APPLICATION OF PORTABLE X-RAY FLUORESCENCE SPECTROMETRY TO PROBLEMS IN VOLCANOLOGY

Portable X-ray fluoresence (PXRF) technology has now reached the point where, for some major and trace elements crucial to investigating the generation, transport and eruption of magma, data quality rivals that obtained from full-size wavelength-dispersive XRF instruments. The advantages of PXRF lie not only in the capability to analyze rocks in the field in real time, but also in screening sample sets for more detailed analysis, and in the ability to rapidly analyze large numbers of samples (in either field or lab) with minimal preparation at little cost. We foresee a future in which studies of individual volcanic rock bodies are populated by thousands of analyses, rather than tens to a few hundreds, opening the way to rigorous statistical treatment of large data sets. We discuss two particular applications:

1. The relationship of the compositional architecture of a chemically zoned ash-flow sheet (the Bandelier Tuff, NM) to its source magma chamber. Extraction patterns of magma from large silicic systems during super-eruptions were numerically modeled by several investigators in the 1980s and early 1990s, but detailed application of these models to real-world examples of zoned tuffs has been hampered by inadequate sampling density. This disconnect can be remedied by intensive sampling and rapid PXRF analysis.

2. Chemical correlation of flood basalts and characterization of their internal variability. Geochemistry has been an essential component of mapping the the Columbia River Basalts because of the similar appearance that many units present in the field. In this case, the advantages of real-time or near real-time field analyses are obvious. Nonetheless, an under-appreciated feature of the Columbia River Basalts is that some flows are chemically heterogeneous. Much exploratory work remains to be done in this area. Portable XRF is ideally suited to rapidly screen samples in the field (or in core) to identify internally variable flows for further investigation by WDXRF and other analytical techniques.