Thirlwall, M.F., Gee, M.A.M., Taylor, R.N. and Murton, B.J.
Mantle components in Iceland and adjacent ridges investigated using double-spike Pb isotope ratios
Geochimica et Cosmochimica Acta, 68, (2), . (doi:10.1016/S0016-7037(03)00424-1).
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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 207Pb/204Pb for any given 206Pb/204Pb, expressed by negative [Delta]207Pb (-0.8 to -3.5) in all but four Icelandic samples. Most samples also have elevated 208Pb/204Pb (strongly positive [Delta]208Pb), which combined with their negative [Delta]207Pb is very unusual in MORB worldwide. The negative [Delta]207Pb is interpreted as a consequence of evolution in high-[mu] mantle sources for the last few hundred Ma. The region of negative [Delta]207Pb appears to correspond with the region of elevated 3He/4He, suggesting that both lithophile and volatile elements in melts from the whole region between 56 and 70[deg]N 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]207Pb and less positive [Delta]208Pb 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 206Pb isotope signatures ~170 Ma ago if present in their sources. The heterogeneity in 207Pb/204Pb is not however significantly reduced at 170 Ma, and the negative present day [Delta]207Pb cannot be supported by the low [mu] observed in depleted lavas from Iceland or the adjacent ridges. Since [mu] 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.
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