A Combined Transmission Electron Microscope and Electron Microprobe Study of Bushveld Pyroxenes from the Bethal Area

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Abstract
Ca-rich and Ca-poor pyroxenes present in the Bushveld rocks of the Bethal area display well developed exsolution textures typical of slowly cooled mafic intrusions. This gave rise to poor reproducibility in electron microprobe analyses of the same pyroxene grain, as well as results which departed from the bulk composition of the original homogeneous mineral. EMMA-4 was used together with the electron microprobe to establish the composition of the constituent phases in exsolved pyroxenes. The data showed that microprobe analyses carried out with a defocused beam were equivalent to the bulk composition of the pyroxenes. Microprobe analyses obtained using a focused beam were found to approach closely the bulk composition of pyroxenes only when the exsolution density reached 90 lamellae per millimetre. Transmission electron microscope examination of microstructures in ion-thinned samples of pyroxenes at 100 kV and 1000 kV showed that the exsolution mechanism in Ca-rich and Ca-poor pyroxenes was one of heterogeneous nucleation. Subsequent growth took place by means of the migration of ledges along the (100) plane. Pigeonite inversion was also shown to occur in iron-rich Ca-poor pyroxene exsolution lamellae in augite. Fractionation trends established for the Bethal pyroxenes from microprobe analyses indicated an overall range from Fs14En84Wo2 to Fs60En31Wo9 in the Ca-poor pyroxene and Fs7En50Wo43 to Fs36En27Wo37 in the Ca-rich pyroxene. Comparison of pyroxene fractionation trends from the western, eastern and Bethal areas of the Bushveld suggests that crystallization took place under different conditions of pressure and temperature.

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