Thermal expansion of mantle mineral inclusions in diamonds

The aim of this PhD thesis is to provide new constraints on the thermoelastic behavior of some minerals phases frequently occurring as inclusions in diamonds. Knowledge of the thermoelastic properties of minerals is a fundamental requirement to enable the estimate of the entrapment pressures for host-inclusion pairs using elastic geobarometry. As it will be shown in the present study precision and accuracy in determining the thermoelastic coefficients can strongly affect the results in terms of calculated entrapment pressures. The new measurements performed on pyroxenes and garnets will be used here also to illustrate the importance of using consistent and homogeneous compressibility and thermal expansion dataset determined using the same measurements protocols on a single sample. This approach allows us to obtain a complete set of thermoelastic coefficients to reliably describe the behavior of a crystalline phase at high-P and T. A new micro-furnace developed by our research group here at the Department of Earth and Environmental Sciences at the University of Pavia has been described . The furnace allows to perform in situ single-crystal X-ray diffraction experiments at high temperature up to 1200K. The combined use of a K-type small diameter thermocouple together with mineral phases with well characterized lattice expansion (allowed us to determine a very accurate temperature calibration from room temperature to about 1273 K. Furthermore, we could evaluate thermal gradients and stability while performing a simulated experiment. Thermal expansion behavior from room T to 1073 K of two Pbca orthopyroxene samples has been investigated, donpeacorite and enstatite. The investigation has been performed by single-crystal X-ray diffraction. Data demonstrated that there is no difference in the volume thermal expansion coefficient as a function of composition. The negligible effect of the Fe-Mn substitution on the bulk thermal expansion allows a strong simplification when elastic geobarometry is applied to orthopyroxenes. In fact, even though the compressibility effect is still not known, it is certain that the nearly identical thermal expansion coefficients will not affect the calculated entrapment pressure (Pe). The elastic behaviour of synthetic single crystals of grossular garnet has been studied in situ as a function of pressure and temperature separately. The same data collection protocol has been adopted to collect both the pressure-volume (P-V) and temperature-volume (T-V) datasets in order to make the measurements consistent with one another. The consistency between the two datasets allows simultaneous fitting to a single pressure-volume-temperature equation of state (EoS). The evaluation of the possible role of iron oxidation on the lattice parameters thermal expansion for Fe-rich aluminosilicate garnets. The oxidation state of the two garnets recovered after the high-T single crystal X-ray diffraction experiments performed under different fO2 conditions has been evaluated using single-crystal Mössbauer spectroscopy. Mössbauer data confirmed that no oxidation occurs for the Fe-rich garnets up to at least about 1100 K neither for the “no buffer” nor for the “iron-wustite” buffered sample. The thermal expansion data nearly identical (within 2 e.s.d.’s) one to another indicates that there is no evident effect of the different fO2 conditions in the vial on the thermal expansion behavior. The resulting thermal expansion coefficients and bulk moduli have been combined for each composition into a single pressure-volume-temperature equation of state that allowed us to compare the thermoelastic properties measured on the eclogitic garnet with those calculated from the endmembers EoS. The remarkable agreement between the calculated and measured thermoelastic coefficients will enable us to calculate EoS coefficients and in turn entrapment pressures for virtually any aluminosilicate garnet occurring in diamonds.