A myriad of melt inclusions: a 3D analysis of melt inclusions reveals the gas rich magma reservoir of Colli Albani Volcano (Italy)

Melt inclusions are small parcels of magma trapped in crystals, which hold key information about pre-eruptive magmatic conditions, including volatile content and melt chemistry. We focus here on melt inclusions for a nuanced view of the magmatic pre-eruptive state of Colli Albani, a mafic-alkaline ignimbrite forming system in central Italy. Recent years have seen an increased amount of research surrounding the feasibility of using melt inclusions as tracers of pre-eruptive magma volatile content including, namely the concern about measuring trapped CO2 present in vapour bubbles. Here we present synchrotron 3D tomographic scans of over 2000 melt inclusions from 35 pyroxene and leucite crystals from Colli Albani. We show that incorporating 3D information into melt inclusion geometry calculations allows for the development of a novel classification scheme, which we then use to draw inferences about the pre-eruptive evolution of the plumbing system of Colli Albani. We define six types of melt inclusions based on shape, crystallinity, and the characteristics of their vapour bubble. We also identify a strong variability in melt inclusion type proportions with stratigraphy, which ultimately is a ref lection of variation in reservoir residence times, magma ascent rates, and tephra quench rates. Additionally, a large number of melt inclusions have large-volume bubbles, suggesting the magma reservoir was bubble bearing at the time of melt inclusion trapping. We suggest that this is essential to prepare the eruption of large volumes of the low-viscosity magma at Colli Albani.