Coesite is one of the most studied phases in high-pressure mineralogy and petrology as it represents a real high-pressure marker characterized by an extremely simplified chemistry. For this reason, since its discover several scientists tried to use coesite as a geobarometer with the final aim to provide an exact value of crystallization pressure. However, nobody was really able to obtain such information from coesite so far. Considering that this would provide an estimate of pressure of formation for coesite-bearing eclogitic rocks, the use of coesite as a geobarometer could be of extreme help. This master thesis will be focused on the geobarometry of diamond relatively to the coesite-diamond pair starting from the pioneering work published in 2000 by Sobolev et al. on PNAS magazine. In that work the authors applied the socalled “elastic method” to obtain the residual pressure and then to calculate the pressure of formation of diamond and coesite. However, it is today accepted that some thermoelastic parameters of coesite (i.e. thermal expansion and dependence of bulk modulus with temperature) do not allow to obtain reliable values of pressure of formation for the coesite-diamond pair. In this thesis work I have obtained new thermal expansion data on coesite beyond the dependency of the bulk modulus with temperature. This last parameter was measured by using Resonant Ultrasound Spectroscopy at low temperature, whereas the thermal expansion was determined by first-principles methods. My results indicate both that the coesite-diamond pair cannot be considered a reliable geobarometer yet, and that the improved “elastic method”, together with the isomeke calculation, need a more-in-depth study on its range of use. In addition, it was demonstrated that the pioneering work by Sobolev et al. (2000) reported wrong calculations on the pressure of formation of coesite.

Thermoelastic parameters of coesite: implications for diamond geobarometry

Anzolini, Chiara
2014/2015

Abstract

Coesite is one of the most studied phases in high-pressure mineralogy and petrology as it represents a real high-pressure marker characterized by an extremely simplified chemistry. For this reason, since its discover several scientists tried to use coesite as a geobarometer with the final aim to provide an exact value of crystallization pressure. However, nobody was really able to obtain such information from coesite so far. Considering that this would provide an estimate of pressure of formation for coesite-bearing eclogitic rocks, the use of coesite as a geobarometer could be of extreme help. This master thesis will be focused on the geobarometry of diamond relatively to the coesite-diamond pair starting from the pioneering work published in 2000 by Sobolev et al. on PNAS magazine. In that work the authors applied the socalled “elastic method” to obtain the residual pressure and then to calculate the pressure of formation of diamond and coesite. However, it is today accepted that some thermoelastic parameters of coesite (i.e. thermal expansion and dependence of bulk modulus with temperature) do not allow to obtain reliable values of pressure of formation for the coesite-diamond pair. In this thesis work I have obtained new thermal expansion data on coesite beyond the dependency of the bulk modulus with temperature. This last parameter was measured by using Resonant Ultrasound Spectroscopy at low temperature, whereas the thermal expansion was determined by first-principles methods. My results indicate both that the coesite-diamond pair cannot be considered a reliable geobarometer yet, and that the improved “elastic method”, together with the isomeke calculation, need a more-in-depth study on its range of use. In addition, it was demonstrated that the pioneering work by Sobolev et al. (2000) reported wrong calculations on the pressure of formation of coesite.
2014-09-16
Coesite, diamond, geobarometry, thermoelastic
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.12608/18202