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Fe-rich Dunite Xenoliths from South African Kimberlites: Cumulates from Karoo Flood Basalts.
Авторы: REHFELDT T., Jacob D., Carlson R.
2007 г.
Journ.Petrol.
Fe-rich dunite xenoliths within the Kimberley kimberlites comprise olivine neoblasts with minor elongated, parallel-oriented ilmenite, and rarely olivine porphyroclasts and spinel. Compared with typical mantle peridotites, olivines in the Fe-rich dunites have lower forsterite (Fo87^89 ) andNiOcontents (1300^2800 ppm) , which precludes a restitic origin for the dunites. Chrome-rich spinels are remnants of a metasomatic reaction that produced ilmenite and phlogopite.Trace element compositions differ between porphyroclastic and neoblastic olivine, the latter having higherTi, V , Cr and Ni and lower Zn, Zr and Nb contents, documenting their different origins.The dunites have high 187 Os / 188 Os ratios (0.11~0.15) that result in young model ages for most samples, whereas three samples show isotopic mixtures between Phanerozoic neoblasts and ancient por- phyroclastic material. Most Fe-rich dunite xenoliths are interpreted to be recrystallized cumulates related to fractional crystallization of Jurassic Karoo flood basalt magmatism, whereas the porphyroclasts are inter- preted to be remnants from a much earlier (probably Archaean Ventersdorp) magmatic episode.The calculated parental magma for the most primitive olivine neoblasts in the Fe-rich dunites is similar to low-Ti Karoo basalts. Modelling the crystal fractionation of the inferred parental magma with pMELTS yields element fractionation trends that mirror the element variation of primitive low-Ti Karoo basalts.
Файл: 647.pdf
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