Dunit-, Wehrlit- und Pyroxenit-Xenolithe als mögliche Kumulate der Archaischen Ozeankruste
Final Report Abstract
This project dealt with dunite, wehrlite and websterite xenoliths from kimberlites of the Kimberley cluster that are, compared to the well studied peridotite xenoliths, rare among the mantle xenolith suite in the Kimberley kimberlites of the Kaapvaal craton in Southern Africa. On grounds of mineralogy, mineral major and trace element and oxygen isotope compositions, as well as whole-rock and olivine separate Re-Os isotope systematics, we were able to distinguish two petrogenetically different ultramatic xenolith suites, one comprising Fe-rich dunites with predominantly Mesozoic Re-depletion ages and the other comprising Mg-rich dunites, wehrlites and websterites with mostly Archaean Re-depletion ages. Fe-rich dunite xenoliths within the Kimberley kimberlites comprise olivine neoblasts with minor elongated, parallel-oriented ilmenite, and rarely olivine porphyrodasts and spinel. Compared with typical mantle peridotites, olivines in the Fe-rich dunites have lower forsterite and NiO contents, which precludes a restitic origin. Chrome-rich spinels are remnants of a metasomatic reaction that produced ilmenite and phlogopite. Olivine trace element compositions differ between porphyrodasts and neoblasts, documenting their different origins. The dunites have high 187Os/188Os ratios that result in young model ages for most samples, whereas few samples show isotopic mixtures between Phanerozoic neoblasts and ancient porphyrodastic material. Most Fe-rich dunite xenoliths are interpreted to be recrystallized cumulates related to fractional crystallization of Jurassic Karoo flood basalt magmatism, whereas the porphyrodasts are interpreted to be remnants from a much earlier (probably Archaean Ventersdorp) magmatic episode. Mg-rich dunite, wehrlite and websterite xenoliths were originally residues of extensive melt extraction and experienced varying amounts and types of melt re-enrichment The mett depletion event, dated by Re-Os isotope systematics at 2.9 Ga or older, is evidenced by the high Mg# of silicate minerals, high Cr# of spinel and mostly low whole-rock Si02, CaO and AI2O3 contents. Shortly after melt depletion, websterites were formed by reaction between depleted peridotites and silica-rich melt derived by partial melting of eclogite before or during cratonization. The melt-peridotite interaction converted olivine into orthopyroxene. All three Mg-rich xenolith types have secondary metasomatic clinopyroxene and garnet, which occur along olivine grain boundaries and have an amoeboid texture. As indicated by the preservation of oxygen isotope disequilibrium in the minerals and trace-element concentrations in clinopyroxene and garnet, this metasomatic event is probably of Mesozoic age and was caused by percolating alkaline basaltic melts. This melt metasomatism enriched the xenoliths in CaO, AI2O3, FeO and high-fietd-strength elements, and might correspond to the Karoo magmatism at 200 Ma. The websterite xenoliths experienced both the orthopyroxeneenrichment and dinopyroxene-garnet metasomatic events, whereas dunite and wehriite xenoliths only saw the later basaltic melt event, and may have been situated further away from the source of melt migration channels. The comparison of Kimberley dunite, wehrlite and websterite xenoliths, representing the Kaapvaal Craton, with one Homestead dunite xenolith, representing the Wyoming mantle lithosphere, showed that, similar processes have influenced the petrogenesis of cratonic mantle xenoliths, such as melt depletion and multiple metasomatic enrichment But the chronology of these processes and the composition of fluids or melts influencing the major and trace element composition of the mantle are different between the Kaapvaal Craton and the Wyoming Craton. Additionally, loss of cratonic mantle lithosphere of the Wyoming Craton may have influenced the recent petrogenetic history of the Homestead dunite xenolith.
Publications
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Rehfeldt, T., Foley, S.F., Jacob, D,E. (2004): Dunite-pyroxenite xenoliths from South African kimberlites: Former cumulates of Archean oceanic crust? - Geochimica et Cosmochimica Acta 68 (11), p. A608-A608,
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Rehfeldt, T., Foley, S.F., Jacob, D.E. (2006): Restoration of premetasomatic protolith compositions in mantle xenoliths. - Geochimica et Cosmochimica Acta 70 (18), p. A523-A523. Goldschmidt Conference, Melbourne/Australia.
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Rehfeldt, T., Foley, S.F., Jacob, D.E., Carlson, R.W. (2007): Characterizing Fe-rich dunite xenoliths as cumulates of Phanerozoic and Archaean flood basalt magmatism, - Goldschmidt Conference, Cologne/Germany.
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Rehfeldt, T., Jacob, D.E., Carlson, R.W., Foley, S.F. (2007): Fe-rich dunite xenoliths from South African kimberlites: cumulates from Karoo flood basalts. Journal of Petrology 48, 1387-1409,
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Rehfeldt, T., Foley, S.F., Jacob, D,E., Carlson, R.W,, Lowry D, (2008); Contrasting types of metasomatism in dunite, wehrlite and websterite xenoliths from Kimberley. South Africa. Geochimica at Cosmochimica Acta, 72, 5722-5756. (Siehe online unter: doi: 10,1016/j,gca.2008.08.020)
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Rehfeldt, T., Jacob, D.E., Foley, S.F., Carlson, R.W., (2008): Spatial differences and chronology of metasomatism in depleted mantle xenoliths beneath Kimberley, South Africa, International Kimberlite Conference, Frankfurt a.M./Germany, extended abstract.
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Foley, S.F., Buhre, S., Rehfeldt, T. (2003): Pyroxenite and dunite xenoliths as metamorphosed cumulates from the Archean lower ocean crust, - International Kimberlite Conference, Vancouver/Canada, 2,PI2,
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Rehfeldt, T., Jacob, D.E., Carlson, R.W., Foley, S.F. (2006): Fe-rich dunite xenoliths from Kimberley, South Africa: Cumulates of fractional crystallisation of low-Ti Karoo basalts, - International Conference on Continental Volcanism, Guangzhou/China,
