Depleted Iceland mantle plume geochemical signature: Artifact of multicomponent mixing?

Abstract

The geochemical variations in basalts from Iceland and the Reykjanes Ridge have for a long time been attributed to mixing between the depleted mid‐ocean ridge basalt (MORB) asthenosphere source and the enriched Iceland mantle plume (Schilling, 1973; Hart et al., 1973; Sun et al., 1975). In contrast, the occurrence of some Iceland basalts with relatively depleted incompatible trace element ratios and low Pb and Sr isotope ratios compared to normal MORB (NMORB) have been interpreted to indicate that the Iceland mantle plume transports both geochemically enriched and depleted material from the lower mantle into the upper mantle and that the depleted MORB asthenosphere source does not contribute significantly to the generation of the Iceland basalts (Thirlwall et al., 1994; Thirlwall, 1995; Hards et al., 1995; Kerr, 1995; Kerr et al., 1995; Fitton et al., 1997; Hardarson and Fitton, 1997; Hardarson et al., 1997; Kempton et al., 1998, 1999; Nowell et al., 1998). As a follow‐up to this controversy, we show on the basis of new Nb, Zr, Y, La, and Sm elemental abundances and Hf isotope ratios reported here, and published trace element and isotope data for Iceland and the North Atlantic MOR (50°–79°N), that the apparent distinction between Iceland basalts and MORB can readily be accounted for by a three‐component mixing model involving two incompatible trace element enriched components, one with relatively high, the other with low 206Pb/204Pb ratios, and the usual surrounding depleted MORB mantle source as the third component. The radiogenic 206Pb‐rich component represents the hot Iceland mantle plume source, while the enriched but low‐206Pb/204Pb EM1‐like component most likely represents entrained subcontinental lithospheric material embedded in the North Atlantic depleted MORB source. The incompatible element and isotope ratio variations do not require a depleted Iceland plume (DIP) component, nor do they exclude the presence of a MORB depleted asthenosphere source component for some of the Iceland basalts, particularly in recent time.