Deep roots of the Earth's volatile cycle through inclusions in diamonds

About This Grant

Volatile cycles between Earth’s atmosphere, lithosphere, cryosphere and hydrosphere play a critical role in planet evolution. Carbon, oxygen and hydrogen are the dominant volatile elements, and Earth’s mantle is the largest reservoir of these elements. One of the most important volatiles on Earth is water. Water exerts key controls over the properties of magma, mantle convection, the composition of the Earth’s atmosphere, and the evolution of life. This project will investigate water in the mantle transition zone (410–660 km depth) and deeper. A small proportion of diamonds, carried to the surface by rare and rapid volcanic processes, encapsulate minerals during their growth. These minerals are among the deepest direct samples available of the Earth's mantle. The project will use a collection of diamonds hosting such minerals to determine the water concentrations of the mantle where the diamonds formed. The project will also expand a large interactive exhibit in the Minerals Hall of the American Museum of Natural History into a web-based version on a platform for children and youth, called OLogy, and hosted by the museum. The new web version will be tested in upper elementary and middle school classrooms. Water is one of the most important volatiles on Earth. This project will investigate water in the deep regions of the planet, such as the mantle transition zone (410–660 km) and the lower mantle (660–2900 km), by studying mineral inclusions in diamonds. A small proportion of diamonds, carried to the surface by kimberlite pipes, contain inclusions of minerals that were encapsulated by the diamond during its growth. The main goals of this project are: 1) to identify the flux of water transported by subducted slabs into the deep subcontinental mantle; 2) to estimate the water budget in the mantle transition zone and the transition zone–lower mantle boundary, and to assess water concentrations in deep mantle minerals such as majoritic garnet and ferropericlase; 3) to investigate whether water is responsible for major redox (reduction–oxidation) reactions in the deep mantle. The project will investigate majoritic garnet and ferropericlase inclusions in diamonds from ultra-deep sources, such as the Amazonian craton in Brazil. Specifically, the research will identify and characterize mineral inclusions in a set of diamonds from Juina, Brazil, using optical microscopy, Fourier Transform Infrared and Raman spectroscopy, as well as cutting-edge computed tomography. Some majoritic garnets will also be studied by synchrotron Mössbauer spectroscopy, and phases that are difficult to identify will be examined by synchrotron X-ray diffraction. Some of the majoritic garnet and ferropericlase inclusions will be used for water analysis. The largest inclusions and those covering a wide range of compositions will be selected for water analysis by nanoSIMS. While sufficient standards exist for the analysis of majoritic garnets, there are currently no standards for ferropericlase. Therefore, this project will also experimentally synthesize standards for water analysis in ferropericlase. Given the unique nature of the studied diamonds, there is strong potential for side projects arising from newly discovered and exposed mineral inclusions in Juina diamonds. 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.