Testing Models of Magma Generation in Warm-slab Subduction Zones: A Case Study of Volatiles in the Cascade Arc

该项目的目标是更好地了解华盛顿州、俄勒冈州和加利福尼亚州北部喀斯喀特山脉火山下地球内部的岩浆是如何产生的。该地区的火山活动是由俯冲过程引起的，在俯冲过程中，太平洋西北部近海的洋壳沿着大陆边缘沉入地球？？S地幔。俯冲作用也是该地区可能发生的大地震的原因。随着下沉板块的升温，它释放出H2O和其他挥发性物质？元素(氯、S、氟、碳)，这些元素反过来导致覆盖的地幔岩石熔化，产生岩浆，为喀斯特火山提供养分。虽然俯冲过程发生在世界许多地区(例如安第斯山脉)，但级联系统的一个显著特征是，当俯冲洋壳开始俯冲时，它在地质上是年轻的。这会导致地壳变得更热，并在俯冲地壳仍然很浅的时候，在它位于火山之下之前，导致H2O和其他挥发物的释放。因此，主要的挑战是了解H2O和其他挥发物在形成喀斯喀特火山下面的岩浆中所起的作用，以及如果挥发物是重要的，它们是如何从俯冲的洋壳移回地幔产生岩浆的。这个项目将检验关于挥发分再循环和岩浆生成的一些基本观点。俯冲带，如瀑布弧形。该项目将涉及对原始瀑布岩浆中的H2O、其他挥发物、流体可移动元素以及稳定和放射性同位素示踪剂的新测量。许多测量将在熔体包裹体上进行，熔体包裹体是困在深处生长的斑晶中的硅酸盐熔体的小斑点。这种包裹体不受岩浆向地球S表面移动时的低压脱气作用的影响，因此提供了喷发前岩浆中溶解挥发分含量的记录。该项目中获得的数据将提供有关地幔和板岩温度、岩浆产生过程以及级联弧下挥发分来源的新信息。该项目涉及与美国、加拿大和日本的其他科学家合作，进行一些专门的化学分析。另一项合作工作将涉及对瀑布下的地幔楔形流动模式进行地球动力学建模，以便将新的地球化学数据与地球物理结果相结合。

The goal of this project is to better understand how magma is produced inside the Earth beneath the volcanoes in the Cascade mountains of Washington, Oregon, and northern California. Volcanism in this region is caused by the process of subduction, in which oceanic crust offshore of the Pacific northwest sinks back into the Earth?s mantle along the continental margin. Subduction is also responsible for the very large earthquakes that can occur in this region. As the sinking plate heats up, it releases H2O and other ?volatile? elements (Cl, S, F, C), which in turn cause the overlying mantle rocks to melt, producing magma that feeds the Cascade volcanoes. While the process of subduction occurs in many regions around the world (e.g., the Andes), a distinctive feature of the Cascades system is that the subducted oceanic crust is geologically young when it starts to subduct. This causes the crust to be hotter and should result in release of H2O and other volatiles when the subducted crust is still quite shallow, before it lies beneath the volcanoes. The main challenge, then, is to understand what role H2O and other volatiles play in creating magma beneath the Cascade volcanoes, and if volatiles are important, how they move from the subducted oceanic crust back into the mantle to produce magma.This project will test some fundamental ideas about volatile recycling and magma generation in ?warm-slab? subduction zones such as the Cascades arc. The project will involve new measurements of H2O, other volatiles, fluid mobile elements, and stable and radiogenic isotope tracers in primitive Cascade magmas. Many of the measurements will be done on melt inclusions, which are small blobs of silicate melt trapped inside of growing phenocrysts at depth. Such inclusions are not affected by low pressure degassing as magma moves towards the Earth?s surface, and thus provide a record of the pre-eruptive dissolved volatile contents in magma. The data obtained in this project will provide new information on mantle and slab temperatures, magma production processes, and the sources of volatiles beneath the Cascades arc. This project involves collaborations with other scientists in the U.S., Canada, and Japan for some of the specialized chemical analyses that will be done. Another collaborative effort will involve geodynamic modeling of mantle wedge flow patterns beneath the Cascades so the new geochemical data can be integrated with geophysical results.