Collaborative Research: Geochemical Processes in Forearc Peridotites: Depletion, Enrichment, and Melt Reactions in the Mantle Wedge

俯冲过程中地幔楔的地球化学通量问题对我们认识弧火山作用至关重要，是全球地球化学通量研究的一个重要方面。这是“地球地球化学参考模型”（GERM）倡议和美国国家科学基金会“边缘”倡议的“俯冲工厂”重点确定的一级问题之一。研究这个问题的一种方法是检查由俯冲带上方的岩浆作用形成的地壳下面的岩石圈地幔的露头。该地幔在一定程度上代表了上覆地壳提取的来源，其矿物学和成分反映了随着时间的推移对其产生影响的过程，包括熔体提取、流体相富集以及随后与来自地幔下部的熔体的相互作用。这种方法的主要优点是，大片的超俯冲岩石圈通常暴露在许多蛇绿岩的底部，使我们能够直接检查它们的组成，并且在比目前任何活动系统更大的长度范围内。本项目重点研究保存在原方济各俯冲带上方岩石圈地幔中的地球化学通量，以加利福尼亚海岸山脉蛇绿岩下的辉锌矿和暗粒岩构造岩为代表。目标是限制这些通量的性质和范围，通过整个岩石的主要元素和微量元素分析，通过使用电子和离子束技术的矿物分析，以及通过超纯的、手工挑选的矿物分离物的同位素分析来记录。我们将提出的主要问题包括熔体提取的累积程度和被提取的熔体的性质，SSZ楔块中流向地幔的流体通量的性质、来源和程度，以及熔体提取后地幔-熔体相互作用的性质和程度（例如，岩石圈下来源的熔体的添加，或这种熔体与先前耗尽的橄榄岩的反应）。岛弧火山源区的地球化学演化是一个重要的问题，不仅对地球的化学收支有影响，而且对通常与岛弧火山活动有关的矿床的起源、地球内部挥发性元素向大气的转移以及这些过程发生的时间尺度也有影响。虽然许多类地行星上都有火山，但板块构造和弧火山作用是地球独有的，是行星演化的重要组成部分。

The question of geochemical flux in the mantle wedge during subduction is critical to our understanding of arc volcanism, and forms an important aspect of the global geochemical flux. This is one of the first order problems identified by the "Geochemical Earth Reference Model" (GERM) initiative, and by the "subduction factory" focus of the NSF "MARGINS" initiative. One way to study this problem is to examine outcrops of lithospheric mantle that underlie crust known to have formed by magmatism above a subduction zone. This mantle represents, in part, the source from which the overlying crust was extracted, and its mineralogy and composition reflect the processes that have affected it through time, including melt extraction, fluid phase enrichment, and subsequent interactions with melt derived from lower in the mantle. The primary advantage of this approach is that large tracts of supra-subduction lithosphere are commonly exposed at the base of many ophiolites, allowing us to examine their composition directly and on larger length scales than is currently possible in any active system. This project focuses on the geochemical flux preserved in lithospheric mantle above the proto-Franciscan subduction zone, as represented by harzburgite and dunite tectonites that underlie the Coast Range ophiolite of California. The goal is constrain the nature and extent of these fluxes, as documented by whole rock major element and trace element analyses, by mineral analyses using electron and ion beam techniques, and by isotopic analyses of ultra-pure, hand-picked mineral separates. Major questions we will pose include the cumulative extent of melt extraction and the nature of the melt extracted, the nature, source, and extent of fluid flux to the mantle in the SSZ wedge, and the nature and extent of mantle-melt interactions subsequent to melt extraction (e.g., addition of melt from sub-lithospheric sources, or reaction of this melt with the previously depleted peridotites). The geochemical evolution of the source region for island arc volcanics is an important problem with implications not only for the chemical budget of the Earth, but also for the origin of ore deposits that are commonly associated with island arc volcanism, the transfer of volatile elements from inside the Earth into the atmosphere, and the time scales over which these processes occur. While volcanoes are found on many terrestrial planets, plate tectonics and arc volcanism are unique to Earth and represent an important component of planetary evolution.