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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.% H 2 O i ) 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.% H 2 O i 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 H 2 O i plays an important role in modulating the distribution of eruptible melt in upper crustal reservoirs. Reservoirs of felsic and water-poor magma ( H 2 O i ) 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.

Ciomadul is the youngest volcano in eastern-central Europe. Although its last eruption occurred at ca. 30 ka, there are independent indications for a high-crystallinity magma reservoir persisting beneath the volcano until present. In order to further test the hypothesis of long-lived melt presence and to better constrain the nature and timescales associated with the subvolcanic magma storage system, over 500 zircon U-Th and U-Pb spot ages (crystal interiors and outer surfaces) were interpreted from dacitic rocks of the most productive eruptive period (the Young Ciomadul Eruptive Period; YCEP, 160-30 ka). Zircon surface ages from lava dome and pumice samples range from ca. 600 ka up to the youngest eruption event at 30 ka. They form a continuous age distribution and some single crystals reveal significant age zonation (>150 kyr difference from core to rim). The oldest zircon ages of YCEP overlap with the last eruption events of the Old Ciomadul Eruptive Period (1000–330 ka). The zircon age spectra, combined with textural data, point to a prolonged (several 100's kyr) residence in a highly crystalline mush state. The range in zircon crystallization temperature (from ∼750 °C to the solidus at ∼680 °C) is consistent with the results of thermometry on amphibole and plagioclase from felsic crystal clots, which represent crystal mush fragments. To maintain magma reservoir for such a long time above solidus, continuous magma input by deeper recharge is required. Zircon crystallization model calculations constrained by thermal modelling imply an average rate of magma input of about 1.3 × 10−4 km3/yr over 2 Myr. Such estimate allows us to calculate an extrusive/intrusive ratio of 1:25–1:30. The model calculations suggest that a crystal mush zone of about 35 km3 is still present within the subvolcanic magma reservoir. Importantly, the Ciomadul plumbing system thus remains thermally primed and renewed magma injection could lead to rapid reawakening and eruption of the apparently inactive volcano.

Magmatic systems are the engines driving volcanic eruptions and the source of fluids responsible for the formation of porphyry-type ore deposits. Sudden variations of pressure, temperature and volume in magmatic systems can produce unrest, which may culminate in a volcanic eruption and/or the abrupt release of ore-forming fluids. Such variations of the conditions within magmatic systems are commonly ascribed to the injection of new magma from depth. However, as magmas fractionating at depth or rising to the upper crust release CO2-rich fluids, the interaction between carbonic fluids and H2O-rich magmas stored in the upper crust (CO2 flushing), must also be a common process affecting the evolution of subvolcanic magma reservoirs. Here, we investigate the effect of gas injection on the stability and chemical evolution of magmatic systems. We calculate the chemical and physical evolution of magmas subjected to CO2-flushing using rhyolite-MELTS. We compare the calculations with a set of melt inclusion data for Mt. St. Helens, Merapi, Etna, and Stromboli volcanoes. We provide an approach that can be used to distinguish between melt inclusions trapped during CO2 flushing, magma ascent and decompression, or those affected by post-entrapment H2O-loss. Our results show that CO2 flushing is a widespread process in both felsic and mafic magmatic systems. Depending upon initial magma crystallinity and duration of CO2 input, flushing can either lead to volcanic eruption or fluid release. We suggest that CO2 flushing is a fundamental process modulating the behaviour and chemical evolution of crustal magmatic systems.

The eruptive dynamics of volcanic systems are largely controlled by the viscosity of deforming magma. Here we report the results of a series of high-temperature, high-pressure experiments at conditions relevant for volcanic conduits (250 MPa confining pressure and temperature between 500 °C and 900 °C) that were undertaken to investigate the rheology of magma with crystal fractions varying between 0.5 and 0.8 (50 to 80 wt.%) at different strain-rate conditions. The experiments demonstrate that the presence of crystals increases the relative viscosity (ratio between the viscosity of the mixture and the viscosity of the melt phase) of magmas and additionally induces a decrease of the relative viscosity with increasing strain-rate (shear thinning, non-Newtonian behavior). The experimental results, combined with existing data at low crystal fractions (0–0.3), were used to develop a semi-empirical parameterization that describes the variations of relative viscosity for crystal fractions between 0 and 0.8 and accounts for the complex non-Newtonian rheology of crystal-bearing magmas. The new parameterization, included into numerical models simulating the magma ascent dynamics, reveals that strain-rate-dependent rheology significantly modifies the dynamic behavior inside volcanic conduits, particularly affecting the magma fragmentation conditions.