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经地壳向地表转移的重要场所。北美西部的喀斯喀特弧位于俯冲带之上，包括美国邻近地区一些最活跃的火山遗址。以前对小瀑布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，地球化学阵列的原始玄武岩爆发在彼此接近表明，异质地幔域可能存在的规模比以前认识到的要小得多。 此外，许多符合“原始”定义的玄武岩也具有某些地壳或岩浆作用的典型地球化学特征。该项目将在两所PUI（威斯康星州奥什科什大学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小规模的研究问题，可以用地球化学技术来回答，从而为本科生的研究生学习和STEM职业做好准备。这项工作是变革性的，因为它对瀑布和其他复杂弧的未来工作具有重要意义，在这些弧段中，单个火山中心的大规模变化，该项目的目标是利用Poison Lake链（PLC）中一套玄武岩的成分变化来识别和表征次卡斯卡迪亚地幔中的小尺度非均匀性位于加州拉森火山中心（LVC）附近。对PLC地球化学变化的详细研究将揭示LVC、Cascades和其他复杂俯冲带系统下地幔多样性的本质。由于PLC原始玄武岩（~110-100 ka和50 km 2）代表的时间和空间有限，这项工作是独一无二的，因为它只集中在成分的变量。建议的工作集中在两个工作假设：1）小规模的不均匀性存在于地幔中，并代表那些PLC玄武岩没有被处理;和2）各种后地幔过程可以在其他PLC玄武岩，这并不代表直接的地幔熔体成分识别。地球化学和岩石学组成的最原始的PLC玄武岩将被用来表征不均匀的地幔域和化学变化的规模（假设1）。从PLC修改（不太原始）的样品的成分将提供洞察地壳过程，可能会修改地幔熔体从地幔到表面的遍历（假设2）。 通过这种方式，该项目将产生反映地幔不均匀性的规模和组成以及随后基于LVC非常孤立的部分的壳内修改过程的模型。

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.