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Title: The chromite-garnet peridotite assemblages and their role in the evolution of the mantle lithosphere
Author: Ivanic, Tim
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2007
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Newlands and Bobbejaan kimberlites, South Africa, contain suites of highly chromian, garnet-rich peridotites amongst their xenolith population and an investigation of these xenoliths has been targeted because there is an overlap of mineral compositions with the garnet-chromite-olivine paragenesis found as inclusions in diamonds. A high proportion of garnets and chromites in these rocks plot in the diamond facies fields on Cr2O3-CaO and Cr2O3-MgO wt. % plots respectively. However, it has also been found that many Cr-rich assemblages are clinopyroxene-bearing (iherzolitic) as well as harzburgitic. Many samples have garnets with inclusions of serpentine ± chromite (+ clinopyroxine in iherozilitic samples) whose arrangement is sometimes indicative of exsolution or annealed exsolution textures. Modal reconstructions reveal that the original crystals were garnets of higher chromium content. A particularly strong chromite-clinopyroxene association is seen included within garnet in most iherozolitic samples; a feature not reported in other garnet iherzolites. Most the garnets have strongly developed zonation patterns, which are a result of diffusion towards the matrix (external zonation) followed by zonation towards inclusions (internal zonation). Cr-A1 and Mg-Ca inter-diffuse in both types of zonation, Ti may also be strongly zoned, whereas Fe is not distinctly zoned in any sample. External zonation is modelled to have occurred on an order of magnitude greater timescale than internal zonation (~20 Ma compared to ~2 Ma using DMg = 10-20m2/s). Final P-T estimates on clinopyroxine inclusion-garnet boundaries indicate 40-50 kb and 900-1100°C, whereas core garnet compositions yield pressures and temperatures consistently a little higher. Garnet-matrix clinopyroxene P-T estimates yield higher temperatures than the final internal ones. Overall the samples plot on a relatively cool continental steady state geotherm (equivalent to 38 mW/m2 conductive geotherm). In order to achieve the level of Cr-saturation in these rocks, repeated melt extraction is required in the spinel stability field. Since Cr stabilises spinel relative to garnet at high pressures, the rocks must have been buried to depths well in excess of 90 km (30 kb) for A1-garnet peridotite facies before the observed exsolution occurred. External zonation appears to have happened whilst garnet was in equilibrium with inclusions so any exsolution (forming the inclusions) represents relatively early decompression to > 50 kb (i.e. greater than the final pressure estimate – see above). External zonation in garnet is the result of equilibrium with the matrix which is predominantly along predictable compositional trajectories. Few samples, however, display evidence for equilibration at the rims with Ca-Ti rich metasomatic fluid thought to have percolated though the matrix. Finally, internal zonation is the result of late P-T re-equilibration with inclusions subsequent to any metasomatic effects from > 50 to 40-50 kb. Initial bulk REE patterns were essentially incorporated into a pre-exsolution, high-Cr garnet and later modified by exsolution of pyroxene.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available