Impact of Varied Slab Thermal Structure on Subduction-Related Fluxes and Source Regions of Northern Cascadia Basalt Magmas

俯冲带岩浆具有非典型地幔熔体的地球化学特征。它们独特的地球化学来源仍然存在争议，因为从俯冲板块向上覆地幔输送元素的过程的性质存在相当大的不确定性。一个核心问题是确定特征地球化学特征中有多少是由运输过程产生的，有多少是纯粹由俯冲物质的成分造成的。另一个因素是地幔楔体的内在特征。正确认识这些不同因素的重要性，对于尝试俯冲带元素的质量平衡，从而确定循环进入地幔的大陆物质的流量是至关重要的。卡斯卡迪亚俯冲系统北部的基性岩与许多会聚的边缘火山岩套具有相同的构造和地球化学关系，但它们发生在地球物理条件良好的“热”俯冲环境中，这表明：(1)俯冲板块的热状态沿会聚边缘变化；(2)添加到地幔楔中的俯冲成分的性质和/或地幔亏损状态可能随火山前缘的走向而变化；(3)玄武岩熔岩的俯冲特征可能反映了主要在弧前地区活动的过程。由于板片熔融可能发生在热的俯冲条件下，这些火山岩提供了一个独特的机会来评估流体与板片熔体(沉积物和/或玄武岩地壳)对上覆地幔弧源区化学收支的贡献的程度和性质。这个项目将利用原始到近原始玄武岩的常量元素、微量元素和同位素组成来：(1)详细说明熔岩中弧形平行的地球化学变化的性质和对地幔楔体内熔融过程的影响；(2)在引入板片来源的通量之前，评估地幔来源的耗竭和不均质性的程度；(3)量化火山前缘玄武岩中记录的俯冲通量(流体或熔体)；以及(4)确定板块热结构对地幔-板片相互作用的程度和性质的影响。这些结果将为识别与俯冲有关的通量(特别是沉积物熔体)的性质和来源提供关键信息，并将对所提出的中深地震的发生与俯冲板块本底会聚边缘的变质反应之间的关系产生重要制约。

GreenEAR-0106900Subduction zone magmas have geochemical signatures atypical of mantle-derived melts. The source of their distinctive geochemistry remains controversial, as considerable uncertainty exists about the nature of processes that transport elements from the subducted plate to the overlying mantle. A central issue is determining how much of the characteristic geochemical signature results from transport processes and how much is due simply to the composition of subducted materials. Another factor is the intrinsic characteristics of the mantle wedge. Correctly identifying the importance of these different factors is critical in attempting mass balance of elements at the subduction zone and hence, in determining the flux of continental material recycled into the mantle.Basalts of the northern Cascadia subduction system share tectonic and geochemical relationships with many convergent margin volcanic suites, but occur within a geophysically well-constrained "hot" subduction environment for which it can be shown that (1) the thermal state of the subducted plate varies along the convergent margin; (2) the nature of the subduction component added to the mantle wedge and/or the state of mantle depletion likely changes along strike of the volcanic front; and (3) the subduction signature of the basaltic lavas probably reflects processes operating primarily within the forearc region. Because slab melting may occur under 'hot' subduction conditions, these volcanic rocks provide a unique opportunity to evaluate the extent and nature of fluid versus slab melt (sediment and/or basaltic crust) contributions to the chemical budget of arc source regions in the overlying mantle.This project will utilize major-element, trace-element, and isotopic compositions of primitive to near-primitive basalts to: (1) detail the nature of arc-parallel geochemical variations in the lavas and implications for melting processes within the mantle wedge; (2) evaluate the degree of mantle source depletion and heterogeneity prior to introduction of slab-derived fluxes; (3) quantify subduction fluxes (fluids or melts) recorded in the basalts along the volcanic front; and, (4) establish the influence of slab thermal structure on the extent and nature of mantle-slab interactions.The results will provide crucial information for identifying different factors controlling the nature and origin of subduction-related fluxes (in particular, sediment melts), and will place important constraints on proposed relationships between generation of intermediate-depth earthquakes and metamorphic reactions in the subducted plate benearth convergent margins.