M.A.M. Gee

Mantle components in Iceland and adjacent ridges investigated using double-spike Pb isotope ratios

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.

Isotopic constraints on the influence of the Icelandic plume

Abstract Thermally buoyant mantle, in the form of a plume, rises beneath Iceland creating a major topographic anomaly along the Mid-Atlantic Ridge and in the surrounding ocean basin. However, the influence of the Iceland plume on the composition of lavas erupted on adjacent ridges remains a contentious issue. Trace element systematics and radiogenic isotope ratios of Sr, Nd and Pb suggest that the plume influences a region 1200 km in length. In contrast, the3He anomaly associated with Iceland closely corresponds to the 2400 km ridge section affected by thermal uplift. We present evidence that the Sr, Nd and Pb isotope signature of the Iceland plume is in fact as widespread as its thermal and3He anomalies. Results imply that much of the source of North Atlantic ridge basalts has been contaminated by lateral outflow of asthenosphere from the Icelandic plume. Consequently, estimates of the average composition of mid-ocean ridge basalt (MORB) sources are likely to be biased by including data from plume-contaminated regions. True MORB values, and perhaps upper mantle geochemistry, can be constrained only by considering data untainted by plume asthenosphere.

Axial magma reservoirs located by variation in lava chemistry along Iceland's mid-ocean ridge

A linear, axis-parallel, array of glacial and postglacial basalt samples from the elevated mid-ocean ridge in southwest Iceland, the Reykjanes Peninsula, shows three MgO lows and two MgO highs at 40 km intervals. Similar patterns are observed with other fractionation indices, e.g., Cr. These along-axis variations in elements affected by fractional crystallization are interpreted as evidence for segment-scale variation in crustal residence times arising from the focusing of magmatic activity at regular intervals along this elevated mid-ocean ridge. In the majority of the samples, Nb/Zr, generally considered to be unaffected by crystal fractionation, does not show a systematic variation with MgO. Lavas with unusually low Nb/Zr, erupted at the end of the last glaciation, are the only exception. These low-Nb/Zr lavas are generally restricted to the MgO highs, resulting in a wider range of lava Nb/Zr in these areas than in the MgO lows. It is proposed that low-Nb/Zr melts are available along the entire ridge section at all times, but are modified before eruption by mixing with melts that are more enriched in incompatible elements. Crustal processes at this ridge axis are governing the distribution of chemistry associated with the mantle.

Dyke-diatreme transition in monogenetic volcanoes: insights from the Hillier Bay volcanic complex, Western Australia

The factors controlling phreatomagmatism and diatreme formation are still poorly constrained and understood. Here, we describe the field relationships between mafic intrusions and volcaniclastic deposits observed at Hillier Bay, Western Australia, and discuss the implications for the formation of monogenetic basaltic volcanoes involving both phreatomagmatic and magmatic eruption phases. The Hillier Bay volcanic complex consists of a series of basaltic sheeted dykelets and larger dykes injected within the metamorphic basement. Volcaniclastic lithologies also occur, usually trapped between basaltic dykes and the basement. These vary between mixtures of juvenile basaltic fragments, metamorphic basement fragments and quartz/feldspar sand in different relative amounts. Based on the textures of the clastic lithologies, we argue that initial phreatomagmatic phases resulted from sequential injections of thin dykelets due to ascending magma struggling to open a path to the surface. The relatively lower magma/water ratio maximises the efficiency of premixing leading to phreatomagmatic explosions. Later, as the main body of magma reaches the shallow conduit through wider dykes, magma overcomes water availability, and the eruption style switches to magmatic/effusive. This implies that magma flux may be a determining factor controlling eruption style in at least some monogenetic volcanoes.