Geochemical and geophysical constraints on the causes of widespread active volcanism in the Galápagos Archipelago

这项拟议的研究重点放在地球上火山最活跃的地区之一--太平洋的加拉帕戈斯群岛。上一次大喷发是在2009年4月，费尔南迪纳靠近假定的高温异常中心(也被称为热点或地幔热柱)。地球物理和地球化学研究结果表明，火山活动是地幔热柱与一个主要的海洋扩张中心并置的结果。加拉帕戈斯扩张中心的薄岩石圈(约7公里)和热柱的高温(约1400oC)的结合导致了随着热柱的上升而大量的地幔熔融。加拉帕戈斯群岛是研究在这一动态构造环境中发生的复杂的质量转移和化学混合过程的理想场所，因为地幔热柱和海洋扩展中心足够接近，可以相互作用，但仍然能够作为不同的物理实体存在。加拉帕戈斯群岛的火山活动的分布和组成不同于传统的热点地区(如夏威夷)：没有线性的海洋岛链，活动火山活动在远离羽流上升区的地方广泛存在。最近的一项高分辨率区域地球物理研究(2007年发表)显示了一个低地震速度的异常区，从热点向扩张中心延伸。最大的异常是在圣地亚哥以下约70公里的深度；圣地亚哥是一个位于群岛中心和羽流外围的岛屿。异常的原因尚不清楚；它可能代表温度的变化和/或强烈的地幔上涌和熔融地带。圣地亚哥是该群岛的第四大岛，但发表的地球化学分析很少。该岛最近一次火山喷发发生在1795年、1897年和1904年。有限的地球化学数据表明，该岛西部火山喷发的熔岩与东部火山喷发的熔岩成分不同，距离约20公里。其中一些变化可能是次火山岩浆室作用的结果，但在地幔熔融过程中很容易分配到岩浆中的元素比例很大，这表明该岛要么位于厚度从西到东变化很大的岩石圈之上，要么表明下面的对流地幔具有不同的成分。这项提议寻求资金，对圣地亚哥的熔岩进行高精度的地球化学分析，在那里，国际和平研究所已经进行了系统的样本收集。这些熔岩的化学成分的数值模拟以及已公布的加拉帕戈斯其他火山的数据将被用来限制地幔下部熔融的成分、温度和压力。结果将与根据地震速度数据计算的温度结合起来，使用新发表的方程，以揭示在烟柱上升流轴和扩散中心之间运行的物理过程，从而导致广泛的火山活动。从地幔上升带向扩张脊延伸的大量熔体的存在，将对这两个动力系统并列时发生的质量传输的性质产生重要的限制。这项拟议的研究将建立在PI之前对地幔熔融的研究基础上。这是新的，因为它将产生第一个综合的地球化学和地球物理模型，以解释加拉帕戈斯群岛广泛存在的活火山活动的原因。这将有助于我们理解更难到达的环境中火山活动的原因，例如中大西洋海脊，并增加对确定过去地质历史中发生过羽流和海脊相互作用的地方的限制。这将反过来增加我们对全球规模的深部地幔和地球表面过程的了解。

The proposed research focuses on one of the most volcanically active regions on Earth, the Galápagos Archipelago in the Pacific Ocean. The last major eruption was in April 2009 at Fernandina which is close to the postulated centre of a high-temperature anomaly (known also as a hot spot or mantle plume). Results of geophysical and geochemical studies have shown that the volcanism results from the juxtaposition of a mantle plume with a major oceanic spreading centre. The combination of thin lithosphere (~7 km) at the Galápagos Spreading Centre and high temperature (~1400oC) of the plume results in a large amount of mantle melting as the plume upwells. The Galápagos Archipelago is an ideal place to study the complex mass-transfer and chemical-mixing processes that occur in this dynamic tectonic setting because the mantle plume and oceanic spreading centre are sufficiently close to interact but still able to remain as distinct physical entities. The distribution and composition of volcanism in the Galápagos archipelago is different to that associated with conventional hotspots (such as Hawaii): there is no linear chain of ocean islands and active volcanism is widespread away from the zone of plume upwelling. A recent high-resolution regional geophysical study (published in 2007) has shown an anomalous zone of low seismic velocity, extending away from the hot spot towards the spreading centre. The anomaly is greatest at a depth of ~ 70 km beneath Santiago; an island located in the centre of the archipelago and on the periphery of the plume. The cause of the anomaly is unknown; it may represent a change in temperature and/or a zone of intense mantle upwelling and melting. Santiago is the fourth largest island in the archipelago but there are very few published geochemical analyses. The most recent volcanic eruptions on the island were in 1795, 1897 and 1904. The limited geochemical data indicates that lavas erupted from volcanoes in the west of the island are of different composition to those in the east, a distance of <20 km. Some of this variation may be a consequence of sub-volcanic magma chamber processes but large ranges in ratios of elements which readily partition in to magmas during mantle melting suggest that the island is located either above lithosphere that varies dramatically in thickness, from west to east, or that the underlying convecting mantle has a varied composition. This proposal seeks funding to undertake high-precision geochemical analyses on lavas from Santiago where systematic sample collection has already been done by the PI. Numerical modelling of the chemistry of these lavas together with published data for other Galápagos volcanoes will be used to constrain the composition, temperature and pressure of melting of the underlying mantle. The results will be combined with temperatures calculated from seismic velocity data, using newly published equations, to shed light on the physical processes that are operating between the axis of plume upwelling and the spreading centre, causing widespread volcanism. The presence of a large body of melt extending from the zone of mantle-plume upwelling towards the spreading ridge would place important constraints on the nature of mass transport that occurs when these two dynamic systems are juxtaposed. The proposed research will build on the PI's previous research on mantle melting. It is novel because it will produce the first integrated geochemical and geophysical model to account for the causes of widespread active volcanism in the Galápagos Archipelago. This will aid our understanding of the causes of volcanic activity in much less accessible settings, such as the Mid-Atlantic Ridge, and increase constraints on identifying where plume-ridge interactions have occurred in the geological past. This will in turn will increase our knowledge of global-scale deep-mantle and Earth surface processes.