Orthopyroxenes 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. 

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... 

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.

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... 

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. 

In detail, however, the picture is not so simple. All mantle peridotites (whether massive peridotites or xenoliths) are metamorphic rocks that have had a complex subsolidus history after melt extraction ceased. As well as subsolidus recrystallization, peridotites have undergone enormous amounts of strain during their emplacement in the lithosphere. Massive peridotites show modal heterogeneity on the scale of centimeters to meters, caused by segregation of the chromium-diopside suite of dikes, which are then folded back into the peridotite as deformation continues. The net result is more or less diffuse layers or bands in the peridotite, which may be either enriched or depleted in the material of the chromium-diopside suite, i.e., in climopyroxene and orthopyroxene in various proportions, minor spinel, and sulfide. This process should cause approximately linear correlations of elements versus MgO, broadly similar to, but not identical with, those caused by melt extraction. Indeed, there is... 

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. 

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.

A) Cratonic/circum-cratonic xenoliths erupted by Kimberlite-related volcanics Al Coarse Mg-rich, Often abundant. Mostly harzburgites and Uierzolites with varying low-T peridotites but low modal diopside and garnet. Wide range of orthopyroxene... 

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