Experimental Investigations on Elastic Properties of Davemaoite and the Iron Carbide-Water Reactions in the Mantle

Abstract

Geophysics has provided a comprehensive overview of the composition and structure of Earth's interior, revealing a silicate mantle and metallic core. In the mantle, CaSiO3 perovskite (davemaoite) poses a challenge due to its enigmatic nature. Simultaneously, the deep volatile cycle raises controversy, considering the unexpectedly high carbon concentration in the mantle and the unidentified light elements contributing to the core's density deficit. The first project (Chapter 3) delves into the structure and density of Ti-bearing davemaoite. As the third and second most abundant phase in the pyrolytic mantle and subducted mid-ocean ridge basalt, respectively, davemaoite plays a crucial role as a Ti-hosting phase. In Chapter 3, we scrutinized the structure and density of Ti-bearing davemaoite (Ca(Si,Ti)O3) under relevant mantle pressure conditions. Surprisingly, Ti-bearing davemaoite was found to be less dense than its pure CaSiO3 counterpart. Given this lower density, we postulate that Ti-bearing davemaoite could contribute to slab stagnation and the formation of large low shear velocity provinces. The second project (Chapter 4) focuses on reactions between volatile-bearing species in the mantle. By investigating Fe3C-H2O and Fe7C3-H2O using a multi-anvil press at 15-23 GPa, 1000-1600°C, with Mg(OH)2 as a water source, we discovered diamond formation through the iron carbide-water reaction, confirmed by electron microprobe and Raman spectroscopy. We propose that diamonds might have formed and been retained in the silicate portion of Earth if Fe-C alloy droplets interacted with water in the early magma ocean. Furthermore, the iron carbide-water reaction could persist, generating diamonds on the slab's surface in the modern mantle, a factor to consider when calculating specific water quantities delivered to the core through slab subduction. The third project (Chapter 5) explores the direct reaction between iron carbide and water ice at ~20-60 GPa using a laser heating diamond anvil cell. The results indicate diamond formation through a direct reaction between iron carbide and water ice, as confirmed by XRD and Raman spectrometry. This suggests that diamonds can form at various depths whenever iron carbide encounters a water source. However, the data reveal a complex assembly of reaction products primarily due to system disequilibrium, prompting the need for future work with higher heating temperatures and longer durations. In summary, Chapter 3 proposes that Ti-bearing davemaoite may contribute to slab stagnation and the formation of large low-shear velocity provinces. Chapters 4 and 5 unveil that diamonds can form in the mantle when iron carbide encounters water sources, providing new insights into our understanding of the deep volatile cycle.