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E309: Micro-to macro-scale evolution of continental mantle (Lead Supervisor: Sally Gibson, Earth Sciences)

Supervisors: Sally Gibson (Earth Sciences) and Graham Pearson (University of Alberta)

Importance of the area of research concerned:

The nature and timescales of garnet formation in the subcontinental lithospheric mantle (SCLM) are important to our understanding of how Earth’s rigid outer shell has evolved and stabilised since the Archean. This is because garnet greatly influences the density -- and hence the long-term stability -- of Archean subcratonic lithosphere. These thick (150 to 220 km) rigid blocks acted as sites of numerous subsequent accretion events and have also been a major influence on locations of continental break-up, i.e. they are major players in the building of continents. The widespread occurrence of pyrope garnet in the SCLM remains one of the ‘holy grails’ of mantle petrology. The paradox is that garnet often occurs in mantle lithologies (dunites and harzburgites) which represent residues of major melting events (up to 40 %) whereas experimental studies on fertile peridotite suggest this phase should be exhausted by <20 % melting. Nevertheless, there is too much garnet in the lithospheric mantle for it to be only a residue of low pressure melting.

Project summary:

Garnets commonly found in mantle peridotite suites have diverse compositions that are typically in equilibrium with high-pressure, small-fraction, mantle melts and represent metasomatic enrichment of the lithospheric mantle following cratonisation. There is, however, rare textural and geochemical evidence that pyrope garnets form by isochemical exsolution from orthopyroxene: such an origin is confirmed by experimental studies1 and recent trace element data2. The increase in density associated with this phase change has important implications for the stability of cratonic mantle. Many of the samples are associated with diamondiferous kimberlites and, in addition to constraining the chemical and physical processes involved in mantle garnet formation, a major goal of the project is to determine when exsolution occurred and its relationship to major tectonic and melting events, e.g. the global cratonization and diamond forming event that occurred ~ 2.7 billion years ago and more recent kimberlite genesis, associated with impacts of mantle plumes on the base of sub-cratonic lithosphere.

What the student will do:

The student will undertake a systematic and detailed microstructural and analytical study of garnet-bearing xenoliths and megacrysts from localities across the Kaapvaal craton. Major-element concentrations of co-existing phases will be analysed by electron microprobe and incompatible-trace element concentrations by laser-ablation inductively-coupled mass spectrometry (LA-ICP-MS). PT estimates will be calculated to assess depths of exsolution and also to calculate geothermal gradients. Lu-Hf and Re-Os isotope analyses3 by MC-ICP-MS will be used together with geo-thermobarometry to model the evolution of the lithospheric mantle beneath the Kaapvaal craton.

Please contact the lead supervisor directly for further information relating to what the successful applicant will be expected to do, training to be provided, and any specific educational background requirements.

References:

Canil, D. Experimental evidence for the exsolution of cratonic peridotite from high-temperature harzburgite. Earth Planet. Sci. Lett. 106, 64–72 (1991).

Gibson, S. A., McMahon, S. C., Day, J. A. & Dawson, J. B. Highly-refractory lithospheric mantle beneath the Tanzanian Craton: evidence from Lashaine pre-metasomatic garnet-bearing peridotites. J. Petrol. 54(8):1503-1546.

Pearson, D. G., Carlson, R. W., Shirey, S. B., Boyd, F. R. & Nixon, P. H. Stabilisation of Archaean lithospheric mantle: A ReOs isotope study of peridotite xenoliths from the Kaapvaal craton. Earth Planet. Sci. Lett. 134, 341–357 (1995).

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