Isotopic and geochemical studies of nodules in kimberlite have implications for the lower continental crust

Lower crustal xenoliths from the Calcutteroo kimberlitic pipes in South Australia show a surprisingly diverse range in Nd and Sr isotopic compositions and rare earth element (REE) abundances. Mafic granulite and garnet–clinopyroxenite xenoliths have measured εNd(0) values between +6.1 and −7.9, both light (L)REE enrichments and depletions, low Rb/Sr ratios (0.005–0.040) and relatively high 87Sr/86Sr ratios of from 0.70675 to 0.70919. A felsic xenolith has compositions more typical of upper crustal rocks with εNd(0) = −21.4, LREE enrichments, Rb/Sr = 1.47 and 87Sr/86Sr = 0.85987. An approximate correlation also exists between Sm/Nd versus 143Nd/144Nd and Rb/Sr versus 87Sr/86Sr indicating major intracrustal differentiation in the lower crust at ∼2,400 Myr. This produced extremely low Rb/Sr ratios, and both relative LREE enrichments (low Sm/Nd) and depletions (high Sm/Nd). If these results are typical, then they indicate that the lower crust has higher 87Sr/86Sr and more positive εNd(0) values than previous estimates1–5. This may require revision of total crustal compositions and crust–mantle evolutionary models1–5.

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