The dynamics of magma storage and ascent beneath an active arc volcano (Villarrica, Chile)

沿着收敛的板块边缘形成的火山弧产生了地球上大部分的陆上火山活动，也是历史上规模最大、最具破坏性的火山爆发的原因。在弧形处，俯冲板块上方的地幔中产生的岩浆上升穿过岩石圈，在那里被储存和化学处理，然后再进入地表喷发，通常是大型平流层火山。了解控制这些岩浆何时以及如何最终离开其地壳储层喷发的因素，以及喷发可能的规模，仍然是火山学的中心目标，并且应该有助于更准确地预测未来的喷发。该项目将采用地球化学和岩石学方法，对智利南部（比利亚里卡）一座弧型火山喷发的岩浆的喷发前过程、储存条件和/或起源进行研究，该火山曾产生过多次不同规模的冰川后喷发。维拉里卡火山是智利最活跃的火山之一，也是南部火山带（SVZ）的一部分。沿SVZ的火山活动受到构造的强烈影响，而Villarrica位于与区域走滑断层系统（Liquine Ofqui断裂带）相关的断裂系统上。这座火山还经历了广泛的晚更新世冰川作用，其峰顶地区仍然永久被冰覆盖。维拉里卡冰川后的活动从小体积喷发事件到大的形成火成岩的火山碎屑喷发不等。尽管火山喷发的幅度如此之大，但值得注意的是，这些事件中的大多数涉及相对基性岩浆，典型的是玄武岩-玄武岩-安山岩成分。因此，与那些较小的低强度喷发相比，维拉里卡的大爆发并不涉及经历了大量储存和/或分离的岩浆。同样值得注意的是，虽然较小的喷发通常在成分上彼此不同，但大喷发往往在成分上相对均匀，这意味着构成岩浆系统的离散熔体批次的有效混合，这些熔体批次为更常见的较小喷发提供了物质。此外，大地测量学研究表明，维拉里卡周围地表变形的最大范围（据推测与岩浆运动有关）与火山中心偏移了几公里，这意味着至少有一些岩浆在地壳中横向移动。这项研究将包括对维拉里卡火山喷发的岩浆生成和储存条件的比较研究。维拉里卡火山喷发的规模在小锥形成事件和大火山碎屑喷发之间变化很大。你将在智利进行实地考察，对几次火山喷发的沉积物进行取样，并选择涵盖尽可能广泛的火山喷发规模。在对这些样品进行岩石学表征后，您将开始一个旨在限制这些熔体起源以及爆发前储存条件的分析程序。

Volcanic arcs, formed along convergent plate margins, produce most of Earth's subaerial volcanic activity and are responsible for the largest and most devastating historical eruptions. At arcs, magma generated in the mantle above the subducting slab ascends through the lithosphere, where it is stored and chemically processed before feeding eruptions at the surface, typically from large stratovolcanoes. Understanding the factors that govern when and how this stored magma ultimately leaves its crustal reservoir(s) to be erupted, and what size that eruption is likely to be, remains a central goal of volcanology and should facilitate more accurate forecasts of future eruptions. This project will employ geochemical and petrological methods to constrain pre-eruptive processes, storage conditions and/or origins of magmas erupted from an arc volcano in southern Chile (Villarrica) that has produced numerous post-glacial eruptions of varying magnitude. Villarrica is one of Chile's most active volcanoes and forms part of the southern Volcanic Zone (SVZ) group of volcanoes. Volcanism along the SVZ has been strongly influenced by tectonics and Villarrica lies along a fracture system associated with a regional strike-slip fault system, the Liquine Ofqui fault zone. The volcano has also experienced extensive late-Pleistocene glaciation and its summit region remains permanently ice-covered. Postglacial activity at Villarrica has varied from small-volume eruptive events to large ignimbrite forming pyroclastic eruptions. Despite this range in eruption magnitude, it is notable that the majority of these events involve relatively mafic magmas, typically basaltic to basaltic-andesitic in composition. Large explosive eruptions of Villarrica do not therefore involve magmas that have undergone significant extents of storage and/or fractionation compared to those feeding smaller lower-intensity eruptions. It is also notable that while smaller eruptions typically vary in composition from each other, large volume eruptions tend to be relatively compositionally homogenous, implying efficient mixing of the discrete melt batches that constitute the magma system feeding the more common smaller eruptions. In addition, geodetic studies show that maximum extents of ground deformation around Villarrica, inferred to be linked to magma movement, are offset from the centre of the volcano by several kms, implying that at least some of the magmas feeding the volcano move laterally through the crust. This research will involve a comparative study of the conditions of magma generation and storage associated with eruptions of Villarrica that vary in size between small cone forming events to large pyroclastic eruptions. You will undertake fieldwork in Chile to sample deposits from several eruptions, selected to cover as wide a range of eruption magnitudes as possible. After petrographic characterization of these samples, you will embark on an analytical programme designed to constrain the origin of these melts as well as the condition of pre-eruptive storage.