Original paper

On the origin of EM-I end-member

Lustrino, Michele; Dallai, Luigi


On the basis of geochemical data available for oceanic basalts distinct mantle reservoirs have been identified during the last decades. Among these reservoirs, the EM–I end-member (Enriched Mantle type I) shows almost unique geochemical characteristics. The definition of the EM–I end member and the petrogenetic processes related to its origin are still debated: the geochemical composition of EM–I basalts has been interpreted in terms of crust-mantle interaction during 1) mantle recycling of subducted crustal material; 2) detachment of lithospheric keels in over-thickened regions; 3) thermal erosion by anomalously hot mantle. Key features of the EM–I end-member are: low uranogenic Pb ratios (206Pb/204Pb 0.85), and 208Pb/206Pb (>2.08). EM–I basalts show apparent or inexistent Ba, Pb, Eu or Sr anomalies in primitive mantle-normalized diagrams and have Ba/Nb ratios ranging from 3.5 to 47.4, Ce/Pb from 1.2 to 24.6, Nb/U from 10.5 to 71.8, Sr/ Nd from 6.2 to 36.4, and Eu/Eu* from 0.83 to 1.25. On these basis, the origin and evolution of the EM–I end-member cannot be explained by a single petrological model, and even the definition of elemental and isotopic parameters is controversial.