Plagioclase peridotite xenoliths

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... 

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. 

In the oceanic setting, spinel Iherzolite xenoliths from Pali (Hawaii) have olivine 5 0 values of 5.09-5.12 per mil, typical of olivines from other oceanic and continental mantle rocks (Ducea et al., 2002). In contrast, olivines from plagioclase peridotites are enriched by 0.5 per mil. This is interpreted to be due to the formation of plagioclase by reaction with or crystallization from melts intruding the Pacific lithospheric mantle. 

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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