Abstract
High precision Sr-Nd isotope ratios together with Pb isotope ratios
corrected for mass fractionation using a double spike are reported
for an extensive suite of late Quaternary to Recent lavas of Iceland,
the Kolbeinsey and Reykjanes Ridges, and a small number of basalts
from further south on the Mid-Atlantic Ridge. Compared with global
MORB, the Icelandic region is distinguished by having low Pb-207/Pb-204
for any given Pb-206/Pb-204, expressed by negative Delta(207)Pb (-0.8
to -3.5) in all but four Icelandic samples. Most samples also have
elevated Pb-208/Pb-204 (strongly positive Delta(208)Pb), which combined
with their negative Delta(207)Pb is very unusual in MORB worldwide.
The negative Delta(207)Pb is interpreted as a consequence of evolution
in high-g mantle sources for the last few hundred Ma. The region
of negative Delta(207)Pb appears to correspond with the region of
elevated He-3/He-4, suggesting that both lithophile and volatile
elements in melts from the whole region between 56 and 70degreesN
are dominantly sourced in a plume that has incorporated recycled
Palaeozoic ocean crust and unradiogenic He, probably from the deep
mantle. At least four mantle components are recognized on Iceland,
two with an enriched character, one depleted and one that shows some
isotopic affinities to EM1 but is only sampled by highly incompatible-element-depleted
lavas in this study. Within restricted areas of Iceland, these components
contribute to local intermediate enriched and depleted components
that display near binary mixing systematics. The major depleted Icelandic
component is clearly distinct in Pb isotopes from worldwide MORB,
but resembles the depleted mantle source supplying the bulk of the
melt to the Kolbeinsey and southern Reykjanes Ridges. However, an
additional depleted mantle source is tapped by the northern Reykjanes
Ridge, which with very negative Delta(207)Pb and less positive Delta(208)Pb
is distinct from all Icelandic compositions. These components must
mostly mix at mantle depths because a uniform mixture of three Icelandic
components is advected southward along the Reykjanes Ridge. Despite
strong covariation with isotope ratios, incompatible trace element
ratios of Icelandic magmas cannot be representative of old mantle
sources. The observed parent-daughter ratios in depleted and enriched
Icelandic lavas would yield homogeneous Sr, Nd, Hf and Pb-206 isotope
signatures similar to170 Ma ago if present in their sources. The
heterogeneity in Pb-207/Pb-204 is not however significantly reduced
at 170 Ma, and the negative present day Delta(207)Pb cannot be supported
by the low mu observed in depleted lavas from Iceland or the adjacent
ridges. Since A is higher in melts than in their sources, it follows
that all the depleted sources must be residues from <170 Ma partial
melting events. These are thought to have strongly affected most
incompatible trace element ratios. Copyright (C) 2004 Elsevier Ltd.
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