Ophiolites isotopic ratios

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. 

Muehlenbachs K. (2001) Ophiolites as faithful records of the oxygen isotope ratio of ancient seawater. Abstr. Progr.— Geol. Soc. Am. 33(6), 225. 

MuehlenbachsK., Fumes H., Fonneland H. C., and HellevangB (2003) Ophiolites as faithful records of the oxygen isotope ratio of ancient seawater the Solund-Stavfjord Ophiolite Complex as a Late Ordovician example. In Ophiolites in Earth History (eds. Y. Dilek and P. T. Robinson). Geol. Soc. of London, Spec. Publ. (in press). 

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