Eva Hartung, Luca Caricchi , Gregor Weber

The segregation and accumulation of felsic melts, from crystallising crustal magma reservoirs, is essential for the chemical evolution of the crust and is a phenomenon preceding some of the largest eruptions on Earth. The physical properties of residual melt and magma and the time over which the conditions remain appropriate for melt extraction are important factors controlling the efficiency of melt extraction and the distribution of melt in magma reservoirs. Here we focus on the initial water content (H2Oi) of magma as it affects both the physical properties of the residual melt and the timescales at which conditions remain appropriate for melt extraction during progressive magma crystallisation. We use rhyolite-MELTS simulations to evaluate the physical evolution of crystallising granodioritic (or dacitic) hydrous magma (i.e. ≥1 wt.% H2Oi) at shallow depth at 200 MPa. To constrain the solidification timescales of reservoirs containing magmas with initially different water content, we perform 2.5D thermal modelling. We combine these results with calculations of melt extraction velocity by compaction and hindered settling to identify the optimal conditions at which melt segregation occurs. These calculations suggest that hydrous felsic magmas that attain water saturation after 40 wt.% crystallisation (rheological locking point) are best suited for melt extraction. Once water-saturation is achieved, the rate of release of latent heat of crystallisation and with it the time magma spends within a given temperature interval increases while the viscosity of the residual liquid and crystal-liquid density contrast remain favourable for melt segregation. We first test our findings on the Takidani pluton (Japan) because it shows evidence of residual melt segregation from crystallising magma, and is associated with caldera-forming eruptions. We finally generalise our results to crustal magma reservoirs containing hydrous felsic magmas. Our results suggest that if segregation starts at rheological locking (i.e. crystallinity of 40¬–50 wt.%) upper crustal reservoirs of ≥100 km3 granodioritic (i.e. dacitic) magma with >2 wt.% H2Oi can produce large melt-rich caps at the top of partially crystallised magma reservoirs in few hundreds to few thousands of years. The formation of separate melt lenses becomes more likely when segregation of melt starts at crystallinities >0.6. Our results suggest that H2Oi plays an important role in modulating the distribution of eruptible melt in upper crustal reservoirs. Reservoirs of felsic and water-poor magma (<2 wt.% H2Oi) tend to be associated with the formation of isolated pockets of crystal-poor and eruptible magma, which could account for the often-observed geochemical heterogeneity of the products of large caldera-forming eruptions in the Snake River Plane. The limited dimensions of these eruptible magma pockets make their detection by geophysical methods challenging.