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The subduction of serpentinized slabs is the dominant process to transport “water” into Earth's mantle, and plays a pivotal role for subduction dynamics. Antigorite, the most abundant serpentine mineral in subduction settings, may imprint a seismic signature on serpentinized slabs, making them seismically distinguishable from the dry, non‐serpentinized ones. However, the complete single‐crystal elasticity of antigorite has not been experimentally constrained at high pressures, hindering the use of seismological approaches to detect serpentinization in subducting slabs. Here, we report the full elastic stiffness tensor of antigorite by single‐crystal Brillouin spectroscopy and X‐ray diffraction up to 7.71(5) GPa. We use our results to model seismic properties of antigorite‐bearing rocks and show that their seismological detectability depends on the geometrical relation between seismic wave paths and foliation of serpentinized rocks. In particular, we demonstrate that seismic shear anisotropy shows low sensitivity to serpentinization for a range of relevant geometries., Plain Language Summary: The subduction of serpentinized slabs plays a key role in the deep recycling of water into the Earth's interior. Antigorite is the main serpentine mineral in subducting slabs, and the most important carrier of water. Antigorite‐bearing rocks are predicted to have a distinct seismic signature, potentially allowing them to be detected with seismological approaches. However, our current knowledge on seismic properties of antigorite‐bearing rocks is limited, mostly hampered by a lack of experimental constraints on single‐crystal elasticity of antigorite at relevant pressures. In this study, state‐of‐the‐art techniques were employed to produce the first experimental description of the complete high‐pressure elasticity of antigorite single crystals. Our experimental data set was implemented in the modeling of seismic properties of antigorite‐bearing rocks at pressures relevant for subduction. Our results were used to discuss the relation between seismic wave path and shear wave anisotropy in serpentinized slabs, and challenge the use of shear wave splitting as a proxy for serpentinization in slabs., Key Points: Single‐crystal elasticity of antigorite at high pressures is determined by Brillouin spectroscopy and X‐ray diffraction experiments. Seismic signature of serpentinized slabs is constrained in a relevant composition‐pressure space. Serpentinization in slabs may be undetectable through shear wave anisotropy., Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659, EC Horizon 2020 Framework Programme http://dx.doi.org/10.13039/100010661, https://doi.org/10.6084/m9.figshare.20348748, https://doi.org/10.6084/m9.figshare.20348781

The detection and study of high pressure minerals either remotely through seismology or in natural specimens can provide important constraints on physical and chemical properties occurring at normally inaccessible conditions, such as during planetary impact events or deep inside planets. For four and a half billion years, countless impact events have shattered the Moon's surface, leaving a unique record of impact craters. Understanding the nature, and estimating the ages of the largest lunar craters was among the main goals of the Apollo missions. However, despite the large number of samples collected, the ages of the largest craters are still debated. 40Ar/39Ar ages constrained in lunar samples may be biased by subsequent thermal events, hampering our current understanding of the Moon's collisional history. A viable way to evaluate this possibility is to evaluate the behaviour of lunar regolith under shock compression. In this thesis, scanning and transmission electron microscope techniques are used to constrain shock conditions recorded in a regolith breccia, by a detailed description of shockinduced microtextures and mineralogical assemblages. I present the first observation of natural ferropericlase in a lunar rock. My observations suggest that the lunar ferropericlase formed as a result of shock-induced incongruent melting of olivine, a phenomenon found previously only in experiments. Furthermore, I estimated the pressure – temperature evolution of the shock event. Our results indicate that because of its porous nature, the lunar regolith can experience elevated temperatures even during low magnitude impacts. Based on these ndings, we suggest that a more accurate estimate of the ages of the main collisional episodes of the Moon's surface requires a reevaluation of the current 40Ar/39Ar constrains. Subduction of altered oceanic slabs and hydrous sediments control the input of water into the deep Earth's interior. During subduction, hydrous materials are exposed to increasing pressures and temperatures, which causes a chain of prograde metamorphic reactions to occur. Previous experimental investigations indicate that water, bound as hydroxyl groups, can be passed between hydrous phases and consequently delivered by subduction to the deepest portions of the Earth's mantle. Seismological surveys provide information on the seismic structures that characterize subducting scenarios, however, an accurate interpretation of the hydration state is achievable only through experimental constraints on the possible seismic signatures of these hydrous phases. In this thesis, I conducted two projects with the aim of characterizing the single-crystal elasticity of phase E and -(Al,Fe)OOH, two hydrous phases relevant for the delivery and stabilization of water in the Earth's deep interior. In the case of phase E, experimental methodologies were used for the synthesis of single crystals, and an accurate chemical characterization was achieved with state-of-the-art analytical techniques. Brillouin spectroscopy and X-ray diffraction analysis were employed to determine the full elastic tensor and unit-cell parameters, respectively. I found that phase E has very low aggregate velocities, signi cantly lower than those of other minerals expected to be stable at the same pressure and temperature conditions. By combining my findings with previous experimental investigations, aggregate velocities of subducted rocks were evaluated assuming different hydration states. These results imply that if present, phase E is capable of significantly lowering seismic wave velocities, raising the possibility that this hydrous phase could be detected remotely allowing hydrated regions of the deep mantle to be mapped. By performing Brillouin spectroscopy and X-ray diffraction measurements in a diamondanvil cell, the structure and elastic properties of -(Al,Fe)OOH have been examined up to pressures where a second order phase transformation occurs from the P21nm space group to Pnnm. The elastic tensors of both the P21nm and Pnnm structures were constrained experimentally. In addition, by tracking the intensity attenuation of selected reflections we were able to tightly constrain the transition pressure. Our findings are in agreement with previous investigations on the aluminium end member, suggesting that the incorporation of Fe3+ has a limited effect on the P21nm to Pnnm phase transition. Both X-ray diffraction and Brillouin spectroscopy results show that, prior to the transition into the Pnnm phase, the P21nm -(Al,Fe)OOH phase experiences an elastic softening. This softening is associated with a change in the hydrogen bond configuration from asymmetric (P21nm) to disordered (Pnnm). Similar changes can be expected in other hydroxide minerals, suggesting that the elastic softening may be a common precursor of hydrogen bond symmetrization.