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 may extend to sequences of tephras erupted by an individual volcano.

Comparison of tephra layers from sources including peats or lacustrine deposits 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 C-14, 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.

Geochemical fingerprinting of tephras to match them with source eruptions can be 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, Fe, Mn), with each analysis requiring 6 minutes or less. Thus geochemical fingerprinting of tephra horizons by EPMA represents a very precise and cost effective dating tool.

Analysis of trace elements such as Rb, REE and U is achieved by ion microprobe analysis, where additional chemical data are required to enable discrimination between tephra layers. Ion beam diameters are around 20 microns.

Recent developments in tephra analysis at Edinburgh (Hayward, in review) using our Cameca SX100 instrument - ideally configured for tephra analysis - have enabled analysis of very fine-grained, distal tephra grains down to <10 microns across without chemical modification by the electron beam during analysis.

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).