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

拟议中的研究重点是地球上火山活动最活跃的地区之一，太平洋上的Galápagos群岛。上一次大喷发发生在2009年4月的费尔南迪纳火山，它靠近一个假定的高温异常中心（也被称为热点或地幔柱）。地球物理和地球化学研究的结果表明，火山作用是地幔柱与一个主要的海洋扩张中心并置的结果。Galápagos扩张中心的薄岩石圈（~7 km）和羽流的高温（~1400oC）共同作用，导致了羽流上涌过程中大量的地幔熔融。Galápagos群岛是研究这种动态构造环境下复杂的质量传递和化学混合过程的理想场所，因为地幔柱和海洋扩张中心足够接近，可以相互作用，但仍然能够保持不同的物理实体。Galápagos群岛上火山活动的分布和组成与传统热点（如夏威夷）不同：没有线性的海洋岛屿链，活火山活动在远离羽流上升流带的地方广泛存在。最近的一项高分辨率区域地球物理研究（发表于2007年）显示了一个低地震速度的异常带，从热点向扩散中心延伸。异常在圣地亚哥下方~ 70 km处最大；位于群岛中心和烟羽边缘的一个岛屿。异常的原因尚不清楚；它可能代表温度的变化和/或一个强烈的地幔上涌和融化的区域。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。岛上最近的火山喷发分别发生在1795年、1897年和1904年。有限的地球化学资料表明，该岛西部火山喷发的熔岩与东部火山喷发的熔岩组成不同，距离小于20公里。其中一些变化可能是次火山岩浆房过程的结果，但在地幔融化过程中，容易分解到岩浆中的元素比例的大范围变化表明，该岛要么位于厚度从西到东变化很大的岩石圈之上，要么位于底层的对流地幔具有不同的组成。该提案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PI已经进行了系统的样本收集。这些熔岩的化学性质的数值模拟，连同其他Galápagos火山的已发表数据，将被用来限制底层地幔融化的成分、温度和压力。这些结果将与根据地震速度数据计算出的温度结合起来，利用新发表的方程，阐明在羽流上涌轴和扩散中心之间运行的物理过程，这些过程导致了广泛的火山活动。从地幔柱上升流区向伸展脊延伸的大量熔融体的存在，将对这两个动力系统并置时发生的质量运输的性质产生重要的限制。拟议的研究将建立在PI先前对地幔融化的研究基础上。它是新颖的，因为它将产生第一个综合地球化学和地球物理模型，以解释Galápagos群岛广泛活跃的火山活动的原因。这将有助于我们理解在难以接近的环境中（如大西洋中脊）火山活动的原因，并增加对确定过去地质中羽状脊相互作用发生的地方的限制。这反过来又将增加我们对全球范围内深部地幔和地球表面过程的认识。

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.