Eclogite xenoliths from the Premier kimberlite, South Africa: Geochemical evidence for subduction origin

A suite of mantle eclogite hosted within the Premier kimberlite on the Kaapvaal craton can be classified on the basis of Na2O content in garnets as group I type, although textures are ambiguous. No accessory phases of note occur, but rutile and phlogopite are found in a few samples. Clinopyroxenes show variable light rare element (LREE) enrichment (La/Ybn = 2–48), and the garnets are strongly LREE depleted relative to chondrites (La/Ybn = <0.04). Four pyroxenite samples include both garnet clinopyroxenite and garnet orthopyroxenite; clinopyroxenes in these samples are strongly LREE enriched (La/Ybn = 57–65). Calculated equilibration temperatures of the eclogites range from 999 ± 32 to 1168 ± 14° C with an average temperature of 1102 ± 37° C, assuming a pressure of 50 kbar. Relative to a shield geotherm of 40mW/m2, these temperatures suggest a sampling depth of 135 to 165 km. A single, calcium-rich sample gives an equilibration temperature of 1296 ± 32° C at the same assumed pressure. Calculated equilibrium temperatures and pressures for the garnet pyroxenites are 887 to 987° C and 26 to 39 kbar (clinopyroxenite) and 1135 to 1156° C and 48 ± 2 kbar (orthopyroxenite). Reconstituted bulk rock compositions of the eclogites indicate the presence of low- and high-MgO groups. The MgO-poor eclogites (8 to10.5 weight % MgO) have jadeite-rich clinopyroxenes and except for lower silica contents are similar to mid-ocean ridge basalts in major element composition, with slight negative Euanomalies (Eu/Eu*=0.83 to 0.96), indicative of (low-P) plagioclase fractionation. The MgO-rich eclogites (13.6 to 18 weight % MgO) are similar in composition to oceanic gabbro. In combination the geochemical data suggest that the Premier eclogite suite represents a fragment of a once composite oceanic crustal section; the protolith to the low-MgO eclogites was recycled oceanic crustal layer two metabasalt, which experienced silicic melt loss during subduction; the protolith to the high-MgO suite was oceanic crustal layer three cumulate gabbro/pyroxenite

Hotspot trails in the South Atlantic controlled by plumes and plate tectonic processes

The origin of hotspot trails ranges controversially1 from deep mantle plumes rising from the core-mantle boundary2 to shallow plate cracking. But these mechanisms cannot explain uniquely the scattered hotspot trails on the 2,000 km-wide southeast Atlantic hotspot swell3, which projects down to one of the Earth’s two largest and deepest regions of slower-than-average seismic wave speed – the Africa Low Shear Wave Velocity Province, which marks a massive thermo-chemical ‘pile’ at the core-mantle boundary4,5,6. Here we use 40Ar/39Ar isotopic ages – and crustal structure and seafloor ages – to show that age progressive hotspot trails formed synchronously across the swell, consistent with African plate motion over plumes rising from the stable edge of a Low Shear Wave Velocity Province. We show also that hotspot trails formed initially only at spreading boundaries at the outer edges of the swell until roughly 44 million years ago, when they started forming across the swell, far from spreading boundaries in lithosphere that was sufficiently weak (young) for plume melts to reach the surface. We conclude that if plume melts formed synchronous age progressive hotspot trails wherever and whenever they could penetrate the swell lithosphere then hotspot trails in the South Atlantic are controlled by an interplay between deep plumes and the motion and structure of the African plate

The oxygen isotope composition of Karoo and Etendeka picrites: High δ18O mantle or crustal contamination?

Olivine and orthopyroxene phenocrysts from picrite and picrate basalt lavas and dykes (Mg# 64-80) from the Tuli and Mwanezi (Nuanetsi) regions of the ~180 Ma Karoo Large Igneous province (LIP) have δ18O values that range from 6.0 to 6.7 ‰ (Fig. 1), suggesting that they crystallized from magmas having δ18O values about 1 to 1.5 ‰ higher than expected in an entirely mantle-derived magma. Olivines from picrite and picrite basalt dykes from the 135 Ma Etendeka LIP of Namibia and Karoo-age picrite dykes from Dronning Maud Land, Antarctica do not have such elevated δ18O values. The Etendeka picrites show good correlations between δ18O value and Sr-, Nd- and Pb-isotope ratios that are consistent with previously proposed models of crustal contamination (e.g. Thompson et al., 2007). Explanations for the high δ18O values in Tuli/Mwenezi picrites are limited to (i) alteration, (ii) crustal contamination, and (iii) derivation from mantle with an abnormally high δ18O. The lack of variation in olivine and orthopyroxene δ18O values, together with the lack of correlation between mineral and whole-rock δ18O values are not consistent with alteration being the cause of high δ18O values. The high δ18O values in selected olivine cores have been confirmed by SIMS, and aggressive cleaning of crystals with HF makes no difference to the δ18O value obtained. Average εNd and εSr values of -8 and +16, and high concentrations of incompatible elements such as K are typical of picrites from the Mwanezi (Nuanetsi) region, which have been explained by a variety of models that range from crustal contamination to derivation from the ‘enriched’ mantle lithosphere. The primitive character of the magmas combined with the lack of correlation between δ18O values and radiogenic isotope composition and MgO content or Mg# are inconsistent with crustal contamination, and lend weight to arguments in favour of an 18O-enriched mantle source having high incompatible trace element concentration and enriched radiogenic isotope composition. Although elevated initial Sr isotope ratios, εNd values of -8, and δ18O values about 1 ‰ higher than expected for mantle-derived magma are also a feature of the Bushveld mafic and ultramafic magmas, it is unlikely that a long-lived 18O-enriched mantle source would have survived for nearly 2 Ga. Incorporation of crustal material into the mantle by subduction or delamination of the lower crust are the most likely mechanisms for enriching the mantle in 18O