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Abstract

Modern in situ analytical methods can determine isotopic ratios in solid materials with sampling masses in the low nanogram (LA-ICPMS) to mid-picogram (SIMS) mass ranges with repeatabilities at, or even below ± 0.1 ‰ (1sd). The proper use of such analytical technologies is, however, constrained by a near total absence of geological rMs that have been adequately characterized for their heterogeneity at such small sampling scales. In many cases the heterogeneity component determined at the “bulk” scale (micrograms to milligrams) is assumed to be valid at the smaller sampling scale, often in the absence of any rigorous in situ testing. Those characterizations that have evaluated the fine scale heterogeneity of a candidate rM have often resorted to comparing the observed repeatability to that presumed for the given analytical method. In the cases where these two values are similar it has been asserted that a given material is “homogeneous” at the micrometer scale. Our research applies statistical methods to SIMS data for quantifying isotopic heterogeneity at the sub-picogram sampling scale, meaning that the analytical uncertainty intrinsic to our laboratory method must be quantified using analysis of variance (ANOVA) and then subtracted during data evaluation. Our work has focused on developing a metrologically rigorous rM for δ 18 O in quartz, for which suitable materials already exist at the bulk sampling scale. Our Cameca 1280-HR SIMS instrument has been able to provide an analytical repeatability down to ± 0.092 ‰ (1sd, n = 100) on silicates. Hence, the uncertainty for the δ 18o value of the rM, including its heterogeneity component, should be known at significantly better than this level. Only under such conditions can the full capability of this technology be exploited in a metrologically rigorous fashion. A second objective of our work is to develop a strategy for predicting sample heterogeneity at larger sampling masses based on data obtained at the picogram sampling scale. We utilize the modelling of the newly defined ‘heterogeneity factor’ as a function of scale, as previous established with elemental measurements at the macroscopic measurement scale (r amsey et al. 2013). Here we make use of not only the 18 O-/ 16 O- ratios observed by SIMS, but also the grain size distribution of the material being tested. This second component is important in order to understand the risks involved when only a single or small number of grains/fragments of an rM can be used for calibrating an analytical series.