Lying in Wait? Evolution of Dacite Beneath Santa Maria Volcano, Guatemala

富含二氧化硅和挥发分的安山岩和英安质岩浆的起源是一个基本的问题，对于理解大陆地壳如何演化，世界上有多少活火山成长，以及充分认识这些潜在危险的火山所构成的危险是一个基本的问题。这个项目的主要目的是提高我们对这种岩浆起源和演化的物理和化学过程的理解。该项目将专注于危地马拉的圣玛丽亚火山，这是一座相对简单的火山，其历史与圣海伦斯火山的历史如出一辙，包括1902年超过8立方公里的英安质火山灰和浮岩的爆炸性喷发，以及随后数十年的熔岩穹顶挤出。通过将历史性喷发的记录与玄武岩熔岩流中化学变化的日期记录联系起来，我们的研究将提供对典型俯冲带火山一生中玄武岩-安山岩-英安岩岩浆演化的前所未有的长期评估。玄武岩熔岩流在72,000至36,000年前喷发，形成了12,400英尺高的火山。这是一个独特的机会，不仅可以量化导致1902年灾难性喷发的过程，还可以量化这些过程的时间尺度和动力学。研究项目的动机是最近的理论和数值模拟，即岩浆在地壳深处数十公里处是如何在周围岩石中冷却的。这些模型预测，与传统的岩浆短暂储存在火山下方浅层的想法不同，由地幔熔融形成的玄武岩岩浆可能会被困在下地壳数千年，慢慢失去热量，在上升到地表之前结晶形成富含挥发性物质的英安岩。在威斯康星大学麦迪逊分校的实验室里，该团队在几个分析方面取得了进展，包括：使用钾-氩衰变时钟的40Ar/39Ar变种对非常年轻的熔岩流进行年龄测定，应用短暂的钍和铀放射性同位素来确定岩浆在喷发前多久开始结晶，以及二次离子质谱仪测量结晶矿物中捕获的熔体微小包裹体中水和二氧化碳的浓度。使用这些方法研究圣玛丽亚火山的熔岩流和火山灰沉积的分析将使我们能够追踪火山和下面的岩浆体增长的速度，英安岩浆开始失热和结晶的深度，以及是否全新的一批源自地幔的玄武岩岩浆进入地壳引发了1902年10月导致数千名居民死亡的大规模喷发。此外，在危地马拉和其他中美洲国家，数十座类似的大型火山在过去数千年里喷发，或今天仍在活动，将附近数万人置于危险之中。更好地了解控制这些火山长期增长的过程，特别是如何形成富含挥发性物质的英安岩浆--可能隐藏了数万年之久--并在没有任何预警的情况下猛烈喷发，是一个首要目标。作为该项目的一部分，研究人员将促进与危地马拉火山学家和文职当局的合作，并让一名研究生参与进来，该研究生将攻读博士学位，同时获得实地方法、实验室分析和科学交流方面的专业知识。

The origin of silica- and volatile-rich andesitic and dacitic magma is a question fundamental to the understanding of how continental crust evolves, how many of the world's active volcanoes grow, and for fully appreciating the hazards posed by these potentially dangerous volcanoes. The main aim of this project is to improve our understanding of the physical and chemical processes by which this kind of magma originates and evolves. This project will focus on Santa Maria Volcano, Guatemala, a relatively simple volcano whose history mirrors that of Mount St. Helens, including explosive eruption of more than 8 cubic kilometers of dacitic ash and pumice in 1902, followed by decades of lava dome extrusion. By linking the record of historic eruptions to a well-dated record of chemical changes in the basaltic lava flows that erupted to form the 12,400 foot high volcano between 72,000 and 36,000 years before the present, our study will provide an unprecedented long-term evaluation of basaltic-andesitic-dacitic magma evolution over the lifetime of a typical subduction zone volcano. This is a unique opportunity to quantify not only the processes responsible for the cataclysmic 1902 eruption, but the time scales and kinetics of these processes as well.The research project is motivated by recent theoretical and numerical modeling of how magma cools within its surrounding rocks when lodged tens of kilometers deep within the crust. These models predict that, rather than the conventional idea of magma stored briefly at shallow depths below the volcano, basaltic magma formed by melting in the Earth's mantle may become trapped within the lower crust for thousands of years, lose heat slowly and crystallize to form volatile-rich dacite before ascending to the surface. In laboratories at the University of Wisconsin-Madison, the team has made advances on several analytical fronts including: age dating of very young lava flows using the 40Ar/39Ar variant of the potassium-argon decay clock, application of short-lived radioactive isotopes of thorium and uranium to determine how long before eruption a magma begins to crystallize, and secondary ion mass spectrometry to measure the concentration of water and carbon dioxide in tiny inclusions of melt trapped within the crystallizing minerals. Analyses using these approaches to study lava flows and ash deposits from Santa Maria Volcano will enable us to track the rates at which the volcano and underlying magma bodies grew, the depth at which the dacite magma began to lose heat and crystallize, and whether, or not, the arrival of a brand new batch of mantle-derived basaltic magma into the crust triggered the massive eruption that killed several thousand inhabitants in October, 1902. Moreover, in Guatemala and other Central American countries, dozens of similarly large volcanoes have erupted in the last few thousand years, or are active today, putting tens of thousands of nearby people at risk. Understanding better the processes that control long-term growth of these volcanoes, and in particular how volatile-rich dacitic magma forms - perhaps deeply hidden for tens of millenia - and erupts violently with little warning, is an overarching goal. As part of this project, the researchers will catalyze collaboration with Guatemalan volcanologists and civilian authorities and involve a graduate student who will pursue the PhD while acquiring expertise in field methods, laboratory analysis, and scientific communication.