The nature of sub-volcanic alteration is usually only observable after erosion and exhumation at old inactive volcanoes, via geochemical changes in hydrothermal fluids sampled at the surface, via relatively low-resolution geophysical methods or can be inferred from erupted products. These methods are spatially or temporally removed from the real subsurface and thus provide only indirect information. In contrast, the ICDP deep drilling of the Mt Unzen volcano subsurface affords a snapshot into the in situ interaction between the dacitic dykes that fed dome-forming eruptions and the sub-volcanic hydrothermal system, where the most recent lava dome eruption occurred between 1990 and 1995. Here, we analyse drill core samples from hole USDP-4, constraining their degree and type of alteration. We identify and characterize two clay alteration stages: (1) an unusual argillic alteration infill of fractured or partially dissolved plagioclase and hornblende phenocryst domains with kaolinite and Reichweite 1 illite (70)-smectite and (2) propylitic alteration of amphibole and biotite phenocrysts with the fracture-hosted precipitation of chlorite, sulfide and carbonate minerals. These observations imply that the early clay-forming fluid was acidic and probably had a magmatic component, which is indicated for the fluids related to the second chlorite-carbonate stage by our stable carbon and oxygen isotope data. The porosity in the dyke samples is dominantly fracture-hosted, and fracture-filling mineralization is common, suggesting that the dykes were fractured during magma transport, emplacement and cooling, and that subsequent permeable circulation of hydrothermal fluids led to pore clogging and potential partial sealing of the pore network on a timescale of ~ 9 years from cessation of the last eruption. These observations, in concert with evidence that intermediate, crystal-bearing magmas are susceptible to fracturing during ascent and emplacement, lead us to suggest that arc volcanoes enclosed in highly fractured country rock are susceptible to rapid hydrothermal circulation and alteration, with implications for the development of fluid flow, mineralization, stress regime and volcanic edifice structural stability. We explore these possibilities in the context of alteration at other similar volcanoes.

通常仅在老的不活火山被侵蚀和发掘后，通过相对低分辨率的地球物理方法，通过地表取样的热液的地球化学变化，才可以观察到亚火山蚀变的性质，或者可以从爆发产物中推断出亚火山蚀变的性质。这些方法在空间上或时间上都从真实地下移除，因此仅提供间接信息。相比之下，ICDP对Unzen山火山地下的深孔钻探提供了形成穹顶喷发的大堤与次火山热液系统之间原位相互作用的快照，其中最近的熔岩穹顶爆发发生在1990年至2005年之间。 1995年。在这里，我们分析了孔USDP-4中的钻芯样品，限制了它们的蚀变程度和类型。我们确定并表征了两个黏土蚀变阶段：（1）具有高岭石和Reichweite 1伊利石（70）-蒙脱石的断裂或部分溶解的斜长石和角闪石斑晶域的不寻常的藻化蚀变充填物（2）闪石和黑云母斑晶的质子化蚀变与亚氯酸盐，硫化物的裂缝导致的沉淀和碳酸盐矿物。这些观察结果表明，早期的粘土形成流体是酸性的，并且可能具有岩浆成分，根据我们稳定的碳和氧同位素数据，这表明与第二亚氯酸盐-碳酸盐阶段有关的流体。堤坝样品中的孔隙度主要为裂缝带，并且充填裂缝矿化作用很普遍，这表明堤坝是在岩浆运输，定位和冷却过程中破裂的，从最后一次喷发停止后约9年的时间尺度上，随后热液的渗透性循环导致了孔堵塞和潜在的孔网部分封闭。这些观察结果与证据相符，即中间的含晶体岩浆在上升和就位时容易破裂，使我们提出，封闭在高裂隙性乡村岩石中的弧形火山易受快速热液循环和蚀变的影响，这对开发具有重要意义。流体流动，矿化，应力状态和火山大厦的结构稳定性。我们在其他类似火山的变迁中探索了这些可能性。与证据相符的证据表明，含中间晶体的岩浆在上升和就位时容易破裂，导致我们提出，高裂隙围岩中封闭的弧形火山易受快速热液循环和蚀变的影响，对流体流动的发展有影响，矿化，应力状态和火山大厦的结构稳定性。我们在其他类似火山的变迁中探索了这些可能性。与证据相符的证据表明，含中间晶体的岩浆在上升和就位过程中容易破裂，导致我们提出，封闭在高度裂隙的乡村岩石中的弧形火山易受快速热液循环和蚀变的影响，对流体流动的发展具有影响，矿化，应力状态和火山大厦的结构稳定性。我们在其他类似火山的变迁中探索了这些可能性。应力状态和火山大厦的结构稳定性。我们在其他类似火山的变迁中探索了这些可能性。应力状态和火山大厦的结构稳定性。我们在其他类似火山的变迁中探索了这些可能性。