Orogenic lherzolite complexes studied by 87Rb- 87Sr: A clue to understand the mantle convection processes?

Abstract

Ultramafic materials such as lherzolites found in orogenic lherzolite bodies are suitable materials to investigate mantle processes. We present here an extensive K, Rb, Sr and Sr isotopic study on 70 samples, whole rocks and diopsides, for lherzolites and pyroxenites from four bodies of orogenic lherzolites in the Western Mediterranean: Lherz, Freychinède, Lanzo, Beni Bousera. The more striking feature is the complexity of the data which cannot be explained by simple models of partial melting and (or) contamination of homogenous mantle. The scatter of the results of whole rocks may partly be due to further contamination, especially in some pyroxenites. Lherzolite diopsides show great heterogeneities on a small scale. At least two partial melting events separated by about 109 years are required to create these heterogeneities. K, Rb, Sr concentrations and Rb/Sr ratios confirm the residual character of the lherzolites and show that pyroxenites also often have a residual (or cumulate) character, and cannot be direct partial melting products of the lherzolites. Orogenic lherzolites bodies are interpreted as “sandwiches” of slightly depleted mantle parts of different origins coupled together as sheets by mechanical stirring in the convective cell and layered by injected partial melting liquids. In this model, pyroxenitees may have had different histories, for example some may have suffered selective partial melting whereas others have not. This model implies that the oceanic mantle is progressively more and more depleted and that it is mixed, and recycled. It is opposed to models of irreversible differentiation of the mantle, without any recycling of the differentiated oceanic crust, and to models of “catastrophic” event of differentiation. This model also implies that dating such orogenic lherzolite bodies is meaningless.