Kathryn M. Fiess
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M. Sc. Thesis
The Rare-Earth Element Geochemistry of Ultramafic Nodules from Southern African Kimberlites
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A pre-irradiation chromatographic technique for use in conjunction with neutron activation analysis was successfully developed in our laboratory to study low ( 10 times chondrites) concentrations of rare earth elements (REE`s) in rocks. The estimated accuracy of the analytical procedure is +8 wt.% at the 1 level. This thesis presents the details of this analytical technique and the results obtained from its application to the study of low concentrations of REE`s in ultramafic nodules from kimberlite Pipe 200 (PTH suite) in Lesotho and the Bulfontein and Koffiefontein kimberlite pipes (Kimberley suite) in South Africa. Rare earth element abundances in clinopyroxene (Cpx) and garnet (Ga) separated from a garnet chromic lherzolite (PTH 207) and in a sample of the Pipe 200 kimberlite, also determined using this method, are reported. Bulk-rock major element abundance trends in both suites of nodules are compared with the analyzed bulk-rock REE abundance trends. Additionally, the results of REE mass balance calculations for nodules of the PTH suite and the REE composition of the hypothetical liquid in equilibrium with sample PTH 207, Cpx and Ga are evaluated.
All of the nodules have light and middle REE fractionated REE patterns which in some cases exhibit variable degrees of heavy REE depletion. Coherent chemical trends were only observed in the PTH suite of nodules and are characterized by decreasing bulk-rock CaO, All2O3, and HREE abundances with increasing bulk-rock 100 Mg/(Mg + Fe) and 100 Cr/(Cr + Al). Poor REE mass balances were obtained for nodules from the PTH suite. Additionally, the liquid in equilibrium with sample PTH 207, Cpx and Ga has REE abundances similar to the Pipe 200 kimberlite.
It is tentatively suggested that the analyzed suites of nodules represent depleted mantle wall rock incorporated and metasomatized by rapidly ascending kimberlitic magmas. Various other models for the petrogenetic/metasomatic history of these nodules are also discussed.
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Supervisor:聽 D. B. Clarke