Vertically Extensive Magma Reservoir Revealed From Joint Inversion and Quantitative Interpretation of Seismic and Gravity Data

Recent advances in our understanding of arc magmatic systems indicate that melt is stored for long periods in low‐melt fraction crystal mushes and that eruptible magma reservoirs are short‐lived and are assembled rapidly by buoyancy‐induced instabilities and draining of the crystal mush. Many aspects of their architecture remain unclear, particularly in relation to their geometry and shallow melt distribution. We investigate the storage of melt below the active Soufrière Hills Volcano (SHV), Montserrat, using joint geophysical inversion combined with a quantitative interpretation approach based on rock physics. We jointly inverted active‐source P‐wave traveltimes and gravity anomalies to derive coincident 3‐D models of P‐wave velocity and density to a depth of 8 km. Comparative analysis of the active SHV and extinct Centre Hills volcano and effective elastic medium computations allow us to constrain temperature, melt fraction, and melt geometry. A continuous column of partial melt is inferred beneath SHV, at 4–8 km depth. Melt fraction is ~6% (ranging from 3 to 13% depending on melt geometry) and is maximum at 5–6 km depth. When under‐recovery of the low‐vP volume is taken into account, the melt fraction is revised to ~17% (ranging from 11 to 28%). Analysis of vP/density cross plots indicates that the melt distribution is best represented by low‐aspect ratio geometries. These likely span a multiscale spectrum ranging from grain‐scale inclusions and fractures to 100‐m‐scale dykes and sills. Our results confirm the concept of vertically extensive crystal mush including one or multiple more melt‐rich layers.

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