Short eruption window revealed by absolute crystal growth rates in a granitic magma

The potential for cataclysmic volcanic eruptions depends on the volume of magma stored in shallow crustal reservoirs and the amount of time over which magma can accumulate without cooling and crystallizing to form a pluton of solid rock. Magma reservoir volume and longevity are, in turn, controlled by the flux of new magma into the system and the crystal content of the reservoir. To understand why some magmas erupt, whereas others solidify in the crust, the timescales for crystal growth and upper-crust magma residence must be determined from both erupted volcanic rocks and intruded plutonic rocks. However, our understanding of these timescales is largely restricted to volcanic rocks only and measurements from plutonic rocks are missing. Here we use U-Pb geochronological dating of zircon crystals sampled from a seven-million-year-old upper crustal pluton in Elba, Italy. The zircon crystals were found as inclusions within the cores and rims of 6-8-cm-long potassium-feldspar megacrysts and constrain the rate of megacryst growth to 0.2-1.1 1/4m yr -1. We combine the measured growth rates with petrological observations and phase-equilibrium modelling to show that the transition from eruptible magmas to immobile granitic mush and pluton formation occurred in just 10-40 thousand years. This short time window for a potential eruption implies that some magmas reside in upper crustal reservoirs for only a brief period before eruption.