The Spatial Distribution of Garnets and Pyroxenes in Mantle Peridotites: Pressure-Temperature History of Peridotites from the Kaapvaal Craton

Abstract

cratonic peridotites of the Kaapvaal preserve a temperature–pressure We present a new method for textural analysis of mineral associations path consistent with an origin in an Archean subduction zone. that uses digital back-scattered electron and X-ray images obtained with the electron microprobe to determine the spatial properties of minerals on a two-dimensional surface of the rock at different scale lengths. We determine modal amounts and average grain sizes of KEY WORDS: craton; mantle xenoliths; microprobe; P–T history; spatial each mineral in the thin section without resorting to ellipsoidal analysis; peridotite approximations of grain boundaries, and investigate the spatial relationship of mineral pairs. The method is used to characterize nine mantle xenoliths exhumed within kimberlite pipes in South Africa and to test whether the pyroxenes are spatially correlated INTRODUCTION with the garnets. The spatial association of these minerals is used Mantle xenoliths exhumed within kimberlite pipes in to develop a model for the evolutionary history of the Kaapvaal South Africa may provide clues to the formation and peridotites. The observed distributions can be explained by a two- evolution of the depleted mantle that makes up the stage model. In stage 1, harzburgitic residues are produced by large Kaapvaal craton. One hypothesis that has been proposed extents of partial melting at shallow depths (>60–90 km) and is that the peridotite originally resides deep in the mantle high temperatures (>1300–1400°C). The melting process leading (at depths of perhaps 300 or 400 km) and is transported to this depletion occurs in the garnet stability field where garnet, upwards into the cratonic lithosphere (>180 km depth), clinopyroxene and olivine are consumed and orthopyroxene and where re-equilibration occurs and clinopyroxene (cpx) liquid are produced. The Kaapvaal sample suite shows modal and unmixes from the garnet phase (Haggerty & Sautter, compositional variations consistent with a progressive melt depletion 1990). An alternative idea is that the garnet lherzolites event. In stage 2, the residuum is dragged down to greater depths were originally high-temperature harzburgites that oriby mantle corner flow adjacent to a subducted slab. The most ginated at depths between 100 and 250 km and that both depleted harzburgites descend to 140–160 km depth and are cooled. the cpx and garnet subsequently exsolved from the AlThe least depleted harzburgites end up at shallower depths. The rich orthopyroxene (opx) when the rocks cooled and reresulting stratigraphy is the opposite of what would be expected for equilibrated (Cox et al., 1987). If exsolution of minerals a preserved mantle melt column and is consistent with inversion of has indeed occurred, then a spatial relationship between the melt column as it was dragged around the wedge corner and grains of minerals that were formerly dissolved in each cooled by the subducted slab. The cooling process causes clinopyroxene other should be apparent in the present-day xenoliths. In particular, the first model implies a spatial relationship and garnet to exsolve from the orthopyroxene. Therefore, the depleted