SEM ELEMENT MAPPING: STRENGTHS, LIMITATIONS AND APPLICATIONS TO PETROGRAPHY OF VOLCANIC ROCKS

摘要

Pre-eruptive magma processes can be investigated through groundmass crystal textures ofvolcanic rocks. Crystal size distributions (CSD) are a means of quantifying textural data andcan provide insights into crystallisation kinetics. Traditionally, backscattered electron imagesfrom scanning electron microscopes (SEM) are used to image groundmass crystals which arethen segmented manually by tracing crystal outlines. This is labour intensive and commonlylimits investigations to a single crystal phase. With the onset of silicon drift detectors inenergy-dispersive X-ray spectrometers it has become much quicker to acquire X-ray elementmaps using the SEM. Here we show how such maps can be used to generate textural data byclassifying each pixel as a crystal, glass or vesicle based on chemical composition. Thismethod enables all phases to be segmented and enables concurrent acquisition of multiphaseCSD, crystallinity and mineral mode data. We apply the method to 17 samples erupted fromMount St Helens, USA during the 1980 to 1986 eruptive period. Variations in CSD intercepts(n0, nucleation density) and slopes (characteristic length or Gτ, the product of growth rate (G)and residence time (τ)), crystallinity and mineral modes are used to show differences inpre-eruptive magma processes between volcanic rocks (pumices, cryptodomes and domes)from summer 1980 and post-summer 1980.