Cross-Arc Geochemical Variations in the Southern Washington Cascades

人们普遍认为，火山弧熔岩的化学成分反映了俯冲组分在其原始地幔源区的印记，俯冲带物质转移的程度取决于俯冲几何形状、板片的热状态、俯冲沉积物的类型和数量、俯冲洋壳的年龄和蚀变程度等因素。 为了解决逐渐加热的俯冲板和其挥发性成分的释放的影响，建议分析原始玄武岩熔岩从第四纪南部华盛顿瀑布的跨弧断面。 这SZ是一个最温暖的已知的，这弧代表了一个重要的端员的例子，板派生的贡献是最小的和地幔楔的特点可能更容易辨别。 另一方面，先前测量的某些地球化学参数的跨弧变化（例如，B/Zr，？11B，铅同位素）表明小，但显着的板输入下的弧前和火山前缘范围内的弧后区域以下的贡献可以忽略不计。 这种对比提供了一个机会，定量定义板贡献一个明确的楔形组合物。 在拟议的研究中，熔融包裹体（MI）在橄榄石斑晶的组合物将被确定，这样的包裹体更有可能保留原始比例的挥发性成分比挤压熔岩，从而更有代表性的原始岩浆的组合物。 分析将包括主要和微量元素成分，以及挥发物（H2O，CO2）含量。 工作将集中在少数代表性的熔岩，从位置跨越弧的宽度，其特征在于以前的主要和微量元素和Sr-Nd-Pb同位素组成。 这一全面的数据将使我们能够解决卡斯卡迪亚板中分析元素的相对保留性，并评估由于与岩浆上升通过地幔楔和地壳的地球化学分馏对岩浆成分的影响。 我们的目标是了解和量化的原始岩浆中的挥发性和流体流动成分的空间变异性，并评估的假设，在板的温度变化（从模型计算的传导加热和地幔对流）在很大程度上决定了弧岩浆源的板派生的贡献的成分和幅度。

LeemanEAR-0003612It is widely accepted that chemical compositions of volcanic arc lavas reflect the imprints of subducted components on their primary mantle sources, and that the extent of material transfer in subduction zones [SZs] depends on such factors as subduction geometry, thermal state of the slab, the type and amount of sediments subducted, and age and degree of alteration of the underthrust oceanic crust. To address the effects of progressive heating of a subducting slab and the release of its volatile constituents it is proposed to analyze primitive basaltic lavas from a cross-arc transect of the Quaternary southern Washington Cascades. This SZ is one of the warmest known, and this arc represents an important end member example where slab-derived contributions are minimal and characteristics of the mantle wedge may be more easily discerned. On the other hand, previously measured cross-arc variations in certain geochemical parameters (e.g., B/Zr, ?11B, Pb isotopes) indicate small but significant slab inputs below the forearc and volcanic front ranging to negligible contributions below the backarc region. This contrast affords an opportunity for quantitative definition of slab contributions to a well-defined wedge composition. In the proposed study, compositions of melt inclusions (MIs) in olivine phenocrysts will determined; such inclusions are more likely to retain original proportions of volatile components than extruded lavas, and thus more representative compositions of primitive magmas. Analyses will include major and trace element compositions, and volatile (H2O, CO2) contents. Work will focus on a small number of representative lavas, from locations spanning the width of the arc, that have been previously characterized for major and trace element and Sr-Nd-Pb isotopic compositions. This comprehensive body of data will allow us to address the relative retentivity of the analyzed elements in the Cascadia slab, and to evaluate effects on magma compositions due to geochemical fractionations associated with magma ascent through mantle wedge and crust. Our goal is to understand and quantify the spatial variability of volatile and fluid-mobile constituents in primitive magmas, and to evaluate the hypothesis that temperature variations in the slab (as calculated from models of conductive heating and mantle convection) largely dictate the composition and magnitudes of slab-derived contributions to arc magma sources.