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发生在地球深处的过程的研究。然而，几乎所有对地球深部的研究都是间接的。例如，地球深部过程是从火山喷发的岩浆中推断出来的，火山喷发的岩浆不是直接的地球深部样品，而是反映了地幔岩石中最容易熔化的部分，而地球深部的物理性质是从地震触发的地震波的行为中推断出来的。另一方面，钻石是非常特殊的，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以下140公里的深处，来自循环的热的富含碳和水（C-O-H）的流体，这些流体通常被捕获为“流体包裹体”。这些包裹体代表了我们唯一的地球深部流体直接样品，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。本研究将测量来自不同大陆的金刚石中流体包裹体的化学和同位素组成。结果将记录它们的起源、钻石的年龄以及地表观测到的历史与地球深部发生的过程之间的关系。富含碳和水（C-O-H）的流体在地球深部形成钻石并被困在钻石中，为研究地球深部提供了独特的机会。但到目前为止，还没有任何技术能够提供可靠的年龄限制这样深的C-O-H流体。这些流体具有高氦浓度，并富含微量元素，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捕获的流体的年龄（产生钻石形成年龄），并限制它们对不同岩石圈省的影响的时间。通过对三种类型C-O-H流体（碳酸岩型、碳酸盐型和盐水型）的He同位素比值和微量元素组成的综合分析，可以确定深部C-O-H流体的来源，例如，它们是通过俯冲作用从地表引入的，还是起源于地幔。这项研究将为地球深部的C-O-H流体提供新的见解。此外，它将对我们理解深层碳和水循环以及地幔和地壳之间挥发物的全球循环产生重要影响，这些循环对地球表面的生命具有根本影响。

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.