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

收敛边缘岩浆作用的主流模式涉及地幔熔融，这是对海洋岩石圈俯冲（逆冲）板块脱水所产生的流体的响应。然而，由于喀斯喀特弧与年轻海洋岩石圈的缓慢俯冲有关，它是已知最温暖的俯冲带之一。对于这种极端的端部构件情况，很可能是俯冲板块在下降到亚弧深度时发生了广泛的变质和脱水，弧下板块衍生流体的供应很低，温度甚至可能高到足以促进这些深度板块的直接熔化。在这种情况下，熟悉的俯冲带“通量熔化”模式可能并不严格适用。然而，喀斯喀特弧的特点是大量的玄武岩岩浆活动。要解释这一谜团，似乎需要其他机制和/或岩浆来源，而这一悖论的解决方案可能会为世界范围内原始弧岩浆的起源提供新的线索。我们的项目侧重于玄武岩岩浆活动，因为这种熔岩可能携带有关支撑火山弧岩浆活动的基本地幔过程的相关信息。我们要解决的一个基本问题是，在这种背景下，源自板块的流体在多大程度上有助于岩浆的生成。在华盛顿瀑布的南部，原始的玄武岩熔岩在整个弧的宽度上喷发，缺乏来自于板块的化学特征。根据这一观察，我们推断地幔楔的大部分受到的板块贡献可以忽略不计。相反，在加利福尼亚北部喀斯喀特山脉（例如，沙斯塔山地区），原始熔岩似乎明显水化，普遍的解释表明，板块衍生的流体确实对该地区的岩浆形成起了重要作用。我们建议进行一项比较研究，利用敏感的地球化学示踪剂（Be和B同位素、流体流动微量元素和放射性成因同位素（Sr、Pb和Os））来研究这两个地区的板块贡献的性质和程度。如果后一区域的岩块贡献显著，我们可以根据这些组成参数更好地定义该特征的起源和组成。如果不是这样，我们将研究其他情景（如减压融化）来解释这些环境下原始岩浆的特征和起源。这项研究将更好地定义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相关的外部强迫函数的影响。学术价值：这项工作将为喀斯喀特山脉提供对岩浆多样性的成因、不同熔融过程的相对贡献、地幔楔内成分多样性的影响，以及最终支撑这种岩浆活动的热结构和过程的更深入理解。这些知识可能很难从更典型的、较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训练和经验。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乔治大学地球科学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。通过参加国内和国际会议来转移这种知识，将有助于许多研究趋同边缘动态的科学家的总体利益。

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.