Volcán Quizapu, Chile, is an under-monitored volcano that was the site of two historical eruptions: an effusive eruption in 1846–1847 and a Plinian eruption in 1932, both of which discharged ∼5 km3 (dense rock equivalent) of lava and/or tephra. The majority of material erupted in both cases is trachydacite, nearly identical for each event. We present H2O-saturated, phase equilibrium experiments on this end-member dacite magma, using a pumice sample from the 1932 eruption as the main starting material. At an oxygen fugacity (fO2) of ∼NNO + 0·2 (where NNO is the nickel–nickel oxide buffer), the phase assemblage of An25–30 plagioclase + amphibole + orthopyroxene, without biotite, is stable at 865 ± 10 °C and 110 ± 20 MPa H2O pressure (PH2O), corresponding to ∼4 km depth. At these conditions, experiments also reproduce the quenched glass composition of the starting pumice. At slightly higher PH2O and below 860 °C, biotite joins the equilibrium assemblage. Because biotite is not part of the observed Quizapu phase assemblage, its presence places an upper limit on PH2O. At the determined storage PH2O of ∼110 MPa, H2O undersaturation of the magma with XH2Ofluid = 0·87 would align Ptotal to mineral-based geobarometry estimates of ∼130 MPa. However, XH2Ofluid < 1 is not required to reproduce the Quizapu dacite phase assemblage and compositions. A second suite of experiments at lower fO2 shows that the stability fields of the hydrous silicates (amphibole and biotite) are significantly restricted at NNO – 2 relative to NNO + 0·2. Additional observations of Quizapu lava and pumice samples support the existing hypothesis that rapid pre-eruptive heating drove the effusive 1846–1847 eruption, with important refinements. We demonstrate that microlites in the end-member dacite lavas are consistent with in situ crystallization (during ascent), rather than transfer from an andesite. In one end-member dacite lava, newly identified reverse zoning in orthopyroxene and incipient destabilization of amphibole are consistent with small degrees of heating. Our work articulates a clear direction for future Quizapu studies, which are warranted given the active nature of the Cerro Azul–Descabezado Grande volcanic axis.

中文翻译：

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智利基扎普火山下英安岩岩浆储存的实验限制

智利基萨普火山是一座监测不足的火山，曾发生过两次历史性喷发：1846-1847 年的一次喷发和 1932 年的一次普林尼式喷发，两次喷发都喷出约 5 平方公里（相当于致密岩石）的熔岩和/或拉面。在这两种情况下喷发的大部分物质都是粗英石，每次事件几乎相同。我们使用 1932 年喷发的浮石样品作为主要起始材料，对这种端元英安岩岩浆进行了 H2O 饱和相平衡实验。在 ∼NNO + 0·2 的氧逸度 (fO2) 下（其中 NNO 是镍-镍氧化物缓冲液），An25-30 斜长石 + 闪石 + 斜方辉石的相组合（不含黑云母）在 865 ± 10 °C 下稳定和 110 ± 20 MPa H2O 压力 (PH2O)，对应于约 4 km 深度。在这些条件下，实验还再现了起始浮石的淬火玻璃成分。在稍高的 PH2O 和低于 860 °C 时，黑云母加入平衡组合。因为黑云母不是观察到的 Quizapu 相组合的一部分，它的存在对 PH2O 设置了上限。在确定的约 110 MPa 的储存 PH2O 下，XH2Ofluid = 0·87 的岩浆的 H2O 欠饱和将使 Ptotal 与约 130 MPa 的基于矿物的地质气压估计值保持一致。然而，XH2Ofluid ＜1 不需要重现 Quizapu 英安岩相组合和成分。在较低 fO2 下的第二组实验表明，相对于 NNO + 0·2，水合硅酸盐（闪石和黑云母）的稳定性场在 NNO – 2 时受到显着限制。对 Quizapu 熔岩和浮石样本的额外观察支持了现有的假设，即快速喷发前的加热推动了 1846-1847 年的喷发，并进行了重要的改进。我们证明了端元英安岩熔岩中的微晶石与原位结晶（上升期间）一致，而不是从安山岩转移。在一个端元英安岩熔岩中，新发现的斜方辉石反向分区和闪石的初期不稳定与小程度的加热一致。我们的工作为未来的 Quizapu 研究指明了明确的方向，鉴于 Cerro Azul-Descabezado Grande 火山轴的活跃性，这是有必要的。我们证明了端元英安岩熔岩中的微晶石与原位结晶（上升期间）一致，而不是从安山岩转移。在一个端元英安岩熔岩中，新发现的斜方辉石反向分区和闪石的初期不稳定与小程度的加热一致。我们的工作为未来的 Quizapu 研究指明了明确的方向，鉴于 Cerro Azul-Descabezado Grande 火山轴的活跃性，这是有必要的。我们证明了端元英安岩熔岩中的微晶石与原位结晶（上升期间）一致，而不是从安山岩转移。在一个端元英安岩熔岩中，新发现的斜方辉石反向分区和闪石的初期不稳定与小程度的加热一致。我们的工作为未来的 Quizapu 研究指明了明确的方向，鉴于 Cerro Azul-Descabezado Grande 火山轴的活跃性，这是有必要的。