PB AND SR ISOTOPIC DISEQUILIBRIUM BETWEEN ORTHOPYROXENE AND PLAGIOCLASE IN THE CRITICAL ZONE, BUSHVELD, S. AFRICA

Orthopyroxene and plagioclase grains from orthopyroxenites and norites were drilled with a micromill to extract Pb and Sr isotopic information from the Lower and Upper Critical Zones in the eastern portion Bushveld Intrusive Complex. Previous work suggests that the Pb isotopes in plagioclase grains are not in equilibrium with the sulfides in the Merensky Reef; however, that work did not specifically address the isotopic equilibrium between plagioclase and orthopyroxene. Calculated initial isotopic ratios suggest that these minerals were not in Pb isotopic equilibrium, and only the uppermost Upper Critical Zone samples appear to approach Sr isotopic equilibrium, within error. In the Pb system, orthopyroxene grains have initial isotopic ratios that appear to be consistently more radiogenic. Plagioclase in these samples commonly has an initial ⁸⁷Sr/⁸⁶Sr ratio that is higher than orthopyroxene, e.g. for one mineral pair plagioclase had an initial ⁸⁷Sr/⁸⁶Sr 0.70591(± 0.00001) and orthopyroxene was 0.70487 ( ±0.00017). Excluding analytic error and alteration, several other processes can be called upon to explain these data. Two possibilities are crystallization during the introduction of isotopically different magma, or isotopic exchange with a fluid migrating through the crystal pile, perhaps one with a crustal component.

This method demonstrates that some of the important rock-forming minerals in the Bushveld appear to be in isotopic disequilibrium. This would suggest that whole-rock isotopic analysis could be misleading, as it may be a weighted, isotopic average. Extraction of a single phase, such as plagioclase, could also be misleading, as other phases, such as orthopyroxene, may not share a common isotopic signature. This technique allows for the extraction of more complicated histories from these rocks, which could lead to a better understanding of the processes responsible for the genesis of chromite and PGE ores.