Eclogite xenoliths from the Lace kimberlite, Kaapvaal craton: From convecting mantle source to palaeo-ocean floor and back

• Published in: Earth and planetary science letters 2015-12, Vol.431, p.274-286
• Main Authors: Aulbach, S., Viljoen, K.S.
• Format: Article
• Language: English
• Subjects: Archaean oceanic crust recycling; atmosphere oxygenation; Crusts; Eclogite; Evolution; Hafnium; kimberlite–borne eclogite xenoliths; Lead (metal); Mantle; mantle potential temperature; mantle redox; Melts; oxygen fugacity; Rare earth metals
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2015.08.039

•Trends in REE, HFSE, Eu⁎ suggest low-P picrite protoliths for Lace mantle eclogites.•Protoliths formed by spinel peridotite melting at P∼2.5 GPa and moderate TP∼1420°C.•Metamorphism induced loss of a partial melt from rutile eclogite at average F∼0.2.•V/Sc indicates fO2 at 1 GPa=FMQ−1.7±1.1, lower than MORB source.•Recycled Lace eclogites cannot generate radiogenic isotope ratios of OIB sources. Major- and trace-element compositions of eclogite and pyroxenite xenoliths of ≥2.5 Ga age (in situ Pb–Pb data on clinopyroxene) from the Lace kimberlite on the Kaapvaal craton were investigated in order to constrain: (1) the nature and evolution of their protoliths; (2) the extent to which they preserve information on the state of the asthenospheric mantle source that gave rise to their low-pressure protoliths; and (3) the effect of their deep recycling on the radiogenic isotope evolution of the convecting mantle. Their elemental relationships are consistent with low-pressure fractionation of olivine ± plagioclase and clinopyroxene during oceanic crust formation, whereby the residual melt was enriched in rare-earth elements (REE), high field-strength elements and Y, producing inverse correlations of ΣREE with the size of Eu- and Sr-anomalies. LREE-depletion may indicate loss of on average 20% of a partial melt upon subduction and metamorphism (eclogitisation) of oceanic crust, which did not, however, contribute to juvenile growth of continental crust. The eclogites have median Sm/Nd (0.40) and Lu/Hf (0.27) similar to Depleted Mantle, and lower U/Pb (0.02) and Th/Pb (0.02). If deeply subducted, these rocks cannot explain unradiogenic Nd and Hf, and radiogenic Pb isotope compositions in the sources of some modern ocean island basalts. Low incompatible trace-element contents similar to picrites, and Yb concentrations at a given TiO2 content similar to modern MORB, indicate derivation of the protoliths by average melt fractions of ∼0.20–0.25 that left a spinel peridotite residue at pressures ≤2.5 to 3.0 GPa. This shallow intersection of the peridotite solidus suggests moderate Archaean ambient mantle potential temperatures of ≤1420 to 1470 °C. Samples filtered for clinopyroxene fractionation and metasomatism have V/Sc (4.7±1.2; n=11) indicating lower fO2 (−1.9 relative to the fayalite–magnetite–quartz buffer=ΔFMQ) than modern MORB. This is in part due to the higher average melt extraction pressure (∼1.5 GPa) during formation of their crustal protoliths. Extrapolation