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

目前流行的会聚边缘岩浆作用模式是，由于大洋岩石圈俯冲(俯冲)板块的脱水作用而产生的流体增加，导致地幔熔融。然而，由于级联弧与年轻的大洋岩石圈的缓慢俯冲有关，它是已知的最热的俯冲带之一。对于这种极端的末端成员情况，俯冲板块很可能在下降到弧下深度时发生了广泛的变质和脱水，弧下板片衍生的流体供应很少，温度甚至可能高到足以促进这些深度的板片直接熔化。在这种情况下，熟悉的俯冲带“熔剂熔化”模式可能并不严格适用。然而，喀斯喀兹弧的特点是大量的玄武岩岩浆活动。为了解释这一谜团，似乎需要替代的机制和/或岩浆来源，而这一悖论的解决方案可能为世界范围内原始弧状岩浆的起源提供新的线索。我们的项目侧重于玄武岩岩浆作用，因为这种熔岩可能携带有关支撑火山弧状岩浆作用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，板岩衍生的流体对岩浆生成的贡献程度有多大。在华盛顿瀑布南部，缺乏板岩化学特征的原始玄武岩熔岩在整个弧线上喷发。从这一观测中我们推断，地幔楔体的大部分得到了可以忽略不计的板片贡献。相比之下，在加利福尼亚州北部的喀斯喀兹(例如，Mt.沙斯塔地区)，原始熔岩似乎被显著水化，普遍的解释表明，板岩衍生的流体确实对该地区的岩浆形成做出了重大贡献。我们建议进行一项对比研究，以调查这两个地区之下板片贡献的性质和范围，使用板片衍生流体的灵敏地球化学示踪剂-Be和B同位素、流体可移动的微量元素和放射成因同位素(Sr、Pb和Os)。如果板条的贡献在后一区域是重要的，我们可以更好地定义关于这些成分参数的签名的起源和组成。如果情况并非如此，我们将研究其他情景(例如，减压熔融)，以解释这些背景下原始岩浆的特征和来源。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有关的外部强迫作用的影响。智力上的优点：这项工作将为喀斯特地区提供更深入的理解，了解岩浆多样性的原因、不同熔融过程的相对贡献、地幔楔体内成分多样性的影响，以及最终支撑这种岩浆作用的热结构和过程。这种知识可能很难从更典型的、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乔治地球科学研究所的研究人员之间的合作将促进智力交流，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，将有助于研究汇聚边际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.