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

富含二氧化硅和挥发物的安山岩和英安岩岩浆的起源是了解大陆地壳如何演变，世界上有多少活火山生长，以及充分认识这些潜在危险火山所造成的危害的基础问题。该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。该项目将重点关注危地马拉的圣玛丽亚火山，这是一座相对简单的火山，其历史与圣海伦火山相似，包括1902年爆发了超过8立方公里的英安质火山灰和浮石，随后是数十年的熔岩圆顶挤出。通过将历史喷发记录与玄武岩熔岩流化学变化的准确记录联系起来，我们的研究将提供一个前所未有的长期评估，在一个典型的俯冲带火山的一生中，玄武岩-安山岩-英安岩岩浆演化。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。该研究项目的动机是最近的理论和数值模拟，研究岩浆在地壳深处数十公里时如何在其周围的岩石中冷却。这些模型预测，与传统的岩浆在火山下浅层短暂储存的想法不同，由地幔融化形成的玄武岩岩浆可能会被困在地壳下层数千年，缓慢地失去热量，在上升到地表之前结晶形成富含挥发物的英安岩。在威斯康星大学麦迪逊分校（University of Wisconsin-Madison）的实验室里，该团队在几个分析前沿领域取得了进展，包括：用40Ar/39Ar的钾氩衰变钟来测定非常年轻的熔岩流的年龄，用钍和铀的短寿命放射性同位素来确定岩浆在爆发前多久开始结晶，用二次离子质谱法来测量结晶矿物中熔体的微小包裹体中水和二氧化碳的浓度。利用这些方法对圣玛丽亚火山的熔岩流和火山灰沉积物进行分析，将使我们能够追踪火山和下面岩浆体的生长速度，英安岩岩浆开始失去热量和结晶的深度，以及是否有一批全新的幔源玄武岩岩浆进入地壳，引发了1902年10月造成数千人死亡的大规模喷发。此外，在危地马拉和其他中美洲国家，几十座类似的大型火山在过去的几千年里爆发过，或者今天仍在活跃，使成千上万附近的人处于危险之中。更好地了解控制这些火山长期生长的过程，特别是富含挥发物的英安质岩浆是如何形成的——可能深埋了数万年——并在几乎没有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民间当局的合作，并让一名研究生参与其中，该研究生将攻读博士学位，同时获得现场方法、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.