Magmatic systems can remain dormant for tens of thousands of years, creating a misleading perception of extinction that complicates hazard forecasting. To identify drivers of protracted quiescence, we integrate geochemical, isotopic, and zircon geochronological data comprising over 1,250 crystallization ages from 31 eruptions at Methana, an active volcano near Athens, Greece. This record allows us to link eruptive activity, magma reservoir evolution, and mantle-source variations over 700,000 years. Here, extended repose correlates with increased metasomatism of the mantle wedge by slab-derived components. The longest quiescence at Methana (>100,000 years) coincides with substantial magma production that was preferentially trapped in the crust. We attribute this trapping to the generation of superhydrous melts (>6 wt% H₂O) from a highly metasomatized mantle. These volatile-rich magmas undergo water saturation and crystallize during ascent, preventing eruption. Such trapping mechanisms can grow large magma reservoirs and may enable transitions from small stratovolcanoes to highly hazardous, caldera-forming systems.

Study Design

Fresh rock samples were collected from 31 of the 34 mapped units of the Methana volcano for dating and geochemical analyses that allow us to track the evolution of the volcanic system in time. GPS coordinates are available in Supplementary File 1. The samples were cut into billets and the crystals detached from the matrix glass using high voltage selective fragmentation (SELFRAG), a batch processing technique that exposed the sample (300-500 g of material) to electrical pulses of 115-125 kV. The material (50-100 g per sample) was subsequently separated into light and heavy fractions by heavy liquid separation using a sodium polytungstate solution with a density of 2.89 g/cm³. Zircon and plagioclase crystals, including ilmenite for the basaltic andesite sample, were handpicked under a stereographic microscope and prepared for dating and isotopic analyses by mounting them in epoxy resin, followed by grinding with aluminum-oxide paper and polishing with diamond suspension. A piece from each rock sample was also cut for bulk-rock XRF analyses. The methodological details of the analyses, which were all performed in the laboratories of ETH Zürich, Switzerland, are provided below. The data, including reference materials, are provided in the supplementary electronic files (Supplementary Files 1-3).

U-Th and U-Pb dating

For zircon dating, we ran a preliminary U-Th survey on 6 crystals from each sample, based on which we selected the preferred methods: U-Th disequilibrium dating for units generally younger than 200 ka, and U-Pb dating for older units, which were then analysed in separate sessions. For both methods, dating was performed with a Laser Ablation Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) setup, consisting of a Resonetics Resolution S155 laser ablation system connected to a Thermo Element XR sector-field mass spectrometer. The ablation spots were set to a diameter of 29 μm, and the laser was configured to a repetition rate of 5 Hz and a fluence of 2.5 J cm^-2. The ²³⁰Th/²³²Th, ²³⁸U/²³²Th and ²³⁸U/²³⁰Th ratios and the ²⁰⁷Pb/²³⁵U and ²⁰⁶Pb/²³⁸U ratios, respectively, were measured. Ablation time was 40 seconds, following a gas blank acquisition of 30-40 seconds. Zircon crystals varied in size, but were mostly between 50 and 200 µm. Smaller grains typically accommodated only a single U-Th or U-Pb ablation spot, while larger crystals allowed us to target both rims and cores, to capture both younger and older crystallization domains.

For U-Th disequilibrium dating, gas blank corrected intensities and uncertainties on the ratios were obtained using the SILLS software (Guillong et al., 2008). The complete data reduction followed the methodology described in Guillong et al. (2016) and consists of correcting for interferences on ²³⁰Th caused by ablation-induced polyatomic zirconium oxides, for the abundance sensitivity of ²³²Th on ²³⁰Th using monazite measurements, and for the relative sensitivity factor based on zircon 91500 (Wiedenbeck et al., 1995) and NIST 612 glass. Secular equilibrium zircon reference materials, including Plesovice (Sláma et al., 2008), Fish Canyon Tuff (Wotzlaw et al., 2013), GJ-1 (Jackson et al., 2004), and AUSZ7-1 (Kennedy et al., 2014), were measured repeatedly throughout the sessions to show the accuracy of the corrections (Supplementary File 2).

To calculate individual crystallization ages based on U-Th isotopic data, we relied on the two-point isochron method, where isochrons are constructed for each zircon based on the equilibrium melt, which in this case was assumed to be the groundmass glass of each unit (Harangi et al., 2015; Popa et al., 2020a). The isotopic compositions of the groundmass glasses were analysed with the same setup as above, but with laser spots of 163 μm, a repetition rate of 10 Hz, and a fluence of 3.5 J cm^-2. Data reduction was performed similarly to zircon, but without the zirconium polyatomic oxide correction, which is not necessary in this case. The reference materials used are ATHO-G (Jochum et al., 2000), BCR-2G (Jochum et al., 2005), BHVO-2G (Jochum et al., 2005), and T1-G (Jochum et al., 2000).

For the U-Th disequilibrium of ilmenite crystals, we used the method described in Keller et al. (2022). The laser spots were varied according to crystal size between 100 and 257 μm, the repetition rate was 20 Hz, and the ablation duration was 30 s, with 40 s of gas blank measurement. The same corrections as for the isotopic analyses of groundmass glass were performed, using ilmenites in secular equilibrium from the Fish Canyon Tuff, as well as rutile and titanite in secular equilibrium, and glass (GSD-1G and GSE-1G, Jochum et al., 2005) for the relative sensitivity factor correction and as validation reference material. Data available in Supplementary File 2.

For U-Pb dating, the data reduction was done with the Iolite software (Paton et al., 2011) using the U-Pb Geochronology reduction scheme with VisualAge (Petrus et al., 2012), which includes a downhole fractionation routine and uses GJ-1 (Jackson et al., 2004) as primary reference material. Additional corrections, namely for common lead and initial Th-disequilibrium, were performed using IsoplotR (Vermeesch et al., 2018), which also yielded the corrected crystallization ages. For the Th-disequilibrium correction, the initial isotopic activity ratio is equivalent to the Th/U-fractionation factor between the crystal and the melt, and we used a nominal value of 0.2 (Sakata et al., 2017). Some points were not concordant on the Tera-Wasserburg plot due to small amounts of common Pb, and we based the common-lead correction on the isochron intercept whenever the ratios defined clear isochrons in the concordia space. Otherwise, we used the Stacey-Kramers model (Stacey and Kramers, 1975) to determine the ²⁰⁷Pb/²⁰⁶Pb (0.8356) of the present day. The secondary standards that were run throughout the measurement sessions include AUSZ8-10 (Lukács et al., 2021), AUSZ7-5 (von Quadt et al., 2016), AUSZ7-1 (Kennedy et al., 2014), Penglai (Li et al., 2010), 91500 (Wiedenbeck et al., 1995), Plesovice (Sláma et al., 2008), and Temora (Black et al., 2003). Data available in Supplementary File 2.

Hf and Sr isotopes

Hafnium isotope compositions of zircon (Supplementary File 3) were determined by LA-MC-ICP-MS, coupling an Australian Scientific Instruments RESOlution 193 nm ArF excimer laser to a Nu Instruments Plasma 2 multicollector inductively coupled plasma mass spectrometer. The measurements were carried out with a spot size of 50 µm over 40 s, an energy fluence of 4 Jcm⁻² and a repetition rate of 5 Hz. To correlate age and Hf isotope data, we placed the Hf spot directly overlapping the initial U-Th or U-Pb crater, which expanded the diameter of the original pit from 29 µm to 50 µm, at a similar ablation depth of ~15 µm. Because the Hf analysis does not preferentially ablate at the base of the original crater, significant mass‑fractionation effects are not expected. Mud Tank zircon was used as the primary reference material. Raw data were reduced using Iolite 4 and the Hf isotopes data reduction scheme. The radiogenic ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratio was corrected for mass bias and interferences of ¹⁷⁶Yb and ¹⁷⁶Lu, determined using the mass-bias corrected ratios ¹⁷⁶Yb/¹⁷³Yb = 0.79502 and ¹⁷6Lu/¹⁷⁵Lu = 0.02656. Initial Hf isotopic composition was calculated using the measured ¹⁷⁶Hf/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf ratios, the crystallization age obtained for the corresponding ablation spot, and the decay constant of Söderlund et al. (2004) for ¹⁷⁶Lu (1.867∙10^–11). Uncertainty on the Hf isotopic composition is reported as 2 SE and is calculated by quadratic addition of within-run 2 SE analytical uncertainty with the average 2 SD reproducibility on the initial ¹⁷⁶Hf/¹⁷⁷Hf ratios of zircon reference materials. εHf (t) was calculated using the CHUR parameters of Bouvier et al. (2008). GJ-1 (Morel et al., 2008), Temora (Wu et al., 2006), and Plesovice (Sláma et al., 2008) validation reference materials were analysed together with the sample unknowns to test the method accuracy. Analyses with a high Yb/Hf ratio (¹⁷⁶Yb/¹⁷⁷Hf >0.1) were discarded.

Strontium isotope compositions of plagioclase were determined using the same LA-MC-ICP-MS setup as above, following the procedure outlined in Pimenta Silva et al. (2023). Plagioclase was analysed with a repetition rate of 8-10 Hz, an energy density of 3.5-4.0 J cm^-2, and a spot size of 100 μm for the unknowns and for most standards, including Hrappsey 14-2, but smaller for the plagioclase standards AMNH-107160 and G29958 (Mulder et al., 2023). Plagioclase standards and unknowns were ablated for 40 s, preceded by 30 s of gas blank measurements, and followed by 30 s of sample washout. The following masses were monitored: 88, 87, 86.5, 86, 85.5, 85, 84.5, 84, 83.5, 83 and 82. Krypton corrections on masses 84 and 86 were performed by on-peak baseline correction. The mass bias was corrected using an exponential law and a reference ⁸⁶Sr/⁸⁸Sr ratio of 0.1194. The intensity of ⁸⁵Rb and a fixed ⁸⁷Rb /⁸⁵Rb = 38.562 were used to correct the ⁸⁷Rb interference on ⁸⁷Sr. Ca dimer and argide interferences were monitored at masses 82 and 83 and corrected accordingly. No correction for isobaric interference of doubly charged REE was required due to negligible REE contributions, never exceeding background values. The data were reduced using the Iolite 4 software (Paton et al., 2011).

Total Sr signals varied between ~1.1 and 2.7 V for plagioclase unknowns. Analytical accuracy and instrumental drift were evaluated by repeated ablation (every ~15 measurements of plagioclase unknowns) of isotopically homogenous plagioclase standards with G29958 employed as primary reference material. All data are reported relative to G29958 ⁸⁷Sr/⁸⁶Sr of 0.707551 (Mulder et al., 2023) via standard bracketing. The weighted mean ⁸⁷Sr/⁸⁶Sr for AMNH-107160 and Hrappsey 14-2 are consistent with solution analyses of the same material reported by Mulder et al. (2023; Supplementary File 3). No correction for ⁸⁷Sr ingrowth was applied due to the very young ages (<1 Ma) of the examined samples, coupled with low ⁸⁷Rb/⁸⁶Sr (generally <0.05). Measurements of ⁸⁴Sr/⁸⁶Sr for unknowns and standards are consistent with the accepted value of ~0.0565. Data screening was based on total signal (>1 V), ⁸⁴Sr/⁸⁶Sr, and Rb/Sr. Anomalous parts (e.g., high Rb/Sr) of the spectra were removed from integration.

Bulk-rock chemical analyses

For determining bulk-rock compositions, unaltered rocks (lavas and pumices) were powdered, dehumidified at 110°C for 24h, devolatilized at 900°C for 2 h, and fused into glass discs at 1200°C, after mixing with a Lithium-Tetraborate flux. The bulk-rock analyses were performed with a PANalytical AXIOS wavelength-dispersive X-ray fluorescence spectrometer. The trace element analyses were done on the same fused glasses, with a laser ablation system consisting of a 193 nm ArF-Excimer (Geolas) laser connected to an NexION 2000 ICP mass spectrometer. The glasses were ablated over 100 μm for 40 s at a repetition rate of 10 Hz. Each sample was measured 3 times, and the results were averaged (Supplementary File 1).

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