Modeling the Merensky Reef,Bushveld Complex,Republic of South Africa |
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Authors: | Alan E Boudreau |
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Institution: | (1) Division of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, NC 27708, USA |
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Abstract: | The Merensky pegmatoid (normal reef) in the western Bushveld Complex is commonly characterized as a pyroxene-rich pegmatoidal
unit with a base that is enriched in chromite and platinum-group element-bearing sulfides overlying a leuconorite footwall.
Models for its formation have ranged from those that view it as entirely a magmatic cumulate succession to those that have
suggested that it is a zone of volatile-induced remelting. The consequences of the latter interpretation are investigated
using the numerical modeling program IRIDIUM, which links diffusive and advective mass and heat transport with a phase equilibration
routine based on the MELTS program. The initial system consists of a simple stratigraphic succession of a partially molten
leuconorite overlain by a partially molten pyroxenite, both initially at 1,190°C and 2 kbar. 2 wt% of a volatile fluid composed
of 75 mol% H2O, 20 mol% CO2 and 5 mol% H2S is then added to the lower 20 cm of the pyroxenite. The system is then allowed to evolve under conditions of chemical diffusion
in the liquid. The addition of the volatile components results in a modest increase in the amount of melt in the pyroxenite.
However, chemical diffusion across the leuconorite–pyroxenite boundary leads to more extensive melting at and below the boundary
with preferential loss of opx from the underlying leuconorite, preferential re-precipitation of sulfide and chromite and concentration
of the PGE at this boundary. These results mimic actual mineral and compositional profiles across the Merensky pegmatoid and
illustrate that long-term diffusion process can effectively produce mineralogical and compositional layering not present in
the original assemblage. |
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Keywords: | Bushveld Complex Merensky Reef Platinum group elements Geochemical modeling |
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