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

俯冲带是火山活动、大陆地壳生长和物质从地幔通过地壳转移到地表的重要场所。北美西部的喀斯喀特弧位于俯冲带上方，包含了美国毗连地区一些最活跃的火山遗址。先前对喀斯喀特岩浆演化的研究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阵列彼此靠近喷发表明，非均质地幔域的存在规模可能比以前认识的要小得多。此外，许多符合“原始”定义的玄武岩还具有典型的地壳或岩浆作用的地球化学特征。该项目将在两所州立大学（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的广泛组成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的职业生涯做好准备。这项工作具有变革性，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，在这些工作中，单个火山中心的大规模变化，跨越弧段，沿着整个弧系统进行研究。该项目的目标是通过加利福尼亚Lassen火山中心（LVC）附近的Poison Lake链（PLC）一套玄武岩的成分变化来识别和表征亚卡斯卡迪亚地幔的小尺度非均质性。对PLC地球化学变化的详细研究将有助于揭示LVC、Cascades和其他复杂俯冲带系统下地幔多样性的本质。考虑到PLC原始玄武岩（~110-100 ka和50km2）所代表的有限时间和空间，这项工作是独特的，因为它只关注成分的变量。建议的工作主要集中在两个工作假设：1)地幔中存在小规模的非均质性，并以未加工的PLC玄武岩为代表；(2)在其他PLC玄武岩中可以识别出各种后地幔过程，这些过程不代表直接的地幔熔体成分。最原始的PLC玄武岩的地球化学和岩石学组成将用于表征非均质地幔域及其化学变化的规模（假设1）。来自PLC的修改（不太原始）样品的组成将提供对地壳过程的深入了解，这些过程可能会改变从地幔到地表的地幔熔体（假设2）。通过这种方式，该项目将产生反映地幔非均质性的规模和组成的模型，以及基于LVC非常孤立的部分的后续壳内修饰过程。

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.