The Fate of Earth's Plates: Sublithopsheric Diamond Constraints on Recycling in Earth’s Mantle Transition Zone

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Earth’s mantle can be divided into an upper mantle and a lower mantle separated by a region known as the “mantle transition zone” occurring between 410 and 660 km depths. The active process of plate tectonics brings cold, seawater-altered, and volatile-element-rich slabs of ocean floor rock into and through this region where they are thought to stall and heat up. Upon heating, the plates release water, hydrogen, carbon, boron, and iron into fluids. These fluids migrate from the slab into the surrounding mantle and cause diamonds to crystallize in regions that constantly produce Earth’s deepest and most energetic earthquakes. This process, known as recycling, has been occurring on Earth for billions of years and is thought to have profound effects on the geochemical composition of the deep mantle. To understand this region of the mantle better, work will be carried out with sophisticated and sensitive laboratory microanalysis of tiny mineral inclusions. These mineral inclusions co-crystallized with and are encased in rare rough diamonds and the offcuts of the largest and most flawless facetted diamonds known to exist. While many manifestations of the recycling process of plate tectonics are visible from Earth’s surface, the importance of this work lies in it being the only way to trace, with actual samples, the deepest aspects of plate tectonics and thus the outcome of the recycling process at depth. The broader impacts of the project include training of early-career scientists, development and sharing of new analytical methods, development of knowledge for use in diamond schools, short courses, and textbooks, and massive public interest on how the world’s most valuable gemstones were created. Sublithospheric diamonds (diamonds forming in the convecting mantle below the subcontinental lithospheric mantle) have been recognized to form in or near the mantle transition zone on the basis of their distinctive high-pressure, retrogressed mineral inclusion assemblages. Two ‘families’ of sublithospheric diamonds can be distinguished by the two types of subducted oceanic lithosphere from which they arose: carbonated oceanic crust leading to diamonds that crystallize from carbonatitic melts, or serpentinized mantle leading to diamonds that crystallize from released metallic liquids or aqueous, supercritical fluids. Samples of both sublithospheric types are available for this work from the Juina, Brazil area; from the Letseng Mine, Lesotho; and other sources. A combination of microanalytical techniques on mineral grains in-situ (confocal Raman, synchrotron micro-tomography, SEM, EPMA, and SIMS) and bulk isotopic techniques on extracted mineral inclusions (MC-ICPMS and N-TIMS) will be used to analyze selected diamonds and their mineral inclusions for their major element compositions, volatile species (H2, CH4, OH), and stable (13C, 56Fe) and radiogenic (Os) isotopic compositions. In particular, 56Fe and 187Os/188Os isotopic compositions will be used to see if the iron oxide phases in the carbonatitic diamonds and the metallic inclusion bearing diamonds are related to the same source of iron. If so, a direct connection between fluids mobilized in the mantle versus crust would be revealed. The selected diamonds, their included mineral assemblages and their associated fluid species also will be characterized to obtain petrogenetic constraints. The fundamental goal of the research is to unravel fluid evolution / melting processes occurring in the slab at mantle transition zone depths by tracking the crust versus mantle portions of the slab as it decarbonates, dehydrates, and releases fluids. How the fluids are released, how they migrate, and what roles they play in modifying the composition of the deep mantle into which they are released are primary questions. A secondary goal is to relate potential effects on the mantle to the compositional variability that is evident in volcanic rocks at Earth’s surface. Of interest here are the mineralogical transport mechanisms for carbon, water, and other components during deep subduction and the chemical transformations in the slab plus the ambient mantle that accompany the recycling process. The proposed research will follow subduction to depths far beyond normal plate tectonic surface observables and will lead to a clearer understanding of the basic geodynamic processes of deep recycling and the creation of deep-seated mantle heterogeneity.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.