UWG-2, A GARNET STANDARD FOR OXYGEN ISOTOPE RATIOS: STRATEGIES FOR HIGH PRECISION AND ACCURACY WITH LASER HEATING

Abstract

UWG-2 is a new garnet standard for oxygen isotope analysis prepared from a single large porphyroblast that was homogeneous (+/- 0.21%%) at the millimeter-scale before grinding. The δ18O value of UWG-2 has been determined in seven laboratories using either a laser probe system or externally heated Ni reaction vessels. The raw laser probe value is 5.74%% at the University of Wisconsin. If all data are normalized to NBS-28 = 9.59%%, then the UW value (5.89%%) is in good agreement with the average of all labs (5.78%%). There is no significant difference between garnet analyses made with the two techniques, nor among labs using different wavelengths of IR laser. UWG-2 is available for interlaboratory comparison, and for assessing the performance of microanalytical techniques including laser probes and ion microprobes with a recommended value of δ18O = 5.8%% SMOW. Multiple, daily analyses of UWG-2 at the University of Wisconsin provide an accurate evaluation of all components in our laser-probe, mass-spectrometer system, and allow analytical problems to be rapidly identified. With this standardization, the accuracy of a single laser probe analysis is better than +/- 0.10%%. Over 1000 analyses of UWG-2 have been made. The average of all uncorrected δ18O values is 5.74 +/- 0.15%% (1 sd). The precision on a single day averages +/- 0.07%% and is frequently better than 0.05%%. The uncertainty in the mean for all analyses is +/-0.005%% (1σ). A small drift of the daily average over time results from inevitable changes in the vacuum line which require careful attention and maintenance.