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Journal of Petrology Volume 42 Number 12 Pages 2215-2229 2001
© Oxford University Press 2001
The Spatial Distribution of Garnets and Pyroxenes in Mantle Peridotites: Pressure–Temperature History of Peridotites from the Kaapvaal Craton
DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139, USA
We present a new method for textural analysis of mineral associations 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 each mineral in the thin section without resorting to ellipsoidal 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 with the garnets. The spatial association of these minerals is used to develop a model for the evolutionary history of the Kaapvaal peridotites. The observed distributions can be explained by a two-stage model. In stage 1, harzburgitic residues are produced by large extents of partial melting at shallow depths (
60–90 km) and high temperatures (1300–1400°C). The melting process leading to this depletion occurs in the garnet stability field where garnet, clinopyroxene and olivine are consumed and orthopyroxene and liquid are produced. The Kaapvaal sample suite shows modal and compositional variations consistent with a progressive melt depletion event. In stage 2, the residuum is dragged down to greater depths by mantle corner flow adjacent to a subducted slab. The most depleted harzburgites descend to 140–160 km depth and are cooled. The least depleted harzburgites end up at shallower depths. The resulting stratigraphy is the opposite of what would be expected for a preserved mantle melt column and is consistent with inversion of the melt column as it was dragged around the wedge corner and cooled by the subducted slab. The cooling process causes clinopyroxene and garnet to exsolve from the orthopyroxene. Therefore, the depleted cratonic peridotites of the Kaapvaal preserve a temperature–pressure path consistent with an origin in an Archean subduction zone.
KEY WORDS: craton; mantle xenoliths; microprobe; P–T history; spatial analysis; peridotite
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