LASER OXYGEN-ISOTOPE ANALYSIS OF SILICATE OXIDE GRAIN SEPARATES - EVIDENCE FOR A GRAIN-SIZE EFFECT/

285 oxygen isotope analyses of grain separates of quartz, biotite, mus covite, aegirine, actinolite, olivine and chromite were carried out by laser fluorination using BrF5. Two of the eleven samples analyzed are oxygen isotope standards (NBS-28 and NBS-30) with internationally agr eed delta(18)O-values. The delta(18)O-values of all other samples were previously determined by conventional fluorination. Laser oxygen isot ope analyses were performed on chips, calibrated size separates and po wdered aliquots of each sample. The results indicate that fine-grained (<250 mu m) and powdered samples of most minerals yield laser delta(1 8)O-values that are systematically and reproducibly lower than the con ventional delta(18)O-values. A continuous series of five size fraction s, ranging from >500 to <75 mu m, was analyzed for two quartz samples. The results show a progressive decrease of laser delta(18)O-values wi th decreasing grain size. The delta(18)O-value of the finest size frac tion is, on average, 0.8-1.0 parts per thousand lower than the convent ionally measured delta(18)O-value. Also, 22 analyses of quartz standar d NBS-28, which grain size is 120-250 mu m, yielded an average laser d elta(18)O-value of +8.8 +/- 0.2 parts per thousand, i.e. 0.8 parts per thousand lower than its true value. Similarly, powdered aliquots (gra in size <75 mu m) of all minerals but actinolite and aegirine gave las er delta(18)O-values that were 0.5-2.0 parts per thousand lower than c onventional values. For all minerals, except muscovite and chromite, l aser analyses of size fractions >250 mu m yielded correct delta(18)O-v alues, suggesting that the grain size of 250 mu m may constitute a thr eshold. Detailed examination of the data indicates that this unexpecte d depletion cannot be attributed to contamination during powder, loss of material during fusion (low yields), intragrain heterogeneity of th e samples, atmospheric contamination, oxygen isotope fractionation dur ing the fluorination reaction or during O-2-CO2 conversion. In additio n, laser beam size, which is shown to have an influence on the laser d elta(18)O-values measured for some minerals, is not responsible for th e observed depletion. At this time, we do not have a complete understa nding of the cause(s) of the observed grain size effect. As a conseque nce, it is advisable that investigators performed preliminary tests be fore analyzing fine-grained or powdered minerals by the laser fluorina tion technique.