Mixing of supernatant and interstitial fluids in the Rhum layered intrusion

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Abstract
An alternative explanation for the occurrence of chrome-spinel layers in the Eastern Layered Series of the Rhum intrusion is suggested by extreme concentrations of chrome-spinel in small-scale structures in the layer at the unit 7–8 boundary (Brown, 1956). These take the form of downward pointing cones several centimetres across and deep, and lined or wholly filled with chrome-spinel; lamination in the underlying allivalite exhibits quaquaversal dips around these cones. By comparing these structures to fluid escape structures in clastic sediments, it is proposed that spinel is the product of mixing and reaction of upward moving interstitial liquid and more primitive liquid newly emplaced in the chamber. Further evidence for the presence of a second liquid during spinel crystallization is provided by spherical silicate inclusions within spinel grains. Complex zoning in feldspars in the underlying allivalite suggests that the newly emplaced primitive liquid was able to penetrate the crystal mush on the intrusion floor.