The Systematics of Helium in Diamond-forming Metasomatic Mantle Fluids

金刚石交代地幔流体中氦的系统学

基本信息

批准号：
1725323
负责人：
Cornelia Class
金额：
$ 36.58万
依托单位：
Columbia University
依托单位国家：
美国
项目类别：
Continuing Grant
财政年份：
2017
资助国家：
美国
起止时间：
2017-07-15 至 2023-06-30
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1725323&HistoricalAwards=false
关键词：
Systematics Helium Diamond forming Metasomatic

项目摘要

Our understanding of how the Earth operates through plate tectonics is to a large extent due to research on processes that occur in the deep Earth. However, almost all studies of the deep Earth are indirect. For example, deep Earth processes are inferred from magmas erupting in volcanoes, which are not direct deep earth samples but rather reflect the most easily melted fraction of mantle rocks, and physical properties of the deep earth are inferred from the behavior of seismic waves triggered by earthquakes. Diamonds, on the other hand are very special in that they offer unique "windows" to look directly at deep parts of the Earth. They form at depths 140 km beneath the Earth's surface, from circulating hot carbon- and water-rich (C-O-H) fluids, which are often captured as "fluid inclusions". These inclusions represent our only direct samples of deep Earth fluids, reflecting the physical strength and the chemically inert nature of diamonds. The few published studies on deep C-O-H fluids conclude they are cycled between the Earth's surface and interior. This study will measure the chemistry and isotopic compositions of fluid inclusions in diamonds from different continents. The results will document their origin, the ages of the diamonds, and the relationships between the history observed at the surface and the processes occurring in the deep earth.Carbon- and water-rich (C-O-H) fluids, that form diamonds in the deep earth and are trapped in the diamonds, offer unique opportunities to investigate deep Earth possesses. But until now no technique has been able to provide reliable age constraints on such deep C-O-H fluids. These fluids have high helium concentrations and are enriched in trace elements, including U and Th. This study utilizes the radioactive U-Th-He decay system to determine ages of the fluids trapped in diamonds (yielding the diamond formation ages), and to constrain the timing of their impact on different lithospheric provinces. By combining the He isotope ratios with trace element compositions in the three types of C-O-H fluids (carbonatitic, silicic, and saline), this study will constrain the sources of C-O-H fluids in the deep Earth, for example, whether they were introduced from the surface by subduction or whether they originate in the mantle. The study will provide new insights on C-O-H fluids in the deep earth through time. Moreover, it will have important impacts on our understanding of the deep carbon and water cycle and the global circulation of volatiles between mantle and crust, which have fundamental impacts on life at the Earth's surface.

我们对地球如何通过板块构造运行的理解在很大程度上归功于对地球深处发生的过程的研究。然而，几乎所有对地球深处的研究都是间接的。例如，地球深层过程是根据火山喷发的岩浆推断出来的，这些岩浆不是直接的地球深层样本，而是反映了地幔岩石最容易熔化的部分，而地球深层的物理性质是根据火山爆发引发的地震波的行为推断出来的。地震。另一方面，钻石非常特别，因为它们提供了独特的“窗口”，可以直接观察地球深处。它们形成于地表以下 140 公里深处，由循环的富含碳和水 (C-O-H) 的热流体形成，这些流体通常被捕获为“流体包裹体”。这些内含物代表了我们唯一的地球深层流体直接样本，反映了钻石的物理强度和化学惰性。少数已发表的关于深层 C-O-H 流体的研究得出结论，它们在地球表面和内部之间循环。这项研究将测量来自不同大陆的钻石中流体包裹体的化学成分和同位素组成。结果将记录它们的起源、钻石的年龄，以及地表观察到的历史与地球深处发生的过程之间的关系。富含碳和水的 (C-O-H) 流体，在地球深处形成钻石并被困在钻石中，为调查地球深处的资源提供了独特的机会。但到目前为止，还没有任何技术能够对如此深的 C-O-H 流体提供可靠的年龄限制。这些流体具有高氦浓度，并且富含微量元素，包括 U 和 Th。这项研究利用放射性 U-Th-He 衰变系统来确定钻石中流体的年龄（得出钻石形成年龄），并限制其对不同岩石圈区域的影响时间。通过将He同位素比值与三种类型的C-O-H流体（碳酸岩、硅质和盐水）中的微量元素成分相结合，这项研究将限制地球深处C-O-H流体的来源，例如它们是否是从地表引入的俯冲作用或它们是否起源于地幔。这项研究将为地球深处的 C-O-H 流体随时间变化提供新的见解。此外，它将对我们对深层碳和水循环以及地幔和地壳之间挥发物的全球循环的理解产生重要影响，从而对地球表面的生命产生根本性的影响。