Abstract

The cyclic unit that hosts the Merensky reef is attributed to the entry of a new pulse of bronzite-saturated magma into a chamber containing a melt that was crystallizing plagioclase. Mixing of these magmas produced a hybrid magma that was superheated with respect to both bronzite and plagioclase. This magma melted the plagioclase cumulates that formed the floor of the chamber, and because the products of erosion were light, they were swept away by turbulent convection and replaced by hot uncontaminated magma, allowing erosion to continue.Thermochemical erosion, leading to the release of a light fluid, has been modeled in a series of experiments in which a block of ice was melted by an overlying salt solution. Two features of the experiments are noteworthy: first, melting produced a pitted texture on the surface of the ice, similar to the dimpled contact between the Merensky reef and the underlying plagioclase cumulates, and second, small holes in the ice rapidly expanded into larger holes which were circular in plan and rounded in section. The similarity between these structures and the potholes suggests that they formed by similar processes. It is proposed that the erosion in the larger potholes was arrested at olivine-rich layers because the products of thermochemical erosion were dense and could not convect away from the melting surface.

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