The nature and origin of upper mantle heterogeneity beneath the Mid-Atlantic Ridge 33-25°N: A Sr-Nd-Hf isotopic perspective

We present new Sr-Nd-Hf isotopic data on mid-ocean ridge basalts from two ridge segments (OH-1, OH-3) between the Oceanographer and Hayes fracture zones at the Mid-Atlantic Ridge 33-35°N to constrain the nature and origin of upper mantle heterogeneity beneath the Mid-Atlantic Ridge. Together with the major and trace elements data (Niu et al., 2001), the new Sr-Nd-Hf isotopic data illustrate that the mantle sources of these lavas comprise three components, i.e., a depleted mantle with high radiogenic Hf isotopic compositions (ADM), an incompatible elemental enriched component with radiogenic Sr and un-radiogenic Nd-Hf isotopic compositions (E-type I), and an incompatible element depleted component with variably enriched Sr-Nd-Hf isotope compositions (i.e., E-type II). The ADM and E-type I components may be understood as representing ancient mantle melting residues and metasomatic veins developed at the base of the thickening oceanic lithosphere, respectively. The unique E-type II component is best explained as recent mantle melting residues of a geochemically enriched mantle, probably associated with the Azores mantle plume to the north. Taking together, we interpret the E-MORB dominated OH-1 lavas as resulting from a relatively higher degree melting of a mantle source composed of ADM matrix and E-type I material, whereas the N-MORB dominated OH-3 lavas resulting from lower degree melting of the mantle source primarily comprising ADM and E-type II components. Such a petrological and geochemical understanding explains the contrast in crustal thickness, ridge morphology and tomography, and mantle Bouguer anomalies between the two ridge segments