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. G roup I xenoliths are commonly diamondiferous while Group II xenoliths are diamond-free. Both xenolith varieties may contain graphite. Isotop ic character is to some degree correlated with major- and trace-elemen t chemistry. Group II samples with Ca-poor garnet have clinopyroxenes with radiogenic Sr-87/Sr-86 (0.705-0.709) and the least radiogenic Nd- 143/Nd-144 (0.5122-0.5125). Group I eclogites with higher Ca and Fe in garnets have less radiogenic Sr-87/Sr-86 (0.702-0.7066) and bulk-Eart h or higher Nd-143/Nd-144 ratios. Group I eclogites have more radiogen ic Pb-206/Pb-204 (18.6-19) than Group II xenoliths (16.5-18.6). In con trast, Group II xenoliths have more variable and, in some cases, more radiogenic Pb-208/Pb-204 (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 e clogite xenoliths are much lower than in Group II samples. All the Gro up II xenoliths are inferred to be enriched in light rare-earth elemen ts while Group I xenoliths are probably characterised in many cases by light rare-earth element depletion. Constituent garnet and clinopyrox ene in both Group I and II eclogite xenoliths are essentially in isoto pic equilibrium at the time of pipe emplacement. Mineral as well as ca lculated whole-rock Nd-143/Nd-144 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 a ges derived from the much lower Nd-143/Nd-144 compositions of the Grou p II eclogite xenoliths range from 661 to 1248 My, with an average cli nopyroxene 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 m odelled 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 dep letion. Their radiogenic Sr isotope character could be due to interact ion of the melt with metasomatised lithosphere, or might be a superimp osed metasomatic signature.