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

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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.