Cross-Arc Geochemical Variations in the Southern Washington Cascades

华盛顿南部瀑布的跨弧地球化学变化

• 批准号: 0003612
• 负责人: William Leeman
• 金额: $ 2.5万
• 依托单位: William Marsh Rice University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-03-01 至 2004-02-29
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0003612&HistoricalAwards=false
• 关键词: Cross; Arc; Geochemical; Variations; Southern

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.

Leemanear-0003612IT被广泛接受，火山弧熔岩的化学成分反映了其主要地幔源上的俯冲成分的印记，并且俯冲带中物质转移的程度[Szs]取决于俯冲几何，slab slab and trabecters of Slab and comeperions and Trabers and comeply and trabe and trabe and trab and trabe and trab and trab and trab and trab and trab and trab and trab and s and sallab and temiment and trab and s and trab and s and。地壳。 为了解决俯冲板进行逐步加热的影响，并提出了其挥发性成分的释放，它提议分析来自华盛顿南部Quaternary Southern Cascades的跨弧形样带的原始底层熔岩。 该SZ是已知最温暖的人之一，该弧代表了一个重要的最终成员示例，在该示例中，平板衍生的贡献很小，并且可以更容易地识别地幔楔的特征。 另一方面，以前测量的某些地球化学参数（例如B/ZR，？11B，PB，PB同位素）的跨ARC变化表明，低于前臂和火山界以下的较小但显着的平板输入，范围为倒数后区域以下可忽略的贡献。 这种对比为定量定义了对明确定义的楔形组成的定义定义的机会。 在拟议的研究中，将确定橄榄型表晶中熔体夹杂物（MIS）的组成；与挤出的熔岩相比，这种夹杂物更有可能保留挥发性成分的原始比例，从而更有代表性的原始岩浆组成。 分析将包括主要元素和痕量元素组成以及挥发性（H2O，CO2）内容。 工作将集中于少数代表性的熔岩，这些熔岩从跨越弧宽度的位置，这些熔岩以前是针对主要和痕量元素以及SR-ND-PB同位素组成的表征的。 这种全面的数据将使我们能够解决卡斯卡迪亚平板中分析元素的相对保留，并通过通过地幔楔和壳的岩浆上升与岩浆上升相关的地球化学分馏，评估对岩浆成分的影响。 我们的目标是理解和量化原始岩浆中挥发性和流体摩托成分的空间变异性，并评估以下假设：平板中温度变化（根据导电加热和地幔对流模型计算得出），在很大程度上决定了对slab衍生的贡献和巨大贡献的贡献。