1. Reconstructing the petrogenesis and PTt history of Paleoarchean gneisses from the East Pilbara Terrane
Study of Paleoarchean terranes (3.6 Ga-3.2 Ga) provides invaluable information on the early Earth. During this time period up to 50% of the present day volume of continental crust was generated. The characteristic Archean dome and keel crustal architecture has been suggested to form during distinctly different tectonic regimes compared to the modern Earth. Partial convective overturn has been invoked to explain dome and keel crustal architecture, where low-density felsic material
ascends from depth and high-density mafic material descends to depth. The interfaces between granitic domes and mafic dominant greenstone belts therefore represent the ideal locations to investigate the formation and subsequent stabilisation of dome and keel crustal architecture.
This project will focus on the >3.5 Ga marginal gneisses of the Southern Muccan Dome where a variety of different gneissic components have been observed. Example images of the felsic gneisses that formed the focus of the Wiemer et al (2018) Nature Geoscience manuscript 'Earth's oldest stable crust in the Pilbara Craton formed by cyclic gravitational overturns' are presented here.
The project will integrate field studies with microstructure, petrochronology and thermobarometry to develop a model for the origin of the different components and to test the cyclic gravitational overturn model.
The successful candidate will perform field mapping in structurally complex and remote locations and to use a variety of mineralogical and geochemical techniques including LA-ICPMS dating (zircon, titanite, rutile & mica), XRF, XRD and EPMA (at CARF QUT). A manual driver's license is required and ideally some 4wd experience.
Wiemer, D, Schrank, C., Murphy, D., Wenham, L., Allen, C., 2018. Earth's oldest stable crust in the Pilbara Craton formed by cyclic gravitational overturns. Nature Geoscience 11: 357-361
For further information, please contact: Dr David Murphy (firstname.lastname@example.org)
2. Exploring the significance of spectacular coarse-grained garnet and orthopyroxene clusters in South African mantle xenoliths
The mantle roots of Archaean cratons are composed of highly depleted olivine-rich peridotites. Kimberlites that erupt through these roots can bring interesting peridotite samples to the surface. A peculiar feature of the Kaapvaal craton's root is that it contains more orthopyroxene and garnet (and less olivine) than what could be expected from predictions of melting models. In the Kimberley diamond fields, piles of historically mined xenoliths are available for study.
Amongst these are spectacularly coarse-grained xenoliths up to nearly 1 meter in diameter. Where they are garnet bearing, they exhibit peculiar 5-10 cm structures of 'clots' or 'megacrysts' often composed of intergrown orthopyroxene and garnet. An example of an orthopyroxene megacryst with garnet exsolution lamellae was on the front page of the April 2018 issue of Geology (see Tomlinson et al., 2018).
This project will undertake the first full-scale petrological study of the clotted peridotites, working from large (>50 cm) slabs cut through the xenoliths. The currently favoured explanation for the garnet and pyroxene growth is the interaction of highly-magnesian melt with highly depleted olivine-rich residual peridotite, requiring much better textural analysis of mineral associations than what is currently known.
The successful candidate will be using a combination of digital image analysis, thin section petrology, electron microprobe and laser-ablation microanalysis and thermodynamic modelling to unlock the secrets of these spectacular rocks. The candidate is expected to spend significant time in Kimberley and must have a driver's license. Strong petrological skills and/or experience with in situ micro-analysis would be desirable attributes.
Tomlinson, E.L., Kamber, B.S., Hoare, B.C., Stead, C.V., Ildefonse, B., 2018. An exsolution origin for Archean mantle garnet. Geology, 46(2): 123-126.
For more information, please contact: Dr. Balz Kamber (email@example.com)