Ophiolites mantle rocks

We shall hereafter follow the current terminology. It is convenient to distinguish three main types of mantle occurrences and to examine them in the following sequence (i) orogenic peridotite massifs, (ii) ophiolitic mantle rocks, and (iii) oceanic peridotites. 

The wide diversity of the isotopic compositions in orogenic peridotites, and the anomalously depleted composition of several ophiolitic and abyssal peridotites have strong implications on the small-scale structure of the convective mantle, as well as on mantle processes such as decompression partial melting of mantle rocks, the formation of oceanic lithosphere and the thermomechanical and chemical erosion of lithospheric mantle by upwelling asthenosphere. We briefly review some of these important issues below. 

As noted above, isotope decouphng between oceanic mantle and crust was observed both in ophiolites (Claesson et al., 1984 Gopel et al., 1984 Jacobsen et al., 1984 Brouxel and Lapierre, 1988 Rampone et al., 1996, 1998) and in abyssal rocks (Snow et al., 1994 Kempton and Stephens, 1997 Salters and Dick, 2002). In most examples, the mantle rocks are distinguished from the oceanic crust by more depleted isotopic compositions, generally reflected by higher Nd/ Nd values. However, the interpretations proposed for the ophiolites and for the abyssal peridotites are markedly different. 

Seafloor spreading eventually pushes oceanic crust into subduction zones where the hydrothermal sediments and rock are recycled back into the mantle. A small fraction of these deposits is uplifted, or obducted, onto land. These rescued deposits are termed ophiolites. Because of their metal enrichments, they serve as major ore bodies and have been mined for various precious metals, such as copper, for thousands of years. 

Fig. 1. Primitive mantle (PM) normalized whole rock PGE profiles for stratiform chromitites from the Stillwater Complex and for podiform chromitites from the mantle section of Thetford Mines Ophiolite (TMO). Note the similarity between IPGE contents of chromitites from these different tectonic settings. D.L.= detection limit.

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. 

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