The Systematics of Helium in Diamond-forming Metasomatic Mantle Fluids

我们对地球如何通过板块构造运行的理解在很大程度上是由于对地球深处发生的过程的研究。然而，几乎所有关于地球深处的研究都是间接的。例如，深部地球过程是从火山喷发的岩浆中推断出来的，这些岩浆不是直接的深部地球样本，而是反映了最容易熔化的地幔岩石部分，深部地球的物理性质是从地震引发的地震波的行为中推断出来的。另一方面，钻石非常特别，因为它们提供了独特的“窗口”，可以直接看到地球的深处。它们形成于地球表面下140公里深处，由循环的热的富碳和富水(C-O-H)流体形成，这些流体通常被捕获为“流体包裹体”。这些包裹体是地球深部流体的唯一直接样本，反映了钻石的物理强度和化学惰性。少数发表的关于深层C-O-H流体的研究得出结论，它们是在地球表面和内部之间循环的。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和同位素组成。这些结果将记录它们的起源、钻石的年龄，以及在地表观察到的历史与地球深处发生的过程之间的关系。在地球深处形成钻石并被困在钻石中的富碳和富水(C-O-H)流体为研究地球深处拥有的钻石提供了独特的机会。但到目前为止，还没有一种技术能够对如此深厚的C-O-H流体提供可靠的年龄限制。这些流体具有较高的氦浓度，并富含U和Th等微量元素。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)，并限制它们对不同岩石圈省份的影响的时间。通过结合碳酸盐岩、硅质和盐岩三种类型C-O-H流体的氦同位素比值和微量元素组成，这项研究将限制地球深部C-O-H流体的来源，例如，它们是通过俯冲从地表引入的，还是起源于地幔。这项研究将提供有关地球深处随时间推移的C-O-H流体的新见解。此外，它将对我们理解深层的碳和水循环以及地幔和地壳之间挥发分的全球循环产生重要影响，这对地球表面的生命具有根本性的影响。

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.