Abstract. Experimental phase equilibria have been investigated on three medium-K silicic andesite (60-61 wt % SiO2) samples from Mount Pelhe at 2-4 kbar, 850-1040øC, under both vapor-saturated CO2-free and vapor-saturated CO2-bearing conditions. Most experiments were crystallization experiments using dry glasses prepared from the natural rocks. Both normal-and rapid quench experiments were performed. Two ranges of oxygen fugacity (fo2) were investigated: NNO (Ni-NiO buffer) to NNO + 1 and NNO + 2 to NNO + 3. At 2 kbar for moderately oxidizing conditions, plagioclase (pl) and magnetite (mt) are the liquidus phases, followed by low-Ca pyroxene (opx); these three phases coexist over a large temperature (

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Decompression experiments have been conducted to simulate syn-eruptive crystallization in the volcanic conduit, in order to infer magma ascent rates and conditions during domeforming and Plinian eruptions of silicic arc volcanoes. The experiments were carried out starting from Mt Pelée rhyolitic interstitial melt (76 wt. % SiO 2 ) and consisted in three consecutive steps: hydration, decompression, and annealing. Hydration (saturated and undersaturated) was performed at 850°C and 200 MPa and followed by isothermal decompression, either linearly or stepwise, to a final pressure, P f , of 30 or 5-10 MPa. Decompression rates range from 0.003 to 25 MPa/min (decompression durations of 15 min to 40 days). Two samples were cooled by 25°C and 50°C during a 3-days step at P f . Subsequent to decompression, the samples were held up to 15 days at P f . The experiments generated three types of crystals: pre-, syn-, and post-decompression crystallization. The experiments basically differ from previous studies in that they are specifically designed to discriminate crystal nucleation from growth and to evaluate the influence of pre-decompression crystals on the decompression-induced crystallization.The effects of pre-decompression crystals, decompression rate, undercooling (P f ), and terminal cooling have been determined on plagioclase nucleation, growth, morphology, and composition. The main results i) suggest a positive correlation between decompression rate and the number density of plagioclases nucleated at P f and ii) highlight the effect of predecompression crystals in further decompression-induced crystallization.The relations between the decompression conditions and the plagioclase characteristics have been used to infer Mt Pelée eruption dynamics, suggesting that i) Plinian magmas ascend from the reservoir within less than 1 hour (1-10 m/s), ii) dome and block-and-ash flows magmas ascend within more than 2-5 days, giving time for syn-decompression crystallization around pre-existent microlites, iii) dome magmas evidence long stagnation and cooling at low pressure, and iv) surge magmas ascend without significant crystallization (within less than ~4 days) and massively nucleate plagioclase at very low pressure. The extent and violence of dome destruction may depend on the size/age of the dome, with large/old domes favouring mildly-explosive BAFs, whereas small/young protodomes may generate highly-explosive surges.

International audienceAmphibole is widely employed to calculate crystallization temperature and pressure, although its potential as a geobarometer has always been debated. Recently, Ridolfi et al. (Contrib Mineral Petrol 160:45-66, 2010) and Ridolfi and Renzulli (Contrib Mineral Petrol 163:877-895, 2012) have presented calibrations for calculating temperature, pressure, fO2, melt H2O, and melt major and minor oxide composition from amphibole with a large compositional range. Using their calibrations, we have (i) calculated crystallization conditions for amphibole from eleven published experimental studies to examine the problems and the potential of the new calibrations; and (ii) calculated crystallization conditions for amphibole from basaltic-andesitic pyroclasts erupted during the paroxysmal 2010 eruption of Mount Merapi in Java, Indonesia, to infer pre-eruptive conditions. Our comparison of experimental and calculated values shows that calculated crystallization temperatures are reasonable estimates. Calculated fO2 and melt SiO2 content yields potentially useful estimates at moderately reduced to moderately oxidized conditions and intermediate to felsic melt compositions. However, calculated crystallization pressure and melt H2O content are untenable estimates that largely reflect compositional variation in the crystallizing magmas and crystallization temperature and not the calculated parameters. Amphibole from Merapi's pyroclasts yields calculated conditions of ~200-800 MPa, ~900-1,050 °C, ~NNO + 0.3-NNO + 1.1, ~3.7-7.2 wt% melt H2O, and ~58-71 wt% melt SiO2. We interpret the variations in calculated temperature, fO2, and melt SiO2 content as reasonable estimates, but conclude that the large calculated pressure variation for amphibole from Merapi and many other arc volcanoes is evidence for thorough mixing of mafic to felsic magmas and not necessarily evidence for crystallization over a large depth range. In contrast, bimodal pressure estimates obtained for other arc magmas reflect amphibole crystallization from mafic and more evolved magmas, respectively, and should not necessarily be taken as evidence for crystallization in two reservoirs at variable depth