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Mantle evolution is governed by continuous depletion by partial melting and replenishment by recycling oceanic and continental crust. Several important unknowns remain, however, such as the extent of compositional variability of the residual depleted mantle, the timescale, mass flux and composition of recycled oceanic and continental crust. Here, we investigate the Ce-Nd-Hf isotope systematics in a globally representative spectrum of mid ocean ridge and ocean island basalts. Using a Monte Carlo approach for reproducing the observed Ce-Nd-Hf isotope variation shows that the type and age of depleted mantle and recycled crust have the dominant influence on the slope, scatter, and extent of the modeled Ce-Nd-Hf isotope array. The model results suggest a relatively young (

There is debate to what degree mid-ocean ridge basalts (MORB) can be used to infer the composition of the sub-ridge mantle and whether the upper mantle contains components that are difficult to recognize by observations on MORB alone. Here we report trace element and isotope data on a fossil mantle section exposed in the ophiolitic Lanzo South massif, Italy. Our findings show the existence of clinopyroxenes in geochemically depleted melt-infiltrated harzburgites with an extreme radiogenic Hf-isotopic composition but MORB-like Nd isotopic compositions. The simplest way of explaining the depleted trace elements in combination with the decoupled isotopic compositions is that the harzburgites were infiltrated by a melt with isotopic compositions inherited from an anciently (>1 Ga) depleted source. The existence of melts from ultra-depleted sectors of the asthenosphere allows constraining of the configuration of the heterogeneities in the sub-ridge mantle. The parallel arrays in Hf–Nd isotope composition space of different Mid-Ocean ridge segments imply the ubiquitous presence of anciently depleted residual mantle as the matrix in which more enriched components occur in pockets.

The evolution of the mantle melting regime and the process of refertilization beneath cold lithosphere remain ambiguous at ultraslow spreading ridges. Few previous studies were concerned with the temporal variations of mantle melting indicated by peridotites on the flank of a single ridge segment. Here, we present in-situ major and trace element analyses of harzburgites from the Dragon Bone ridge segment both from near the spreading axis and from rift mountains up to tens of kilometers from the ridge axis. We propose that the Dragon Bone mantle had been depleted anciently at higher pressures, prior to the recent ascent under the ridge at lower pressures, and only a limited volume of melt was generated during recent melting beneath the ridge. This interpretation is consistent with the high degree of depletion of the Dragon Bone harzburgites with little trapped melt, and thin discontinuous igneous crust on the seafloor. Comparison of peridotites from near-ridge locations with those further out on the flanks indicates that the melting regime has progressively shrunk over time creating at present a nearly amagmatic segment. Peridotites from segment ends are seldom affected by late-stage refertilization, while those from the segment center are highly depleted mantle residues (resembling those from fast spreading ridges) that sustained syn-melting metasomatism.

The upper mantle is a heterogeneous mixture of refractory and recycled crustal domains. The recycled portions, more fertile and thus preferentially melted, dominate the composition of the basalts erupted on the surface, whereas the imprint of melting of the refractory counterparts is more difficult to discern from the basalt chemistry. Contrasting radiogenic isotopic signatures of mid-ocean ridge basalts and oceanic mantle, however, show that Hf isotope ratios may provide hints for melting of refractory source materials despite ubiquitous magma mixing during ascent and stalling in the crust. This property may allow identifying contributions from depleted mantle materials unseen in other isotope systematics in basalts. Here, we show that basalts from Mohns and Knipovich ridges, two >500-km long oblique super-segments in the Arctic Atlantic, have distinctly high Hf isotope ratios, not mirrored by comparatively high Nd and low Sr and Pb isotope ratios. These compositions can be explained if a highly depleted asthenospheric mantle melts beneath this section of the Arctic Mid Atlantic Ridge. We argue that this depleted source consists of high proportions of ancient (>1 Ga), ultra-depleted mantle, previously drained of enriched components before being re-melted in its current location following a recent ridge-jump, allowing the identification of ultra-depleted mantle components in the arctic subridge mantle.

To understand the dynamics of solid mantle upwelling and melting in the Hawaiian plume, we present new major and trace element data, Nd, Sr, Hf, and Pb isotopic compositions, and 238U–230Th–226Ra and 235U–231Pa–227Ac activities for 13 Haleakala Crater nepheline normative basanites with ages ranging from ∼900 to 4100 yr B.P. These basanites of the Hana Volcanics exhibit an enrichment in incompatible trace elements and a more depleted isotopic signature than similarly aged Hawaiian shield lavas from Kilauea and Mauna Loa. Here we posit that as the Pacific lithosphere beneath the active shield volcanoes moves away from the center of the Hawaiian plume, increased incorporation of an intrinsic depleted component with relatively low 206Pb/204Pb produces the source of the basanites of the Hana Volcanics. Haleakala Crater basanites have average (230Th/238U) of 1.23 (n = 13), average age-corrected (226Ra/230Th) of 1.25 (n = 13), and average (231Pa/235U) of 1.67 (n = 4), significantly higher than Kilauea and Mauna Loa tholeiites. U-series modeling shows that solid mantle upwelling velocity for Haleakala Crater basanites ranges from ∼0.7 to 1.0 cm/yr, compared to ∼10 to 20 cm/yr for tholeiites and ∼1 to 2 cm/yr for alkali basalts. These modeling results indicate that solid mantle upwelling rates and porosity of the melting zone are lower for Hana Volcanics basanites than for shield-stage tholeiites from Kilauea and Mauna Loa and alkali basalts from Hualalai. The melting rate, which is directly proportional to both the solid mantle upwelling rate and the degree of melting, is therefore greatest in the center of the Hawaiian plume and lower on its periphery. Our results indicate that solid mantle upwelling velocity is at least 10 times higher at the center of the plume than at its periphery under Haleakala.

Approximately coeval, relatively primitive (∼5–10% MgO with exception of a trachyandesite) alkaline mafic dikes and sills at or near Spanish Peaks, CO are divided into relatively sodic and potassic varieties on the basis of K 2 O/Na 2 O. Many of these dikes are true lamprophyres. In spite of variable alkali element ratios, the alkaline rocks share a number of geochemical similarities: high LIL element contents, high Ba and similar Sr, Nd and Hf isotope ratios near that of Bulk Earth. One important difference is that the potassic rocks are characterized by lower Al 2 O 3 contents, typically less than 12 wt.%, than the sodic dikes/sills which typically have more than 13 wt.% Al 2 O 3 , and this difference is independent of MgO content. We attribute the distinct Al 2 O 3 contents to varying pressure during melting: a mica-bearing, Al-poor vein assemblage for the potassic magmas melted at higher pressure than an aluminous amphibole-bearing vein assemblage for the sodic magmas. Remarkable isotopic and trace element similarities with approximately contemporaneous, nearby Rio Grande rift-related basalts in the San Luis Valley, indicate that the magmatism at Spanish Peaks was rift-related, and that lithosphere sources were shared between some rift magmas and those at Spanish Peaks. High Zn/Fe ratios in the Spanish Peaks mafic rocks point to a clinopyroxene- and garnet-rich source such as lithosphere veined by pyroxenite or eclogite. Lithospheric melting was possibly triggered by foundering of cool, dense lithosphere beneath the Rio Grande rift during the initiation of rifting with the potassic parent magmas generated by higher pressure melting of the foundered lithosphere than the sodic parent magmas. This process, caused by gravitational instability of the lithosphere (Elkins-Tanton, 2007) may be common beneath active continental rifts.

The mobility of metals in soils and subsurface aquifers is strongly affected by sorption and complexation with dissolved organic matter, oxyhydroxides, clay minerals, and inorganic ligands. Humic substances (HS) are organic macromolecules with functional groups that have a strong affinity for binding metals, such as actinides. Thorium, often studied as an analog for tetravalent actinides, has also been shown to strongly associate with dissolved and colloidal HS in natural waters. The effects of HS on the mobilization dynamics of actinides are of particular interest in risk assessment of nuclear waste repositories. Here, we present conditional equilibrium binding constants (Kc, MHA) of thorium, hafnium, and zirconium-humic acid complexes from ligand competition experiments using capillary electrophoresis coupled with ICP-MS (CE- ICP-MS). Equilibrium dialysis ligand exchange (EDLE) experiments using size exclusion via a 1000 Damembrane were also performed to validate the CE-ICP-MS analysis. Experiments were performed at pH 3.5-7 with solutions containing one tetravalent metal (Th, Hf, or Zr), Elliot soil humic acid (EHA) or Pahokee peat humic acid (PHA), and EDTA. CE-ICP-MS and EDLE experiments yielded nearly identical binding constants for the metal- humic acid complexes, indicating that both methods are appropriate for examining metal speciation at conditions lower than neutral pH. We find that tetravalent metals form strong complexes with humic acids, with Kc, MHA several orders of magnitude above REE-humic complexes. Experiments were conducted at a range of dissolved HA concentrations to examine the effect of [HA]/[Th] molar ratio on Kc, MHA. At low metal loading conditions (i.e. elevated [HA]/[Th] ratios) the ThHA binding constant reached values that were not affected by the relative abundance of humic acid and thorium. The importance of [HA]/[Th] molar ratios on constraining the equilibrium of MHA complexation is apparent when our estimated Kc, MHA values attained at very low metal loading conditions are compared to existing literature data. Overall, experimental data suggest that the tetravalent transition metal/-actinide-humic acid complexation is important over a wide range of pH values, including mildly acidic conditions, and thus, these complexes should be included in speciation models.

The isotopic composition of species-specific atmospheric mercury (Hg) was investigated in the coastal environment of Grand Bay, Mississippi, USA. Atmospheric mercury species (Hg 0 (g) , Hg II (g) and Hg (p) ) were collected individually, and analyzed for isotopic composition. Gaseous elemental Hg (Hg 0 (g) ) displayed δ 202 Hg ranging from − 3.88‰ to − 0.33‰. Aerosol Hg (Hg (p) ) displayed intermediate δ 202 Hg ranging from − 1.61‰ to − 0.12‰, while reactive gaseous Hg (Hg II (g) ) displayed positive δ 202 Hg ranging from + 0.51‰ to + 1.61‰. Significant positive mass-independent fractionation (MIF) was observed in Hg (p) (∆ 199 Hg = + 0.36‰ to + 1.36‰), while Hg 0 (g) displayed negative MIF (∆ 199 Hg = − 0.41‰ to − 0.03‰) and Hg II (g) displayed intermediate MIF (∆ 199 Hg = − 0.28‰ to 0.18‰). Positive MIF of 199 Hg and 201 Hg measured in Hg (p) is consistent with significant in-aerosol photoreduction. Significant MIF of 200 Hg was observed in all Hg species with Hg 0 (g) displaying negative ∆ 200 Hg values of − 0.19‰ to − 0.06‰ while Hg II (g) and Hg (p) displayed positive ∆ 200 Hg values of + 0.06‰ to + 0.28‰, which are similar to ∆ 200 Hg values reported by Gratz et al. (2010). These results suggest that isotope tracing of each atmospheric Hg species may be feasible during important atmospheric processes such as wet and dry deposition.

The proposed origins for the Enriched Mantle I component are many and various and some require an arbitrary addition of an exotic component, be it pure sediment or an enriched melt from the subcontinental lithosphere. With Pitcairn, Walvis Ridge is the ‘type-locality’ for the Enriched Mantle I (EMI) component. We analyzed basalts from DSDP Site 525A, Site 527 and Site 528 on the Walvis Ridge with the aim to constrain the history of its source. The isotopic compositions we measured for the three sites overlap with the values obtained by Richardson et al. (1982a) and extend towards less radiogenic Sr and more radiogenic Pb and Nd isotopic compositions. We used our new trace element and radiogenic isotope (Hf, Nd, Pb and Sr) characterization in combination with the literature data to produce the simplest possible model that satisfies the trace element and isotopic constraints. Although the elevated 207Pb/204Pb with respect to 206Pb/204Pb predicts an ancient origin for EMI, none of the proposed origins had modeled it as such. The data is consistent with the EMI composition being formed by the addition of a melt to a mantle with bulk Earth-like composition followed by melt extraction of a low degree melt. The timing of these two events is such that the metasomatism has to have taken place prior to 4 Ga and the subsequent melt removal before 3.5 Ga. This confirms the expectation of an ancient character for the EMI component. The Walvis Ridge data shows two distinct two component mixing trends: one formed by the less enriched Site 527 and Site 528 basalts and one formed by the Site 525A basalts. The two trends have the EMI endmember in common. The less depleted end of the Site 527–Site 528 basalts is FOZO-like and can be explained by the addition of a recycled component (basaltic oceanic crust plus sediment). This recycled component was altered during subduction. The sense and magnitude of the chemical fractionation resulting from the subduction alteration are in agreement with dehydration experiments on basalts and sediment. Compared to other EMI like basalts the Walvis Ridge basalts have flatter REE patterns and show less fractionation between large ion lithophile and heavy REE elements. Using the isotopic compositions as constrains for the parent–daughter ratios we were able to model the trace element patterns of the basalts as melting between 5 and 10% for Site 525A and between 10 and 15% for the depleted end of the Site 528–Site 527 array. In all cases a significant portion of melting takes place in the garnet stability field.

The background and applications of high resolution sector field inductively coupled plasma mass spectrometry (HR-ICP-MS) as a detector for capillary (CE) and gel electrophoretic separations are reviewed. Notable progress has been made in the fields of bioinorganic and environmental (geo-) chemistry. Metallomics, the study of metal species interactions and functions in biological systems, puts substantial technical demands on speciation analysis. The combination of high species resolving power (CE) and high sensitivity-high mass resolving power (HR-ICP-MS) provides a solid base to meet such demands.

The Emperor Seamount-Hawaiian island chain is thought to originate from melting of a heterogeneous mantle plume, but the composition of the plume has always been inferred from the compositions of the erupted lavas. It has been suggested that recycled (i.e. previously subducted) basaltic oceanic crust (with or without sediments) is part of this plume and responsible for the enriched isotopic compositions of the Hawaiian lavas. Here we present the first combined Hf–Os isotopic study on peridotite mantle xenoliths from the island of Oʻahu, Hawaiʻi. The clinopyroxene and spinel major element compositions overlap the global range of compositions of abyssal peridotites, suggesting that the Oʻahu peridotites are samples from the oceanic mantle and residues of variable degrees of melting. The peridotite xenoliths from Salt Lake Crater have both highly unradiogenic Os (down to 187Os/188Os = 0.1138) and radiogenic Hf (up to eHf = 114) isotopic compositions and minimum rhenium-depletion ages up to ∼ 2 Ga. Such extreme Hf and Os isotopic compositions are not observed in Hawaiian lavas and are far removed from the composition of the depleted mantle, as it is sampled by mid oceanic ridge basalts and abyssal peridotites. Importantly, both Hf and Os isotopic compositions correlate with degree of depletion, suggesting that they are related to an ancient melting event. In contrast, peridotites from the Pali and Kaau vents have Os and Hf isotope ratios consistent with an origin from the ∼ 100 Ma Pacific lithosphere that lies beneath the Hawaiian islands. We suggest that the Salt Lake Crater peridotites are fragments of an ancient (> 2 Ga) depleted and recycled mantle lithosphere that is part of the upwelling Hawaiian plume. Such depleted peridotites have higher solidus temperature than other more fertile mantle components, so that their contribution to the erupted lavas compositions is minor, if any. The recognition of such isotopically and compositionally depleted materials within the Hawaiian plume suggests that depleted components are more common in mantle plumes than previously realized.

We present the first comprehensive major, trace element and Hf, Nd and Sr isotope investigation of clinopyroxene and garnet mineral separates from a set of garnet clinopyroxenite xenoliths from the Salt Lake Crater, Oahu, Hawaii. These xenoliths occur in the posterosional Honolulu Volcanics Series lavas and represent some of the deepest samples from the oceanic mantle lithosphere. Our study shows that the Salt Lake Crater pyroxenites represent high pressure (>20 kb) accumulates from melts similar (but not identical) to the erupted Honolulu Volcanics, and unlike MORB or E-MORB-type melts. All clinopyroxene-garnet mineral pairs in these xenoliths show, within error, zero-age Lu-Hf and Sm-Nd isotope systematics. These pyroxenites have relatively radiogenic Hf isotope compositions (for a given Nd) and define a distinct steep slope (3.3) in eHf-eNd isotope space, similar to the Honolulu Volcanics but unlike other ocean island basalts (OIB). These compositions require an end-member component that falls above the OIB array in Nd-Hf space. This component is different than present-day MORB-mantle and it is best explained by an old depleted oceanic lithosphere. We suggest that this depleted component most likely represents a recycled depleted lithosphere that is intrinsic to the Hawaiian plume. In this respect, the Hawaiian plume is sampling both the enriched portion of a subducted oceanic crust (basalt and sediments) as well as the depleted lithospheric portion of it. This suggests that, at least for Hawaii, the whole subducted oceanic slab package has retained its integrity during subduction and subsequent mixing and storage in the mantle, probably in the order of a billion years, and that the plume is sampling the full range of these compositions.

This article provides a review of the use of modern mass spectrometry (MS) for quantitative and qualitative measurements of organic phosphorus compounds in nature. Included is a brief discussion of recent developments in large molecule mass spectrometry, focusing on time-of-flight (TOF) and ion cyclotron resonance (ICR) mass analysis techniques, as well as electrospray (ESI) and inductively coupled plasma (ICP) ionization. The use of ICP with high-resolution mass spectrometry for quantitative measurements of total phosphorus and as a detector coupled to HPLC and CE for defining organic phosphorus speciation is demonstrated using results from a study of phosphorus cycling in a treatment wetland. Qualitative identifications of individual phosphorus compounds by ultrahigh resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is demonstrated using dissolved organic phosphorus isolated from this same wetland.

The following Comment addresses the issueconcerning the accuracy of the thallium-corrected Pbisotope data from Theistareykir, NE Iceland (Strackeet al., 2003), and the drill core recovered by theHawaii Scientific Drilling Project (HSDP; Blichert-Toft et al., 2003). Baker et al. (2004) claim that,because of improper mass-spectrometry techniques,these data sets are inaccurate to a degree that by farexceeds the errors reported in these two papers.Baker et al. (2004) dismiss the Theistareykir(Stracke et al., 2003) and HSDP data (Blichert-Toftet al., 2003) and assert that the Pb isotope ratiosproduced in Lyon are more than 3000 ppm offcompared to the claimed reproducibility of less thanabout 500 ppm (Stracke et al., 2003; Blichert-Toft etal., 2003; Albare`de et al., 2004). Baker et al’s. (2004)arguments are based on a comparison of Copenhagenand Lyon data for solution and rock standards and onebcommonQ sample. The Baker et al. (2004) compar-ison is, however, erroneous because,1. The only solution standard (98B) analyzed by boththe Copenhagen and Lyon groups shows anexcellent agreement (103, 109, 174 ppm on

Sm–Nd and Lu–Hf isotopic data are presented for 19 chondritic meteorites: six carbonaceous chondrites, five L-chondrites, seven H-chondrites, and a single enstatite chondrite. The primary goal of the study is to better define the Bulk Silicate Earth (BSE) reference values for Hf isotopes. Except for one sample with lower Sm/Nd, the Sm–Nd data define a cluster around the accepted reference values for chondrites and terrestrial planets, giving a mean 147Sm/144Nd of 0.1960±0.0005, and a mean 143Nd/144Nd of 0.512631±0.000010 (uncertainties are two standard errors). It seems appropriate to retain the presently accepted Sm–Nd reference parameters, 147Sm/144Nd=0.1966 and 143Nd/144Nd=0.512638 (when fractionation-corrected to 146Nd/144Nd=0.7219).Lu–Hf isotopic data are not clustered, but spread along an approximate 4.5-Ga isochron trend, with a range of 176Lu/177Hf from 0.0301 to 0.0354. The data are similar to many of the samples of chondrites presented by Bizzarro et al. [Nature 421 (2003) 931], but lack the range to lower Lu/Hf shown by those authors. Our chondrite data define a regression line of 4.44±0.34 Ga when 1.867×10−11 year−1 is used for the decay constant of 176Lu [Science 293 (2001) 683; Earth Planet. Sci. Lett. 219 (2004) 311–324]. Combining our data with the main population of analyses from Bizzarro et al. [Nature 421 (2003) 931] yields 4.51±0.24 Ga. Unless samples of eucrite meteorites and deviating replicates of chondrites with 176Lu/177Hf less than 0.030 are employed, no combination of the main population of chondrite Lu–Hf data yields a regression with sufficiently low error to constrain the decay constant of 176Lu. Sample heterogeneity seems to hinder the acquisition of reproducible Lu–Hf analyses from small, manually ground pieces of chondrites, and we suggest that analysis of powders prepared from large volumes of meteorite will be needed to adequately characterize the Lu–Hf isotope systematics of chondritic reservoirs and of BSE. Our results for carbonaceous chondrites show higher average 176Lu/177Hf and 176Hf/177Hf than ordinary chondrites, and the mean of carbonaceous chondrites also coincides with replicate analyses of a powder representing a large volume of meteorite, the Allende powder from the Smithsonian Institution. Use of the carbonaceous chondrite mean for BSE Lu–Hf characteristics results in a BSE Hf–Nd point that lies well within the array of terrestrial compositions, and leads to plausible initial eHf values for Precambrian rocks. An improved objective resolution of meteorite data and of meteoritic models for the Earth needs to occur before BSE can be established for Lu–Hf.

We present a detailed geochemical investigation on the Hf, Nd and Sr isotope compositions and trace and major element contents of clinopyroxene mineral separates from spinel lherzolite xenoliths from the island of Oahu, Hawaii. These peridotites are believed to represent the depleted oceanic lithosphere beneath Oahu, which is a residue of a MORB-related melting event some 80–100 Ma ago at a mid-ocean ridge. Clinopyroxenes from peridotites from the Salt Lake Crater (SLC) show a large range of Hf isotopic compositions, from ϵHf=12.2 (similar to the Honolulu volcanics series) to extremely radiogenic, ϵHf=65, at nearly constant 143Nd/144Nd ratios (ϵNd=7–8). None of these samples show any isotopic evidence for interaction with Koolau-type melts. A single xenolith from the Pali vent is the only sample with Hf and Nd isotopic compositions that falls within the MORB field. The Hf isotopes correlate positively with the degree of depletion in the clinopyroxene (e.g. increasing Mg#, Cr#, decreasing Ti and heavy REE contents), but also with increasing Zr and Hf depletions relative to the adjacent REE in a compatibility diagram. The Lu/Hf isotope systematics of the SLC clinopyroxenes define apparent ages of 500 Ma or older and these compositions cannot be explained by mixing between any type of Hawaiian melts and the depleted Pacific lithosphere. Metasomatism of an ancient (e.g. 1 Ga or older) depleted peridotite protolith can, in principle, explain these apparent ages and the Nd–Hf isotope decoupling, but requires that the most depleted samples were subject to the least amount of metasomatism. Alternatively, the combined isotope, trace and major element compositions of these clinopyroxenes are best described by metasomatism of the 80–100 Ma depleted oceanic lithosphere by melts products of extensive mantle–melt interaction between Honolulu Volcanics-type melts and the depleted lithosphere.

We measured trace element and Rare Earth Element (REE) contents of clinopyroxenes (cpx) in peridotites from ophiolite complexes from the Hellenic Peninsula: Vourinos, Pindos, Othris (Greece), and Bulqiza (Albania). Compared to abyssal peridotites the Hellenic peridotites have a highly depleted mineralogy (

We have measured partition coefficients for U, Th, REE, and high field strength element for orthopyroxene–liquid (3) and garnet–liquid (9) pairs from 2.4 to 2.8 GPa and for clinopyroxene–liquid (6) from 1.2 to 2.8 GPa. These are the first measurements of partition coefficients from experiments that are close to multiple saturation with an anhydrous lherzolite assemblage, similar to what is believed to be the source of primary melts formed beneath mid-ocean ridges. In determining the phase compositions of the anhydrous lherzolite we found that the composition of clinopyroxene on the anhydrous lherzolite solidus changes from high-Ca to low-Ca with increasing pressure. Garnet and clinopyroxene partition coefficients for the elements we studied (U, Th, Nb, Hf, Y, Zr, Ce, Nd, Sm, Er, Yb and Lu) are dependent on pressure, temperature, and composition; although crystal composition appears to be the dominant control on pyroxene coefficients. The garnet partition coefficients most applicable to lherzolite melting are significantly higher than those from previous studies, whereas the clinopyroxene partition coefficients are significantly lower. These new partition coefficients in combination with the new phase equilibria relax the constraints from uranium–thorium disequilibria on mantle porosities in the melting region and reconcile the implications of excess 230Th with melting models based on other geochemical and geophysical parameters [Kelemen et al., A review of melt migration processes in the adiabatically upwelling mantle beneath oceanic spreading ridges, Philos. Trans. R. Soc. London Ser. A 355 (1997) 283-3182; Toomey et al., Tomographic constraints on upper mantle structure beneath the MELT region of the East pacific Rise, Trans. AGU 78 (1997) 705]. With these new partition coefficients significant 230Th excess can be created with residual porosities of up to 1%.

The similarity of the Lu–Hf and Sm–Nd isotope system during most mantle differentiation processes makes the combination of 176Hf/177Hf and 143Nd/144Nd a very sensitive indicator of a select number of processes. This paper present new Hf-isotope data for a large number of ocean islands and examines the Hf–Nd–Pb isotope relations of oceanic volcanics. Except for HIMU islands, St. Helena and Tubaii, the Hf and Nd isotope ratios in ocean island basalts (OIB) are extremely well correlated. It is argued that crustal recycling (by either continental or oceanic sediments) most likely did not cause the Hf–Nd variations. The correlated 176Hf/177Hf–143Nd/144Nd variations in OIB most likely represent the time integrated fractionations which are the result of melting in the presence of garnet. The Hf-isotope systematics of HIMU-type OIB are consistent with these basalts representing recycled oceanic crust and thus support the earlier hypothesis on the origins of HIMU basalts. Chondrites form an array that is at high angle with the OIB array. This allows a choice in the 143Nd/144Nd and 176Hf/177Hf values for chondritic bulk earth. With a choice of bulk earth at the extreme end of the OIB array the shift of OIB to higher 176Hf/177Hf can be explained by either isolation of a significant amount of basalts from the mantle for several billions of year or by fractionation and isolation of small amounts (

We present Hf and Pb isotopic data, and chemical compositions of the outermost layers of marine ferromanganese deposits of different types (hydrogenous and hydrothermal) with a worldwide distribution. The Hf isotopic compositions display a broad range and refine previously reported regional differences as follows: Atlantic Ocean ɛHf = −4to+2, Indian Ocean ɛHf = +2to+4, Pacific Ocean ɛHf = +3to+10. The most radiogenic Hf isotopic compositions in the Pacific samples are for hydrothermal manganese deposits that also have low 207Pb204Pb, demonstrating that this signature reflects a contribution from hydrothermal venting of Hf leached from oceanic volcanic rocks rather than from riverine inputs, volcanic ash, or eolian dust. Hafnium concentrations in the deposits increase from 20 ppb to 20 ppm with decreasing ɛHf, The Hf and Pb isotopic compositions for ferromanganese crusts define an apparent mixing trend between literature values of average continental crust and MORB. The range in ɛHf for ferromanganese crusts is narrower than it is for 206Pb204Pb compared to the differences in isotopic composition of the sources of Hf and Pb. This is consistent with Hf having a longer residence time than Pb. The concentration of Hf in ferromanganese crusts has been found to co-vary with growth rate, and inversely correlates with Hf isotopic compositions. Hf isotope ratios may be used to determine not only the source of Hf, but possibly the source of Fe and Mn. Measurements of ɛHf and Hf concentrations in nodule tops, bottoms and associated sediments show that the ɛHf of nodules is sensitive to sedimentary oxic and sub-oxic diagenetic processes and thus most nodules may not reliably reflect the isotopic composition of Hf in seawater.

Dissolved Zr and Hf distributions have been determined for five stations located across the shelf break in the northeastern Atlantic Ocean on the Celtic Approaches. For stations in deep water, the range in Zr concentrations has been found to be 70–180 pmol/kg, and the range in hafnium concentrations 0.4–1.1 pmol/kg. The distributions with depth are indicative of a nutrient-like scavenging-regeneration behavior in seawater, although the dissimilarity with silicate and nitrate for samples below the nutricline suggest that other mechanisms are also important in defining the oceanic distribution of these elements. In contrast, the concentrations of Zr and Hf at the station located closest to land are highest in surface waters and decrease with depth, although salinity does not indicate dilution of seawater with freshwater with high Zr and Hf concentrations. The ratio of dissolved Zr to Hf in seawater is higher and more variable than is found for the majority of terrestrial rocks, or predicted from thermodynamic considerations. Fractionation due to small differences in their interactions with particulate material is implicated.

This paper presents new Hf and Nd isotope and trace element data for mid-ocean ridge basalts. The intention of this study was to investigate whether melt parameters derived from the isotopic systematics of MORB ( δ (Lu) (Hf) and δ (Sm) (Nd) ) support the variations in depth and degree of melting as inferred from the major elements and ridge depth; that is, the ‘local’ and ‘global’ trend [1]. Variation on a ‘local’ scale is defined as variation within a ridge segment, while the ‘global’ scale variation is defined as variation between ridge segments. Both on a ‘local’ and a ‘global’ scale, δ (Lu) (Hf) and δ (Sm) (Nd) show consistent variations with the major element melt parameters, Na 8 and Fe 8 . In addition, the δ melt parameters vary consistently with ridge depth. Basalts from some of the deepest ridges, Australian-Antarctic Discordance and Mid-Cayman Rise, show evidence that a relatively large proportion of the basalt was generated in the presence of garnet. A melting model is derived using best available estimates for a large number of parameters that influence the chemistry of MORBs. With this melting model, the additional constraints of δ (Lu) (Hf) and δ (Sm) (Nd) further define the melting regime beneath the ocean ridges. The onset of melting for shallow ridges, like Kolbeinsey Ridge, starts deeper than for deep ridges, the aggregated melts from the Kolbeinsey Ridge sample a columnar shaped melting regime. Basalts from deep ridges represent a smaller degree of melting, which was generated in a triangular-shaped melting regime. The translation of δ (Lu) (Hf) and δ (Sm) (Nd) in absolute values for degree and depth of melting for individual ridge segments is hampered by uncertainties in the details of the heterogeneity of the MORB reservior.

Lavas from several major bathymetric highs in the eastern Indian Ocean that are likely to have formed as Early to Middle Cretaceous manifestations of the Kerguelen hotspot are predominantly tholeiitic; so too are glass shards from Eocene to Paleocene volcanic ash layers on Broken Ridge, which are believed to have come from eruptions on the Ninetyeast Ridge. The early dominance of tholeiitic compositions contrasts with the more recent intraplate, alkalic volcanism of the Kerguelen Archipelago. Isotopic and incompatible-element ratios of the plateau lavas are distinct from those of Indian mid-ocean ridge basalts; their Nd, Sr, 207Pb204Pb and 208Pb204Pb isotopic ratios overlap with but cover a much wider range than measured for more recent oceanic products of the Kerguelen hotspot (including the Ninetyeast Ridge) or, indeed, oceanic lavas from any other hotspot in the world. Samples from the Naturaliste Plateau and ODP Site 738 on the southern tip of the Kerguelen Plateau are particularly noteworthy, with ϵNd(T) = − 13 to −7, (87Sr86Sr)T=0.7090 to 0.7130 and high 207Pb204Pb relative to 206Pb204Pb. In addition, the low-ϵNd(T) Naturaliste Plateau samples are elevated in SiO2 (> 54 wt%). In contrast to “DUPAL” oceanic islands such as the Kerguelen Archipelago, Pitcairn and Tristan da Cunha, the plateau lavas with extreme isotopic characteristics also have relative depletions in Nb and Ta (e.g., ThTa, La Nb > primitive mantle values); the lowest ϵNd(T) and highest ThTa and La Nb values occur at sites located closest to rifted continental margins. Accepting a Kerguelen plume origin for the plateau lavas, these characteristics probably reflect the shallow-level incorporation of continental lithosphere in either the head of the early Kerguelen plume or in plume-derived magmas, and suggest that the influence of such material diminished after the period of plateau construction. Contamination of asthenosphere with the type of material affecting Naturaliste Plateau and Site 738 magmatism appears unlikely to be the cause of low-206Pb04Pb Indian mid-ocean ridge basalts. Finally, because isotopic data for the plateaus do not cluster or form converging arrays in isotope-ratio plots, they provide no evidence for either a quickly evolving, positive ϵNd, relatively high-206Pb204Pb plume composition, or a plume source dominated by mantle with ϵNd of −3 to ∼ 0.

The Lamont—Doherty Geological Observatory (LDGO) Isolab 54 is a double focussing isotope ratio mass spectrometer that allows the measurement of thermal ions produced on a hot filament, (thermal-ionization mass spectrometry (TIMS)), secondary ions produced by sputtering a sample using a primary ion beam, (secondary ion mass spectrometry (SIMS)), and sputtered neutrals resonantly ionized using laser radiation, (sputter-induced resonance ionization mass spectrometry (SIRIMS)). Sputtering is carried out using an Ar primary beam generated in a duoplasmatron and focussed onto the sample using a two-lens column. Resonance ionization is accomplished using a frequency-doubled dye laser pumped by an excimer laser. The Isolab's forward geometry analyzer, consisting of an electrostatic followed by a magnetic sector, allows the simultaneous collection of different isotopes of the same element. This instrument is the first to have a multicollector that contains an ion-counting system based on a microchannel plate as well as traditional Faraday cups. A second electrostatic sector after the multicollector is equipped with an ion-counting Daly detector to allow high abundance sensitivity for measurements of large dynamics range. Selectable source, collector, α and energy slits on the instrument allow analyses to be made over a range of mass resolving powers and analyzer acceptances. Recent applications of the instrument have included the analyses of U by TIMS, Hf, Th and Re by SIMS and Re and Os by SIRIMS.

This paper presents Nd- and Hf-isotope data for oceanic volcanics and island arc basalts in order to (1) further examine the correlation between the two isotopic systems, (2) determine 176Hf/177Hf of the mantle components as described by Zindler and Hart (1986) [1], and (3) assess the origin of HFSE depletions in calc-alkaline volcanics. Ocean island basalts (OIB) form an array on a 143Nd/144Nd versus 176Hf/177Hf diagram, whereby the mantle endmembers as defined in SrNdPb isotopic space can be extended to the Hf system. For MORB, however, the decoupling of Hf and Nd isotopes requires a variable MORB endmember. Melting models in which Lu/Hf and Sm/Nd are fractionated during the formation of the MORB reservoir can explain the complete range in Nd and Hf isotopes in MORB as long as variable amounts of both diopside and garnet are present during melting. A model in which a MORB-type mantle is contaminated with high 176Hf/177Hf, high 143Nd/144Nd material is also consistent with the isotope data. This high 176Hf/177Hf component may be generated by high Lu/Hf caused by metasomatism with a melt/fluid which was in equilibrium with phases with a high D for Hf like perovskite. The existence of high-field-strength-element (HFSE) depletions has often been thought to be a widespread characteristic of subduction-related volcanism (Gill, 1981) [2]. However, a literature survey shows that Ti, Zr and Hf depletions are not a ubiquitous a feature in calc-alkaline volcanics (see also White and Patchett, 1984 [3] and Woodhead, 1989 [4]), and that, except for Zr and Hf, the HFSE (Nb, Ta, Ti, Zr, and Hf) behave independently from each other. 176Hf/177Hf ratios in island arc lavas are not related to the degree of Hf/REE variation, strongly suggesting that the HfZr depletion is less than 250 m.y. old. The HFSE variations are not compatible with generation during melting nor are the HFSE variations solely due to the addition of a slab-derived component. However, the available data are compatible with the variations being a reflection of a HFSE-depleted mantle source of calc-alkaline volcanics. The HFSE depletions in this sub-arc mantle have to be less than 250 m.y. old.

We report here the widespread occurrence of mantle peridotites in both suboceanic and subcontinental lithosphere that show trace element characteristics similar to those of island arc volcanics. These anhydrous peridotites, which reside at high levels in the mantle, cannot generate nor be in equilibrium with mid-ocean ridge or ocean island basalts, and have complex geochemical histories involving partial melting, melt extraction and metasomatism.

87 Sr/ 86 Sr, major and trace element data are presented for calc-alkaline lavas from Santorini and Milos, Aegean Sea, Greece. 87 Sr/ 86 Sr ratios of fresh lavas from Santorini range from 0.70472 to 0.70509 whereas those of fresh lavas from Milos lie in the range 0.70540–0.70620. Altered lavas from both islands have higher 87 Sr/ 86 Sr ratios (up to 0.70573 and 0.70662 respectively). The range of 87 Sr/ 86 Sr ratios of fresh lavas from Santorini is only just greater that the maximum analytical uncertainty and is thus considered to reflect that of the parental magmas from which the lava series was derived (by fractional crystallization at low pressures). For the analysed Milos lavas (dacite-rhyolite) there is no correlation between 87 Sr/ 86 Sr and any major or trace element concentration and this, together with the fact that the 87 Sr/ 86 Sr ratios are similar to those of basaltic andesites and andesites from Aegina and Methana in the northwestern part of the arc, likewise indicates that the 87 Sr/ 86 Sr ratios are identical to those of the parental magmas from which the lava series was derived (also by fractional crystallization). The relatively high 87 Sr/ 86 Sr ratios of the Santorini and Milos parental magmas indicate that terrigenous material has played a role in their genesis. This could be either via the subduction process or by assimilation of lower crustal material during ascent of mantle-derived magmas. The weight of the available evidence favours the latter possibility, but, even so, there are some indications that unusual mantle source material was involved in the genesis of the magmas.

Volcanic activity on Santorini started in Upper Pliocene times and is still in progress (the last eruption occurred in 1950). At least seven eruptive centers were active at various times, and some centers were active contemporaneously. The overall chemical variations displayed by lavas and pyroclastics from the volcanic complex are largely explicable in terms of fractional crystallization involving removal of the observed phenocryst phases, together with variable amounts of magma mixing and assimilation. Comparison of the chemistry of volcanic products from different eruption centers indicates long-term variations in magma chemistry (progressive depletion in LIL elements and enrichment in compatible elements with time) that cannot be explained in terms of fractionation, mixing and assimilation. Such variations are attributed to processes occurring within the upper mantle. Trace element data require that the primitive lavas erupted on Santorini were derived from an upper mantle source which was relatively depleted in LIL elements (compared with other calc-alkaline volcanics) and it is proposed that the source region contained a (dominant) refractory component which had experienced earlier events and a component derived from the subducted slab. As magmatism progressed, the contribution to the source region from the slab decreased, due to either (1) cessation of subduction and hence a decrease in the proportion of slab-derived material in the source region; (2) a rise in geothermal gradient during melting so that the earliest melts contained a higher proportion of (low-melting) slab derived material; or (3) a combination of these processes. We conclude that it is possible to “see through” intracrustal processes and to evaluate the processes which occur in the upper mantle beneath island arcs and we also conclude that the time-integrated history of the mantle wedge above the subducted slab beneath island arcs must be taken into account when constructing viable models for are magmatism.