Data for: Effusive-explosive transitions of water-undersaturated magmas. The case study of Methana Volcano, South Aegean Arc

Data for: Methana Volcano, South Aegean Volcanic Arc Bulk-rock major and trace elemental compositions: Rock-powders were dehumidified (24h, 110˚C), devolatilized (2h, 850˚C) and fused at 1100˚C after mixing with a Lithium-Metaborate flux. The disks were analyzed for major elements with a PANalytical AXIOS wave-length dispersive X-ray fluorescence spectrometer (WDXRF). The trace element analyses were performed on the same disks using laser-ablation inductively-coupled plasma mass spectrometry (LA-ICPMS). We used a 193 nm ArF-Excimer (Geolas) laser connected to a NexION 2000 ICP mass spectrometer. We ablated 3 points on each sample at a diameter of 100 μm for 40 seconds, with a repetition rate of 10 Hz. The 3 points were averaged for each sample. We used the SILLS data reduction software of Guillong et al. (2008), with NIST610 as a primary standard and the SiO2 content of each sample as internal standard. The basaltic andesite enclave data was acquired with a similar method by V. Dietrich in the 1980s’ and is integrated with the new dataset. For these particular analyses, a Philips PW 1404 sequential XRF spectrometer was used, at the Federal Institute for Material Testing in Dübendorf, Switzerland. Groundmass glass major elemental compositions: The compositions of the matrix glasses were obtained by using an EDS-calibrated JEOL JSM-6390 LA Scanning Electron Microscope (SEM), equipped with a Thermo Fisher NORAN NSS7 EDS system with LaB6 filament, 30 mm2 silicon-drift detector and Faraday-cup for calibration. The main advantage of using this setup is the possibility of measuring compositions over wide areas (e.g. 50 μm diameter). This allows us to automatically average the microlite and interstitial glass compositions, which is paramount for estimating the pre-eruptive chemistry of the melts in lavas. Hence, we can automatically eliminate the late-stage differentiation effect of post-eruptive microlite crystallization. Mineral compositions: Amphibole, pyroxenes and plagioclase were analyzed with a JEOL JXA-8200 Electron Probe Microanalyzer (EPMA) equipped with 5 spectrometers, using an energy of 15 kV and a 20 nA focused beam. The calibration was performed on mineral (crystalline) standards. We used a peak-background correction with measurement times varying between 20-40 s per peak. The calibration setup and measurement times were slightly different for plagioclase. For example, we preferred anorthite and albite standards for the calibration of major elements and we increased the measurement times up to 100 s per peak for the key trace elements (Fe, Mg, Ti). We acquired 9% of the plagioclase data with the calibrated SEM described earlier (using a focused beam), during microprobe downtime. The results are perfectly comparable with the EPMA dataset. All analyses were performed in the laboratories of ETH Zürich, unless otherwise stated.