Ophiolitic peridotites

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. 

Secondary alteration processes are also responsible for element deviations, particularly the strong depletion of calcium and sodium relative to aluminum in some abyssal peridotites, as well as part of the scattered variations of silicon, magnesium, calcium, and sodium. Serpentinized, orogenic and ophiolitic peridotites may also be strongly depleted in calcium and sodium. However, these samples are rare in the selected database for peridotite massifs, which includes only moderately serpentinized samples. 

Figure 13 Chondrite-normalized abundances of REEs in representative ophiolitic peridotites from the Internal Ligurides, New Caledonia, Oman, and Cuba (whole-rock analyses). Data from Rampone et al. (1996), Prinzhofer and Allegre (1985), Godard et al. (2000) (reproduced by permission of Elsevier from Earth Planet. Sci. Lett. 2000, 180, 133-148), and Proenza et al. (1999), respectively. Normalizing values after Sun and McDonough (1989).

The abyssal peridotites from the Izu-Bonin-Mariana Forearc resemble the ophiolitic peridotites. They are, nevertheless, distinguished by more prominent enrichments in alkaline and alkaline-earth elements (rubidium, caesium, strontium, and barium), possibly reflecting a supra-subduction imprint (Parkinson and Pearce, 1998). Except for one sample, they lack the niobium enrichment relative to FREE that is observed in the Ronda and... 

Several Nd-Sr isotopic studies were also devoted to ophiolitic peridotites, and a more restricted number of studies to oceanic peridotites. The early studies have reported anomalously high Sr/ Sr values (e.g., Bonatti et al., 1970) in these rocks and concluded that they were genetically unrelated to oceanic cmst (see references in Menzies and Murthy, 1978). More recent studies... 

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. 

Rampone E., Piccardo G. B., Vannucci R., Bottazzi P., and Zanetti A. (1994) Melt impregnation in ophiolitic peridotites an ion microprobe study of clinopyroxene and plagioclase. Min. Mag. 58A, 756-757. 

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