The transition from alkali basalts to kimberlites: Isotope and trace element evidence from melilitites |
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| Authors: | C Alibert A Michard F Albarède |
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| Institution: | 1. Centre de Recherches Pétrographiques et Géochimiques and Ecole Nationale Supérieure de Géologie, B.P. 20, F-54501, Vandoeuvre, France
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| Abstract: | Eighteen Cenozoic melilitite samples from Spain, France, West Germany and Czechoslovakia have been analyzed for major and trace elements (including REE) together with their Sr and Nd isotopic compositions. Leaching experiments produced significant shifts of their87Sr/86Sr ratio indicative of a contamination by a crustal component. Most samples fall within the Sr-Nd mantle array with ?Nd values in the 1.5–6 range. These values are considered as minimum for the melilitite mantle source hence demonstrating its time integrated LRE depletion. The Ni and Cr contents of the samples are typical of primary magmas and exclude extensive crystallization of olivine and pyroxene in a closed system. However, the chemical relationships suggest that dilution of the liquids by mafic minerals of the conduits during their ascent has been important. The REE patterns show some variations which are interpreted by this dilution effect. Once normalized to Yb they are closely similar and perfectly distinguishable from those of alkali basalts and kimberlites. All of these rocks have Ce/Yb ratios which are high but distinctive for each rock type: 40 to 200 times the chondritic ratio for kimberlites, 20 to 30 for melilitites, 8 to 15 for alkali basalts. As contamination is likely to have modified somewhat the isotopic characteristics of most of these rocks, there is no overwhelming evidence that their source is chemically different. The Ba and Rb contents together with the REE patterns of the melilitites would constrain the degree of melting to be very small (<0.2%). The calculation of batch melting and steady zone refining models suggests that kimberlites, melilitites and alkali basalts may have been derived by equilibration of deep melts with different upper mantle levels characterized by decreasing garnet/clinopyroxene ratios. The strongly incompatible elements are enriched in the melt during its ascent by leaching of the wall rocks. For the steady zone refining model, the degree of melting concept loses its significance and the difficult requirement of extracting small liquid fractions from a molten source disappears. Within the frame of this model, the preenrichment of the kimberlite, melilitite and alkali basalts source in incompatible elements by metasomatic fluids is no longer necessary. |
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