The Pudahuel Ignimbrite is a rhyolitic, crystal poor deposit associated with the formation of the Diamante caldera in the Southern Volcanic Zone of the Andes. This Quaternary ignimbrite is the result of a massive eruption in what is now a densely populated area. The goal of this study is to understand the origin of this caldera-forming eruption by determining the pre-eruptive magmatic conditions through analytical and experimental approaches. The FeTi oxide geothermometer and the plagioclase hygrometer were used to determine the temperature-X conditions that produce the glass, feldspar, and FeTi oxide compositions observed in the natural sample. The results were combined with a composition-dependent water solubility model to constrain the magma storage pressure. This methodology was extended to temperatures up to 850 °C to define a band of P-T-HO conditions that satisfy the natural plagioclase and glass compositional pairs, from 700 °C at 310 MPa to 850 °C at 85 MPa. Phase equilibrium experiments were performed at conditions within the band at HO-vapor saturated and undersaturated conditions using a representative pumice sample as the starting material. The purely analytical approach, based on FeTi oxide geothermometry and plagioclase hygrometry of the pumice phases, suggests pre-eruptive temperatures of 717 ± 7 °C, P of 200–360 MPa and between 6.9 and 7.6 wt% HO. The natural phase assemblage and the glass composition was best reproduced by experiments performed at low temperatures (700–750 °C) and high P (>275 MPa, more likely 300–350 MPa), with water contents of at least 6 wt%. A compositional match between synthetic and natural glasses occurred in some HO-undersaturated experiments, suggesting that the melt was not necessarily saturated with HO-rich fluid. Overall, our work highlights that the source region related to the caldera-forming Pudahuel ignimbrite was relatively deep (> 10 km). This depth, combined with the low crystallinity of the magma, and the possibility of an HO-undersaturated melt, suggest that caldera formation was likely initiated by an external trigger rather than internal pressurization due to volatile buildup in the reservoir. The storage conditions determined in this study lay the groundwork for continuing work to understand the Maipo volcanic complex and the future hazards it poses.

中文翻译：

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破火山口形成的火山喷发的储存条件：来自智利中部 Pudahuel 流纹岩火凝灰岩的见解（南纬 32° 10′）

Pudahuel Ignimbrite 是一种流纹岩、贫晶矿床，与安第斯山脉南部火山带的 Diamante 破火山口的形成有关。这种第四纪火山灰岩是现在人口稠密地区大规模喷发的结果。这项研究的目的是通过分析和实验方法确定喷发前的岩浆条件，以了解火山口形成喷发的起源。FeTi 氧化物地温计和斜长石湿度计用于确定产生在天然样品中观察到的玻璃、长石和 FeTi 氧化物成分的温度-X 条件。结果与成分相关的水溶性模型相结合，以限制岩浆储存压力。该方法扩展到高达 850 °C 的温度，以定义满足天然斜长石和玻璃成分对的 PT-HO 条件范围，从 310 MPa 下的 700 °C 到 85 MPa 下的 850 °C。使用代表性浮石样品作为起始材料，在 H2O2 蒸气饱和和不饱和条件下进行相平衡实验。基于 FeTi 氧化物地温测量和浮石相斜长石湿度测量的纯分析方法表明，喷发前温度为 717 ± 7 °C，P 为 200–360 MPa，H2O 含量在 6.9 至 7.6 wt% 之间。自然相组合和玻璃成分最好通过在低温（700-750°C）和高P（>275 MPa，更有可能300-350 MPa）、含水量至少为6 wt%的条件下进行的实验来再现。在一些 H2O 不饱和实验中，合成玻璃和天然玻璃之间的成分匹配，表明熔体不一定被富含 H2O 的流体饱和。总体而言，我们的工作强调，与形成火山口的 Pudahuel 火凝灰岩有关的源区相对较深（> 10 公里）。这个深度，加上岩浆的低结晶度，以及 H2O 不饱和熔体的可能性，表明破火山口的形成可能是由外部触发因素引发的，而不是由于储层中挥发物积聚而导致的内部加压。本研究确定的储存条件为继续了解迈坡火山群及其未来造成的危害奠定了基础。