title: High Pressure Minerals in the Earth and Moon : Understanding the Lunar Impact History and Earth's Deep Water Cycle
Creators: name: Satta, Niccolò
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: DHMS
subject: water
subject: high pressure
subject: elasticity
subject: Brillouin spectroscopy
subject: lunar regolith
subject: Moon
subject: Apollo 15
subject: 500

description: 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.