Rhenium-osmium isotope measurements of selected chromitites from the Critical Zone of the Bushveld igneous complex reveal that a significant component of radiogenic Os was present in the magma when it formed 2.06 Ga ago. Measured 187Os/188Os ratios increase with stratigraphic height, and individual layers have similar values across the complex. Initial 187Os/188Os ratios for chromitites from the western Bushveld decrease from 0.140 at the LG to 0.129 at the MG2 chromitite, increasing to 0.153 at the UG2 chromitite. The inflection at MG2/MG3 is the boundary between the lower and upper Critical Zone, consistent with previous mineralogical and geochemical studies indicating a shift in magma composition at this point. Chromite separates have γOs values of ∼ 12-35 at 2.06 Ga and are too radiogenic for a purely mantle derivation. Assimilation of 5% mafic granulitic lower crust in a tholeiitic parent magma can explain the radiogenic values. Pd/Ru ratios for the intergranular and chromite separate components of each layer differ by orders of magnitude, but relative shifts from layer to layer are similar. Combined with the Re-Os data, the results suggest that PGE were concentrated into chromitites by magmatic or early deuteric processes that were specific to each layer. Post-layering, upward mobilization of PGE by hydrothermal processes probably did not play a significant role in PGE enrichment.
ASJC Scopus subject areas
- Geochemistry and Petrology