Understanding Hydrogen in Diamonds: A Foundation for Tracing the Origin of Earth’s Water

The source of Earth’s water is a fundamental question in planetary science. Natural diamonds, which form deep within the Earth’s mantle, provide an exceptional archive for the exploration of primordial hydrogen owing to their ability to capture and store hydrogen-related defects for geological timescales. This thesis discusses the formation and evolution of hydrogen in natural diamonds through studying signals in the three-phonon region of the Fourier-transform infrared (FTIR) spectra due to hydrogen(H)-nitrogen(N)-vacancy(V) complexes, commonly referred to as VxNyHz defects. Using a dataset composed of natural diamonds with a variety of N-aggregation states, has enhanced our understanding of how hydrogen is incorporated as an interstitial atom or ion, how H forms structural defects, and how such defects evolve alongside N-aggregation processes with progressive mantle annealing. Quantitative analysis of the positions and relative intensity of H-related IR peaks has revealed a systematic evolution of H-related defects as a function of N-aggregation state, expressed here as %IaA. Analysis of H-related signal in the three-phonon region of diamonds with low to intermediate %IaA shows gradual development towards fewer and more stable VxNyHz defects with increasing %IaA. The data suggests a sequence of defect evolution in which less stable, early-stage H defects progressively combine with other defects towards more thermodynamically favorable VxNyHz defects during diamond residence in the mantle. Strong correlations between specific H-related peaks point to structural relations, whereas differences in the onset and intensity of different H-related peaks provide insights into how H-defects evolve with time and temperature. These findings suggest that natural diamonds behave as robust repositories of H capable of preserving primordial H signatures during mantle residence periods of millions to billions of years. Diamonds, by maintaining H in stable VxNyHz, defects, might contain H isotopic signatures that reflect the Earth’s early volatile inventory. Such work emphasizes the promise of diamond as a geochemical archive for investigating the origins of terrestrial water.