Magma Residence and Recharge in Thick Continental Crust: Volcan Maipo, Andean Southern Volcanic Zone

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。该项目旨在确定大陆地壳在过去约86000年在Volcán Maipo喷发的一套熔岩的形成过程中的性质，该火山是安第斯山脉南部火山带（SVZ）最北端的火山之一。自公元1600年以来发生的六次最严重的火山灾害中，有五次发生在俯冲带，包括1985年北安第斯山脉的内华达德尔鲁伊斯火山喷发，造成2.5万人死亡。安第斯山脉SZV最近的火山活动包括智利的Llaima和chait<e:1>的喷发。在休眠了一万年之后，chaitan火山的爆发迫使4000多人撤离，这提醒我们必须时刻注意安第斯山脉的火山活动。Volcán迈波距离人口稠密的智利圣地亚哥和阿根廷门多萨不到150公里，这座火山系统古代喷发的火山灰构成了现代城市圣地亚哥的基础。火山本身位于一个受欢迎的国家公园内，距离一条国际天然气管道不到10公里。高人口密度、对公共设施的威胁以及历史上的火山活动使得对这座火山的研究非常有意义。了解大陆地壳在改变俯冲带岩浆的组成和运移中的作用，将为了解喀斯喀特火山所造成的危害提供信息，喀斯喀特火山目前威胁着包括西雅图和波特兰在内的美国西北部主要大都市地区。这项拟议中的研究将资助一名博士生三年，帮助培养新一代美国科学家。这项研究还支持了一名早期职业生涯的美国女科学家和一名资深阿根廷女科学家之间的合作。一般认为，岩浆是在俯冲带由地幔楔的流体熔融作用产生的，形成富镁玄武岩和玄武岩安山岩。然而，在俯冲带喷发的大部分熔岩是安山岩到英安岩的演化。提出的工作将检验这样一种假设，即大部分化学分异发生在下地壳热区，随后在喷发前的上地壳岩浆室中，通过岩浆补给的重复循环，某种程度的持续分异发生。研究的具体假设为：1)地幔源玄武岩岩浆向玄武岩安山岩分异发生在地壳下部；2)在地壳深部热区，石榴石和斜长石的分异结晶作用使玄武岩安山岩向英安岩不断分化；3)岩浆在地壳浅室中结晶，是通过多次来自下地壳系统的新鲜岩浆的补给，直到火山喷发。从玄武岩安山岩到英安岩的一整套岩石的主元素、微量元素和同位素数据将有助于验证假设(1)和假设(2)，而斜长石斑晶的主元素、微量元素和Sr同位素分带的原位分析将有助于验证假设(3)，同时也有助于确定发生在地壳浅岩浆房和地壳深部热区的分化的相对数量。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The proposed project seeks to determine the nature of continental crust involvement in the generation of a suite of lavas erupted over the past ~86 thousand years at Volcán Maipo, one of the northernmost volcanoes of the Southern Volcanic Zone (SVZ) of the Andes. Five of the six worst volcanic disasters that have occurred since 1600 AD have occurred at subduction zones, including the 1985 eruption of Nevado del Ruiz in the Northern Andes that claimed the lives of 25,000 people. Recent volcanic activity in the Andean SZV includes eruptions at both Llaima and Chaitén in Chile. The eruption of Chaitén after ten thousand years of dormancy necessitated the evacuation of more than 4,000 people, reminding us that we must be mindful of volcanic processes in the Andes at al times. Volcán Maipo is located within 150 km of the populous cities of Santiago del Chile and Mendoza, Argentina, and the ash from an ancient eruption of this volcanic system underlies the modern city of Santiago. The volcano itself lies within a popular national park, and less than 10 km from an international gas pipeline. The combination of high population density, threat to public utilities, and historic volcanic activity make study of this volcano acutely pertinent. Understanding the role of continental crust in modifying the composition and transport of magma at subduction zones will provide information that may be of use in understanding the hazards posed by the Cascades volcanoes, which currently threaten major metropolitan areas of the American Northwest including Seattle and Portland. The proposed study will support a PhD student for 3 years, helping to prepare a new generation of American scientists. The study also supports collaboration between an early-career female US scientist and a senior female Argentine scientist.It is generally understood that magmas are generated at subduction zones by fluid-fluxed melting of the mantle wedge to generate Mg-rich basalts and basaltic andesites. However, the bulk of the lavas erupted at subduction zones are evolved andesites to dacites. The proposed work will test the hypothesis that the bulk of chemical differentiation occurs in a lower crustal hot zone, followed by some degree of continued differentiation through repeated cycles of magma recharge in an upper crustal magma chamber prior to eruption. The specific hypotheses to be addressed are: 1) Differentiation from mantle-derived basaltic magma to fractionated basaltic andesite occurs in the lowermost crust; 2) Continuous differentiation from basaltic andesite to dacite proceeds in the deep crustal hot zone by fractional crystallization of garnet and plagioclase; 3) Magma crystallizes in the shallow crustal chamber through multiple episodes of recharge by fresh magma from the lower crustal system until the time of eruption. Whole rock major element, trace element, and isotopic data for a suite of rocks ranging from basaltic andesites to dacites will help to address hypotheses (1) and (2), and in situ analysis of major element, trace element, and Sr isotope zonation in plagioclase phenocrysts will help to address hypothesis (3), while also helping to determine the relative amounts of differentiation taking place in the shallow crustal magma chamber and the deep crustal hot zone.