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

Eclogite xenoliths from Orapa can be accurately classified as Group I or Group II on the basis of Na₂O in garnet and K₂O 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 ⁸⁷Sr/ ⁸⁶Sr (0.705-0.709) and the least radiogenic ¹⁴³Nd/¹⁴⁴Nd (0.5122-0.5125). Group I eclogites with higher Ca and Fe in garnets have less radiogenic ⁸⁷Sr/⁸⁶Sr (0.702-0.7066) and bulk-Earth or higher ¹⁴³Nd/¹⁴⁴Nd ratios. Group I eclogites have more radiogenic ²⁰⁶Pb/²⁰⁴Pb (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 ²⁰⁸Pb/²⁰⁴Pb (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 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 ¹⁴³Nd/¹⁴⁴Nd 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.