Origins of the ultramafic nodules from some kimberlites of northern Lesotho and the Monastery Mine, South Africa

Abstract

Temperature-depth estimates for a group of lherzolite nodules from the kimberlites of northern Lesotho and the Monastery Mine, South Africa, suggest that these rocks have come from the depth interval 100–200 km. Lherzolites coming from depths greater than about 150 km are severely deformed and have porphyroclastic or mosaic textures. The degree of deformation is progressive with depth. Sheared dunites which are believed to be cumulates and a suite of megacrysts of garnet, pyroxene, and ilmenite also appear to have come from depths below 150 km. Lherzolites originating at shallower depths have granular textures. Harzburgites and spinel peridotites also have granular textures and, on this basis, are believed to have originated with the granular lherzolites. The spinel phase in the garnet-free peridotites and harzburgites is commonly chromite rather than aluminous spinel, and thus it is difficult to be sure whether or not there is a garnet peridotite rlarr2gspinel peridotite reaction in this mantle section. Evidence provided by the sheared lherzolites, the megacryst suite, and regularities in the occurrence of phlogopite suggest that the top of the low-velocity zone may have been at a depth of 150 km when these kimberlite were erupted.