Plagioclase Glomerocrysts: taking the pulse of volcanic eruptions

Volcanic eruptions result from complex interplay of magmatic processes including the ascent and storage of magma in the crust, volatile exsolution and crystallization. The dynamics of crystallization in shallow subvolcanic systems and the consequences of interaction between ascending magmas and gases remain largely unconstrained. Mineral clusters, or glomerocrysts, provide a novel tool to infer magmatic processes occurring in vertically integrated subvolcanic systems. Here, we target cm-wide glomerocrysts formed of radially oriented plagioclase from Tolbachik volcano, Kamchatka, Russia. We use glass inclusion data, mineral textures and diffusion chronometry on olivine and plagioclase and show that cm-size glomerocrysts can grow in days to weeks in a dynamic and melt-rich system prior to or during volcanic eruption. Rhythmic CO2-fluxing by degassing of deep-seated magmatic reservoirs controls the crystallization and resorption of plagioclase and might well lead to significant overpressure in shallow subvolcanic systems, thereby controlling the rhythmicity of volcanic eruptions. Radially oriented plagioclase glomerocrysts represent the volcanic counterpart of radial magmatic layering found in shallow magmatic plutons, namely plagioclase-dominated comb layers and orbicular rocks (McCarthy and Müntener, 2016). Long timescales and slow cooling are mechanisms generally required to form well-developed magmatic layering according to fundamental and long-standing tenets of igneous layered intrusions (Wager and Brown, 1963, McBirney and Noyes, 1979). We argue however that vertical, radial magmatic layering dominated by plagioclase crescumulates found in shallow subvolcanic systems and sampled as mineral clusters in erupted products should be reassessed as monitors of fluctuating P-T-H2O-CO2 conditions on eruptive timescales.