AbstractOne of the most controversial and unresolved questions regarding the evolution of the Earth is the question of when did modern subduction-driven plate tectonics begin? Although the magmatic record implies that subduction-driven plate tectonics began in the Archean c. 3 Ga, the lack of high pressure, low temperature rocks in the pre-Neoproterozoic metamorphic record suggests that the onset of modern subduction occurred after c. 800 Ma. To offer a way forward with the debate, and to look beyond the Neoproterozoic for evidence of such events, this thesis aims to develop a novel way of utilising rutile, a robust accessory mineral that is stable over a large P-T range and commonly found within a wide variety of high grade metamorphic rocks. In this contribution, rutiles from Neoproterozoic blueschists, eclogites and ultrahigh-pressure terranes (Syros, Greece and the Western Alps), and Archean ultrahigh-temperature granulites (Antarctica), have been investigated for mineral inclusions using electron probe microanalysis and Raman spectroscopy. Trace element analysis of rutile has also been carried out using laser ablation inductively coupled plasma mass spectrometry to determine metamorphic temperatures of formation using Zr-in-rutile geothermometry.
In the blueschists and eclogites, rutile is shown to contain numerous inclusions of high-pressure minerals such as glaucophane, lawsonite, omphacite and garnet, as well as diagnostic ultrahigh-pressure minerals, including the first reported occurrence of exceptionally preserved monomineralic coesite in rutile from the Dora-Maira massif, Western Alps. The chemical comparison between inclusion and matrix phases show that inclusions generally represent peak metamorphic assemblages; although prograde phases not found in the matrix have also been identified implying that rutile grows continuously during prograde burial and traps mineralogic evidence of this evolution. Pressure-temperature estimates obtained from mineral inclusions using conventional geothermobarometry and average pressure-temperature calculations, when used in conjunction with Zr-in-rutile geothermometry of the host rutile, can provide additional constraints on the metamorphic conditions of the host rock. Furthermore, the preliminary investigation of mineral inclusions within detrital rutiles from river sediments collected in the Po Plain, Western Alps and their comparative chemistry to inclusion and matrix phases within in-situ samples from the nearby Sesia Zone and Monviso metaophiolite, demonstrates the potential of using mineral inclusions in sediment provenance analysis.
In the Archean granulites, rutile grains which occur in contact with or adjacent to zircon are found to record temperatures lower than expected as post-peak fluid-rock interaction resulted in significant zircon recrystallization and the variable resetting of zirconium concentrations in rutile. However, rutile grains found as inclusions within orthopyroxene have been shielded from post-peak diffusional resetting and record UHT conditions. In addition, rutile is found to contain inclusions of kyanite, sillimanite, quartz and corundum, demonstrating that rutile has the capacity to preserve mineralogic evidence of ultra-high temperature metamorphism. While the discovery of inclusions in Archean granulites demonstrates that rutile can preserve evidence of metamorphism in old crust, inclusions were not present in all samples leading to new questions as to the formation and growth of rutile.
Overall, this study demonstrates that rutile is an excellent repository for mineral inclusions and that the study of mineral inclusions in rutile may profoundly change how we investigate and recover evidence of both high-pressure and high-temperature events in both detrital populations and partially retrogressed samples.
|Date of Award||Jul 2016|
|Supervisor||Craig Storey (Supervisor) & Mike Fowler (Supervisor)|