Stable and radiogenic isotope study of eclogite xenoliths from the Orapa kimberlite, Botswana

Abstract

Eclogite xenoliths from Orapa can be accurately classified as Group I or Group II on the basis of Na 2O in garnet and K 2O in clinopyroxene. Group I xenoliths are commonly diamondiferous while Group II xenoliths are diamond-free. Both xenolith varieties may contain graphite. Isotopic character is to some degree correlated with major- and trace-element chemistry. Group II samples with Ca-poor garnet have clinopyroxenes with radiogenic 87 Sr 86 Sr (0.705–0.709) and the least radiogenic 143 Nd 144 Nd (0.5122–0.5125). Group I eclogites with higher Ca and Fe in garnets have less radiogenic 87 Sr 86 Sr (0.702–0.7066) and bulk-Earth or higher 143 Nd 144 Nd ratios. Group I eclogites have more radiogenic 206 Pb 204 Pb (18.6–19) than Group II xenoliths (16.5–18.6). In contrast, Group II xenoliths have more variable and, in some cases, more radiogenic 206 Pb 204 Pb (36.6–39.3) than Group I xenoliths (38.3–38.4). The Sr, Sm, Nd and Pb concentrations of minerals in Orapa Group I eclogite xenoliths are much lower than in Group II samples. All the Group II xenoliths are inferred to be enriched in light rare-earth elements while Group I xenoliths are probably characterised in many cases by light rare-earth element depletion. Constituent garnet and clinopyroxene in both Group I and II eclogite xenoliths are essentially in isotopic equilibrium at the time of pipe emplacement. Mineral as well as calculated whole-rock 143 Nd 144 Nd compositions of most of the Group I eclogites are too close to bulk-Earth and depleted-mantle estimates in order to obtain useful model age information. Depleted-mantle model ages derived from the much lower 143 Nd 144 Nd compositions of the Group II eclogite xenoliths range from 661 to 1248 My, with an average clinopyroxene model age of 908 My and an average whole-rock model age of 1016 My. On the basis of an observed covariation of O and Sr isotopic compositions the entire Orapa Group I eclogite xenolith suite can be modelled as mixtures of oceanic basalt with or without a few percent of ocean floor sediment. The Group II xenoliths might have crystallised from a melt which derives from a protolith with time-averaged LREE depletion. Their radiogenic Sr isotope character could be due to interaction of the melt with metasomatised lithosphere, or might be a superimposed metasomatic signature.