A Re—Os isotope study of ultramafic xenoliths from the Matsoku kimberlite

Ultramafic xenoliths erupted within the Matsoku kimberlite pipe display a range of petrographic features that have been interpreted as the products of melt injection or infiltration and metasomatic processes. We have analysed Re and Os concentrations and the Os isotope composition of these samples with the aim of investigating how such processes affect the Re—Os system. In particular, we have focused on the relationships between: (1) veins, dykes and sheets, that are thought to record melt injection or infiltration processes; (2) the coarse peridotite wallrocks to the veins, dykes and sheets, which usually do not show modal metasomatism but have been affected by low-volume melt infiltration; and (3) common coarse peridoties which show little, if any, evidence of modal metasomatism or melt infiltration. tite samples (i.e., groups 2 and 3 above) all display low 187Os/188Os and thus mimic the signature that is now well established for the sub-continental mantle lithosphere. We demonstrate a relationship between Os isotope composition and modal mineralogy that suggests a link between Re depletion and extraction of a basaltic component to establish the refractory compositions of lithospheric mantle samples. Model age calculations suggest a mean age of melt extraction older than 2200 Ma. velopment of ilmenite-rutile-phlogopite-sulphide metasomatic assemblages does not appear to affect strongly Re/Os. By contrast, pyroxenite dykes have substantially higher 187Os/188Os which is broadly supported by elevated Re/Os. High Re/Os is consistent with the pyroxenites having formed by precipitation from melts intruded into the lithospheric mantle. Such melts, as is observed for the dykes, would also be expected to carry a low Sm/Nd signature which with time would develop negative εNd. Wallrock peridotites and other rocks affected only by low-volume melt migration also show low Sm/Nd but with accompanying low Re/Os, thereby demonstrating the potential for the generation of the low 143Nd/144Nd, low 187Os/188Os signature which appears to be characteristic of sub-continental lithospheric mantle.