Lherzolites composition

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. 

Most of the orogenic spinel Lherzolites also contain small amounts of amphibole ( 1 %) textu-rally equilibrated with the peridotite minerals. These amphiboles have been ascribed to the infiltration of melts/fluids in mantle conditions (Fabries et al., 1991), but their presence is not associated with noticeable chemical enrichments (Vannucci et al., 1995). A possible explanation is that the liquid/rock ratio was so low that the fluid composition was buffered by the LREE-depleted composition of the peridotites. Alternatively, these amphiboles might represent the products of a late crystallization stage during fertilization of the peridotites by LREE-depleted basaltic melts. 

Frey F. A., Shimizu N., Leinbach A., Obata M., and Takazawa E. (1991) Compositional variations within the lower layered zone of the Horoman peridotite, Hokkaido, Japan constraints on models for melt-solid segregation. In Orogenic Lherzolites and Mantle Processes, Spec. Vol. J. Petrol, (eds. M. A. Menzies, C. Dupuy, and A. Nicolas). Oxford University Press, Oxford, UK, pp. 211-227. 

Ionov D. A., and Weis D. (2002) Hf isotope composition of mantle peridotites first results and inferences for the age and evolution of the hthospheric mantle. Abstract, 4th Int. Workshop on Orogenic Lherzolites and Mantle Processes, Samani, Japan, pp. 56—57. 

FIGURE 3.13 The composition of the subcontinental lithosphere plotted on a mantle composition Mg/Si-Al/Si weight ratio diagram. Labeled triangles A Archaean lithosphere Pr Proterozoic lithosphere Ph-sp Phanerozoic spinel lherzolite Ph-grt, Phanerozoic garnet lherzolite (data from Griffin et al.( 2003, see Table 3.4). Other symbols as in Fig. 3.9. 

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