High spatial resolution electron probe microanalysis of tephras and melt inclusions without beam-induced chemical modification (original) (raw)
Related papers
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.
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.
INTERLABORATORY COMPARISON OF ELECTRON PROBE MICROANALYSIS OF GLASS GEOCHEMISTRY
Tephrochronology commonly relies upon grain-discrete analysis of glass shards to reveal the subtle geochemical differ- ences 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 maxi- mized, and that rigorous standardization procedures must be followed. In this paper, the performance of five electron micro- probe 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.
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.
Chemical Geology, 2010
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 > 20,000 cps, relative standard deviations (%RSDs) in the Thorsmörk tephra are 5–10% — approximately twice those achieved in the MPI-DING glasses (3–5%) at the same conditions. Rhyolitic pumice and fiamme from the Thorsmörk ignimbrite are compositionally homogenous. The proximal deposit also contains subordinate basalt scoria, therefore the deposit is bimodal. The Thorsmörk rhyolite correlates with the North Atlantic Ash Zone 2 (NAAZ2) tephra described in a marine sediment core (Lacasse and Garbe-Schönberg, 2001, JVGR 170, 113–147).►Data reduction by averaging is the most appropriate for natural tephra analysis. ►Signal intensity is the limiting factor for tephra analysis, dictating the minimum spot size. ►The Thorsmörk ignimbrite comprises homogenous rhyolitic pumice and subordinate basaltic scoria. ►The rhyolitic Thorsmörk shows a good statistical correlation with North Atlantic Ash Zone 2.
Improving the reliability of microprobe-based analyses of andesitic glasses for tephra correlation
The Holocene, 2007
Andesite tephras have not been used in tephrostratigraphic studies to the same extent as rhyolitic units because they are seen as being chemically more complex than the latter. They are particularly difficult to 'fingerprint' owing to apparent heterogeneity in glass analyses from single andesitic layers. Using case study tephras from two andesite volcanoes in New Zealand, Mts Taranaki and Ruapehu, glass chemical heterogeneity is shown to be due predominantly to (1) differing particle types generated during closedand open-vent phases of single eruption sequences, resulting in a broad range of glass compositions, and (2) contaminated glass microprobe analyses by various proportions of microlite phase(s). A simple evaluation procedure using least-squares mixing calculations is presented to classify glass data sets for hybrid analyses and to estimate the proportions of the main contaminant microlite phase. By employing particle morphology studies as well as the glass-analysis evaluation procedure, variations in andesitic glass compositions can be significantly reduced. Hence this approach shows promise for enabling the use of some andesitic tephras for tephrostratigraphic correlation. This may facilitate the addition of a new degree of resolution in tephrostratigraphic records.