The Systematics of Helium in Diamond-forming Metasomatic Mantle Fluids
DESCRIPTION: 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.