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

躺着等待？

• 批准号: 0738007
• 负责人: Bradley Singer
• 金额: $ 30.64万
• 依托单位: University of Wisconsin-Madison
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-01-01 至 2011-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0738007&HistoricalAwards=false
• 关键词: Lying; Wait; Evolution; Dacite; Beneath

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.

富含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一个基本问题。 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利亚火山，这是一座相对简单的火山，其历史反映了圣海伦斯山，包括1902年超过8立方公里的英安岩灰和浮石的爆炸性喷发，随后是数十年的熔岩圆顶挤出。 通过将历史喷发的记录与玄武岩熔岩流中化学变化的日期明确的记录联系起来，这些熔岩流在72，000年至36，000年前喷发形成了12，400英尺高的火山，我们的研究将提供一个前所未有的长期评估在典型俯冲带火山的生命周期内，玄武安山岩英安质岩浆的演化。 这是一个独特的机会，不仅可以量化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，即岩浆在地壳内数十公里深处时如何在周围岩石中冷却。 这些模型预测，与传统的岩浆在火山下方的浅层短暂储存的想法不同，在地幔中熔化形成的玄武岩浆可能会被困在下地壳中数千年，缓慢失去热量，并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的实验室中，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40 Ar/39 Ar变体对非常年轻的熔岩流进行年龄测定，应用钍和铀的短寿命放射性同位素来确定岩浆在喷发前多久开始结晶，以及二次离子质谱法来测量结晶矿物中熔融物的微小内含物中的水和二氧化碳的浓度。 使用这些方法来研究圣玛利亚火山的熔岩流和火山灰沉积物的分析将使我们能够跟踪火山和底层岩浆体生长的速度，英安岩岩浆开始失去热量和结晶的深度，以及是否一批全新的地幔玄武岩岩浆进入地壳引发了10月份造成数千居民死亡的大规模喷发，1902. 此外，在危地马拉和其他中美洲国家，几十座类似的大火山在过去几千年里爆发，或者今天仍然活跃，使附近成千上万的人处于危险之中。 更好地了解控制这些火山长期增长的过程，特别是富含挥发性的英安岩岩浆如何形成-也许深隐藏了数万年-以及在几乎没有警告的情况下猛烈喷发，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政当局的合作，并涉及一名研究生，他将攻读博士学位，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。