We present microstructural and outcrop-scale evidence for melt migration pathways that formed during deformation within high-strain zones cutting through both partially molten (supra-solidus) and solid (sub-solidus) rocks. High-strain zones with more than 10% undeformed felsic or leucocratic material are easily recognized as melt pathways, especially in supra-solidus rocks. However, identifying similar pathways in sub-solidus rocks—or those with very low melt content (1–2%)—is more difficult, as they often lack visible felsic components at the outcrop scale and closely resemble typical mylonite zones, leading to frequent misinterpretation.

Even where large volumes of melt once moved through these zones, direct microstructural evidence can be subtle or only detectable under the microscope. When melt-rock interaction is more intense, textural signs may remain faint, but geochemical signatures can be detected at both the micro- and macro-scale. In these settings, melt tends to flow through porous, high-strain networks in a highly reactive manner, producing complex reaction fronts.

We urge geologists to carefully assess whether deformation occurred in the presence of melt, even in zones that appear solid-state. This distinction is critical because: (1) rocks deformed with even small amounts of melt are significantly weaker than those in the solid state; (2) melt-rock interactions can lead to metasomatism; and (3) these zones can channel and sustain prolonged melt movement, contributing to large-scale chemical differentiation of the crust.

The eruption behavior of volcanic systems is governed by the separation of an H₂O fluid phase from a supersaturated hydrous melt during magma ascent. Vesicle number density (VND, mm^-3) is a key parameter used to quantify the efficiency of this fluid-melt separation. While VND typically increases with decompression rate due to nucleation-driven vesicle formation—making it a potential proxy for magma ascent velocity—recent studies suggest that spinodal decomposition, which produces decompression rate-independent VND, may dominate in hydrous alkali-rich melts.

We conducted super liquidus decompression experiments on phonolitic melts under H₂O-saturated and undersaturated conditions, across decompression rates of 0.064–1.7 MPa/s. The samples show consistently high initial logVNDs of ~5.5, regardless of decompression rate, with uniform vesicle diameters of 2–8 µm, supporting spinodal decomposition as the vesicle formation mechanism. As vesicle coalescence begins and progresses, vesicle sizes increase up to ~500 µm and VND decreases significantly, becoming dependent on decompression rate and final pressure. The lowest logVNDs of 0.5–0.8 occur at the slowest rates, while the highest coalesced VNDs with 3.1–3.7 are observed at the fastest rates.

This transition—from a high-VND, decompression rate-independent regime to a coalescence-driven, rate-dependent regime—marks a critical shift in vesiculation dynamics. It reflects a development from a closed, potentially explosive system to a more open, outgassing system favoring effusive eruption. These findings offer vital insights into how degassing processes may influence eruption style and contribute to improved volcanic hazard assessment.

Carbon dioxide (CO₂) is the second most abundant volatile component in magmatic systems after H₂O and plays a crucial role in controlling magma ascent, bubble nucleation, and eruption dynamics. Accurate knowledge of CO₂ solubility and diffusivity in silicate melts is therefore fundamental for understanding and modeling volcanic processes. Despite its significance, experimental constraints on CO₂ diffusion in silicate melts—particularly under hydrous conditions—remain limited.

This study presents new experimental data on CO₂ diffusion in basaltic melts, with a specific focus on the effect of dissolved H₂O. Diffusion couple experiments were conducted at 300 MPa and temperatures between 1200 and 1350 °C using a rapid-quench, internally heated pressure vessel. The resulting glass charges were doubly polished and analyzed using Fourier-transform infrared (FT-IR) micro-spectroscopy to obtain concentration–distance profiles. CO₂ diffusion coefficients (DCO₂) were calculated by fitting these profiles with error functions.

Preliminary results from anhydrous basaltic compositions exhibit clear Arrhenian behavior, with DCO₂ values consistent with previous measurements in Fe-free systems. Ongoing experiments are aimed at quantifying the influence of dissolved H₂O on CO₂ diffusivity, providing new insights into volatile mobility and transport mechanisms in hydrous basaltic melts.

Amphibole is a common hydrous mineral in mafic calc-alkaline and alkaline magmas that typically crystallizes via a peritectic reaction. Due to its flexible crystal structure, amphibole covers a wide range of chemical composition, making it a crucial mineral phase for thermobarometry, oxybarometry, or diffusion chronometry. Up to date, only a limited amount of experimental studies focused on the amphibole peritectic reaction in basaltic to basaltic-andesitic systems, showing that the reaction involves various silicate minerals, such as plagioclase, olivine, orthopyroxene and clinopyroxene. However, although these studies provide first insights, systematic investigations allowing us to predict amphibole crystallization are still missing. The aim of our contribution is to experimentally quantify the amphibole-forming peritectic reaction, and to understand its dependence on various parameters (e.g. pressure or bulk composition). Equilibrium crystallization experiments were performed in internally heated pressure vessels (IHPV) between 200 and 400 MPa and temperatures between 900 and 1000 °C employing varying initial bulk H₂O contents. Oxygen fugacity was buffered at conditions ranging from NNO+2 to NNO+2.3. So far, two different starting compositions were explored: a high-Mg basalt from the Adamello Batholith (Italy), and a magnesian basalt from the Cascades magmatic arc (USA), covering the typical compositional range of mafic arc magmas. Our preliminary results indicate that, minerals assemblages (e.g. plagioclase or orthopyroxene) involved in the amphibole peritectic reaction vary as a function of pressure and bulk system composition. Combining our dataset with literature data, we formulated a preliminary, quantitative model of the amphibole-forming peritectic reaction for calc-alkaline and alkaline magmas.

The Cordón Caulle volcano in the Southern Andean Volcanic Zone in Chile represents an ideal setting to study magmatic processes within an active arc system. Rhyolitic lavas erupted in 2011-2012 host crystal-rich basaltic enclaves which have been interpreted as pieces of an active crystal mush. Interstitial glasses of these mush fragments are compositionally very similar to their host rhyolitic magmas suggesting that a basaltic crystal mush represents the source of the rhyolitic melts, thereby offering a rare possiblity to investigate an active mush system.

In this study, we experimentally test this petrogenetic model by performing partial melting experiments on natural rock powders of basaltic enclave samples employing bulk water contents of 0.5-1.0 wt.%. Experiments were run at 150 MPa in internally heated pressure vessels (IHPV) at temperatures between 750 and 1000 °C and fO₂ buffered between NNO-1 and NNO. Our experimental setup is specifically designed to simulate a crystallisation-driven differentiation mechanism applicable to an in-situ evolving crystal mush, representing a mixture between fractional and equilibrium crystallisation regimes, where the "reactive magmatic system" is continously changing during progressive cooling.

Experimental residual melts define distinct differentiation trends and show a close compositional match with bulk rocks as well as groundmass glasses of the rhyolitic lavas strongly supporting a petrogenetical link between basaltic mush enclaves and host ryholites. Consequently, the generation of highly-evolved liquids in a cooling basaltic crystal mush combined with an efficient residual melt extraction mechanism represents a possible differentiation scenario for the Cordón Caulle volcano and similar volcanic systems.

The Izu-Bonin-Mariana (IBM) fore-arc is a rare example where flux melting of the refractory mantle and the generation and differentiation of boninitic melts can be investigated. A previous study proposed that IBM boninitic magmas were stored at very shallow depths (20–40 MPa) and temperatures > 1100 °C, based on orthopyroxene-liquid and olivine-liquid thermobarometry (1). However, given the strongly hydrous nature of these boninitic magmas (2) (up to 3.2 wt% H₂O in quenched natural glasses), higher pressures seem likely. To constrain the liquid lines of descent and magma storage conditions of IBM boninites, we conducted equilibrium crystallization experiments on evolved high- and low-Si boninites using an internally heated pressure vessel (IHPV). Experiments were performed over a P-T range of 200-400 MPa and 1020-1220 °C with varying H₂O contents.

For the high-Si boninite, orthopyroxene is the liquidus phase. For the low-Si boninite, orthopyroxene is the liquidus phase at low water concentrations while olivine is the liquidus phase at high H₂O contents. Differentiation for both compositions proceeds with clinopyroxene and plagioclase crystallization. Residual melts follow calc-alkaline differentiation trends, evolving to high-Mg andesitic and dacitic compositions. Experimental results demonstrate that the liquid lines of descent of natural IBM boninitic glasses as well as mineral compositions can be reproduced at 200 MPa and bulk H₂O concentrations >2 wt%. We, therefore, conclude that natural boninitic magmas differentiated at higher pressures than previously estimated.

(1) Whattam, S. A. et al. (2020): Am. Mineral. 105 (10)

(2) Coulthard, D. A. et al. (2021): Geochem. Geophys. Geosyst. 22 (1)