Abstract
A detailed crystal chemical study of coexisting olivine, orthopyroxene, clinopyroxene and spinel from selected Victorian (Australia) lherzolite suites was carried out by means of single crystal x-ray diffraction and electron probe microanalysis to obtain actual site occupancies. The aim of this study was primarily to characterise the intracrystalline configurations and related cation ordering on sites in major mantle constituents. The results demonstrate that cation ordering on sites is subject to distinctive crystallographic controls which depend on the petrological evolution of the suite. Mg-Fe2+ ordering in M1–M2 pyroxene sites depends on variations of the smaller cations, mainly Alvi, Ti4+, Fe3+, and related configurations of M 1. Pressuresensitive Alvi is crucial to Fe2+, the more ordered clinopyroxene showing high Alvi configurations which tend to exclude the larger bivalent cations and yield small polyhedral volumes for M 1, M 2, T sites and the unit cell. Conversely, the coexisting orthopyroxene, characterised by lower Alvi configuration and higher M 1 and unit cell volumes, is relatively more disordered. Olivine is consistent with the coexisting clinopyroxene, the more disordered crystals coexisting with more disordered clinopyroxene, while Al-Mg order in the coexisting spinel shows the reverse relationship. Estimated temperatures of apparent equilibration based on current geothermometers are not considered realistic. Assumptions of ideal cation mixing on sites in pyroxene and spinel are not supported.
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Cundari, A., Dal Negro, A., Piccirillo, E.M. et al. Intracrystalline relationships in olivine, orthopyroxene, clinopyroxene and spinel from a suite of spinel lherzolite xenoliths from Mt. Noorat, Victoria, Australia. Contr. Mineral. and Petrol. 94, 523–532 (1986). https://doi.org/10.1007/BF00376343
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DOI: https://doi.org/10.1007/BF00376343