Abstract Most arc volcanoes erupt intermediate to silicic magmas that have sufficient volatile contents to behave explosively. Despite this explosive potential, low-energy effusive eruptions of such viscous water-rich magmas are common occurrences. Hence, predicting the style of the next eruption, with its obvious importance on hazard mitigation, remains one of the main challenges of volcanology. Here, we investigate the changes in eruptive styles at Methana volcano, from the South Aegean Arc. This volcano has generated multiple andesite-to-dacite eruptions, with the last event occurring in historical times (~2250 BP). We focus on 14 eruptive events, 3 being explosive. We compare the petrology and geochemistry of the deposits to reconstruct the magma chamber events that preceded both types of eruptions. We estimate the thermo-chemical conditions that characterized each eruptive event, and highlight the causes that promoted or inhibited magma fragmentation: temperature, pressure, composition, relative volumes of different magma batches, dissolved water content, crystallinity, melt and bulk viscosities. The results indicate that Methana harbors an upper-crustal silicic reservoir stored at ~2 kbar (likely dacitic to rhyodacitic in composition) that is in a highly crystalline state (>50 vol% crystals, mostly uneruptible). All the eruptions are triggered by deeper magma recharge ascending from storage pressures of ~4–5 kbar, leading to some rejuvenation of the shallow silicic mush. The most mafic recharges (~55 wt% SiO2, ~1000 °C, >3 wt% H2O) promote effusive eruptions after mixing and hybridizing the upper-crustal reservoir (hybrids of ~57–64 wt% SiO2, ~900–950 °C, generally 3–4 wt% H2O). Slightly colder, wetter and more differentiated recharges (~62 wt% SiO2; ~900 °C, ~4 wt% H2O) trigger explosive events after having minimal interaction with the upper-crustal reservoir. However, the dissolved water content is not the only factor influencing explosivity, as some of the wettest magmas (>4.5 wt% H2O) generate lava flows. Our data indicate that a key control on effusive-explosive transitions at Methana is the crystallinity of the erupted material. A high crystallinity (40–55 vol%) increases the bulk-viscosity, which results in the magma having a slower ascent velocity. In addition, crystallinity enhances the formation of permeable pathways for the gas. Longer ascent timescales and enhanced permeability in the conduit improve the outgassing potential of the magmas and lead to effusive behavior. The opposite occurs for explosive events. Here, the lower crystallinity of the magmas (30 vol%) translates into lower bulk-viscosities and faster ascent rates, which inhibit outgassing. This case study highlights the importance of obtaining better constraints on the state of subvolcanic magma reservoirs, in particular concerning crystallinity and volatile concentrations, which has to become a priority in the years to come.

中文翻译：

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水不饱和岩浆的喷发-爆炸转变。南爱琴海弧 Methana 火山的案例研究

摘要 大多数弧形火山喷发的硅质岩浆具有足够的挥发性成分，具有爆炸性。尽管具有这种爆炸性的潜力，但这种粘性富含水的岩浆的低能量喷发还是很常见的。因此，预测下一次喷发的风格，对减轻灾害具有明显的重要性，仍然是火山学的主要挑战之一。在这里，我们调查了来自南爱琴海弧的 Methana 火山喷发方式的变化。这座火山产生了多次安山岩到英安岩的喷发，最后一次喷发发生在历史时期（~2250 BP）。我们专注于 14 个爆发事件，其中 3 个是爆炸性事件。我们比较了沉积物的岩石学和地球化学，以重建两种类型喷发之前的岩浆房事件。我们估计了表征每个喷发事件的热化学条件，并强调了促进或抑制岩浆破碎的原因：温度、压力、成分、不同岩浆批次的相对体积、溶解水含量、结晶度、熔体和体积粘度。结果表明 Methana 拥有一个储存在 ~2 kbar（成分可能是英安岩到流纹英石）的上地壳硅质储层，该储层处于高度结晶状态（> 50 vol% 的晶体，大部分是不可爆发的）。所有的喷发都是由从~4-5 kbar 的储存压力上升的更深的岩浆补给引发的，导致浅层硅质泥浆恢复活力。最多的镁铁质补给量（~55 wt% SiO2，~1000 °C，> 3 wt% H2O）在混合和杂交上地壳储层（~57-64 wt% SiO2，~900-950 °C，通常为 3-4 wt% H2O）后促进喷发喷发。在与上地壳储层的相互作用最小后，稍微更冷、更湿和更分化的补给（~62 wt% SiO2；~900 °C，~4 wt% H2O）触发爆炸事件。然而，溶解水含量并不是影响爆炸性的唯一因素，因为一些最湿的岩浆（>4.5 wt% H2O）会产生熔岩流。我们的数据表明，对 Methana 喷发-爆炸转变的关键控制是喷发材料的结晶度。高结晶度 (40–55 vol%) 会增加体积粘度，从而导致岩浆上升速度较慢。此外，结晶度增强了气体可渗透通道的形成。管道中更长的上升时间尺度和增强的渗透性提高了岩浆的除气潜力并导致喷出行为。爆炸性事件则相反。在这里，岩浆的较低结晶度 (30 vol%) 转化为较低的体积粘度和较快的上升速率，从而抑制释气。本案例研究强调了更好地约束次火山岩浆储层状态的重要性，尤其是在结晶度和挥发分浓度方面，这将成为未来几年的优先事项。从而抑制释气。本案例研究强调了更好地约束次火山岩浆储层状态的重要性，尤其是在结晶度和挥发分浓度方面，这将成为未来几年的优先事项。从而抑制释气。本案例研究强调了更好地约束次火山岩浆储层状态的重要性，尤其是在结晶度和挥发分浓度方面，这将成为未来几年的优先事项。