Conference Agenda

Fluids exert a first-order control on the evolution and eruptibility of silicic systems. Far from passive byproducts of crystallization and magma ascent, a volatile phase modulates magma compressibility, rheology, heat transfer and elemental transport across the lifespan of silicic reservoirs, from melt extraction and mush rejuvenation to pegmatite formation and hydrothermal alteration. Here, we synthesize observations from thermomechanical models and diffusion experiments with field studies of Li-rich pegmatite and granite systems to investigate the role of fluids in silicic reservoirs.

Thermomechanical models demonstrate that volatile exsolution and resorption can fundamentally alter the stability of silicic magma chambers. In systems undergoing voluminous recharge, volatile resorption may accelerate pressurization by reducing magma compressibility, thereby expediting destabilization and eruption onset. Such feedbacks illustrate how fluids regulate eruption frequency and magma remobilization in long-lived crystal-rich reservoirs. Constraining fluid transfer rates remains challenging, but diffusivity contrasts in fluid-mobile elements and isotopes, like Li, provide a promising avenue for quantifying fluid transport timescales.

Fluid evolution is equally critical for rare-metal mineralization. High concentrations of incompatible fluid-mobile metals, e.g. Li in pegmatites, require carefully balanced timing between crystallization and fluid saturation. Early fluid saturation may flush incompatible metals from the system prior to extreme enrichment, producing unmineralized aplites and quartz veins, whereas excessively late fluid saturation can trap fluids within low-permeability crystal frameworks. Late hydrothermal alteration can further modify primary Li enrichment through muscovitization and chloritization. Together, these observations emphasize the role of fluids in coupling magma reservoir evolution, eruption initiation, and ore deposit formation across crustal scales.

Nucleation of H₂O vesicles from supersaturated melt is one of the key factors promoting explosive volcanic eruptions. Extensive effort has been undertaken to link vesicle textures found in natural volcanic samples to experimental degassing samples and ultimately to produce predictive degassing models[1]. As is known from classical nucleation theory, the surface tension of the H₂O-bearing melt is the key parameter for vesicle formation[2]. Due to its substantial influence on the energy barrier of nucleation, knowledge of the surface tension is essential for the modeling of degassing processes.

To investigate the vesicle formation behavior of dacitic melt and constrain the surface tension, we performed a thorough experimental decompression campaign. Hydrated melts with 5 wt.% of dissolved H₂O were continuously decompressed from 200 MPa at temperatures of 1423 to 1323 K towards final pressures ranging from 100 to 30 MPa at constant decompression rates between 0.1 to 5 MPa/s.

In contrast to spinodal decomposition of hydrous phonolitic melt [3,4], our experimental results suggest decompression rate dependent vesicle number densities, indicating nucleation as the vesicle formation process in the dacitic melt. We constrained the onset of homogeneous nucleation and evaluated the textural evolution of the melt during further decompression. From these observations, we derived nucleation rates and values for the surface tension.

[1] Toramaru (2006): J. Volcanol. Geotherm. Res., 154, 303-316

[2] Gardner et al. (2013): J. Volcanol. Geotherm. Res., 267, 68-74

[3] Allabar and Nowak (2018): Earth Planet. Sci. Lett., 501, 192-201

[4] Marks and Nowak (2025): Eur. J. Mineral., 37, 385–412

Amphibole is a common hydrous mineral in mafic magmas and is crucial to constrain melt composition, pressure, temperature, and fO₂ during magma evolution. However, to date, few studies have directly compared amphibole crystallization systematics between alkaline and calc-alkaline magmatic systems (e.g., importance of amphibole composition for magmatic evolution, peritectic vs. cotectic crystallization). To fill this gap, we compiled literature data and combined them with new experimental results to build a comprehensive comparative database. Equilibrium crystallization experiments were performed in internally heated pressure vessels using three different starting compositions: two basalts representative of mafic arc magmas (Mg# 69 and 61) and a nephelinite (Mg# 69) representative of alkaline magmatism. Basaltic experiments were conducted in the P-T range 200-400 MPa and 950-1100 °C with variable bulk H₂O contents (1-9 wt.%). Nephelinite runs were conducted at 400 MPa over the identical temperature range under mixed CO₂–H₂O fluid conditions.

Despite crystallizing under similar pressure (100–1000 MPa) and temperature (700–1100 °C) conditions, amphiboles from mafic alkaline and calc-alkaline systems show distinct chemical compositions. Alkaline amphiboles are generally more alkali- (Na and K) and Ti-rich than calc-alkaline amphiboles. Interestingly, alkaline amphiboles already start crystallizing from melts significantly poorer in SiO₂ (minimum 44 wt.%) compared to calc-alkaline melts (minimum 50 wt.%). In contrast, the coexisting melts in both series display broadly the same Mg# range. Our preliminary results highlight the crucial role of amphibole during magmatic processes and provide new insights into how amphibole crystallization differs between alkaline and calc-alkaline compositional series.

Phonolites are highly evolved, SiO₂-undersaturated igneous rocks, which are often peralkaline (molar (Na+K)/Al > 1). Based on their HFSE (Ti, Zr, Nb) mineral assemblages, miaskitic and agpaitic types are distinguished. While the more common miaskitic phonolites typically contain minerals like titanite and zircon, agpaitic varieties crystallize rare halogen-bearing Na-Ca-HFSE minerals, such as minerals of the eudialyte-, wöhlerite- or rinkite-groups. Their formation requires specific conditions such as volatile retention, but the precise controls on their mineralization remain poorly understood.

This study focusses on the under-investigated sub-volcanic trachytes to phonolites of the French Massif Central. The main magmatic phase occurred during the Miocene-Pliocene producing predominantly alkali basaltic to basanitic lavas that cover large parts of the underlying Variscan basement. The area comprises several volcanic sub-provinces, however, phonolitic occurrences are mostly restricted to Cantal (including Sillon Houiller), Monts Dore and Velay, all of which were sampled for this study. The phonolites intruded into the crystalline basement, sometimes, where present, also the basaltic plateau.

Preliminary microprobe data reveal different types of clinopyroxene, some showing complex zoning following different diopside-augite-aegirine trends. These chemical changes, coupled with diverse felsic mineral assemblages ranging from hauyne-bearing to nepheline- and sodalite-rich to alkali feldspar-dominated types suggest distinct crystallization conditions or different volatile contents in the evolving magmas.

Using textural observations, whole-rock geochemistry, and electron probe microanalysis (EPMA) of key minerals (e.g. clinopyroxene, feldspar, SGM, titanite, Na-Ca-HFSE-F-minerals), this study aims to determine what controls these distinct phonolite types, their fractionation paths and the impact of volatile retention on their crystallization.

Marsili Volcano (MV) is a young back-arc spreading centre in the southern Tyrrhenian Sea where two types of subduction-related basalts, low-Ca and high-Ca, with distinct phenocryst assemblages and geochemical signatures have been erupted. High-Ca basalts occur widespread along the edifice, whereas low-Ca basalts are restricted to the northern sector. To investigate the processes controlling magma evolution, we performed a detailed petrological and geochemical study of glomerocrysts, crystal aggregates formed from parental melts of the host lavas, composed of olivine, clinopyroxene, and plagioclase. Their minerals record crystallization across the 12 km-thick oceanic crust at 1020-1220°C, with low to moderate melt H₂O contents (0.5-3.5wt%) increasing from northern to lateral sectors, and moderately to highly oxidizing conditions (+1-+2.9∆QFM), particularly in high-Ca basalts. The glomerocryst assemblages define two distinct liquid lines of descent: olivine then plagioclase fractionation in low-Ca basalts, and olivine followed by clinopyroxene then plagioclase fractionation in high-Ca basalts. Major and trace element systematics of these mineral phases confirm that low-Ca and high-Ca basalts erupted at the northern sector experienced limited differentiation and preserve the signatures of two separate primary melts. In contrast, olivine in high-Ca glomerocrysts from axial and lateral sectors records mixing between low-Ca- and high-Ca-derived magmas, indicating a hybrid origin. Regardless of basalt type, clinopyroxene displays complex major- and trace-element zoning consistent with crystallization in multi-level, sill-like mush bodies, implying that crustal transport and differentiation occur within a vertically distributed sill complex rather than a single shallow magma chamber, supporting oceanic crustal accretion by stacked sill emplacement beneath back-arc spreading centres.