Collaborative Research: On the Origins of Primitive Magmas in the Cascade Volcanic Arc

会聚边缘岩浆作用的流行模型涉及地幔熔融，以响应大洋岩石圈俯冲（俯冲）板块脱水产生的流体的增加。然而，由于喀斯喀特弧与年轻海洋岩石圈的缓慢俯冲有关，因此它是已知最温暖的俯冲带之一。对于这种极端的端部成员的情况下，很可能是俯冲板块广泛变质和脱水，因为它下降到亚弧深度，弧下的板片衍生流体的供应是低的，温度甚至可能高到足以促进直接熔化的板片在这些深度。在这种情况下，熟悉的俯冲带“通量熔化”范例可能不严格适用。然而，喀斯喀特岛弧的特点是大量的玄武岩浆活动。要解释这个谜，似乎需要替代机制和/或岩浆来源，这个悖论的解决方案可能会揭示新的光的起源原始弧magmas. We的项目侧重于玄武岩浆作用，因为这样的熔岩可能携带有关的基本地幔过程支撑火山弧岩浆作用的相关信息。一个基本的问题，我们解决的问题是在何种程度上板派生流体有助于岩浆生成在这种设置。在南部的华盛顿瀑布，原始玄武岩熔岩缺乏板衍生的化学签名已经爆发了整个宽度的弧。从这一观察，我们推断，地幔楔的大部分已收到可以忽略不计的板片的贡献。相反，在北方加州喀斯喀特山脉（例如，山沙斯塔地区），原始熔岩似乎显着水化，和流行的解释表明，板片衍生的流体在这个地区的岩浆形成作出了重大贡献。我们提出了一个比较研究，调查的性质和程度板下的贡献，使用敏感的地球化学示踪剂板派生的流体- Be和B同位素，流体流动的微量元素，放射性同位素（Sr，Pb和Os）。如果板贡献是显着的，在后一个区域，我们可以更好地定义的起源和组成的签名相对于这些组成参数。如果不是这种情况，我们将调查其他情况（例如，减压熔融），以解释这些设置的原始岩浆的特征和起源。这项研究将更好地确定相互竞争的熔融过程之间的相对贡献，并使我们能够解决它们如何受到与俯冲带动力学有关的外部强迫函数的影响。这项工作将使喀斯喀特山脉更深入地了解岩浆多样性的原因、不同熔融过程的相对贡献、地幔楔内成分多样性的影响，以及支撑岩浆作用的热结构和过程。这种知识可能很难从更典型、更冷的俯冲系统中提取出来。更广泛的影响：参与这项工作的研究生和本科生将获得基本的科学培训和经验。莱斯大学、亚利桑那大学和圣路易斯华盛顿大学的研究人员与意大利比萨的地球科学和地质研究所的研究人员之间的合作将促进知识交流，并提供广泛的分析方法。通过参加国家和国际会议转让这一知识，将有助于研究边际趋同动态的许多科学家的总体利益。

The prevailing model for convergent margin magmatism involves mantle melting in response to additions of fluids produced by dehydration of subducting (underthrusting) plates of oceanic lithosphere. However, because the Cascade arc is associated with slow subduction of young oceanic lithosphere, it is one of the warmest subduction zones known. For this extreme end member case, it is likely that the subducting plate is extensively metamorphosed and dehydrated as it descends to subarc depths, that the supply of slab-derived fluids beneath the arc is low, and that temperatures may even be high enough to promote direct melting of the slab at those depths. In this case the familiar subduction zone 'flux melting' paradigm may not strictly apply. Yet, the Cascades arc is characterized by voluminous basaltic magmatism. To explain this enigma, alternative mechanisms and/or magma sources are seemingly required, and the solution to this paradox may shed new light on origins of primitive arc magmas worldwide.Our project focuses on basaltic magmatism because such lavas are likely to carry relevant information concerning fundamental mantle processes underpinning volcanic arc magmatism. A basic question that we address concerns the extent to which slab-derived fluids contribute to magma generation in this setting. In the southern Washington Cascades, primitive basaltic lavas lacking slab-derived chemical signatures have erupted over the entire width of the arc. From this observation we infer that much of the mantle wedge has received negligible slab contributions. In contrast, in the northern California Cascades (e.g., Mt. Shasta area), primitive lavas appear to be significantly hydrated, and prevailing interpretations suggest that slab-derived fluids do contribute significantly to magma formation in this area. We propose a comparative study to investigate the nature and extent of slab contributions beneath both areas, using sensitive geochemical tracers for slab-derived fluids - Be and B isotopes, fluid-mobile trace elements, and radiogenic isotopes (Sr, Pb, and Os). If slab contributions are significant in the latter region, we can better define the origin and composition of that signature with respect to these compositional parameters. If this is not the case, we will investigate other scenarios (e.g., decompression melting) to explain the characteristics and origins of primitive magmas in these settings. This study will better define the relative contributions between competing melting processes, and allow us to address how they are influenced by external forcing functions related to subduction zone dynamics.Intellectual merit: This work will provide, for the Cascades, a deeper understanding of the causes for magmatic diversity, the relative contributions of different melting processes, the influence of compositional diversity within the mantle wedge, and ultimately the thermal structure and processes underpinning this magmatism. This knowledge may be difficult to extract from more typical, cooler subduction systems. Broader impacts: Graduate and undergraduate students involved in this work will gain basic scientific training and experience. Collaboration between researchers at Rice University, University of Arizona, and Washington University at St. Louis, and the Istituto di Geoscienze e Georisorse in Pisa, Italy, will foster intellectual exchange and provide access to a broad range of analytical approaches. Transfer of this knowledge through participation at national and international meetings will contribute to the overall benefit of many scientists studying the dynamics of convergent margins.