INTERLABORATORY COMPARISON OF ELECTRON PROBE MICROANALYSIS OF GLASS GEOCHEMISTRY (original) (raw)
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An inter-laboratory comparison of the electron probe microanalysis of glass geochemistry
Quaternary International, 1996
Tephrochronology commonly relies upon grain-discrete analysis of glass shards to reveal the subtle geochemical differences between tephras from past explosive volcanic eruptions. The use of electron probe microanalysis for this purpose is widely accepted by tephrochronologists. In addition, it is recognized widely that both precision and accuracy must be maximized, and that rigorous standardization procedures must be followed. In this paper, the performance of five electron microprobe centers used in the analysis of glass shards from Leg 152 tephras is compared, using a geochemically homogeneous obsidian secondary standard. The results reveal the compatibility of most of the participants, supporting the comparability of additional glass geochemistry presented within this volume. In recent years the application of distal tephrochronology to stratigraphical problems in the North Atlantic Quaternary record has become increasingly common. Geochemical standardization is vitally important at an early stage in the development of tephrochronological frameworks, and it is hoped that exercises such as that presented here will encourage the production of reliable tephra geochemical data, both on Ocean Drilling Program exercises and wider afield.
2017
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.
2019
The chemical compositions of tephra shards are widely utilised in a myriad of disciplines, including volcanology, petrology, tephrochronology, palaeoecology and climate studies. Previous research has raised concerns over the possible chemical alteration of microscopic (<100 µm) volcanic glass shards through standard extraction procedures, such as the widely used acid digestion method. This study subjects 10 samples of well-characterised volcanic glasses ranging from basalt to rhyolite to three common methods used in the extraction of volcanic material from lake sediments and peats. The major element geochemistry of each sample was analysed and compared with a control group. The results of this test indicate that basaltic and andesitic glasses are highly susceptible to chemical alteration, particularly to the concentrated corrosive materials used in acid and base digestion techniques. PERMANOVA analysis of the variation within groups suggests that the oxides most susceptible to variation are alkalis from groups I and II (K 2 O, Na 2 O, CaO, MgO) and SiO 2 , and the most stable oxides are Al 2 O 3 and FeO. Felsic glasses are considerably less susceptible to alteration by both acidic (HCl, HNO 3 , H 2 SO 4) and alkaline (KOH) digestions. Our findings have important implications for interpreting the geochemistry of volcanic glasses.
Boreas, 2010
: Testing the reliability of the JEOL FEGSEM 6500F electron microprobe for quantitative major element analysis of glass shards from rhyolitic tephra. Electronprobe microanalysis is now widely adopted in tephra studies as a technique for determining the major element geochemistry of individual glass shards. Accurate geochemical characterization is crucial for enabling robust tephra-based correlations; such information may also be used to link the tephra to a specific source and often to a particular eruption. In this article, we present major element analyses for rhyolitic natural glass standards analysed on three different microprobes and the new JEOL FEGSEM 6500F microprobe at Queen's University Belfast. Despite the scatter in some elements, good comparability is demonstrated among data yielded from this new system, the previous Belfast JEOL-733 Superprobe, the JEOL-8200 Superprobe (Copenhagen) and the existing long-established microprobe facility in Edinburgh. Importantly, our results show that major elements analysed using different microprobes and variable operating conditions allow two high-silica glasses to be discriminated accurately.
Geochronology, 2021
Although analyses of tephra-derived glass shards have been undertaken in New Zealand for nearly four decades (pioneered by Paul Froggatt), our study is the first to systematically develop a formal, comprehensive, openaccess reference dataset of glass-shard compositions for New Zealand tephras. These data will provide an important reference tool for future studies to identify and correlate tephra deposits and for associated petrological and magma-related studies within New Zealand and beyond. Here we present the foundation dataset for TephraNZ, an open-access reference dataset for selected tephra deposits in New Zealand. Prominent, rhyolitic, tephra deposits from the Quaternary were identified, with sample collection targeting original type sites or reference locations where the tephra's identification is unequivocally known based on independent dating and/or mineralogical techniques. Glass shards were extracted from the tephra deposits, and major-and trace-element geochemical compositions were determined. We discuss in detail the data reduction process used to obtain the results and propose that future studies follow a similar protocol in order to gain comparable data. The dataset contains analyses of glass shards from 23 proximal and 27 distal tephra samples characterising 45 eruptive episodes ranging from Kaharoa (636 ± 12 cal yr BP) to the Hikuroa Pumice member (2.0 ± 0.6 Ma) from six or more caldera sources, most from the central Taupō Volcanic Zone. We report 1385 major-element analyses obtained by electron microprobe (EMPA), and 590 trace-element analyses obtained by laser ablation (LA)-ICP-MS, on individual glass shards. Using principal component analysis (PCA), Euclidean similarity coefficients, and geochemical investigation, we show that chemical compositions of glass shards from individual eruptions are commonly distinguished by major elements, especially CaO, TiO 2 , K 2 O, and FeO tt (Na 2 O+K 2 O and SiO 2 /K 2 O), but not always. For those tephras with similar glass major-element signatures, some can be distinguished using trace elements (e.g.
Earth System Science Data, 2020
Tephra layers produced by volcanic eruptions are widely used for correlation and dating of various deposits and landforms, for synchronization of disparate paleoenvironmental archives, and for reconstruction of magma origin. Here we present our original database TephraKam, which includes chemical compositions of volcanic glass in tephra and welded tuffs from the Kamchatka volcanic arc. The database contains 7049 single-shard major element analyses obtained by electron microprobe and 738 trace element analyses obtained by laser ablation inductively coupled plasma mass spectrometry on 487 samples collected in close proximity to their volcanic sources in all volcanic zones in Kamchatka. The samples characterize about 300 explosive eruptions, which occurred in Kamchatka from the Miocene up to recent times. Precise or estimated ages for all samples are based on published 39 Ar/ 40 Ar dates of rocks and 14 C dates of host sediments, statistical age modeling and geologic relationships with dated units. All data in TephraKam are supported by information about source volcanoes and analytical details. Using the data, we present an overview of geochemical variations in Kamchatka volcanic glasses and discuss applications of these data for precise identification of tephra layers, their source volcanoes, and temporal and spatial geochemical variations in pyroclastic rocks in Kamchatka. The data files described in this paper are available on ResearchGate at
2019
Tephra layers produced by volcanic eruptions are widely used for correlation and dating of various deposits and landforms, for synchronization of disparate paleoenvironmental archives, and for reconstruction of magma origin. Here we present our original database TephraKam, which includes chemical compositions of volcanic glass in tephra and welded tuffs from the Kamchatka volcanic arc. The database contains 7049 major element analyses obtained by electron microprobe and 738 trace element analyses obtained by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) on 487 samples collected in proximity of their volcanic sources in all volcanic zones in Kamchatka. The samples characterize about 300 explosive eruptions, which occurred in Kamchatka from the Pliocene until historic times. Precise or estimated ages for all samples are based on published 39 Ar/ 40 Ar dates of rocks and 14 C dates of host sediments, statistical age modelling and geologic relationships with dated units. All data in TephraKam is supported by information about source volcanoes and analytical details. Using the data, we present an overview of geochemical variations of Kamchatka volcanic glasses and discuss application of this data for precise identification of tephra layers, their source volcanoes, temporal and spatial geochemical variations of pyroclastic rocks in Kamchatka. The data files described in this paper are available on
The application of LA-ICP-MS to tephrochronology: advantages and methodology
2009
Micron-scale analysis of vesicular volcanic glass can be problematic because thin vesicle walls and junctions limit the area available for analysis, subsurface vesicles limit the vertical thickness available, microcrysts at or below the surface may contaminate glass analyses and some glasses show compositional banding. In addition, distal tephra are very small (10-100 μm) and material may be sparse. We have analysed the MPI-DING reference glasses and natural tephra samples (pumice, scoria and fiamme) from the Thorsmörk ignimbrite (Southern Iceland) using laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS). Three different reduction strategies are used: averaging, uncertainty weighting and log-linear regression. We then assess the data quality achieved using the various strategies. Using our technique we show that the main limiting factor on data quality is precision, particularly for natural tephra analyses. At N 20,000 cps, relative standard deviations (%RSDs) in the Thorsmörk tephra are 5-10%