Garnets peridotite xenoliths

While all spinel-lherzolite facies suites show remarkably similar compositional trends as a function of depletion, some garnet peridotite xenoliths in kimberlites and lamproites from ancient cratonic lithospheric keels show signih-cantly different trends (e.g., see Boyd, 1989 Chapters 2.05 and 2.08). Most of these xenoliths are extremely depleted extrapolation of the trends back to the PM MgO of 36.7% gives similar concentrations of Si02, EeO AI2O3, and CaO to the spinel Iherzolites (O Neill and Palme, 1998) the difference in their chemistry is due to a different style of melt extraction, and not a difference in original mantle composition. 

Figure 6 P-T arrays compiled for garnet peridotite xenoliths from several suites using two-pyroxene thermometry and Al-in-orthopyroxene barometry (Tbkn and Rbkn methods, Table 5). Data sources given in Rudnick and Nyblade with additional data here for Vitim (Ionov et al., 1993a) and Canada (MacKenzie and Canil, 1999 Schmidberger and Francis, 1999). The best-fit line for the Kaapvaal data is plotted in each figure for reference. Intersection of P-T array with mantle adiabats (shaded field) represents an estimate of the thickness of lithosphere at the time of sampling.

Figure 20 Primitive mantle normalized extended PGE patterns (including rhenium) for cratonic whole-rock garnet peridotite xenoliths from the Letseng kimberlite (Lesotho) (sources Irvine, 2002 and Pearson et al., 2004).

Bedini R. M., Blichert-Toft J., Boyet M., and Albarede F. (2002) Lu-Hf isotope geochemistry of garnet-peridotite xenoliths from the Kaapvaal craton and the thermal regime of the lithosphere. Geochim. Cosmochim. Acta (Spec. Suppl.) 66S1, A61. 

Finnerty A. A. and Boyd F. R. (1987) Thermobarometry for garnet peridotite xenoliths a basis for upper mantle stratigraphy. In Mantle Xenoliths (ed. P. H. Nixon). Wiley, Chichester, pp. 381-402. 

Ionov D. A., Ashchepkov I. V., Stosch H.-G., Witt-Eickschen G., and Seek H. A. (1993a) Garnet peridotite xenoliths from the Vitim volcanic field, Baikal Region the nature of the garnet-spinel peridotite transition zone in the continental mantle. J. Petrol. 34, 1141—1175. 

Stern C. R., Saul S., Skewes M. A., and Futa K. (1989) Garnet peridotite xenoliths from the Pali-Aike alkali basalts of southernmost South America. In Kimberlites and Related Rocks. Geol. Soc. Austral. Spec. Publ. No. 14 (ed. J. Ross). Blackwell, Perth, vol. 2, pp. 735-744. 

Zhuravlev A. Z., Laz ko Y. Y., and Ponomarenko A. I. (1991) Radiogenic isotopes and REE in garnet peridotite xenoliths from the Mir kimberlite pipe, Yakutia. Geokhimiya 7, 982-994. 

Kesson S. E. and Ringwood A. E. (1989) Slab-mantle interactions 1. Sheared and refertilized garnet peridotite xenoliths-samples of wadati-benioff zones Chem. Geol. 78, 89-96. 

Boyd, F. R. 1987. High- and low-temperature garnet peridotite xenoliths and their possible relation to the lithosphere-asthenosphere boundary beneath southern Africa. In Nixon, P. H. (ed.) Mantle Xenoliths, Wiley-Interscience, Chichester, 403-412. 

Richardson, S. H., Erlank, A. J. Hart, S. R. 1985. Kimberlite-bome garnet peridotite xenoliths from old enriched subcontinental lithosphere. Earth and Planetary Science Letters, 75, 116-128. 

Frick C. (1973) The sulphides in griquaite and garnet-peridotite xenoliths in kimberlite. Contrib. Mineral Petrol 39, 1-16. 

Cover photo Thin seaion photograph of a garnet peridotite xenolith from the Monastery kimberlite in South Africa. Polarizers are only partially crossed to allow the large garnet in the upper left comer to remain transparent. (Photograph provided by Richard W. Carlson)... 

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