Multiphase petrography of volcanic rocks using element maps: a method applied to Mount St. Helens, 1980–2005

Abstract

Quantitative textural analyses including crystal size distributions (CSDs) provide insights into crystallisation kinetics of magmatic systems. Investigations of volcanic crystal textures often rely on greyscale variations on backscattered electron images to identify crystal phases, which must then be thresholded and/or traced manually, a laborious task, and investigations are typically restricted to a single crystal phase. A method is presented that uses energy-dispersive X-ray element maps to generate textural data. Each pixel is identified as a crystal phase, glass or vesicle according to relative chemical composition enabling concurrent acquisition of multiphase CSD, crystallinity and mineral mode data. Data processing is less time intensive for the operator but considerable instrument time is required to generate element maps. The method is applied to 17 dacite samples from the 1980–1986 and 3 from the 2004–2005 eruptive periods of Mount St. Helens volcano (USA) to provide quantitative insights into multiphase textural evolution. All of the CSDs are curved and concave-up in the standard CSD plot with curvature increasing with plagioclase content. To facilitate comparisons with previous studies, CSDs for microlites (<50 μm length crystals) are approximated as straight lines. The line intercepts and slopes provide information on n 0 (nucleation density) and characteristic length or Gτ (the product of growth rate (G) and residence time (τ)), respectively. These parameters, as well as the total groundmass crystallinity, show distinct differences between explosive deposits from summer 1980 and post-summer 1980 domes. Post-summer 1980 microlite n 0 values are mostly at the lower end of the range of those measured for summer 1980 samples. Total groundmass crystallinities during summer 1980 are between 10 and 30 vol.%, whereas post-summer 1980 crystallinity increases to between 39 and 51 vol.%. The range of n 0 values is similar to those previously published for Mount St. Helens but Gτ is consistently higher. Gτ of a May 1985 sample analysed in this study is approximately 2 μm higher compared with previously published data for the same sample when processed using similar methodologies. Groundmass crystallinity data show similar trends to those previously published for the 1980 to 1986 eruption, increasing sharply after summer 1980 then increasing more gradually during the dome-building phase of the eruption. The effects of varying L, the apparent crystal size, and crystal aspect ratio on resultant CSDs are also investigated. Whilst relative temporal variations in CSDs for a given dataset can be investigated, absolute values from different studies cannot be compared unless methods of data acquisition and processing are exactly the same.