Peridotite xenoliths

Blatter DL, Carmichael ISE (1998) Hornblende peridotite xenoliths from central Mexico reveal the highly oxidized nature of subarc upper mantle. Geology 26 1035-1038 Blundy J, Wood B (2003) Mineral-melt partitioning of uranium, thorium and their daughters. Rev Mineral Geochem 52 59-123... 

Elliott et al. (2003). The data set for unmetasomatized peridotite xenoliths is too sparse to aid much in this controversy at present. As measurement reproducibility betters the 0.5%o barrier, it is just a matter of analyzing the most appropriate samples before such questions can be answered and an accurate estimate can be made of the Li isotopic composition of the mantle, and hence of the bulk Earth. 

Ottonello G. (1980). Rare earth abundance and distribution in some spinel peridotite xenoliths from Assab (Ethiopia). Geochim. Cosmochim. Acta, 44 1885-1901. 

Interesting differences in sulfur isotope compositions are observed when comparing high-S peridotitic tectonites with low-S peridotite xenoliths (Fig. 3.7). Tec-tonites from the Pyrenees predominantly have negative 5 " S-values of around —5%c, whereas low-S xenoliths from Mongoha have largely positive 8 S-values of up to - -l%o. Ionov et al. (1992) determined sulfur contents and isotopic compositions in... 

Beccaluva L, Bianchini G, Coltorti M, Perkins WT, Siena F, Vaccaro C, Wilson M (2001) Multistage evolution of the European lithospheric mantle new evidence from Sardinian peridotite xenoliths. Contrib Mineral Petrol 142 284-297... 

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. 

Jochum K. P., McDonough W. F., Pahne H., and Spettel B. (1989) Compositional constraints on the continental lithospheric mantle from trace elements in spinel peridotite xenoliths. Nature 340, 548-550. 

Ionov D. A., Bodinier J.-L., Mukasa S. B., and Zanetti A. (2002) Mechanisms and sources of mantle metasomatism major and trace element compositions of peridotite xenoliths from Spitsbergen in the context of numerical modelling. J. Petrol. 43, 2219-2259. 

Xu Y.- G., Menzies M. A., Bodinier J.-L., Bedini R.-M., Vroon P., and Mercier J.-C. (1998) Melt percolation-reaction at the lithosphere-plume boundary evidence from the poikiloblas-tic peridotite xenolith from Boree (Massif Central, Frace). Contrib. Mineral. Petrol. 132, 65-84. 

Despite peridotite xenoliths in basalts being recognized for several centuries and comparisons being made to Iherzolite massifs (Lacroix, 1893), it was not until work on garnet peridotites and diamonds in kimberlites by Fermor (1913) and Wagner (1914) that such xenoliths were conceptually associated with a peridotite zone in... 

Xenoliths most commonly show coarse or porphyroclastic textures, or some continuum between the two. The former is defined by a grain size of greater than 2 mm (Table 2) and is commonly equigranular (Figure 1(a)). This texture represents a stable grain size developed under differential stresses that are small and constant over milhons of years in the lithosphere. The grain size in olivine, the most abundant but weakest mineral in peridotite xenoliths, can be a measure... 

Figure 1 Common petrographic textures in peridotite xenoliths. Other textural details given in Table 1. (a) Coarse texture in cross-polarized light of garnet peridotite containing olivine (ol), orthopyroxene (opx), and garnet (gt). (b) Mosaic porphyroclastic texture in plane polarized light of garnet peridotite. Both samples are from the Torrie kimberlite. Slave Province (scale bar is 1 mm).

By definition, peridotites contain greater than 40% olivine with lesser amounts of orthopyroxene and clinopyroxene. An aluminous phase, plagio-clase, spinel, or garnet may be present depending on the pressure of equilibration and defines the facies from which the peridotite xenolith was sampled (Figure 2). Plagioclase-peridotites are generally rare in continental xenolith suites... 

Boyd (1989) compiled modes of major minerals in peridotite xenoliths from on and off-craton localities and compared the results with mantle residues from oceanic mantle represented by... 

Differences between the modes observed in spinel peridotite xenoliths and those predicted by experiment may result because the former are not simple residues, and have had components introduced after their original formation. 

Plagioclase occurs only in peridotite xenoliths sampled from areas of high geothermal gradient and thinned crust, where the plagioclase-facies is stable and can be sampled from beneath the... 

Moho (Figure 2). It may also occur in shallow lithosphere that has been impregnated by melts (Sen and Leeman, 1991). Plagioclase in peridotite xenoliths is typically calcic (An60-90) due to the low Na20 levels of most peridotites. 

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.

In view of the inhomogeneities observed for peridotite outcrops in ophiolites and orogenic massifs (Dick and Sinton, 1979 Chapter 2.04), there is some uncertainty as to how representative the bulk analysis of a centimeter-scale sample is of the volume of mantle sampled by a host magma. Furthermore, most peridotite xenoliths are coarse-textured and some contain spatial associations on the centimeter-scale (see above). Compilations discussed or presented here make no distinction for sample size but larger samples (>500 g) are... 

Figure 8 Mg/(Mg + Fe) versus Mg/Si for (a) off-craton spinel peridotite xenoliths and (b) cratonic garnet and spinel peridotite xenoliths. Arrows mark the oceanic trend (Boyd, 1989, 1997) defined by abyssal peridotites. Also shown are various estimates for primitive upper mantle (polygons from Table 7).

Figure 10 Mean and median AI2O3 (wt.%) for peridotite xenoliths from different settings, compared with samples from known modem tectonic settings (in italics Table 8).

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