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The role of fluid-induced partial melting in magmatic crystal mush in the origin of stratiform chromite deposits of the Bushveld Complex, South Africa

Subject Area Mineralogy, Petrology and Geochemistry
Term from 2018 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 391702990
 
Final Report Year 2022

Final Report Abstract

This multidisciplinary research project combining field observations, mineralogical studies, experimental petrology and stable isotope analyses was aimed at testing the working hypothesis that chromitite layers in mafic-ultramafic layered intrusions, and specifically in the classical case of the Bushveld Complex in South Africa, can be formed by partial melting and replacement of silicate minerals by chromite due to additions of H2O and other volatile components to the resident magma at the contact with pre-existing, partly solidified cumulates. In general, the obtained results support the working hypothesis. Partial melting and replacement of orthopyroxene and plagioclase by chromite and olivine was reproduced in a series of experiments at 0.3 GPa and 1100–1300 °C by additions of variable amounts (2-6 wt.%) of H2O component to the model magma composition, which is believed to be parental for the lower mafic-ultramafic zones of the Bushveld complex. Triple-oxygen and hydrogen isotope ratios of interstitial, late-magmatic phlogopite from three localities of the UG2 chromitite layer imply two-stage contamination of the mantle-derived parental magma, first by as much as 30-40 wt.% of the lower and middle continental crust, and then by hydrous fluids (probably brines) derived from Transvaal sediments.

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