Nd–Hf Isotope Systematics of Megacrysts from the Mbuji-Mayi Kimberlites, D. R. Congo: Evidence for a Metasomatic Origin Related to Kimberlite Interaction with the Cratonic Lithospheric Mantle

Abstract

Garnet and clinopyroxene megacrysts from the Cretaceous (70 Ma) Mbuji-Mayi kimberlites and one garnet megacryst from the lower Oligocene (32 Ma) Kundelungu kimberlites in Democratic Republic of Congo have been investigated for combined Nd and Hf isotope compositions. These megacrysts are thought to result from the metasomatic (re)crystallization of lithospheric mantle peridotites during the infiltration of a proto-kimberlitic melt/fluid. In addition, zircon and baddeleyite megacrysts from the Mbuji-Mayi kimberlites have been investigated for Hf isotope composition. Although baddeleyites are uncommon in kimberlite megacryst suites, their origin is most probably related to that of zircon megacrysts from Mbuji-Mayi. Mbuji-Mayi garnet megacrysts display ranges of eNd(t) from −0.6 to +6.1 and eHf(t) from +6.6 to +12.1; the Kundelungu garnet megacryst has overlapping isotopic compositions (+0.8 and +6.0, respectively). By contrast, Mbuji-Mayi clinopyroxene megacrysts display a more restricted range of eNd(t) values (+2.7 to +4.6) and extend toward lower eHf(t) compositions (+3.0 to +9.1). Mbuji-Mayi zircon and baddeleyite megacrysts have similar eHf(t) values (+6.5 to +7.1 and +6.0 to +8.4, respectively). Differences in initial isotopic composition between garnets on the one hand and clinopyroxenes, zircons, and baddeleyites on the other have been confirmed by various Hf and Nd model ages calculations. Clinopyroxene, zircon, and baddeleyite megacrysts plot close to the worldwide kimberlite field in a combined eHf(t)-eNd(t) plot, which favors recent (i.e. at or shortly before the time of kimberlite eruption) crystallization through interaction between the infiltrating proto-kimberlite melt/fluid and peridotite wall rocks. On the other hand, the wide range of eNd(t) values and the higher eHf(t) values for garnet megacrysts suggest that they have been formed through recrystallization of old garnet-bearing peridotitic protoliths in the subcontinental lithospheric mantle. Calculated rare earth element patterns of liquids in equilibrium with clinopyroxene megacrysts confirm the direct relationship to Group I kimberlites, while those in equilibrium with garnet megacrysts show more variability, which could also reflect that their formation results from more complex processes.