Application of Siderophile Elements to the Study of the Chemical Structure and Mixing History of the Oceanic Mantle

海洋地幔可能占地球质量的60%以上。现代大洋地幔是已知的化学和同位素的不均匀性，主要是通过洋中脊玄武岩（MORB）的研究记录。造成非均质性的过程很可能是由部分熔融（熔体去除）、交代作用（熔体/流体添加）和再循环地壳（通过俯冲）的结合所主导的。这些过程中的每一个的相对影响，过程的时间，以及由此产生的异质性的长度尺度仍然是相当大的辩论的主题。因此，澄清海洋地幔中化学和同位素不均匀性的原因、时间和空间范围，对于完善我们对这一主要陆地储层的当前总体组成的认识，对于更全面地了解地球地幔的化学和同位素组成如何随时间演变，以及对于评估今天海洋地幔对流混合的效率至关重要。为了解决这些问题，将研究四个蛇绿岩复合体（构造侵位的海洋岩石圈薄片）中的海洋地幔剖面。目标蛇类的年龄范围从19.5亿岁到600万年。这项研究将主要集中在检查绝对和相对丰度的亲铁（爱铁）元素在地幔岩石暴露在这些位置。这项研究将寻求回答有关现在和过去的海洋地幔的基本问题，包括：1）海洋地幔的长期，异常熔融耗尽部分的几何形状是什么？2)普通的大洋地幔岩性，如辉石岩和纯橄榄岩，是否具有化学成分和足够的体积，从而与某些具有不寻常成分的火山岩（如一些夏威夷熔岩）的生成有关？3)大洋地幔的关键地球化学参数随时间变化了吗？如果是这样的话，它们是否可以与可能随着时间的推移作用于地幔的过程的速率相关联。 除了回答这些关于海洋地幔化学结构的问题之外，比较近20亿年的蛇绿岩数据将开始记录地幔化学结构的长期变化。

The oceanic mantle may comprise more than 60% of the mass of the Earth. The modern oceanic mantle is known to be chemically and isotopically heterogeneous, primarily documented through studies of mid-ocean ridge basalts (MORB). The processes that resulted in the creation of the heterogeneities were most likely dominated by a combination of partial melting (melt removal), metasomatism (melt/fluid addition) and incorporation of recycled crust (via subduction). The relative impacts of each of these processes, the timing of the processes, and the length scales of the resulting heterogeneities remain topics of considerable debate. Clarifying the causes, timing and spatial extent of chemical and isotopic heterogeneities in the oceanic mantle is, therefore, critical for refining our knowledge of the present bulk composition of this major terrestrial reservoir, for achieving a more complete understanding of how the chemical and isotopic composition of Earth's mantle evolved through time, and for assessing the efficiency by which the oceanic mantle is convectively mixed today. To tackle these issues, sections of the oceanic mantle accessible in four ophiolite complexes (tectonically emplaced slivers of oceanic lithosphere) will be studied. The targeted ophiolites range in age from 1.95 billion years old to 6 million years old. The study will primarily focus on examining the absolute and relative abundances of siderophile (iron-loving) elements in the mantle rocks exposed in these locations. The study will seek to answer fundamental questions about the present and past oceanic mantle including: 1) What is the geometry of long-term, exceptionally melt-depleted portions of the oceanic mantle? 2) Do common oceanic mantle lithologies, such as pyroxenites and dunites, have chemical compositions and sufficient volume to be implicated in the generation of certain volcanic rocks with unusual compositions, for example some Hawaiian lavas? 3) Have key geochemical parameters in the oceanic mantle changed through time? If so, can they be correlated with the rates of processes that may have acted on the mantle through time. In addition to answering these questions about the chemical structure of the oceanic mantle, comparison of data from ophiolites spanning nearly two billion years will begin the process of documenting secular changes in the chemical structure of this mantle.