Mixing of supernatant and interstitial fluids in the Rhum layered intrusion.

Extreme concentrations of chrome-spinel occur in small-scale structures in the layer at the unit 7-8 boundary of Brown (M.A. 14-69) in the eastern layered series of the Rhum intrusion. These structures take the form of downward-pointing cones several cm across and deep, and are lined or wholly filled with chrome- spinel. By comparison with fluid escape structures in clastic sediments, it is suggested that the 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 the underlying allivalite suggests that the newly emplaced primitive liquid was able to operate the crystal mush in the floor of the intrusion.-R.A.H.