Cross-Arc Geochemical Variations in the Southern Washington Cascades

普遍认为，火山弧熔岩的化学成分反映了俯冲成分在其原始地幔来源上的印记，俯冲带[SZ]的物质转移程度取决于俯冲几何形状、板块的热状态、俯冲沉积的类型和数量以及俯冲洋壳的年龄和程度等因素。为了解决俯冲板块的逐步加热及其挥发性组分释放的影响，建议从第四纪华盛顿瀑布南部的弧形断面上分析原始玄武岩熔岩。这个SZ是已知的最热的例子之一，这个弧形代表了一个重要的端元例子，其中板片派生的贡献很小，地幔楔形的特征可能更容易被识别。另一方面，以前测量的某些地球化学参数(如B/Zr、~(11)B、~(11)B)的跨弧变化表明，在弧前和火山前缘下方的板片输入很少，但意义重大，而在弧后区域下方的贡献可以忽略不计。这种对比提供了定量定义板材对明确定义的楔形成分的贡献的机会。在拟议的研究中，将确定橄榄石斑晶中熔融包裹体的成分；此类包裹体比挤压熔岩更有可能保留挥发性成分的原始比例，因此更具原始岩浆成分的代表性。分析将包括主要元素和痕量元素成分，以及挥发性(H2O、CO2)含量。工作重点将放在少数具有代表性的熔岩上，这些熔岩来自整个弧宽的地点，以前已经确定了主要元素和微量元素以及锶-钕-铅同位素组成的特征。这一全面的数据体系将使我们能够解决卡斯卡迪亚板块中被分析元素的相对保留率，并评估与通过地幔楔形和地壳上升的岩浆有关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原始岩浆中挥发性和流体流动组分的空间变异性，并评估板块中的温度变化(根据传导加热和地幔对流模型计算)在很大程度上决定了板块对弧状岩浆源的贡献的成分和大小的假设。

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.