Interpreting Pyroclast Textures: Re-Evaluating Crystal Size Distributions in the Context of New Views of Magmatic Systems

Crystal size distributions (CSDs) are widely employed in studies of volcanic clasts to decipher the nature and timescales of eruptive processes. Since their initial application to geologic systems nearly three decades ago, our view of magmatic systems has evolved in the face of mounting evidence that many volcanic crystals begin their lives in a melt distinct from the one in which they erupt. In contrast to systems in which crystals grow in situ from their host melt (e.g., lava lakes or layered intrusions), pyroclasts are often the product of complex crustal magmatic systems in which rims grow on antecrystic cores and distributed magma bodies may be rapidly reassembled shortly before eruption. This raises questions about the suitability of an interpretative tool that assumes a unimodal distribution of crystal sizes subject to constant kinetic conditions. The form of CSDs in natural volcanic rocks - commonly curved or kinked - bears out the challenges involved. Petrologic experiments simulating crystal growth present one means of untangling the complexities of CSD interpretation in the context of changing views of magmatic systems. A large cache of experimental data are now available to describe crystal growth under controlled conditions (both decompression and cooling) for a range of magmatic compositions. Here we present compiled experimental data that invite opportunities to investigate alternatives to the conventional CSD plot, such as size histograms, textural regime diagrams, and separate quantitative analysis of crystal rims and cores. Our compilations also shed light on the uncertainties associated with quantitative interpretation of CSDs for polymodal crystal populations, or those formed under dynamic conditions (i.e., non-constant growth rate), which likely dominate in shallow volcanic plumbing systems. Our aim is to encourage a more critical, and ultimately more informative, approach to the interpretation of crystal sizes in pyroclasts.