Acapulcoites by Clayton et al. (1992), and confirm the classification of FRO90011 as an additional member of a rare group. The results presented here demonstrate the ability of the laser technique to analyze small samples of meteorite for both delta^17O and delta^18O with a precision suitable for almost all current applications. This technique offers the potential of further reducing the specimen size so that particularly precious material such as inclusions within meteorites and mineral separates may be studied. The high temperatures attainable with the laser should also permit the easy analysis of acid resistant phases, such as oxides. Clayton R.N., Mayeda T.K., Goswami J.N., and Olsen E.J. (1991) Geochim. Cosmochim. Acta 55, 2317-2337. Clayton R.N., Mayeda T.K., and Nagahara H. (1992) Lunar Planet. Sci. (abstract) 23, 231232.
- Pub Date:
- July 1992