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

Ca-rich and Ca-poor pyroxenes present in the Bushveld rocksof the Bethal area display well developed exsolution texturestypical of slowly cooled mafic intrusions. This gave rise topoor reproducibility in electron microprobe analyses of thesame pyroxene grain, as well as results which departed fromthe bulk composition of the original homogeneous mineral. EMMA-4was used together with the electron microprobe to establishthe composition of the constituent phases in exsolved pyroxenes.The data showed that microprobe analyses carried out with adefocused beam were equivalent to the bulk composition of thepyroxenes. Microprobe analyses obtained using a focused beamwere found to approach closely the bulk composition of pyroxenesonly when the exsolution density reached 90 lamellae per millimetre. Transmission electron microscope examination of microstructuresin ion-thinned samples of pyroxenes at 100 kV and 1000 kV showedthat the exsolution mechanism in Ca-rich and Ca-poor pyroxeneswas one of heterogeneous nucleation. Subsequent growth tookplace by means of the migration of ledges along the (100) plane.Pigeonite inversion was also shown to occur in iron-rich Ca-poorpyroxene exsolution lamellae in augite. Fractionation trends established for the Bethal pyroxenes frommicroprobe analyses indicated an overall range from Fs₁₄En₈₄Wo₂to Fs₆₀En₃₁Wo₉ in the Ca-poor pyroxene and Fs₇En₅₀Wo₄₃ to Fs₃₆En₂₇Wo₃₇in the Ca-rich pyroxene. Comparison of pyroxene fractionationtrends from the western, eastern and Bethal areas of the Bushveldsuggests that crystallization took place under different conditionsof pressure and temperature.