Diamond formation in Earth's mantle: Role of C-O-H fluid composition and host rock lithology.
Final Report Abstract
This research explored the formation of diamonds from reduced fluids (C-O-H system) and examined related changes in host rocks deep within Earth’s upper mantle (UM) and transition zone (TZ). These regions are critical for understanding Earth’s seismic and geochemical properties, but they display significant heterogeneity due to processes such as the incomplete reintegration of subducted oceanic crust. Diamonds, as carriers of deep-Earth material, provide unique insights into these processes, but the exact conditions under which diamonds form remain elusive. The project aimed to investigate: (1) The formation of pyroxenites and their role in diamond genesis. (2) The relationship between diamond growth and fluid composition. (2) The potential for diamonds to form independently of host rock composition. Experiments revealed that interaction between siliceous melts and peridotitic mantle material leads to mineral transformations consistent with natural pyroxenite formation. However, despite using advanced capsule designs, diamond formation was not achieved. Instead, metastable graphite was consistently observed, often as idiomorphic platelets. Confocal Raman spectroscopy identified fluid inclusions containing graphite, methane, and water. These results suggest that diamond formation from CH4-rich fluids may depend on the sequestration of hydrogen in olivine and pyroxenes, which has been suggested by Matjuschkin et al. [2020]. The research also highlights the challenges of reproducing natural diamond growth in the laboratory, including kinetic barriers and maintaining stable redox conditions. Although no diamonds were formed, the findings refine our understanding of high-pressure carbon behavior and pave the way for future research.
Publications
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A reversed redox gradient in Earth's mantle transition zone. Earth and Planetary Science Letters, 575, 117181.
Beyer, Christopher; Myhill, Robert; Marquardt, Katharina & McCammon, Catherine A.
