Standardized analysis of juvenile pyroclasts in comparative studies of primary magma fragmentation; 1. Overview and workflow

Abstract

Juvenile pyroclasts, especially in the ash size range, provide important information on primary fragmentation processes, i.e., initial explosive magma fragmentation, and on the state of the magma both prior to and at the point of fragmentation and quenching. There exists an extensive body of literature focusing on the quantification of juvenile particle morphology (shape), internal textures, and surface features spanning several decades; however, a standardized method has yet to emerge for comparative studies. No community-wide consensus currently exists (i) regarding the most representative size fraction(s) to be examined, (ii) on sample preparation procedures (such as whether to use whole-particle silhouettes or 2D cross-sections), (iii) on imaging techniques and image acquisition parameters, or (iv) on the optimal morphometric parameters to measure. Lack of a standardized method precludes robust comparison between different studies and laboratories. We propose here a preliminary “best practices” and workflow for characterization of juvenile pyroclasts, for comparative studies of primary fragmentation. If the community follows such a standardized method, it will become possible to accumulate a large volume of consistent data on juvenile pyroclasts from a range of eruption styles, fragmentation mechanisms, and magma compositions. This will ultimately allow deeper insights into the full panoply of magma-to-pyroclast processes that drive particleproducing volcanic eruptions. One or more “fragmentation diagrams” may eventually be developed to allow different types of magmatic and phreatomagmatic explosive eruptions to be distinguished based on their products.