The Fate of Earth's Plates: Sublithopsheric Diamond Constraints on Recycling in Earth’s Mantle Transition Zone

地球的地幔可以分为上地幔和下地幔，中间有一个被称为“地幔过渡带”的区域，位于410至660公里的深度之间。板块构造的活跃过程将寒冷的、受海水侵蚀的、富含挥发性元素的海底岩石板块带入并穿过这个地区，在那里它们被认为是停滞和升温的。加热后，金属板会释放水、氢、碳、硼和铁到液体中。这些流体从板块迁移到周围的地幔，导致钻石在不断产生地球上最深、最强烈地震的地区结晶。这个被称为循环的过程已经在地球上发生了数十亿年，并且被认为对深部地幔的地球化学组成有深远的影响。为了更好地了解地幔的这一区域，将对微小的矿物包裹体进行复杂而敏感的实验室微量分析。这些矿物包裹体与稀有毛坯钻石共结晶，并被包裹在已知存在的最大、最完美的刻面钻石的边角料中。虽然从地球表面可以看到板块构造循环过程的许多表现形式，但这项工作的重要性在于，它是用实际样品追踪板块构造最深层方面的唯一途径，从而了解深度回收过程的结果。该项目的广泛影响包括培训早期职业科学家，开发和分享新的分析方法，开发用于钻石学校、短期课程和教科书的知识，以及公众对世界上最珍贵的宝石是如何被创造出来的广泛兴趣。岩石圈下钻石（在次大陆岩石圈地幔下方对流地幔中形成的钻石）根据其独特的高压、退变矿物包裹体组合被认为形成于地幔过渡带内或附近。岩石圈下钻石的两个“家族”可以通过它们产生的两种类型的俯冲海洋岩石圈来区分：碳酸化海洋地壳导致钻石从碳酸盐岩熔体结晶，或蛇纹岩化地幔导致钻石从释放的金属液体或含水超临界流体结晶。这两种岩石圈下类型的样品均来自巴西Juina地区；莱索托莱特森矿；还有其他来源。结合矿物颗粒原位微分析技术（共聚焦拉曼、同步加速器显微断层扫描、SEM、EPMA和SIMS）和提取矿物包裹体的大块同位素技术（MC-ICPMS和N-TIMS），将分析选定的钻石及其矿物包裹体的主要元素组成、挥发性物质（H2、CH4、OH）、稳定同位素（13C、56Fe）和放射性成因（Os）同位素组成。特别是，56Fe和187Os/188Os同位素组成将用于确定碳质金刚石和含金属包裹体金刚石中的氧化铁相是否与同一铁源有关。如果是这样，在地幔和地壳中流动的流体之间的直接联系将被揭示出来。还将对选定的钻石、其包含的矿物组合及其伴生流体种类进行表征，以获得岩石成因限制。该研究的基本目标是通过跟踪板块的地壳和地幔部分脱碳、脱水和释放流体的过程，揭示在地幔过渡带深度板块中发生的流体演化/融化过程。这些流体是如何释放的，它们是如何迁移的，以及它们在改变它们被释放到的深部地幔的成分方面发挥了什么作用，这些都是主要的问题。第二个目标是将地幔的潜在影响与地球表面火山岩中明显的成分变化联系起来。这里感兴趣的是在深俯冲过程中碳、水和其他成分的矿物运输机制，以及伴随循环过程的板块和周围地幔的化学转化。拟议的研究将跟踪俯冲到远远超出正常板块构造表面可观测范围的深度，并将更清楚地了解深部再循环的基本地球动力学过程和深部地幔非均质性的产生。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Earth’s mantle can be divided into an upper mantle and a lower mantle separated by a region known as the “mantle transition zone” occurring between 410 and 660 km depths. The active process of plate tectonics brings cold, seawater-altered, and volatile-element-rich slabs of ocean floor rock into and through this region where they are thought to stall and heat up. Upon heating, the plates release water, hydrogen, carbon, boron, and iron into fluids. These fluids migrate from the slab into the surrounding mantle and cause diamonds to crystallize in regions that constantly produce Earth’s deepest and most energetic earthquakes. This process, known as recycling, has been occurring on Earth for billions of years and is thought to have profound effects on the geochemical composition of the deep mantle. To understand this region of the mantle better, work will be carried out with sophisticated and sensitive laboratory microanalysis of tiny mineral inclusions. These mineral inclusions co-crystallized with and are encased in rare rough diamonds and the offcuts of the largest and most flawless facetted diamonds known to exist. While many manifestations of the recycling process of plate tectonics are visible from Earth’s surface, the importance of this work lies in it being the only way to trace, with actual samples, the deepest aspects of plate tectonics and thus the outcome of the recycling process at depth. The broader impacts of the project include training of early-career scientists, development and sharing of new analytical methods, development of knowledge for use in diamond schools, short courses, and textbooks, and massive public interest on how the world’s most valuable gemstones were created. Sublithospheric diamonds (diamonds forming in the convecting mantle below the subcontinental lithospheric mantle) have been recognized to form in or near the mantle transition zone on the basis of their distinctive high-pressure, retrogressed mineral inclusion assemblages. Two ‘families’ of sublithospheric diamonds can be distinguished by the two types of subducted oceanic lithosphere from which they arose: carbonated oceanic crust leading to diamonds that crystallize from carbonatitic melts, or serpentinized mantle leading to diamonds that crystallize from released metallic liquids or aqueous, supercritical fluids. Samples of both sublithospheric types are available for this work from the Juina, Brazil area; from the Letseng Mine, Lesotho; and other sources. A combination of microanalytical techniques on mineral grains in-situ (confocal Raman, synchrotron micro-tomography, SEM, EPMA, and SIMS) and bulk isotopic techniques on extracted mineral inclusions (MC-ICPMS and N-TIMS) will be used to analyze selected diamonds and their mineral inclusions for their major element compositions, volatile species (H2, CH4, OH), and stable (13C, 56Fe) and radiogenic (Os) isotopic compositions. In particular, 56Fe and 187Os/188Os isotopic compositions will be used to see if the iron oxide phases in the carbonatitic diamonds and the metallic inclusion bearing diamonds are related to the same source of iron. If so, a direct connection between fluids mobilized in the mantle versus crust would be revealed. The selected diamonds, their included mineral assemblages and their associated fluid species also will be characterized to obtain petrogenetic constraints. The fundamental goal of the research is to unravel fluid evolution / melting processes occurring in the slab at mantle transition zone depths by tracking the crust versus mantle portions of the slab as it decarbonates, dehydrates, and releases fluids. How the fluids are released, how they migrate, and what roles they play in modifying the composition of the deep mantle into which they are released are primary questions. A secondary goal is to relate potential effects on the mantle to the compositional variability that is evident in volcanic rocks at Earth’s surface. Of interest here are the mineralogical transport mechanisms for carbon, water, and other components during deep subduction and the chemical transformations in the slab plus the ambient mantle that accompany the recycling process. The proposed research will follow subduction to depths far beyond normal plate tectonic surface observables and will lead to a clearer understanding of the basic geodynamic processes of deep recycling and the creation of deep-seated mantle heterogeneity.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.