Earth’s Inner Workings Revealed through Mineral Inclusions in Diamond

Wenz, Michelle Dawn

doi:https://doi.org/10.21985/n2-x0j5-fk04

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Earth’s Inner Workings Revealed through Mineral Inclusions in Diamond

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Mineral inclusions in diamond, brought to the surface through kimberlitic eruptions, provide a unique glimpse into the geochemical inner workings of Earth’s deep interior. Diamonds source a wide range of depths in the mantle. While most diamonds originate from the upper 200 km of the Earth’s mantle, aptly called super-deep diamonds originate from depths ranging between 300 and 1000 km serving as the only natural samples from such depths. By studying mineral inclusions in diamond an insight into the chemical cycling between the Earth’s surface and interior is gained. In the first study, I co-developed a fast high-throughput method for the fast identification of minerals inclusions while they remain encased in diamond with the GeoSoilEnviro Center for Advanced Radiation Sources (GSECARS). Prior to 2011, most mineral inclusions in diamond were limited to destructive methods for identification. By studying these inclusions in-situ high-pressure phases, oxidation states, and remnant inclusion pressures are preserved, providing valuable insight into the geochemical conditions (i.e. redox conditions) and the geochemical recycling of Earth’s deep interior (i.e. recycling of biocritical elements such as H, B, C, P, S, Cl, and Ca). This method is now being used by other researchers at GSECARS, where the new setup is now accessible through the General User Program at the Advanced Photon Source. In the second study, I investigated inclusions in a suite of 121 diamonds from Juína, Brazil a locality known to produce super-deep diamonds from the transition zone (410-660 km) and lower mantle (>660 km). These diamonds were investigated by Fourier Transform Infrared (FTIR) spectroscopy, ultraviolet (UV) imaging, and synchrotron X-ray microtomography to characterize their atomic-scale defects (N, H, and B) and mineral inclusions. A subset of 41 diamonds were selected for study by synchrotron X-ray diffraction to determine the diversity of mineral inclusions, the orientation distribution of the inclusions, and to search for potentially hydrous minerals. A total of 107 mineral inclusions were successfully identified via their lattice parameters while they remained encased in diamond, representing one of the largest catalogs of mineral inclusions in diamonds from a single locality. In the third study, I investigated the first known blue-colored olivine, found included within a Type IaAB triangular macle (twinned) diamond, using a variety of non-destructive techniques including: X-ray microtomography, FTIR Spectroscopy, single-crystal X-ray diffraction, X-ray fluorescence, photoluminescence spectroscopy, UV fluorescence imaging, Raman spectroscopy, Mӧssbauer spectroscopy, visible absorption spectroscopy, and X-ray absorption near-edge structure. UV-VIS absorption spectroscopy and the presence of metallic Fe-Ni alloy within the olivine inclusion suggest trace Cr2+ as the possible cause of the blue color. XANES spectroscopy revealed that the average Cr valence state of the blue olivine was 2.98(3) and did not show a peak at the diagnostic Cr2+ energy. However, if we consider how much Cr2+ could be present based on two times the standard deviation, the valence could be as low as 2.92 (8% Cr 2+ ). Therefore, we cannot rule out reduced Cr as the cause of the unique blue color, but future experimental work is required to determine how much reduced Cr is needed to cause a blue color in olivine.

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