Plagioclase crystal size distribution parameterization: A tool for tracking magma dynamics at Stromboli

Abstract

In this study we parameterize the textural attributes of plagioclase phenocrysts and microlites from nineteen pyroclasts ejected during mild to violent explosions at Stromboli over a timespan of ∼18 years, from 2003 to 2021. By allying kinetic and crystal size distribution principles, we document that the morphological stability of large-sized, euhedral phenocrysts is superimposed on an internal textural heterogeneity due to growth-dissolution phenomena associated with the input rate of hot, H2O-rich recharge magmas rising from depth. As a result, the volumetric plagioclase proportion, dominant size, and number of phenocrysts per unit volume decrease from mild to violent explosions responding to a more efficient magma mixing process via sustained injections of mafic magmas into the shallow reservoir. On the other hand, the crystallization of anhedral plagioclase microlites is controlled by fast growth kinetics taking place in the uppermost part of the conduit during magma acceleration towards the surface. Under such highly dynamic crystallization conditions, the microlite number density closely depends on the increase of melt liquidus temperature via magma decompression and H2O exsolution. This mutualism allows to model the degassing rate and ascent velocity of magma under open-conduit flow regimes for the different eruptive styles, thereby supporting the idea that violent explosions at Stromboli are driven by sustained influxes of recharge magmas favoring strong acceleration (∼12–27 m/s), decompression (∼0.25–0.49 MPa/s), and H2O exsolution (∼0.005–0.01 wt%/s) before magma discharge at the vent.