Tephra geochemistry: a discussion of some persistent analytical problems

Abstract

Distal tephrochronological studies are reliant upon precise and accurate geochemical quantitation. This quantitation can involve difficulties not often encountered in more conventional petrology. Solving these problems is of particular importance in the Holocene (and Late Quaternary) of northwest Europe, where numerous tephra layers of Icelandic origin have been detected in peat bogs and lake and marine sediments. Electron probe microanalysis (EPMA) has proved the most applicable analytical technique when insufficient quantities of tephra are available for bulk analysis. Standard EPMA techniques require fine tuning to the tephra problem as the geochemical instability of glass is a common feature. In many previous studies, geochemical data of avoidably low quality have been produced, and little attention appears to have been directed to modification of EPMA methods. It is suggested that the 'black box' approach to geochemical analysis should be avoided by the consideration of several factors. These include minimization of sodium mobility during analysis, prevention and nonacceptance of low analytical totals, non-normalization of data, and making provision for the acceptable presentation and statistical treatment of data. Amongst tephrochronologists, these suggestions may not receive universal acceptance, but it is hoped that they will serve to promote further discussion on a much neglected topic. Until reliable analytical techniques are universally adopted, there is a real possibility that erroneous tephrochronologies will be established.