Academic literature on the topic 'Elastic geobarometry'

Elastic geobarometry makes use of the contrast in elastic proprieties between host and inclusion crystals to determine the entrapment conditions of the inclusions from the residual stress and strain measured in the inclusion when its host is at ambient conditions. The theoretical basis has been developed extensively in the past few years, but an experimental validation of the method is still required. We performed two syntheses experiments with quartz inclusions in pure almandine garnet at eclogitic conditions. Experiment labelled Alm-1 with synthesis performed at P=3.0 GPa and T=775°C and Alm-2 at P=2.5 GPa and T=800°C. All the experiments have been carried out in a piston-cylinder press. Isolated, fully-enclosed quartz inclusions in the recovered garnets have been then measured using micro-Raman spectroscopy. All fully-buried inclusions exhibit Raman peaks at higher frequencies and wavenumbers than those obtained from quartz crystals at ambient pressure. If these peak shifts are interpreted as a remnant pressures by use of hydrostatic calibrations of the Raman shifts of quartz with pressure, the remnant pressures show a large spread in values and lead to significant errors in back-calculated entrapment pressures, of up to 1.4 GPa for inclusions synthesised at 3.0 GPa. These results confirm that quartz inclusions trapped inside garnet are not subject to hydrostatic pressure. We therefore used the phonon-mode Grüneisen tensors of quartz to calculate the full strain state of each inclusion, from which the full anisotropic stress state can be calculated by using the elastic properties of quartz. The mean residual remnant stress of the inclusions determined in this way show a much smaller spread in values. Entrapment pressures calculated from this mean stress with the isotropic model for host-inclusion systems differ from the known experimental values by less than 0.2 GPa, which is of the order of the combined experimental uncertainties. These results show that the most significant effect of the elastic anisotropy of quartz is on the Raman shifts of the inclusion, and not on the subsequent calculation of entrapment conditions.

Diamonds, and the mineral inclusions they trap during growth, are pristine samples from the mantle that reveal processes in the deep Earth, provided the depth of formation of an inclusion-diamond pair being known. The majority of diamonds are lithospheric, while the depth of origin of super-deep diamonds (SDDs), which represent only 6% of the total, is uncertain. SDDs are considered to be sub-lithospheric, with formation from 300 to 800 km depth, on the basis of the inclusions trapped within them, which are believed to be the products of retrograde transformation from lower-mantle or transition-zone precursors. This Ph.D. project aims to obtain the real depth of formation of SDDs by studying the most common mineral phases enclosed within them by non-destructive methods. We have studied about 40 diamonds with such inclusion phases as CaSiO3-walstromite or ferropericlase using in-house single-crystal X-ray diffraction and micro-Raman spectroscopy as well as field emission gun-scanning electron microscopy, synchrotron X-ray tomographic microscopy and synchrotron Mössbauer source at outside Institutions. In addition, laser-heating diamond-anvil cell experiments were performed on a synthetic Ti-free jeffbenite to determine if the absence of Ti extends the stability field of such mineral compared to previous studies. Finally, elastic geobarometry has been completed both on ferropericlase and CaSiO3- walstromite, in this last case together with thermodynamic and first-principles calculations. One of our principal results suggests that CaSiO3-walstromite may be considered a sub-lithospheric mineral, but retrograde transformation from a CaSiO3-perovskite precursor is only possible if the diamond around the inclusion expands in volume by ~30%. Moreover, high-pressure and high-temperature experiments indicate that Ti-free jeffbenite could be directly incorporated into diamond in the transition zone or uppermost lower mantle and therefore this mineral may represent a high-pressure marker to detect SDDs. Finally, the observation of magnesioferrite exsolutions within ferropericlase, combined with elastic geobarometry results, strengthen the hypothesis that single ferropericlase inclusions might not be reliable markers for a diamond lower-mantle provenance.<br>I diamanti e le inclusioni minerali da essi intrappolate durante l’accrescimento sono campioni inalterati provenienti dal mantello terreste che possono fornire importanti informazioni sull’interno della Terra, a patto di conoscerne la reale profondità di formazione. La maggior parte dei diamanti sono litosferici, mentre la profondità di formazione dei diamanti super-profondi (DSS), che rappresentano solo il 6% del totale, è ancora incerta. Le inclusioni in essi contenute sono ritenute essere i prodotti di trasformazione retrograda da precursori stabili nel mantello inferiore o nella zona di transizione e, sulla base di ciò, si pensa che i DSS si formino in condizioni sub-litosferiche, tra 300 e 800 km di profondità. L’obiettivo di questa tesi è ottenere la reale profondità di formazione dei DSS tramite lo studio non distruttivo delle più comuni inclusioni in essi racchiuse. Abbiamo studiato circa 40 diamanti contenenti CaSiO3-walstromite o ferropericlasio utilizzando la diffrazione a raggi X a cristallo singolo, la spettroscopia micro-Raman, la microscopia elettronica a scansione con sorgente ad emissione di campo, la tomografia a raggi X in luce di sincrotrone e la spettroscopia Mössbauer in luce di sincrotrone. In più, sono stati eseguiti degli esperimenti in cella a incudine di diamante mediante riscaldamento laser sulla jeffbenite sintetica allo scopo di verificare se l’assenza di Ti estende il suo campo di stabilità rispetto a studi precedenti. Infine, la geobarometria elastica è stata applicata sia sul ferropericlasio che sulla CaSiO3-walstromite, in quest’ultimo caso combinata con calcoli termodinamici e ab initio. Uno dei principali risultati suggerisce che la CaSiO3-walstromite sia sub-litosferica, ma che una trasformazione retrograda dalla CaSiO3-perovskite sia possibile solo se il diamante si espande del ~30%. Inoltre, gli esperimenti in alta pressione e temperatura indicano che la jeffbenite povera di Ti sia stabile nella zona di transizione o all’inizio del mantello inferiore, pertanto può essere considerata una fase indicatrice per i DSS. Infine, la presenza di essoluzioni di magnesioferrite nelle inclusioni di ferropericlasio, insieme coi risultati della geobarometria elastica, suggeriscono che tali inclusioni non possano, da sole, rappresentare un’origine dei diamanti nel mantello inferiore.