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

合作研究：喀斯喀特火山弧原始岩浆的起源

• 批准号: 0409423
• 负责人: Cin-Ty Lee
• 金额: --
• 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: 2004
• 资助国家: 美国
• 起止时间: 2004-06-01 至 2008-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0409423&HistoricalAwards=false
• 关键词: Collaborative; Research; Origins; Primitive; Magmas

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.

会聚边缘岩浆作用的流行模型涉及地幔熔化，这是由于海洋岩石圈俯冲（俯冲）板块脱水产生的流体增加而引起的。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。对于这种极端端元的情况，俯冲板块在下降到弧下深度时可能会发生广泛的变质和脱水，弧下源自板片的流体供应量较低，而且温度甚至可​​能高到足以促进板片在这些深度的直接熔化。在这种情况下，熟悉的俯冲带“通量熔化”范式可能并不严格适用。然而，喀斯喀特弧的特点是大量玄武岩岩浆作用。为了解释这个谜团，似乎需要替代机制和/或岩浆源，而解决这个悖论可能会为全世界原始弧岩浆的起源提供新的线索。我们的项目重点关注玄武质岩浆作用，因为此类熔岩可能携带有关支撑火山弧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涉及板片衍生流体在这种情况下对岩浆生成的贡献程度。在华盛顿喀斯喀特南部，缺乏板片化学特征的原始玄武岩熔岩在整个弧线范围内喷发。根据这一观察，我们推断大部分地幔楔受到的板片贡献可以忽略不计。相比之下，在北加州喀斯喀特（例如沙斯塔山地区），原始熔岩似乎含水分很高，普遍的解释表明板片衍生的流体确实对该地区的岩浆形成有显着贡献。我们提出一项比较研究，利用板片衍生流体的敏感地球化学示踪剂（Be 和 B 同位素、流体移动微量元素和放射性同位素（Sr、Pb 和 Os））来调查这两个区域下方板片贡献的性质和程度。如果板层贡献在后一个区域中很重要，我们可以根据这些成分参数更好地定义该签名的起源和成分。如果情况并非如此，我们将研究其他情况（例如减压熔化），以解释这些环境中原始岩浆的特征和起源。这项研究将更好地定义竞争性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。智力价值：这项工作将为喀斯喀特提供对岩浆多样性的原因、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最终支撑这一过程的热结构和过程的更深入的了解。 岩浆作用。这些知识可能很难从更典型、更冷的俯冲系统中提取。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、圣路易斯华盛顿大学以及意大利比萨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来传播这些知识将有助于许多研究收敛边缘动态的科学家的整体利益。