Collaborative Research: Volatile Recycling and Magma Genesis in a Hot, Dry Subduction Zone: A Case Study of the Cascade Arc

俯冲带在控制地壳、水圈、大气和深层地幔之间的化学循环中起着重要作用。在这些环境中，挥发性元素（H、C、F、Cl、S等）介导了质量传递和元素循环，并强烈影响了俯冲带环境中岩浆的生成、搬运和喷发过程。为了研究挥发性元素在单个俯冲体系中的分布、作用及其对传质和熔融的影响，本研究将系统地研究加利福尼亚、俄勒冈和华盛顿喀斯喀特弧原始岩浆熔体包裹体中的主要元素、微量元素和挥发性元素。目的是确定在Cascade弧中发现的不同原始岩浆类型的喷发前挥发性浓度和氧逸度条件，并使用这些数据来检查沿弧和横跨弧的板块和地幔楔衍生成分的组成，并测试当前的熔融和岩浆成因模型。虽然喀斯喀特俯冲带被认为是一个末端“干热”俯冲带，但挥发物在元素循环和岩浆成因中的作用尚不清楚。现有的研究和我们的初步数据表明，水的含量是不同的，一些原始的喀斯喀特岩浆非常“湿”——水的含量高达4%到7%。将采用两种采样策略，首先涉及研究俄勒冈中部瀑布(Lat。~ 43-45度)。俄勒冈高Cascades地区的特点是最近喷发量最大的基性岩浆，并包含弧中发现的所有主要原生岩浆类型（HAOT， obb -类，CAB和shoshonite）。这项工作将与对其他熔岩的研究工作结合起来，这些熔岩来自沿弧选定的经过充分研究的地点。总的来说，这种方法可以用来识别喀斯喀特弧内产生的不同岩浆类型的挥发性成分，并提供对弧内和弧内挥发性丰度变化的见解。

Subduction zones play a major role in governing chemical cycling between the Earth's crust, hydrosphere and atmosphere and the deeper mantle. In these environments volatile elements (H, C, F, Cl, S and others) mediate mass transfer and elemental cycling, and also strongly influence the processes that lead to the generation, transport and eruption of magmas within subduction zone environments. To investigate the distribution and role of volatile elements and their effect on mass transfer and melting in single subduction system, this study will involve a systematic study of major, trace and volatile elements trapped in melt inclusions from primitive magmas in the Cascade arc in California, Oregon and Washington. The aims are to determine pre-eruptive volatile concentrations and oxygen fugacity conditions for the different primitive magma types found in the Cascade arc, and to use this data to examine the composition of slab- and mantle-wedge derived components along and across the arc, and to test current models for melting and magma genesis in subduction zones. Although the Cascade subduction zone is recognized as an endmember "hot and dry" subduction zone, the role of volatiles in element cycling and magma genesis remains unclear. Existing studies and our preliminary data show that water contents vary and that some primitive Cascade magmas are quite "wet" - with H2O up to 4 to 7 wt.%. Two sampling strategies will be pursued, firstly involving study of lavas from along an east-west cross-arc traverse in the Central Oregon Cascades (Lat. ~43-45 degrees). The Oregon High Cascades region is characterized by the highest recent erupted volumes of mafic magma and contains all major primary magma types found in the arc (HAOT, OIB-like, CAB, and shoshonite). This work will be combined with work on other lavas from selected well-studied locations along the arc. Overall this approach will allow use to identify the volatile composition of the range of different magma types produced within the Cascade arc, and provide insight on the variations in volatile abundances across and along the arc.