Tephrological implications of beam size-sample-size effects in electron microprobe analysis of glass shards

• Published in: Journal of quaternary science Vol. 16; no. 2; pp. 105 - 117
• Main Authors: Hunt, John B., Hill, Peter G.
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.02.2001
• Subjects: beam size; electron probe microanalysis; error; glass; sample size; tephra; tephrochronology
• ISSN: 0267-8179; 1099-1417
• DOI: 10.1002/jqs.571

This paper concerns the potential consequences of varying procedures for the determination of tephra geochemistry by electron microprobe. Application of electron probe microanalysis to tephrostratigraphical methods has increasingly facilitated the resolution and refinement of Quaternary chronology associated with records of proxy‐environmental or proxy‐climatic change. The geographical range over which tephras are recovered has expanded significantly with the identification and analysis of crypto (or hidden) tephras in areas far removed from tephra sources. These tephras are dominated by glass shards, which, in many distal environments, may be either small in size (μm) or may be highly pumiceous with low glass:void ratios and thin (<10 μm) shard walls. We demonstrate that reducing the size of the electron beam used to analyse shard geochemistry cannot be used reliably to permit analysis of thin glass walls. This approach distorts the geochemical data, creating analytical differences that may generate inappropriate tephrogeochemical fingerprints. Additional distortion of the geochemical fingerprint in the form of hybrid analyses may be encountered in glass fragments containing micron‐sized crystalline phases such as feldspar. Copyright © 2001 John Wiley & Sons, Ltd.