Clinopyroxenes peridotite xenoliths

Ottonello G. Piccardo G. B., Mazzucotelli A., and Cimmino F. (1978). Clinopyroxene-orthopyroxene major and REE partitioning in spinel peridotite xenoliths from Assab (Ethiopia). Geochim. Cosmochim. Acta., 42 1817-1828. 

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... 

Figure 17 Summary fields of chondrite-normalized REE patterns for whole-rock peridotites and cUnopyroxenes for peridotite xenoliths. Noncratonic whole-rock peridotites are either LREE-depleted (type lA least common) or LREE-enriched (type IB most common). Data sources from Stosch and Seek (1980), Stosch and Lugmair (1986), Menzies et al (1985). Clinopyroxenes from these rocks also show LREE enrichment or depletion. Cratonic peridotite whole rocks are ubiquitously LREE-enriched. Low-T (granular) suite show greater LREE/HREE compared to high-T (sheared) suite and this is reflected in the more LREE-enriched clinopyroxene compositions in the low-T suite. Data sources from Shimizu (1975), Nixon et al. (1981), and Irvine (2002). Low-T whole-rock suite includes 19 samples...

One of the first studies to show this was performed on Kilboume Hole spinel Uierzolites (Jagoutz et al, 1980). Equihbrated neodymium isotopes in orthopyroxene and diopside defined essentially zero age isochrons, consistent with the very recent eruption age of the host volcanic rocks, while strontium isotopes were un-equilibrated. Stolz and Davies (1988) found varying degrees of equihbration between amphibole, clinopyroxene and apatite in peridotite xenoliths from S.E. Australia. Several samples contained coexisting amphibole and clinopyroxene and had almost reached isotopic equilibrium for strontium but displayed disequilibrium relations for lead and neodymium isotopes. This was taken to indicate more rapid diffusion of strontium than lead and neodymium. Some peridotite and eclogite... 

Figure 38 Nd-Sr isotope variation of clinopyroxenes and garnet in peridotite xenoliths. (a) Compares cratonic and noncratonic peridotite xenoliths with continental crust. Inset shows restricted field for oceanic mantle. Arrow points to a peridotite from Lashaine, Tanzania, that lies at an Sr/ Sr value of 0.83. (b) Compares cratonic peridotites from the Kaapvaal, Wyoming, and Siberian cratons.

Zangana N. A., Downes H., ThirlwaU M. F., Marriner G. F., and Bea F. (1999) Geochemical variation in peridotite xenoliths and their constituent clinopyroxenes from ray pic (French Massif Central) implications for the compositions of the shallow lithospheric mantle. Chem. Geol 153, 11-35. 

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... 

MacGregor I. D. and Carter J. L. (1970) The chemistry of clinopyroxenes and garnets of eclogite and peridotite xenoliths from the Roberts Victor Mine, South Africa. Phys. Earth Planet. Int. 3, 391-397. 

Figure 9. Plots of Li and radiogenic isotopes for mantle rocks, (a) 5 Li vs. Sr/ Sr (b) 5 Li vs. Nd/ Nd (c) "Sr/ Sr vs. Pb/ Pb (d) 5"Li vs. Pb/ Pb (Nishio et al. 2003, 2004). Symbols + = south Pacific island basalts (six islands) O = Iherzolite xenolith, Bullenmerri, Australia = Iherzolite xenolith, Sikhote-Alin, Russia (three localities) A = dunite-peridotite-pyroxenite xenolith, Kyushu, Japan (two localities) V = Iherzolite xenolith, Ichinomegata, Japan. The ocean island data are from bulk rocks, the xenolith data are clinopyroxene separates. For explanations of the derivation of radiogenic isotope fields (DM, EMI, EM2, HIMU), see Zindler and Hart (1986). The estimate for Li isotopes in DM is based on MORE. The Li isotopic ranges for the other mantle reservoirs are based on Nishio et al. (2004) and Nishio et al. (2003), but these will require further examination (hence the use of question marks).

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.

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. 

Figures 3(d)-(f) compares modes observed in four well-characterized on-craton xenolith suites (n = 189) with degree of depletion. When compared to the off-craton samples, trends are far more scattered for olivine and orthopyroxene, and a significant population of samples are orthopyr-oxene-rich, as originally remarked by Boyd (1989). The trend for garnet is remarkably regular and uniform, whereas many samples contain far more clinopyroxene than expected for their level of depletion. This excess clinopyroxene may be of exsolution origin, or introduced to the rock after its original formation as a residue (Canil, 1992 Shimizu, 1999 Simon et al, 2003). Comparison with trends expected from peridotite melting models is complicated by the fact that orthopyroxene is replaced at the solidus by a low calcium clinopyroxene, and is a product of the melting reaction at P > 3 GPa (Walter, 1998). The mean and median modes of off-craton and on-craton xenoliths from Figure 3 are summarized in Table 2.

In a given facies, the sodium and titanium content of clinopyroxene shows a negative correlation with depletion (or chromium content) and can increase substantially in metasomatized xenoliths. The substimtion of sodium, chromium, and aluminum has a complex relationship with T, P and degree of depletion but in general, clinopyroxene from higher P samples contains more sodium. Representative analyses for different peridotite facies are shown in Table 4. 

Clinopyroxene shows a range of REE patterns from extremely enriched to very depleted TREE signatures (Figure 22). Noncratonic peridotites are subdivided on the basis of clinopyroxene REE patterns into LREE-depleted (type lA) and LREE-enriched (type IB Menzies, 1983 Figure 17). LREE-enriched type IB pyroxenes are the norm in most suites. LREE-depleted varieties are relatively scarce. Very few clinopyroxenes show simple LREE-depleted REE patterns that can be interpreted solely in terms of melt depletion, i.e., LREE depletion, fiat, unfractionated MREE-HREE patterns (e.g., UM-6 or 2905 Eigure 22). For peridotites that do have LREE-depleted clinopyroxenes, a correlation of HREE with other incompatible trace elements (e.g., yttrium, strontium, zirconium) in xenoliths suites worldwide requires fractional melting to be the principal means of depletion in the mantle (Norman, 2001). 

Few systematic Sr-Nd isotope studies have been performed on ocean island xenolith suites. Ducea et al. (2002) analyzed clinopyroxenes from plagioclase-spinel and spinel peridotites from Pali, (Oahu, Hawaii) and found relatively depleted strontium and neodymium isotope systematics that they interpret as representing their evolution as residues from the extraction of Pacific Ocean crust. Consistent with this is a 61 20Ma errorchron defined by the pyroxene separates that is within error of the 80-85 Ma age of Pacific lithosphere beneath Hawaii. 

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