| Summary: | Detailed investigations of the petrology and geochemistry of the Jagersfontein mantle peridotite xenolith suite have yielded new refinements to models of sub-cratonic mantle petrogenesis. The Jagersfontein peridotitic xenoliths show three distinct xenolith types as recognised by Winterburn (1987): a coarse low-temperature, a coarse medium-temperature, and a deformed high-temperature suite geothermobarometry, using the orthopyroxene-clinopyroxene solvus thermometer, and aluminium in orthopyroxene in equilibrium with garnet barometer of Brey and Kohler (1990), has been used to refine the depth ranges of these suites, and yields a geotherm lacking an inflection at high temperatures. The general major-minor-trace element geochemistry of garnets shows variations related to depth, whilst complex heterogeneities in the composition of single garnets yield evidence of changing geochemistry with time. By combining these types of information a model of geochemical evolution, involving melt metasomatism, is developed in both space and time for the sub-cratonic mantle. Garnets from coarse low-temperature xenoliths are G9 (1herzolitic) and plot is confined region near the base (low Ca-Cr) of the G9 trend on the Cr<SUB>2</SUB>O<SUB>3</SUB>-CaO diagram. Garnets from coarse-medium temperature xenoliths are G10 (harzburgitic) and plot in variable Ca and Cr to the low-Ca side of the G9 trend Garnets from deformed xenoliths are also from 1herzolite assemblages and define the G9 trend, with the garnets from the lowest pressure deformed samples plotting at the high Ca-Cr end of the G9 trend. With increasing depth deformed xenolith garnets plot at progressively lower Ca-Cr, with the deepest samples plotting at the base of the trend. Some garnets in the deformed, and coarse medium-temperature suites possess preserved major and/or trace element heterogeneities formed by metasomatism which diffusion constraints indicate to have occurred within a few million years prior to kimberlite eruption. Geometries of these heterogeneities give clear evidence that peridotite containing these garnets was infiltrated by a melt which caused growth of new garnet, as well as cracking/annealing, and chemical exchange.
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