UWG-2, A Garnet Standard for Oxygen Isotope Ratios: Strategies for High Precision and Accuracy with Laser Heating

Valley, John W., Kitchen, Nami, Kohn, Matthew J., Niendorf, Christopher R., and Spicizza, Michael J. (1995) UWG-2, a garnet standard for oxygen isotope ratios: Strategies for high precision and accuracy with laser heating. Geochimica et Cosmochimica Acta, v. 59, n. 24, p. 5223-5231.

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 d¹⁸O 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 d¹⁸O = 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 d¹⁸O 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‰ (1s ). A small drift of the daily average over time results from inevitable changes in the vacuum line which require careful attention and maintenance.

Figure 1. Uncorrected values of d18O for 1081 analyses of UWG-2 standard on 212 days at the University of Wisconsin. Data are coded according to analyst. The average value for all data is 5.74 ± 0.15‰ (1 standard deviation about the mean, see Table 2) with an uncertainty in the mean of ±0.005‰. Small amounts of systemtic drift about the long term average are seen with the precision on most days better than ±0.1‰. Daily averages, standard deviations and # of analyses are included in Appendix A. (a) Data with uncertainties about the daily mean. (b) Data with uncertainties in the daily mean. Figure 1b shows that drift was significantly larger than uncertainty and that the cause is not related to different analysts. In at least some instances, drift was caused by wear of the laser system and, once identified by these standard analyses, was easily remedied.

Figure 2. A total of 82 UWG-2 and 65 NBS-28 analyses have been made together on 11 days (see Table 1). (a) Data for NBS-28, African Glass Sand. (b) Data for UWG-2, UW-Gore Mountain Garnet #2. Figure 2c shows that analyses of the two standards track each other with an average difference of 3.70 ± 0.06‰

Figure 3. Uncorrected analyses of UWG-1 as a function of sample size. There is no difference with sample size from 0.5 mg to over 4 mg.