کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
6427798 | 1634725 | 2015 | 13 صفحه PDF | دانلود رایگان |
![عکس صفحه اول مقاله: Eclogite xenoliths from the Lace kimberlite, Kaapvaal craton: From convecting mantle source to palaeo-ocean floor and back Eclogite xenoliths from the Lace kimberlite, Kaapvaal craton: From convecting mantle source to palaeo-ocean floor and back](/preview/png/6427798.png)
- Trends in REE, HFSE, Euâ suggest low-P picrite protoliths for Lace mantle eclogites.
- Protoliths formed by spinel peridotite melting at Pâ¼2.5GPa and moderate TPâ¼1420°C.
- Metamorphism induced loss of a partial melt from rutile eclogite at average Fâ¼0.2.
- V/Sc indicates fO2 at 1GPa=FMQâ1.7±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 â¥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 â¼0.20-0.25 that left a spinel peridotite residue at pressures â¤2.5 to 3.0 GPa. This shallow intersection of the peridotite solidus suggests moderate Archaean ambient mantle potential temperatures of â¤1420 to 1470â°C. Samples filtered for clinopyroxene fractionation and metasomatism have V/Sc (4.7±1.2; n=11) indicating lower fO2 (â1.9 relative to the fayalite-magnetite-quartz buffer=ÎFMQ) than modern MORB. This is in part due to the higher average melt extraction pressure (â¼1.5 GPa) during formation of their crustal protoliths. Extrapolation to 1 GPa, similar to the average pressure of present-day MORB generation, yields ÎFMQ of â1.7±1.1, corresponding to Fe3+# of 0.07±0.04. If these results are correct, they suggest an Archaean ambient mantle more reducing than at present, with implications for the speciation of volatiles, which affects the mantle solidus and the composition of volcanic gases. This has implications for the Archaean atmospheric redox evolution and the recycling of carbon and other volatiles.
Journal: Earth and Planetary Science Letters - Volume 431, 1 December 2015, Pages 274-286