Elastic-thermobarometry: methods and applications to ultrahigh pressure metamorphic rocks

Ultra-High Pressure (UHP) metamorphic rocks represent the evidence we have for the detailed reconstruction of tectonic processes, which are the source of major natural hazards like deep-focus earthquakes and volcanic eruptions. Mineral inclusions are often the only proof of ultrahigh pressure metamorphism and their study provides insights into the mechanisms of subduction and subsequent exhumation of metamorphic rocks. The most frequently used approach for this purpose is the application of equilibrium thermodynamics assuming a linear relationship between the inferred Pressure (P) and the depth of formation of metamorphic mineral assemblages. For instance, coesite or diamond-bearing systems suggest that rocks can be exhumed from depths even greater than 100 km. However, a major current controversy is whether high-pressure minerals actually indicate such great depths of subduction or whether they are the result of tectonic overpressure during subduction. If tectonic overpressure are present, coesite- and diamond-bearing rocks could come from shallower domains of the lithosphere and a revaluation of the current knowledge of plate-tectonics would be necessary. So far, there are no available techniques to constrain the amount of deviatoric paleo-stress present during metamorphic processes. A first attempt has been recently proposed combining mineral physics and petrology: the elastic-thermobarometry. The advantage of this technique is that it is not based on the equilibrium thermodynamic assumption but, rather, on the contrast in the elastic properties of two crystals that are constrained within a confined space such as a mineral inclusion and its surrounding host. The analysis of solid inclusions that are fully buried within their hosts by non-destructive techniques, such as Raman spectroscopy or single crystal X-ray diffraction, reveal pressures that can considerably deviate from the external (ambient) one. This is the so-called residual pressure and it arises as a response to the contrast in the thermo-elastic properties between the host and the inclusion if, for example, the entrapment of the inclusion occurred at high P-T conditions. Importantly, the amount of residual pressure is linked to the entrapment pressure and knowing the physical properties of the two crystals (i.e. their equations of state), using theoretical models it is possible to back-calculate the P-T conditions of inclusion entrapment. The current theoretical models for interpreting the residual pressure are based on simplified assumptions and ideal host-inclusion systems (e.g. isotropic elasticity for both the host and the inclusion crystals, shape of the inclusions are spherical and infinite in size for the host crystal). In this regard, this PhD Thesis has two main objectives: (i) to understand, from an experimental point of view, how much the deviations from the ideal host-inclusion system can actually influence the thermobarometric estimates and (ii) to apply the new theoretical and experimental developments of the elastic thermobarometry to a natural case of study. The first point focuses on the use of Raman spectroscopy to measure and determine the strain state of natural (i.e. non-ideal) mineral inclusions while the second one develops the application of this technique to the famous UHP metamorphic rocks of the Dora Maira Massif (Western Alps). This study allowed the development of experimental protocols devoted in selecting reliable mineral inclusions for elastic-thermobarometric purposes. Backbone of this work are zircon inclusions because they represent one of the most common accessory minerals in metamorphic rocks and, furthermore, can give also age information on the metamorphic processes. Finally, the application of the elastic thermobarometric method to a natural case of study, shows the possibility in considering the presence of deviatoric stresses during inclusion entrapment starting from experimental measurements of stress field in host-inclusion mineral systems. Although this last point remains currently difficult to confirm, the aim of this work is also to give some future perspectives that, eventually, can be used to start in describing metamorphic processes to a higher level that has never been envisaged before.