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Journal of Petrology Advance Access originally published online on May 28, 2007
Journal of Petrology 2007 48(7):1387-1409; doi:10.1093/petrology/egm023
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© The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Fe-rich Dunite Xenoliths from South African Kimberlites: Cumulates from Karoo Flood Basalts

Tatjana Rehfeldt1,*, Dorrit E. Jacob1, Richard W. Carlson2 and Stephen F. Foley1

1Mainz University, Department of Geosciences, Becherweg 21, D-55099 Mainz, Germany
2Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad branch road, NW, Washington, DC 20015, USA

RECEIVED FEBRUARY 24, 2006; ACCEPTED APRIL 2, 2007


  Abstract

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) and NiO contents (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 higher Ti, V, Cr and Ni and lower Zn, Zr and Nb contents, documenting their different origins. The dunites have high 187Os/ 188Os 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 porphyroclastic 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 interpreted 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.

KEY WORDS: dunite xenoliths; fractional crystallization; Karoo; large igneous province; pMELTS; Re–Os; trace elements

*Corresponding author. Telphone: 0049-6131-3922370. Fax: 0049-6131-3923070. E-mail: rehfeldt{at}uni-mainz.de


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[Abstract] [Full Text] [PDF]


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