Nd-Sr isotope and REE geochemistry of alkali basalts from the Massif Central, France

Nd and Sr isotopic compositions as well as trace element concentrations have been determined on a suite of alkali basalts from the Massif Central, in France. Samples show a typical enrichment in incompatible elements. In particular, the REE patterns exhibit a strong fractionation characterized by a (La/Yb)sub(N) ratio of about 20. The Ybsub(N) content is about 10 times chondrite. The 143Nd/144Nd ratios exhibit a range from 0.512775 to 0.512989, values quite comparable to those from oceanic island basalts. The 87Sr/86Sr ratios vary between 0.70338 and 0.70458 and are anti-correlated with the Nd isotopic ratio. The isotopic and the trace element (in particular REE) data have been used in order to quantitatively model the genesis of the alkali basalts. Among the several types of models tested here, the most likely one appears to be the model of mantle metasomatism. A semi-quantitative approach shows that the source of alkali basalts from the Massif Central was metasomatized prior to melting. In such a model, the basalts could be produced by rather high degrees of partial melting (such as 10 or 15%) of the metasomatically enriched mantle. (author)