GENERATION OF THE PB ISOTOPIC CHARACTERISTICS OF THE ICELAND PLUME

Existing interpretations of the ph isotopic compositions of recent basaltic lavas from Iceland and adjacent sectors of the Mid-Atlantic Ridge have regarded these as the products of mixing between a single Icelandic plume component and a depleted mantle component similar to that supplying mid-ocean ridge basalts (MORE) away from plume influences. Pb-208-Pb-207 relationships are not consistent with these interpretations and suggest instead that few if any modern Icelandic lavas contain a MORE asthenosphere component. In particular, the low Pb-207/Pb-204 Of north Iceland picrites relative to MORE and the Northern Hemisphere Reference Line is very difficult to explain by mixing of currently proposed mantle components. The offset of the Icelandic data array to lower Pb-207/Pb-204 than MORE on all Pb isotope diagrams might instead be viewed as the product of Pb-206/(204)pb and Pb-208/Pb-204 increases, resulting from moderately high mu during the Phanerozoic, a HIMU character also indicated by K depletion in Iceland plume basalts. This mechanism can generate the Pb isotope compositions of most Icelandic basalts, but not those of Theistareykir picrites, the origin of which remains unclear.