Collaborative Research: Rear-arc Magmatism in the Northern Volcanic Zone of the Andes: Processes and Timescales in a Continental Rear-arc Setting

这项研究的首要目标是研究南美安第斯山脉北部火山带(NVZ)的弧后岩石成因过程。NVZ是这项研究的理想之选，因为它在许多方面与其他研究得很好的弧线相似，但它表现出NVZ独有的特征。这项研究很重要，因为我们有机会了解南美洲从主要受俯冲影响的熔融向MORB型成岩机制过渡的过程。除了主要目标外，还将评估弧后活火山(El Reventado)的岩浆上升和脱气的时间尺度，这对于限制弧后区域岩浆形成的时间尺度是重要的。新的数据集将用于测试现有的弧后岩浆作用模型，同时开发可应用于未来弧后研究的新模型。拟议的研究将包括利用实地研究、分析工作和地球化学建模相结合的方式对厄瓜多尔弧后火山进行区域深入的地球化学研究。岩石成因过程将利用主量元素和微量元素结合放射性同位素(铅、锶、钕)和铀系列在整个岩石和矿物分离上进行表征。将特别注意矿物，以表征熔体包裹体和揭示精细岩石成因指纹的成分变化。时间尺度(即结晶和脱气速率、年龄)将通过测量矿物分离物和整个岩石中的238U-、230Th-、226Ra和210Ph-和210Po核素来确定，使用世界卫生组织以及怀俄明大学和爱荷华州大学的设施。我们将利用加州大学洛杉矶分校的电子探针和二次离子质谱仪设施，对矿物进行严格的表征。这些数据将被用来评估熔体形成的对比模型。这项研究对于了解安第斯大陆弧和一般弧的弧后过程的性质至关重要。NVZ的独特之处在于岩浆成分的多样性(钙碱性和碱性)，南美克拉通边界的后弧形位置，以及俯冲卡内基山脊的(可能)影响。与其他弧后火山的相似性为比较我们的数据和测试模型提供了坚实的基础。铀系核素在火成岩地球化学中的应用已被证明有助于确定弧形火山的熔融和分异时间，最近又用于确定熔融和分异过程。厄瓜多尔目前的这些活火山提供了一个机会，可以使用U系列核素来研究弧后环境中的熔融过程和岩浆时间表。该项目的更广泛影响包括向学生宣传地球科学、促进国际交流和强调科学对社会的重要性。将互动网站作为实地研究的一部分，将把实地体验直接带到美国和厄瓜多尔的课堂上，并为学生提供一种科学发现的感觉，这可能会对他们的生活产生长期和积极的影响。该项目有可能加强K-12学生的科学基础，对以西班牙裔为主的社区的少数族裔学生有很大影响。此外，这笔资金将支持一名研究生，并为加州州立大学洛杉矶分校的一名初级教员提供资金。

The overarching goal of this research is to investigate rear-arc petrogenetic processes in the Northern Volcanic Zone (NVZ) of the South American Andes. The NVZ is ideal for this study because it is similar in many ways to other well-studied arcs, yet it exhibits characteristics that are unique to the NVZ. This research is important because we have the opportunity to learn about the processes that mark the transition from dominantly subduction-influenced melting to the regime of MORB-type petrogenesis in South America. In addition to the primary objective, timescales will be evaluated for magma ascent and degassing in an active, calc-alkaline rear-arc volcano (El Reventador), which is important for constraining the timescales of magmagenesis in the rear-arc region. The new data set will be used to test existing models of rear-arc magmatism, while developing new models that can be applied to future arc research.The proposed research will comprise a regional, in-depth, geochemical study of Ecuadorian rear-arc volcanoes using a combination of field research, analytical work and geochemical modeling. Petrogenetic processes will be characterized using major and trace elements combined with radiogenic isotopes (Pb, Sr, Nd) and U-series on whole rocks and mineral separates. Special attention will be given to minerals in order to characterize melt inclusions and compositional variation that reveal fine-scale petrogenetic fingerprints. Timescales (i.e. crystallization and degassing rates, ages) will be determined by measuring 238U-, 230Th-, 226Ra, and 210Pb- and 210Po nuclides in mineral separates and whole rocks, using facilities at WHOI and the Universities of Wyoming and Iowa. Rigorous characterization of minerals and will be accomplished using electron microprobe and secondary ion mass spectrometry facilities at UCLA. These data will be used to evaluate contrasting models for melt formation.Intellectual Merit. This research is critical for understanding the nature of rear-arc processes in the Andean continental arc and arcs in general. The uniqueness of the NVZ lies in the compositional diversity of magmas (calc-alkaline and alkaline), the location of the rear-arc at the boundary of the South American craton, and the (possible) influence of the subducting Carnegie Ridge. Similarities to other rear-arc volcanoes provide a strong foundation for comparing our data and testing models. The application of U-series nuclides to igneous geochemistry has proven useful for determining melting and differentiation timescales in arc volcanoes, and more recently for identifying melting and differentiation processes. These currently active volcanoes in Ecuador present an opportunity to use U-series nuclides to investigate melting processes and magmatic timescales in a rear-arc setting.Broader Impacts. The Broader Impacts of this project include promoting geoscience to students, fostering international communication and emphasizing the importance of science to society. Incorporation of an interactive website as part of the field research, will bring the field experiences directly into classrooms in the U.S. and Ecuador, and provide students a sense of scientific discovery that could have a long lasting and positive impact on their lives. This project has the potential to strengthen the scientific foundation of K-12 students, with a large impact on minority students in a dominantly Hispanic community. Additionally, this funding will support a graduate student and provide funding for a junior faculty member at California State University, Los Angeles.--