Rex N. Taylor

THERMAL AND CHEMICAL STRUCTURE OF THE ICELAND PLUME

Basaltic lavas, forming thick offshore seaward-dipping reflector sequences (SDRS) and onshore igneous provinces around the North Atlantic margins, represent melting of anomalously hot mantle in the head of the ancestral Iceland plume. Some of these lavas are chemically and isotopically indistinguishable from recent Icelandic basalt, but others more closely resemble basalt erupted at normal segments of mid-ocean ridges (N-MORB). In this paper we show that Icelandic basalt and N-MORB define parallel tight arrays on a plot oflog(Nb/Y) against log(Zr/Y), with N-MORB relatively deficient in Nb. Deficiency or excess of Nb, relative to the lower bound of the Iceland array, may be expressed as ΔNb=1.74+log⁡(Nb/Y)−1.92log⁡(Zr/Y)such that Icelandic basalt has ΔNb > 0 and N-MORB has ΔNb < 0. ΔNb is a fundamental source characteristic and is insensitive to the effects of variable degrees of mantle melting, source depletion through melt extraction, crustal contamination of the magmas, or subsequent alteration. We use new and published Nb, Zr and Y data to identify the mantle sources for Palaeocene and Eocene basaltic lavas erupted around the Atlantic margins in order to deduce the thermal and compositional structure of the head of the ancestral Iceland plume. The results show that the head of the plume was zoned, with an axial zone of Icelandic mantle surrounded by 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 reconcile with the initiation of rifting and SDRS formation through the impact of a large plume head originating solely from the lower mantle. The thick outer shell of hot, depleted upper mantle, which formed more than 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.

Isotopic characteristics of subduction fluids in an intra-oceanic setting, Izu–Bonin Arc, Japan

New radiogenic isotope and trace element data are presented for the volcanic sequences along 600 km of the active Izu–Bonin arc, the Oligocene Izu arc, and their associated rift basins. As with many intra-oceanic island arcs, the Pliocene–Recent Izu–Bonin frontal-arc lavas are highly depleted in Zr, Nb and the rare-earth elements relative to typical mid-ocean ridge basalt (MORB), indicating that the mantle wedge source has undergone a previous episode of melting. Ratios between these elements (such as Nb/Zr and La/Sm), as well as 143Nd/144Nd, do not vary significantly along the length of the frontal-arc. These parameters suggest that each of the arc volcanoes is derived from similar melt fractions of the mantle wedge. However, Ba/Zr, Ba/Rb and 87Sr/86Sr increase along the frontal-arc to the north. This leads us to propose that a variable enrichment in Ba and radiogenic Sr is superimposed on the mantle wedge. Sr–Nd and Pb–Nd isotope variation indicate that both Sr and Pb become more radiogenic after fluid addition. However, Pb isotope ratios do not correlate with increases in Pb concentration or ratios such as Ba/Zr and Nb/Pb. In other words, the Pb isotopic composition of the arc lavas appears to be independent of the amount of Pb introduced by subduction fluids into the mantle source. This buffering of Pb isotopes along the frontal-arc means that the isotopic composition of the lavas is indistinguishable from that of the fluid. Isotopic mixing models presented for the arc are only illustrative of the many plausible combinations of components and quantities. Despite this, we are able to determine that the mantle wedge has isotopic characteristics similar to Indian Ocean MORB, and that the subduction-fluid solute is primarily derived from subducted oceanic basalt with a <2% contribution from subducted sediment. Lavas in the Oligocene Izu arc and fore-arc basin were derived from a mantle wedge of similar composition to the active arc. Despite levels of Pb enrichment comparable to those of the modern arc, the Pb isotopes of the Oligocene volcanics indicate a lower sediment input into the melting region.

An improved method for extracting marine sediment fractions and its application to Sr and Nd isotopic analysis

The radiogenic isotopic composition of both detrital and Fe–Mn fractions in marine sediments can be used in paleoceanography to infer changes of bottom-water circulation. We have examined various chemical techniques for the analysis of Sr and Nd isotope ratios in these fractions and present a robust method that can be used to separate both the Fe–Mn oxides and the detrital fraction from a marine sediment sample for isotopic analysis. Our sequential leaching procedure involves the use of 10% acetic acid, followed by 1 M hydroxylamine hydrochloride in 25% acetic acid to remove the carbonate component and the Fe–Mn oxide fraction, respectively. The applicability of our chemical procedure is illustrated with examples from a marine sediment core raised from the northern Cape Basin, southeast Atlantic Ocean.

The Iceland plume in space and time: a Sr-Nd-Pb-Hf study of the North Atlantic rifted margin

New Sr–Nd–Pb–Hf data require the existence of at least four mantle components in the genesis of basalts from the the North Atlantic Igneous Province (NAIP): (1) one (or more likely a small range of) enriched component(s) within the Iceland plume, (2) a depleted component within the Iceland plume (distinct from the shallow N-MORB source), (3) a depleted sheath surrounding the plume and (4) shallow N-MORB source mantle. These components have been available since the major phase of igneous activity associated with plume head impact during Paleogene times. In Hf–Nd isotope space, samples from Iceland, DSDP Leg 49 (Sites 407, 408 and 409), ODP Legs 152 and 163 (southeast Greenland margin), the Reykjanes Ridge, Kolbeinsey Ridge and DSDP Leg 38 (Site 348) define fields that are oblique to the main ocean island basalt array and extend toward a component with higher 176Hf/177Hf than the N-MORB source available prior to arrival of the plume, as indicated by the compositions of Cretaceous basalts from Goban Spur (∼95 Ma). Aside from Goban Spur, only basalts from Hatton Bank on the oceanward side of the Rockall Plateau (DSDP Leg 81) lie consistently within the field of N-MORB, which indicates that the compositional influence of the plume did not reach this far south and east ∼55 Ma ago. Thus, Hf–Nd isotope systematics are consistent with previous studies which indicate that shallow MORB-source mantle does not represent the depleted component within the Iceland plume [Thirlwall, J. Geol. Soc. London 152 (1995) 991–996; Hards et al., J. Geol. Soc. London 152 (1995) 1003–1009; Fitton et al., Earth Planet. Sci. Lett. 153 (1997) 197–208]. They also indicate that the depleted component is a long-lived and intrinsic feature of the Iceland plume, generated during an ancient melting event in which a mineral (such as garnet) with a high Lu/Hf was a residual phase. Collectively, these data suggest a model for the Iceland plume in which a heterogeneous core, derived from the lower mantle, consists of ‘enriched’ streaks or blobs dispersed in a more depleted matrix. A distinguishing feature of both the enriched and depleted components is high Nb/Y for a given Zr/Y (i.e. positive ΔNb), but the enriched component has higher Sr and Pb isotope ratios, combined with lower eNd and eHf. This heterogeneous core is surrounded by a sheath of depleted material, similar to the depleted component of the Iceland plume in its eNd and eHf, but with lower 87Sr/86Sr, 208Pb/204Pb and negative ΔNb; this material was probably entrained from near the 670 km discontinuity when the plume stalled at the boundary between the upper and lower mantle. The plume sheath displaced more normal MORB asthenosphere (distinguished by its lower eHf for a given eNd or Zr/Nb ratio), which existed in the North Atlantic prior to plume impact. Preliminary data on MORBs from near the Azores plume suggest that much of the North Atlantic may be ‘polluted’ not only by enriched plume material but also by depleted material similar to the Iceland plume sheath. If this hypothesis is correct, it may provide a general explanation for some of the compositional diversity and variations in inferred depth of melting [Klein and Langmuir, J. Geophys. Res. 92 (1987) 8089–8115] along the MAR in the North Atlantic.

Fluid-mantle interaction in an intra-oceanic arc: constraints from high-precision Pb isotopes

Abstract We present new isotopic and trace element data, including high-precision double-spike Pb isotope measurements, for back-arc lavas from the Izu–Bonin arc. Systematic along-arc isotopic variation of lavas has been identified in the Izu–Bonin arc that is coherent between the volcanic front and back-arc. The Sr isotopes are more radiogenic in the north, while Pb isotopes are less radiogenic in the north compared to the central part of the arc. This is particularly apparent in the back-arc seamounts. Decoupled variation of Pb and Sr isotopes cannot be explained by changes in the amount of a single subduction component. Almost parallel but distinct trends on Pb–Pb isotope plots imply differing mantle sources in the northern and central parts of the arc. The decoupling of Sr and Pb isotopic variation for both volcanic front and back-arc can be explained by the presence of two mantle components: a MORB source observed in the back-arc basins of the Philippine Sea Plate and a Pacific MORB-like source. An internally consistent model which explains along- and across-arc isotopic trends can be obtained by assuming mixing between the two mantle sources, but a lesser contribution of Pacific MORB-type source in the northern part of the arc. The source mantle of the rifting-related volcanism is distinct from the back-arc seamount chain volcanism and has a much stronger signature of Pacific MORB. A strong correlation between Sr isotopes and fluid-mobile element enrichment in the volcanic front lavas imply a significant contribution of slab-derived fluid to the source of volcanic front. In contrast, back-arc seamounts and rifting-related volcanism show a more limited influence of a fluid phase contribution to their source. Instead, high Δ7/4 and low 143Nd/144Nd associated with high Th/Ce imply that subducted bulk sediment is an important component in the back-arc. By assuming the regional mantle end-members, the relative contribution of a subduction-related component can be estimated. The magmatism in the back-arc seamounts is estimated to have a 0.2–0.3% bulk sediment addition to the source for both the northern and central parts of the arc. Minimal addition of fluid from altered oceanic crust is also predicted. For the rifting-related magmatism, a lower sediment contribution is predicted. Volcanic front magmatism is compatible with having no direct bulk sediment input, but is likely to involve the contribution of 2–2.5% of a fluid derived from altered oceanic crust and sediment in a mixing ratio of about 99:1.

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.

A new ground-level fallout record of uranium and plutonium isotopes for northern temperate latitudes

Plutonium and uranium isotope ratios can be used to differentiate the sources of nuclear contamination from nuclear weapon establishments (Environ. Sci. Technol. 34 (2000) 4496; Internal Report for AWRE Aldermaston, UK (1961)), weapon fallout (Geochim. Cosmochim. Acta 51 (1987) 2623; Earth Planet. Sci. Lett. 63 (1983) 202; Earth Planet. Sci. Lett. 22 (1974) 111; Geochim. Cosmochim. Acta 64 (2000) 989), reprocessing plants, reactor or satellite accidents (Science 105 (1979) 583; Science 238 (1987) 512) and in addition they provide markers for post-1952 geochronology of environmental systems. A good record of plutonium and uranium isotope ratios of the background resulting from atmospheric nuclear testing is essential for source characterisation studies. Using recently developed mass spectrometric techniques (J. Anal. At. Spectrom. 16 (2001) 279) we present here the first complete records between 1952 and the present day of northern temperate latitude 240Pu/239Pu and 238U/235U atom ratios for atmospheric deposition. Such information was not derived directly during the period of atmospheric testing because suitable mass spectrometric capability was not available. The currently derived records are based on an annual herbage archive and a core from an Alpine glacier. These studies reveal hitherto unseen fluctuations in the 238U/235U atmospheric fallout record, some of which are directly related to nuclear testing. In addition, they also provide the first evidence that plutonium contamination originating from Nevada Desert atmospheric weapon tests in 1952 and 1953 extended eastwards as far as northwestern Europe. The results presented here demonstrate that we now have the capability to detect and precisely identify sources of plutonium in the environment with implications for the development of atmospheric transport models, recent geochronology and environmental studies.

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.

Controls on magmatic degassing along the Reykjanes Ridge with implications for the helium paradox

To consider the 3He characteristics of plume-related lavas, we report a detailed survey of helium isotope (3He/4He) and concentration ([He]) variations along an 800-km transect of the Reykjanes Ridge (RR). 3He/4He ratios vary from 11.0 to 17.6 RA (where RA=air 3He/4He) whereas [He] ranges over three orders of magnitude from >5 cm3 STP/g – in the range of most mid-ocean ridge basalts (MORB) – to lows of 4 ncm3 STP/g. The lowest [He] and intermediate 3He/4He ratios occur along the northern RR (closest to Iceland) where eruption depths are shallow (<1000 m) and water contents of lavas are high (0.3–0.4 wt%). We suggest that low-pressure, pre-eruptive magmatic degassing is extensive in this region with degassed magmas susceptible to addition of radiogenic helium thereby lowering 3He/4He ratios. Along the southern RR, [He] reaches maximum values, and 3He/4He ratios display strong correlations with lead isotopes (206Pb/204Pb) consistent with binary mixing. These correlations indicate that the high-3He/4He plume component has higher absolute abundances of the primordial isotope 3He compared to the source of depleted MORB mantle. This finding implies that the so-called ‘helium paradox’ – the observation that plume-derived oceanic glasses apparently have lower 3He contents than MORB glasses – may be an artifact related to considering lavas (e.g. from Loihi seamount, Hawaii) which have not retained their source volatile inventory as well as those erupted along the southern RR.

Rapid procedure for plutonium and uranium determination in soils using a borate fusion followed by ion-exchange and extraction chromatography

A rapid and highly precise method, which uses a borate fusion, of U and Pu determination in soils, sediments and other materials is described. The chemical separation steps are optimised by using an anion resin column stacked on an extraction chromatography column (Eichrom Industries UTEVA resin). The whole procedure was streamlined to measure 700 soil samples in 10 weeks as part of an urgent environmental monitoring programme. Uranium was measured using thermal ionisation mass spectrometry with a precision of 0.2% at the 95% uncertainty level. Pu was measured using alpha spectrometry with a precision of approximately 10% at the 95% uncertainty level. The method can be applied successfully to a wide range of sample types and other actinide elements (Th, Am, etc.) can also be effectively included in the analytical scheme. The whole method has been critically evaluated by measuring a range of international reference samples.