Widespread Layered Pyroxenites in the Earth's Mantle

地幔中广泛分布的层状辉石岩

• 批准号: 1220440
• 负责人: Veronique Le Roux
• 金额: $ 25.91万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-08-01 至 2016-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1220440&HistoricalAwards=false
• 关键词: Widespread; Layered; Pyroxenites; Earth; Mantle

The Earth's mantle is primarily composed of peridotites, which are magnesium and iron silicate rocks that often contain 5 to 10% heterogeneities by volume. These heterogeneities can be very variable in composition but pyroxene-rich rocks, and in particular websterites (those that have additional calcium, sodium and aluminum in the silicate crystal structure) appear to be the dominant type of heterogeneity associated with peridotites exhumed at the Earth?s surface. No study has so far explained why and how this type of heterogeneity is so widespread in mantle rocks. Importantly, it might be an indication that a ubiquitous process is significantly modifying the composition of the mantle and its derived lavas. A multidisciplinary study will be conducted, which will combine field observations, geochemical and petrophysical analyses of selected websterites from two locations: the Lherz Massif (FR) and the Josephine Peridotite (USA), that are believed to represent subcontinental and subarc mantle, respectively. Trace and major element variations will be measured and modeled in order to constrain the process(es) responsible for the formation of heterogeneities in the mantle. Additionally geodynamical conditions of emplacement of the websterites will be constrained by measuring small-scale variations of lattice-preferred orientations of minerals. The results of this study will benefit a broad community of scientists due to its multidisciplinary approach that can reconcile observations of interest for mantle geochemists, rheology experimentalists and geophysicists.The mantle represents about 80% of the volume of our planet, lying underneath the crust between approximately 30 km and 2900 km depth. A large part of the thermal and chemical variations of the Earth is controlled by the mantle, which locally melts and gives rise to most of the volcanic activity visible at the surface of the planet. In order to better predict volcanic hazards, it is fundamental to constrain what type of material is melting at depth, because the composition of the lava affects the explosivity of volcanic eruptions. In order to answer this question, the composition of the mantle should be well constrained. Exposures of mantle rock at the Earth?s surface are rare but where we can obtain access, we find that many aspects of its composition remain uncertain. We know that the Earth's mantle is not uniform in composition and that it contains heterogeneities, however the nature and the distribution of these heterogeneities are poorly constrained. For instance, there could be blobs of crustal material, recycled in the deep Earth by plate tectonics. This project will focus on the fundamental processes responsible for the formation of these heterogeneities.

地幔主要由橄榄岩组成，橄榄岩是镁和铁的硅酸盐岩石，通常含有5%至10%的体积不均匀性。这些异质性可以是非常可变的组合物，但富含辉石的岩石，特别是镁橄榄石（那些有额外的钙，钠和铝的硅酸盐晶体结构）似乎是主要类型的异质性与橄榄岩挖出在地球？s表面。到目前为止，还没有研究解释为什么以及如何这种类型的不均匀性在地幔岩石中如此普遍。重要的是，这可能表明一个普遍存在的过程正在显着改变地幔及其衍生熔岩的组成。将进行一项多学科研究，其中将结合联合收割机实地观察、地球化学和岩石物理学分析，对来自两个地点的选定的白榴辉岩进行分析：Lherz地块（法国）和Josephine橄榄岩（美国），据信它们分别代表次大陆和弧下地幔。微量元素和主要元素的变化将被测量和建模，以约束的过程（ES）负责在地幔中的非均质性的形成。此外，通过测量矿物的晶格择优取向的小尺度变化，将限制镁橄榄石侵位的地球动力学条件。这项研究的结果将有利于广大科学家，因为它的多学科方法可以协调地幔地球化学家，流变学实验学家和地球物理学家感兴趣的观测结果。地幔占我们地球体积的80%左右，位于地壳下面大约30公里和2900公里深之间。地球的热变化和化学变化的很大一部分是由地幔控制的，地幔局部融化，并引起了地球表面可见的大部分火山活动。为了更好地预测火山灾害，限制什么类型的物质在深处融化是至关重要的，因为熔岩的成分会影响火山爆发的爆炸性。为了回答这个问题，地幔的成分应该得到很好的约束。地幔岩在地球上的暴露？虽然地球表面很罕见，但我们在哪里可以找到它，我们发现它的组成的许多方面仍然不确定。我们知道，地球的地幔是不均匀的组成，它包含的不均匀性，但这些不均匀性的性质和分布的限制很差。例如，可能有一团地壳物质，通过板块构造在地球深处循环。这个项目将集中在负责这些异质性的形成的基本过程。