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

加拉帕戈斯群岛广泛活动火山活动的地球化学和地球物理约束

基本信息

批准号：
NE/H01053X/1
负责人：
Sally Gibson
金额：
$ 6.82万
依托单位：
University of Cambridge
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2010
资助国家：
英国
起止时间：
2010 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FH01053X%2F1
关键词：
Geochemical geophysical constraints causes widespread

项目摘要

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.

拟议的研究重点是地球上火山最活跃的地区之一，即太平洋的加拉帕戈斯群岛，最近一次大规模喷发发生在 2009 年 4 月的费尔南迪纳，该地区靠近假定的高温异常中心（也称为高温异常）。热点或地幔柱）地球物理和地球化学研究的结果表明，火山活动是地幔柱与主要海洋并置的结果。加拉帕戈斯群岛扩张中心的薄岩石圈（约 7 公里）与地幔柱的高温（约 1400 摄氏度）相结合，导致地幔大量融化，加拉帕戈斯群岛是研究的理想场所。在这种动态构造环境中发生的复杂的质量传递和化学混合过程，因为地幔柱和海洋扩张中心足够接近相互作用，但仍然能够加拉帕戈斯群岛火山活动的分布和组成与热点地区（如夏威夷等传统火山活动）不同：没有线性的海洋岛屿链，活跃的火山活动广泛分布在远离羽流上升区域的地方。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异常区，从热点向传播中心延伸。异常现象在圣地亚哥以下约 70 公里的深度处最为严重；位于群岛中心和羽流外围的岛屿。异常原因尚不清楚，可能代表温度和/或区域的变化。圣地亚哥是该群岛的第四大岛，但很少有关于该岛最近火山喷发的地球化学分析。 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<20公里，其中一些变化可能是由于次年火山喷发造成的。火山岩浆房过程，但在地幔熔化过程中容易分配到岩浆中的元素比例变化很大，表明该岛要么位于岩石圈之上，从西到东厚度变化很大，或者底层对流地幔具有不同的成分。该提案寻求资金对圣地亚哥熔岩进行高精度地球化学分析，PI 已经对这些熔岩的化学成分进行了数值模拟并已发表。其他加拉帕戈斯火山的数据将用于限制底层地幔熔化的成分、温度和压力，这些结果将与使用新发布的方程根据地震速度数据计算出的温度相结合，以阐明这一点。在地幔柱上升流轴和扩张中心之间进行的物理过程，导致广泛的火山活动，从地幔柱上升流区向扩张脊延伸的大量熔体的存在将对地幔柱的性质产生重要的限制。当这两个动态系统并置时发生的质量传输将建立在 PI 先前关于地幔熔化的研究的基础上。它是新颖的，因为它将产生第一个综合的地球化学和地球物理模型来解释广泛的原因。加拉帕戈斯群岛的活跃火山活动将有助于我们了解大西洋中脊等交通不便的地区火山活动的原因，并增加对过去地质中羽流与山脊相互作用发生位置的限制。反过来将增加我们对全球范围的深部地幔和地球表面过程的了解。