RUI: Collaborative Research: Mantle Heterogeneities and Crustal Processes of the Cascade Arc: Small Scale Variations with Broad Implications

俯冲带是火山作用、大陆地壳生长以及从地幔穿过地壳到地表的物质转移的重要场所。北美洲西部的喀斯喀特弧位于俯冲带上方，涵盖了美国本土一些最活跃的火山地点。先前对喀斯喀特岩浆演化的研究主要集中在地球化学的大规模复杂性上，使用在那里喷发的单个原始（或地幔衍生）玄武岩来表征地幔。然而，原始玄武岩的地球化学排列彼此非常接近地喷发，这表明异质地幔域的存在规模可能比之前认识的要小得多。 此外，许多符合“原始”定义的玄武岩也具有某些地壳或岩浆加工的典型地球化学特征。该项目将在两个 PUI（威斯康星大学奥什科什分校和加州州立大学奇科分校）进行，每年将培训 3-5 名本科生。 PLC 的广泛成分和小面积将提供许多可以用地球化学技术回答的小规模研究问题，从而为本科生的研究生学习和 STEM 职业做好准备。这项工作具有变革性意义，因为它对喀斯喀特和其他复杂弧的未来工作具有重要意义，在这些复杂弧中，研究单个火山中心、跨弧段和沿整个弧系统的大规模变化。该项目的目标是利用加利福尼亚州拉森火山中心 (LVC) 附近毒湖链 (PLC) 中一组玄武岩的成分变化来识别和表征喀斯喀特下地幔中的小规模异质性。对PLC地球化学变化的详细研究将有助于揭示LVC、喀斯喀特和其他复杂俯冲带系统下方地幔多样性的本质。鉴于 PLC 原始玄武岩（~110-100 ka 和 50km2）所代表的时间和空间有限，这项工作是独一无二的，因为它只关注成分变量。拟议的工作集中于两个工作假设：1）地幔中存在小规模的异质性，并以那些未经加工的PLC玄武岩为代表； 2）在其他PLC玄武岩中可以识别出多种后地幔过程，这些过程并不代表直接的地幔熔融成分。最原始的 PLC 玄武岩的地球化学和岩石学成分将用于表征异质地幔域和那里的化学变化规模（假设 1）。来自 PLC 的改良（不太原始）样本的成分将提供对地壳过程的深入了解，这些过程可能会改变地幔熔体从地幔到地表的穿越过程（假设 2）。 通过这种方式，该项目将产生反映地幔异质性的规模和组成以及基于LVC非常孤立的部分的后续地壳内改造过程的模型。

Subduction zones are important sites of volcanism, continental crustal growth, and mass transfer from the mantle, through the crust, to the surface. The Cascade Arc in western North America is located above a subduction zone and encompasses some of the most active volcanic sites in the contiguous US. Previous investigations of the evolution of magmas in the Cascades focused on large-scale complexities in geochemistry, using individual primitive (or mantle-derived) basalts that erupt there to characterize the mantle. However, the geochemical array of primitive basalts erupted in close proximity to one another suggests that heterogeneous mantle domains may exist on a much smaller scale than previously recognized. Furthermore, many basalts that fit the definition of 'primitive' also have geochemical characteristics typical of some crustal or magmatic processing. The project will be conducted at two PUIs (University of Wisconsin Oshkosh and California State University, Chico) and will train 3-5 undergraduate students per year. The wide range of compositions and small area of the PLC will provide numerous small-scale research questions that can be answered with geochemical techniques, thus preparing undergraduate students for graduate study and careers in STEM. This work is transformative for its important implications for future work in the Cascades and other complex arcs where large-scale variations of a single volcanic center, across arc segments, and along entire arc systems are investigated.The goal of this project is to identify and characterize small-scale heterogeneities in the sub-Cascadian mantle using compositional variations in a suite of basalts in the Poison Lake chain (PLC) near the Lassen Volcanic Center (LVC) in California. Detailed study of the geochemical variations in the PLC will shed light on the nature of mantle diversity beneath the LVC, the Cascades and other complex subduction zone systems. Given the restricted time and space represented by PLC primitive basalts (~110-100 ka and 50km2), this work is unique because it concentrates solely on the variable of composition. Proposed work focuses on two working hypotheses: 1) Small-scale heterogeneities exist in the mantle and are represented by those PLC basalts that have not been processed; and 2) a variety of post mantle processes can be recognized in other PLC basalts, which do not represent direct mantle melt compositions. Geochemical and petrologic compositions of the most primitive PLC basalts will be used to characterize heterogeneous mantle domains and the scale of chemical variation there (Hypothesis 1). Compositions of modified (less primitive) samples from the PLC will provide insight into crustal processes that may modify mantle melts on their traverse from mantle to surface (Hypothesis 2). In this way, the project will result in models that reflect the scale and composition of mantle heterogeneity and subsequent intracrustal modification processes based on a very isolated section of the LVC.