Abstract Despite their generally low volume fraction, Fe Ti oxides have the potential to greatly influence the eruptive style because they lower the supersaturation pressure for heterogeneous bubble nucleation. Once nucleated, bubbles respond fast to pressure changes, fostering rapid expansion and explosive behavior. Yet, oxide microlite quantifications are often absent from data of explosive products. We used new, re-analyzed, and published data to build a compilation of oxide number densities (ONDs) and vesicle number densities (VNDs) of trachytic and calc-alkaline products. Four eruptive styles were selected: 1) Vulcanian explosions from Soufriere Hills volcano, Montserrat, Lascar volcano, Chile, and Kilian volcano, France, 2) blasts from Mt. Pelee volcano, Lesser Antilles, Mount St. Helens, USA, and Merapi volcano, Indonesia, 3) a sub-Plinian explosion from Merapi volcano, and 4) lava dome effusions with intermittent collapse from Soufriere Hills and Mt. Pelee volcanoes. Natural samples were separated into two groups according to the dominant texture of the products of each event: 1) vesicular pumice clasts from explosions with a strong vertical component and 2) dense clasts with diktytaxitic textures from dome collapse event and lateral blast. Group 1 samples are either distributed alongside the 1:1 trend between VND and OND that spans from 1015 to 1017 m−3, or have a constant VND of 1016–16.5 m−3 regardless of OND. A large proportion of oxides (55–100%) touch vesicles. A more variable proportion of vesicles (16–72%) are in contact with oxides because of syn-explosive growth and coalescence. Group 2 samples have ONDs in the same broad range as group 1 samples. We also used new and published data to build a compilation of ONDs and VNDs of five series of experimental decompression of rhyolitic and phonolitic melts. In samples with OND ≈ VND, most bubbles are in contact with more than one oxide and 64–88% of the oxides are in contact with bubbles. Such high levels of connectivity suggest that the role of oxides in controlling bubble nucleation has been underestimated. When VND ≥ OND, nucleation densities of experimental vesicles can be reproduced by heterogeneous nucleation models, which we used to calculate syn-explosive decompression rates from VNDs at Merapi, Soufriere Hills, and Kilian. These rates and textural evidence suggest that the decompression front accompanying these Vulcanian and sub-Plinian explosions is responsible for syn-explosive bubble nucleation. We calculated the average pre-explosive ascent rates necessary to yield the observed ONDs at Soufriere Hills and Merapi volcanoes. The resulting rates, 0.005–20 m/s, overlap considerably with the range of critical ascent rate inferred for the effusive–explosive transition, supporting the hypothesis that this transition is primarily controlled by oxide microlites in the conduit because oxides are a primer for explosive behavior when present in sufficiently high number densities. Focusing on the older eruption of Kilian, for which no observation is available, we infer that pre-explosive ascent rates of >7 × 10−3 m/s were necessary for explosive behavior to occur.

中文翻译：

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氧化物在上升岩浆浅层水泡中的作用

摘要 尽管通常体积分数较低，但 Fe Ti 氧化物有可能极大地影响喷发方式，因为它们降低了异质气泡成核的过饱和压力。一旦成核，气泡就会对压力变化做出快速反应，从而促进快速膨胀和爆炸行为。然而，爆炸性产品的数据中通常没有氧化物微晶量化。我们使用新的、重新分析的和已发布的数据来构建粗体和钙碱性产品的氧化物数密度 (OND) 和囊泡数密度 (VND) 的汇编。选择了四种喷发方式：1) 蒙特塞拉特苏弗里耶尔山火山、智利拉斯卡火山和法国基利安火山的火山爆发，2) 火山爆发。Pelee 火山、小安的列斯群岛、美国圣海伦斯火山和印度尼西亚的默拉皮火山，3）默拉皮火山的亚普林尼亚爆炸，以及 4）苏弗里耶尔山和山间歇性坍塌的熔岩穹窿喷流。皮利火山。天然样品根据每个事件产物的主要质地分为两组：1) 爆炸产生的具有强烈垂直分量的泡状浮石碎屑和 2) 圆顶坍塌事件和侧向爆炸产生的具有直向结构的致密碎屑。第 1 组样本要么沿 VND 和 OND 之间的 1:1 趋势分布，范围从 1015 到 1017 m-3，要么具有 1016-16.5 m-3 的恒定 VND，而与 OND 无关。大部分氧化物 (55–100%) 接触囊泡。由于同爆生长和聚结，更多可变比例的囊泡 (16-72%) 与氧化物接触。第 2 组样品具有与第 1 组样品相同的广泛范围的 OND。我们还使用新的和已发表的数据构建了五个系列的流纹岩和音岩熔体实验减压的 OND 和 VND 汇编。在 OND ≈ VND 的样品中，大多数气泡与一种以上的氧化物接触，64-88% 的氧化物与气泡接触。如此高的连通性表明氧化物在控制气泡成核方面的作用被低估了。当 VND ≥ OND 时，实验囊泡的成核密度可以通过异质成核模型重现，我们用它来计算 Merapi、Soufriere Hills 和 Kilian 的 VND 的同爆减压率。这些速率和结构证据表明，伴随这些瓦肯尼阶和亚普林阶爆发的减压前沿是同爆气泡成核的原因。我们计算了在 Soufriere Hills 和 Merapi 火山产生观察到的 OND 所需的平均爆炸前上升速率。所得速率 0.005-20 m/s 与为喷出-爆炸过渡推断的临界上升速率范围有很大重叠，支持这种过渡主要由管道中的氧化物微晶控制的假设，因为氧化物是爆炸物的引物以足够高的数密度存在时的行为。关注 Kilian 较早的喷发，对此没有观测资料，我们推断 >7 × 10−3 m/s 的爆炸前上升速率是发生爆炸行为所必需的。与为渗出-爆炸转变推断的临界上升速率范围相当重叠，支持这种转变主要由管道中的氧化物微晶控制的假设，因为当氧化物以足够高的数量密度存在时，氧化物是爆炸行为的引物。关注 Kilian 较早的喷发，对此没有观测资料，我们推断 >7 × 10−3 m/s 的爆炸前上升速率是发生爆炸行为所必需的。与为渗出-爆炸转变推断的临界上升速率范围相当重叠，支持这种转变主要由管道中的氧化物微晶控制的假设，因为当氧化物以足够高的数量密度存在时，氧化物是爆炸行为的引物。关注 Kilian 较早的喷发，对此没有观测资料，我们推断 >7 × 10−3 m/s 的爆炸前上升速率是发生爆炸行为所必需的。