Tephra Geochemistry

Tephra vary in chemistry from basaltic (low-silica) through to rhyolitic (high-silica) compositions, reflecting the chemical variation typical of magmas.

Because the geological settings of different volcanic centres differ from one another in detail, even within localised regions, magma and hence tephra compositions produced by different volcanoes have distinct ranges of chemical composition. This chemical distinctiveness extends to sequences of tephras erupted by an individual volcano.

Comparison of tephra layers from sources including peat bogs, soils, lacustrine deposits, ice cores and marine sediment cores with well-characterised proximal tephrostratigraphies from possible source volcanoes enables the identification of specific eruptions within the stratigraphic record. For eruptions that have been dated by absolute methods such as 14C, or by historical observation, the ages of individual layers can be used to create a tephrochronological framework within an area or wider region. The chronological resolution within a tephrostratigraphic sequence is limited only by the accuracy and precision of the absolute dating methods used for individual tephra layers. Even in regions of the world that currently lack detailed characterisation of source volcanoes, tephrostratigraphy remains valuable, providing a framework for correlation between different locations.

Geochemical fingerprinting of tephras is achieved very reliably and rapidly by EPMA. Discrimination between the majority of tephra layers is usually achieved easily by analysis of the 10 major and minor elements present in the magma (Na, Mg, Al, Si, P, K, Ca, Ti, Mn, Fe), with each analysis requiring 6 minutes or less. Most individual tephra layers can be identified reliably with 15-20 analyses. Thus geochemical fingerprinting of tephra horizons by EPMA is both precise and cost effective.

Our Cameca SX100 instrument is ideally configured for tephra analysis. Recent developments in tephra analysis and preparation at Edinburgh (Hayward, 2012; Hall and Hayward 2015) have enabled analysis of very fine-grained, distal tephra grains down to 10-15 microns across.

With the collection of relatively large numbers of analyses (around 50) it can be possible to differentiate between chemically similar tephras from a single volcano and hence to produce very high resolution tephrostratigraphic correlations. Where this is impractical (e.g. where limited tephra grains are available or large numbers of layers require analysis), discrimination of geochemically similar sequences may be achieved via measurement of trace elements such as Rb, REE and U by laser ablation ICP-MS, also available at the School of Geoeciences.

Analysis of the volatile elements S, Cl and F by EPMA is routine at Edinburgh and with very good sensitivity. Such measurements permit understanding of the degassing of sulphur and halogens from magma during ascent from source regions to final eruption and thus the understanding of atmospheric and environmental impacts caused by volcanism. Volatiles such as water and carbon dioxide can be measured by SIMS, which is available at the School of Geosciences.

Various on-line databases of tephra composition and eruption dates, compiled from published literature, are available to assist with the identification of source volcanoes and individual eruptions, (e.g. Tephrabase ).

This article was published on 25 May, 2020