On the origin of EM-I end-member

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 petrogenctic 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 (Pb-206/Pb-204 < 17), slightly radiogenic Sr isotopes (Sr-87/Sr-86 similar to 0.706), unradiogenic Nd (Nd-143/Nd-144 similar to 0.5121), unradiogenic Hf (Hf-176/Hf-177 similar to 0.2826), slightly radiogenic Os (Os-187/Os-188 similar to 0.135-0.145), high Pb-207/Pb-206 (>0.85), and Pb-208/Pb-206 (>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.