The evolution of hydrogen defects in diamonds

Earth is a unique planet as liquid water covers 70% of its surface, but the origin of Earth’s water is still a mystery. Many studies consider an extra-terrestrial origin of water (i.e. water was delivered by hydrated materials such as carbonaceous-chondrites and/or comets that formed in the outer Solar System) as the most accreditable theory. In order to establish what are the best candidates to be the source of water on Earth, the H isotopic composition (D/H ratio) of different extra-terrestrial objects must be analysed and compared to that of Earth’s Ocean. However, there is the possibility that the D/H ratio of Earth’s Ocean may not represent the primordial D/H. The best minerals that one can use for this purpose are diamonds. Diamonds are chemically inert and are resistant to biological, chemical and/or physical alteration. They can be very old, up to 3.5 Ga, and they are mined in almost every continent, so they are capable of sampling the entire mantle worldwide. However, an important factor to consider in order to correctly interpret the D/H of natural diamonds is whether the diffusion rate of H in diamond is fast. In fact, if the diffusion rate of H is fast, then the D/H ratio measured in diamond may not reflect the primordial (H2O-bearing) fluids from which it crystallized. Some recent studies show that the H diffusion rate is high in CVD (Chemical Vapor Deposition) diamonds (see Cherniak et al., 2018), but these studies are based on artificial diamonds, which have different structural/growth properties compared to natural diamonds. In fact, the behaviour of H may be significantly different in natural diamonds compared to CVD diamonds, and moreover, the interaction of H with N-related defects is something that has been largely ignored until now. The aim of this thesis is to better understand the H-related defects present in some natural diamonds. These defects are detected using Fourier Transform Infrared (FTIR) spectroscopy. Raw FTIR spectra are processed using different software to obtain information about the defects related to the nitrogen (N) and H. A comparative analysis was done to understand how the H-related defects change at different N-aggregation states, from the moment of initial diamond formation to the time at which B-centers form after long mantle residence times. In doing so, a history of the diamonds is constructed where particular N-aggregation states are shown to mark the appearance (or disappearance) of IR peaks that correspond to different H-related defects. The results show that the H diffusion rate may be almost zero once H is trapped by N-related defects. In fact, many different H-related defects are present in Type Ib to Type IaB diamonds. In general, the number of unique H-defects decreases as N-aggregation progresses, this suggests that many different H-defects combine to form much fewer, relatively more stable defects, over time. These H-defects in Type IaB diamond, that represent the end state of N-aggregation, are observed as two peaks at 3107 and 3236 cm-1 corresponding to the VN3H and VN4H defects, respectively. In Type IaB diamond, these peaks are much more intense than other H-peaks observed in Type Ib + IaA diamond and thus it is likely that most H-related defects eventually combine (aggregate) to form VN3H or VN4H given enough time at sufficiently high mantle residence temperatures. However my interpretations must be confirmed by high-temperature, high-pressure annealing studies of diamond samples studied here to further constrain how N/H-defects evolve with increasing temperature, pressure and time. Nevertheless, my results represent an important piece of information that must be considered when interpreting the D/H ratio of diamonds measured with the Isotope Ratio Mass Spectrometry or other techniques. Such data, coupled with the age of the diamonds, will provide insights into the primordial D/H ratio of Earth and how it changed through time.