Standardized analysis of juvenile pyroclasts in comparative studies of primary magma fragmentation: 2. Choice of size fraction and method optimization for particle cross-sections

Abstract

The morphological and textural features of juvenile pyroclasts record crucial details on magma conditions at the time of fragmentation. Their study is therefore essential to better understand the dynamics of explosive eruptions. Unfortunately, the absence of a standardized protocol of investigation hinders data reproducibility and comparison among different laboratories. Here we focus on morphometric parameters, 2D crystallinity and 2D vesicularity resulting from cross-section analysis of juvenile particles using backscattered electron imaging, and address the following questions: (i) how to prepare polished epoxy grain mounts, (ii) which pixel density to be used, (iii) how to facilitate image preparation and image analysis, (iv) which sample size is necessary to obtain statistically robust results, and (v) what is the optimum size fraction for analysis. We test juvenile particles in grain size bins ranging from 2–1 mm (− 1 to 0ɸ) to 88–63 µm (+ 3.5 to + 4ɸ), using samples from the 1977 Ukinrek eruption. We find that the required resolution ranges from 75 000 to 10 000 pixels per particle, depending on the size fraction, higher than previously postulated. In the same size ranges, less than 50 grains per size fraction and sample are needed to get robust averages. Based on theoretical, empirical, and practical considerations, we propose 0.71–0.5 mm (+ 0.5 to + 1ɸ) as the optimum size fraction to be analyzed as particle cross-sections in standardized comparative studies of magma fragmentation. We provide a detailed guide for preparing polished epoxy grain mounts and introduce a software package (PASTA) for semi-automated image preparation, image processing, and measurement of morphological and textural parameters.