THERMAL AND CHEMICAL-STRUCTURE OF THE ICELAND PLUME

Basaltic lavas, forming thick offshore seaward-dipping reflector seque nces (SDRS) and onshore igneous provinces around the North Atlantic ma rgins, represent melting of anomalously hot mantle in the head of the ancestral Iceland plume. Some of these lavas are chemically and isotop ically indistinguishable from recent Icelandic basalt, but others more closely resemble basalt erupted at normal segments of mid-ocean ridge s (N-MORB). In this paper we show that Icelandic basalt and N-MORB def ine parallel tight arrays on a plot of log(Nb/Y) against log(Zr/Y), wi th N-MORB relatively deficient in Nb. Deficiency or excess of Nb, rela tive to the lower bound of the Iceland array, may be expressed as Delt a Nb = 1.74 + log(Nb/Y) - 1.92 log(Zr/Y) such that Icelandic basalt ha s Delta Nb > 0 and N-MORB has Delta Nb < 0. Delta Nb is a fundamental source characteristic and is insensitive to the effects of variable de grees of mantle melting, source depletion through melt extraction, cru stal contamination of the magmas, or subsequent alteration. We use new and published Nb, Zr and Y data to identify the mantle sources for Pa laeocene and Eocene basaltic lavas erupted around the Atlantic margins in order to deduce the thermal and compositional structure of the hea d of the ancestral Iceland plume. The results show that the head of th e plume was zoned, with an axial zone of Icelandic mantle surrounded b y a thick outer shell of anomalously hot but compositionally normal N- MORB-source mantle. The zoning is very similar in scale and character to that seen today along the Reykjanes Ridge and is difficult to recon cile with the initiation of rifting and SDRS formation through the imp act of a large plume head originating solely from the lower mantle. Th e thick outer shell of hot, depleted upper mantle, which formed more t han half the volume of the plume head, suggests that at least part of the plume originated in the thermal boundary layer at the base of the upper mantle. (C) 1997 Elsevier Science B.V.