The INternational focus group on Tephrochronology And Volcanism (INTAV) of the International Union for Quaternary Research (INQUA) has conducted an intercomparison of tephrochronology laboratories with electron-beam microanalytical data on volcanic glasses submitted from 27 instruments at 24 institutions in 9 nations. This assessment includes most active tephrochronology laboratories and represents the largest intercomparison exercise yet conducted by the tephrochronology community. The intercomparison was motivated by the desire to assess the quality of data currently being produced and to stimulate improvements in analytical protocols and data reporting that will increase the efficacy of tephra fingerprinting and correlation.Participating laboratories were each supplied with a mount containing three samples for analysis: (1) rhyolitic Lipari obsidian ID3506, (2) phonolitic Sheep Track tephra from Mt. Edziza, British Columbia, Canada, and (3) basaltic Laki 1783 A.D. tephra. A fourth sample, rhyolitic Old Crow tephra, was also distributed.Most laboratories submitted extensive details of their analytical procedures in addition to their analytical results.Most used some combination of defocused or rastered beam and modest beam current to reduce alkali element migration. Approximately two-thirds reported that they routinely analyze one or more secondary standards to evaluate data quality and instrument performance. Despite substantial variety in procedures and calibration standards, most mean concentrations compare favorably between laboratories and with other data. Typically, four or fewer data contributions had means for a given element on a given sample that differed by more than +/-2 standard deviations from the overall means. Obtaining accurate Na 2 O concentrations for the phonolitic tephra proved to be a challenge for many laboratories. Only one-half of the data sets had means within +/-1 standard deviation of the ~8.2 wt% Na 2 O value obtained by other methods. One mean is higher and 14 are lower. Three of the data set means fall below 7 wt% Na 2 O. Most submissions had relative precision better than 1-5% for the major elements. For low-abundance elements, the precision varied substantially with relative standard deviations as small as 10% and as large as 110%. Because of the strong response to this project, the tephrochronology community now has a large comparative data set derived from common reference materials that will facilitate improvements in accuracy and precision and which can enable improved use of published data produced by the participating laboratories. Finally, recommendations are provided for improving accuracy, precision, and reporting of electron-beam microanalytical data from glasses.

We define tephras and cryptotephras and their components (mainly ash-sized particles of glass ± crystals in distal deposits) and summarize the basis of tephrochronology as a chronostratigraphic correlational and dating tool for palaeoenvironmental, geological, and archaeological research. We then document and appraise recent advances in analytical methods used to determine the major, minor, and trace elements of individual glass shards from tephra or cryptotephra deposits to aid their correlation and application. Protocols developed recently for the electron probe microanalysis of major elements in individual glass shards help to improve data quality and standardize reporting procedures. A narrow electron beam (diameter ~35 μm) can now be used to analyze smaller glass shards than previously attainable. Reliable analyses of 'microshards' (defined here as glass shards <32 µm in diameter) using narrow beams are useful for fine-grained samples from distal or ultra-distal geographic locations, and for vesicular or microlite-rich glass shards or small melt inclusions. Caveats apply, however, in the microprobe analysis of very small microshards (~5 µm in diameter), where particle geometry becomes important, and of microlite-rich glass shards where the potential problem of secondary fluorescence across phase boundaries needs to be recognised. Trace element analyses of individual glass shards using laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), with crater diameters of 20 μm and 10 μm, are now effectively routine, giving detection limits well Highlights  Advances in the microanalysis of major, minor, and trace elements of glass shards are reviewed  We evaluate numerical and statistical methods for tephra correlation via glass/crystal analyses  We focus on (1) differences in mean composition of samples or their range; and  (2) sample variance and degree of compositional similarity to establish equivalence or not  We illustrate various statistical methods and data transformations using case studies Wherever possible, such analytical data are very markedly supported and more readily interpreted by the attainment of numerical ages on tephras (Turner et al., 2011b; Green et al., 2014; Damaschke et al., 2017a). Dating techniques applied to tephras include: (i) radiometric, for example radiocarbon (14 C), fission track, luminescence, 40 Ar/ 39 Ar, U-Th-disequilibrium/U-Pb and (U-Th)/He zircon dating (e.g. Biswas et al.,

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Keuhn, S.C., Froese, D.G., Carrara, P.E., Foit Jr, F.F., Pearce, N.J.G., Rotheisler, P. (2009). Major- and trace-element characterization, expanded distribution, and a new chronology for the latest Pleistocene Glacier Peak tephras in western North America. Quaternary Research, 71 (2), 201-216 Sponsorship: NSERC Discovery grant; Alberta Ingenuity New Faculty Award to Duane FroeseThe Glacier Peak tephra beds are among the most widespread and arguably some of the most important late Pleistocene chronostratigraphic markers in western North America. These beds represent a series of closely-spaced Plinian and sub-Plinian eruptions from Glacier Peak, Washington. The two most widespread beds, Glacier Peak ?G? and ?B?, are reliably distinguished by their glass major and trace element abundances. These beds are also more broadly distributed than previously considered, covering at least 550,000 and 260,000 km2, respectively. A third bed, the Irvine bed, known only from southern Alberta, is similar in its major-element composition to the Glacier Peak G bed, but it shows considerable differences in trace element concentrations. The Irvine bed is likely considerably older than the G and B tephras and probably records an additional Plinian eruption, perhaps also from Glacier Peak but from a different magma than G through B. A review of the published radiocarbon ages, new ages in this study, and consideration in a Bayesian framework suggest that the widespread G and B beds are several hundred years older than widely assumed. Our revised age is about 11,600 14C yr BP or a calibrated age (at 2 sigma) of 13,710?13,410 cal yr BP.Peer reviewe

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