Peridotites heterogeneities

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... 

Osmium isotopes currently provide the strongest case for mineral-to-mineral disequilibrium, and for mineral-melt disequilibrium available from observations on natural rocks. Thus, both osmium alloys and sulfides from ophiolites and mantle xenoliths have yielded strongly heterogeneous osmium isotope ratios (Alard et al., 2002 Meibom et al., 2002). The most remarkable aspect of these results is that these ophiolites were emplaced in Phanerozoic times, yet they contain osmiumbearing phases that have retained model ages in excess of 2 Ga in some cases. The melts that were extracted from these ophiolitic peridotites contained almost certainly much more radiogenic osmium and could, in any case, not have been in osmium-isotopic equilibrium with all of these isotopically diverse residual phases. 

Nd-Sr isotopic studies in orogenic peridotites have emphasized their wide isotopic heterogeneity but also showed that the predominant fertile Uierzolite facies are actually fairly homogenous and characterized by depleted, MORB-type isotopic compositions. The low-alumina, deformed (olivine) websterites, that are chemically equilibrated with the peridotites (see Section 2.04.4.2.3), also show a limited range of isotopic variations and are generally indistinguishable... 

Batanova V. G., Suhr G., and Sobolev A. V. (1998) Origin of geochemical heterogeneity in the mantle peridotites from the Bay of Islands OphioUte (Newfoundland, Canada) ion probe study of cUnopyroxenes. Geochim. Cosmochim. Acta 62, 853-866. 

Girardeau J. and Gil Ibarguchi J. I. (1991) Pyroxene-rich peridotites of the Cabo Ortegal Complex (Northwestern Spain) evidence for large-scale upper-mantle heterogeneity. J. Petrol. (Orogenic IherzoUtes and mantle processes) (sp. vol.), 135-154. 

Menzies M. A. (1984) Chemical and isotopic heterogeneities in orogenic and ophiolitic peridotites. In Ophiolites and Oceanic lithosphere (eds. I. G. Gass, S. J. Lippard, and A. W. Shelton). Blackwell, London, UK, pp. 231-240. 

Mukasa S. B., Shervais J. W., Wilshire H. G., and Nielson J. E. (1991) Intrinsic Nd, Pb and Sr isotopic heterogeneities exhibited by the Lherz alpine peridotite massif, French Pyrenees. 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. 117-134. 

Song Y. and Prey P. A. (1989) Geochemistry of peridotite xenoUths in basalt from Hannuoba, Eastern China implications for subcontinental mantle heterogeneity. Geochim. Cosmochim. Acta 53, 97—113. 

Stephens C. J. (1997) Heterogeneity of oceanic peridotite from the western canyon wall at MARK results from site 920. 

Shimizu N., Sobolev N. V., and Yefimova E. S. (1997) Chemical heterogeneities of inclusion garnets and juvenile character of peridotitic diamonds from Siberia. Russian J. Geol. Geophys. 38(2), 356-372. 

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