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

这个项目的目标是更好地了解华盛顿、俄勒冈州和加利福尼亚北部喀斯喀特山脉火山下的岩浆是如何在地球内部产生的。这个地区的火山活动是由俯冲作用引起的，在这个过程中，太平洋西北部近海的海洋地壳下沉回地球。沿着大陆边缘的S地幔。俯冲作用也是该地区可能发生的大地震的原因。随着下沉板的升温，它会释放出水和其他挥发性物质。元素（Cl， S， F， C），这些元素反过来又导致上覆的地幔岩石融化，产生岩浆，为喀斯喀特火山提供燃料。虽然世界上许多地区（如安第斯山脉）都发生了俯冲过程，但喀斯喀特体系的一个显著特征是，当俯冲的海洋地壳开始俯冲时，它在地质上是年轻的。这导致地壳温度升高，并在俯冲地壳还很浅的时候释放出水和其他挥发物，然后才到达火山下面。因此，主要的挑战是了解水和其他挥发物在喀斯喀特火山下形成岩浆的过程中所起的作用，如果挥发物很重要，那么它们是如何从俯冲的海洋地壳回到地幔中产生岩浆的。该项目将测试热板中挥发性物质循环和岩浆生成的一些基本想法。俯冲带，如喀斯喀特弧。该项目将涉及对原始Cascade岩浆中的H2O、其他挥发物、流体流动元素以及稳定和放射性同位素示踪剂的新测量。许多测量将在熔融包裹体上进行，这些包裹体是被困在深处生长的现象晶体中的硅酸盐熔体的小块。当岩浆向地球移动时，这些包裹体不受低压脱气的影响吗？从而提供了喷发前岩浆中溶解挥发物含量的记录。该项目获得的数据将提供有关喀斯喀特弧下地幔和板块温度、岩浆产生过程和挥发物来源的新信息。这个项目包括与美国、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。另一项合作将涉及喀斯喀特山脉下地幔楔流模式的地球动力学建模，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.