Geochemical analysis of extremely fine-grained cryptotephra : new developments and recommended practices

• Published in: Quaternary Geochronology
• Main Authors: Innes, Helen M., Hutchison, William, Burke, Andrea
• Other Authors: Medical Research Council, The Leverhulme Trust, EPSRC, University of St Andrews.School of Earth & Environmental Sciences, University of St Andrews.St Andrews Isotope Geochemistry
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2024
• Subjects: QE Geology; DAS; MCC; QE; ice core
• Online Access: https://hdl.handle.net/10023/30297; https://doi.org/10.1016/j.quageo.2024.101553

Funding: This work was funded by a NERC IAPETUS2 DTP studentship [NE/S007431/1] awarded to H. M. Innes and UKRI Future Leaders Fellowship [MR/S033505/1] awarded to W. Hutchison. A. Burke is supported by a Leverhulme Trust Prize [PLP-2021-167]. The St Andrews EPMA was supported by the EPSRC through grants EP/T019298/1, EP/L017008/1 and EP/R023751/1. Tephrochronology is a powerful tool used to synchronise and date stratigraphic records by accurate and precise geochemical analysis of deposited volcanic glass shards. However, in many distal stratigraphic records (e.g., polar ice cores) tephra shards are often extremely fine-grained (<10 μm). Geochemical characterisation of these shards is challenging because conventional preparation and analytical techniques require highly polished glass areas >5 μm for electron probe microanalysis (EPMA) to ensure high analytical totals and minimise alkali element loss. Recent method developments have put forward alternative approaches to accurately measure major oxides of small shards: a smaller 3 μm diameter beam, overlapping large (20 μm) beam areas onto supporting epoxy resin, and using scanning electron microscopy with energy dispersive spectrometry (SEM-EDS). However, there has been no direct intercomparison of these alternative techniques, which to date have only been tested on a limited range of glass compositions and tephras that are much larger than the extremely fine-grained material found in distal archives. These issues complicate decision making about the best analytical approach to take when faced with small shards. Here, we provide a new workflow protocol for the analysis of <10 μm tephra by determining the accuracy and precision of alternative SEM-EPMA methods. By analysing a variety of glass standards including those prepared to replicate fine-grained ice-core cryptotephras, we show that a 3 μm EPMA beam is suitable for use on all glass compositions provided the beam current is reduced to 1 nA. When glass areas are too small for a 3 μm beam we show that ...