Abstract

Existing interpretations of the Pb 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 (MORB) away from plume influences. 208Pb-207Pb relationships are not consistent with these interpretations and suggest instead that few if any modern Icelandic lavas contain a MORB asthenosphere component. In particular, the low 207Pb/204Pb of north Iceland picrites relative to MORB 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 207Pb/204Pb than MORB on all Pb isotope diagrams might instead be viewed as the product of 206Pb/204Pb and 208Pb/204Pb increases, resulting from moderately high μ 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.

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