Stressed pyroxenite nodules from the Jagersfontein kimberlite

Abstract

SummaryExamination of some pyroxenite nodules from the Jagersfontein kimberlite shows that they have suffered variable deformation followed by different degrees of recovery. Most interes~ting of the nodules is the 'diallage rock', which consists of highly sheared and broken lamellar crystals of diopside containing exsolved enstatite. Petrographic evidence indicates a pre-tectonic exsolution of enstatite and a syntectonic exsolution of pyrope-almandine from enstatite. Deformation occurred in the mantle, prior to incorporation of the nodule in the kimberlite. Compositions of pyroxenes from the diallage rock suggest it re-equilibrated at a temperature of 1000° and at a pressure of about 35 kb. Olivine and phlogopite, accompanied by serpentine, occur only in broken kink-bands in the diallage rock and they are considered to be of secondary origin, precipitated from kimberlite magma at temperatures near 700°