Breadcrust bombs are pyroclasts displaying fractured, dense surfaces enveloping expanded interiors, and are associated with Vulcanian explosions. We document pyroclasts from the 2008–2009 CE eruption of Chaitén (Chile) that are internally as well as externally breadcrusted. The pyroclasts are cut by intersecting micrometer-to millimeter-thick tuffisites with dense glassy walls, which grade into strongly inflated pumiceous material. We find H2O diffusion gradients proximal to the breadcrusted surfaces, such that H2O is depleted from far-field magma (0.68 ± 0.04 wt%) into dense, fractured vein walls (0.2–0.3 wt%), indicating a spatial association between H2O mass transfer within the pyroclast interior and both suppressed vesiculation and breadcrusting. We experimentally confirm that diffusive H2O depletion suppresses bubble growth at shallow conduit conditions. Therefore, we interpret the breadcrust formation to be induced by H2O diffusion and the associated rise in viscosity rather than by cooling in the classical breadcrust-formation models. We posit that a “dehydration quench” is important as degassing continues to very low H2O contents in shallowconduit magma that continues to vesiculate.

中文翻译：

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中面包危机：流纹岩火山碎屑的内部面包皮表面显示脱水淬火

面包屑炸弹是火山碎屑，显示出断裂的、致密的表面包裹着膨胀的内部，并且与火山爆发有关。我们记录了 2008 年至 2009 年 CE 爆发的柴滕（智利）的火山碎屑，这些火山碎屑在内部和外部都有面包屑。火山碎屑通过与密集的玻璃壁相交的微米至毫米厚的凝灰岩切割，这些凝灰岩分级成强烈膨胀的浮石材料。我们发现靠近面包结皮表面的 H2O 扩散梯度，使得 H2O 从远场岩浆 (0.68 ± 0.04 wt%) 耗尽到致密的破裂静脉壁 (0.2-0.3 wt%)，表明 H2O 传质之间存在空间关联在火山碎屑内部，并且都抑制了水泡和面包结皮。我们通过实验证实，扩散的 H2O 消耗抑制了浅管道条件下的气泡生长。因此，我们解释面包皮形成是由 H2O 扩散和相关的粘度升高引起的，而不是经典面包皮形成模型中的冷却引起的。我们假设“脱水淬火”很重要，因为脱气继续使浅管道岩浆中的 H2O 含量非常低，并继续形成泡状。