Olivine peridotite xenoliths

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... 

To illustrate the effects of metasomatism on the silicate mineralogy of peridotite xenoliths it is instructive to compare the modal abundances of garnet and clinopyroxene, expected to be the first minerals to be exhausted during partial melting, with indices of melt extraction such as the mg-number of olivine (Fig. 1). The observed abundances of garnet plus clinopyroxene with olivine... 

Fig. 10. Histograms of Trd ages for on- and off-craton peridotites from southern Africa co-plotted with histograms of olivine mg-numbers. Kaapvaal low-L peridotites xenohths, n = 96. Data from Nixon et al. (1983), Walker et al. (1989), Pearson et al. (1995a), Carlson et al. (1999), Menzies et al. (1999) and Irvine et al. (2002). Lrd ages for peridotite xenoliths from kimberlites from East Griqualand, 170 km from the Lesotho on-craton kimberlites (Pearson et al. 1998) and for peridotites from the Farm Louwrencia kimberlite. Southern Namibia (Pearson et al. 1994 Pearson 1999a). It should be noted that no Archaean Trd ages are observed for off-craton peridotites. Olivine compositional data from Boyd Nixon (1979), Nixon (1987) and Boyd (unpubl. data).

Boyd, F. R. 1996. Origins of peridotite xenoliths major element considerations high pressure and high temperature research on lithosphere and mantle materials. Proceedings of the International School of Earth and Planetary Sciences, Siena, 89-106. Boyd, F. R. 1997. Correlation of orthopyroxene abundance with the Ni-content of eoexisting olivine in cratonic peridotites. EOS Transactions, American Geophysical Union, 78, 746. 

Figure 14. Inter-mineral Fe isotope fractionations among olivine and clinopyroxene from spinel peridotite mantle xenoliths. Data are from Zhu et al. (2002) ( ) and Beard and Johnson (2004) ( ). In the study by Beard and Johnson (2004), the difference in the Fe isotope composition between clinopyroxene and olivine is larger as a function of their 5 Fe values, suggesting disequilibrium fractionation.

Cr-poor variety widespread, locally abundant (e.g.. Monastery). Garnets, clino- and orthopyroxenes, phlogopite and ihnenite most common, zircon and olivine rarer. Debatable whether phlogopite and olivine are members of Cr-poor suite. Wide range in chemistry but Cr-poor, Fe-Ti-rich relative to type I (low-Z) peridotite minerals. Mineral chemistry and estimated equilibration P/Ts overlap those of type V (high-Z) Iherzolites. Some Slave craton Cr-poor megacrysts show mineral chemistry links to type II megacrystalline pyroxenite xenoliths. See review of Schulze (1987). 

The modal abundances of ohvine, orthopyroxene, clinopyroxene and spinel observed for spinel peridotites from six well-characterized olf-craton xenolith suites are plotted against a depletion index in Figures 3(a)-(c). The amount of ohvine correlates negatively with degree of depletion, as expected because olivine is a product of the reaction that produces melt at the solidus (Figure 2) (see Chapter 2.08). The number of samples compiled (n = 143) may not be completely representative, but there is nonetheless a suspicious population gap at 2 wt.% AI2O3. 

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