Vincent J. M. Salters

Trace element partitioning during the initial stages of melting beneath mid-ocean ridges

Abstract 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%.

Hf isotope constraints on mantle evolution

Abstract 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 (

The generation of mid-ocean ridge basalts from the Hf and Nd isotope perspective

Abstract 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.

Mineralogy of the mid-ocean-ridge basalt source from neodymium isotopic composition of abyssal peridotites

Inferring the melting process at mid-ocean ridges, and the physical conditions under which melting takes place, usually relies on the assumption of compositional similarity between all mid-ocean-ridge basalt sources. Models of mantle melting therefore tend to be restricted to those that consider the presence of only one lithology in the mantle, peridotite. Evidence from xenoliths and peridotite massifs show that after peridotite, pyroxenite and eclogite are the most abundant rock types in the mantle. But at mid-ocean ridges, where most of the melting takes place, and in ophiolites, pyroxenite is rarely found. Here we present neodymium isotopic compositions of abyssal peridotites to investigate whether peridotite can indeed be the sole source for mid-ocean-ridge basalts. By comparing the isotopic compositions of basalts and peridotites at two segments of the southwest Indian ridge, we show that a component other than peridotite is required to explain the low end of the 143Nd/144Nd variations of the basalts. This component is likely to have a lower melting temperature than peridotite, such as pyroxenite or eclogite, which could explain why it is not observed at mid-ocean ridges.

Trace and REE content of clinopyroxenes from supra-subduction zone peridotites. Implications for melting and enrichment processes in island arcs

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 (<1% modal cpx) and the cpxs have extremely low concentrations in Ti (30–150 ppm) and heavy REE. The light and middle REE, Zr and Sr contents of the cpxs show enrichments compared to cpx from abyssal peridotites. Our cpx data falls in the field of previous studies on modern arc peridotites ([Parkinson, I.J., Pearce, J.A., Thirwall, M.F., Johnson, K.T.M., Ingram, G., 1992. Trace element geochemistry of peridotites from the Izu–Bonin–Mariana forearc, Leg 125. In: Fryer, P., Pearce, J.A., Stokking, L.B. (Eds.), Proc. Ocean Drilling Program. pp. 487–506; Bonatti, unpubl.]) suggesting that these complexes originated above a subduction zone. Dry melting of an upper mantle source similar to a mid-ocean ridge basalt (MORB) source (e.g., [Johnson, K.T.M., Dick, H.J.B., Shimizu, N., 1990. Melting in the oceanic upper mantle: an ion microprobe study of diopsides in abyssal peridotites. J Geophys. Res. 95, 2661–2678.]) cannot generate the extreme depletions in Ti and heavy rare earth element (HREE) seen in the cpxs from these supra-subduction zone (SSZ) peridotites. We propose that these high degrees of depletion can be achieved during hydrous melting of a MORB-depleted peridotite above a subduction zone. The melting rate of cpx during hydrous melting is expected to be less than in dry melting, while the rate of orthopyroxene (opx) consumption increases. These changes in melting modes allow for larger degrees of total melting and larger depletions in Ti and HREE in the cpx before cpx is exhausted from the residual peridotite. The enrichments in light rare earth element (LREE), MREE, Zr and Sr are best modeled by a constant flux of a slab-derived fluid component to the mantle wedge during melting. Ti and the HREE are not mobile in this fluid and are not enriched in the mantle peridotite. The continuous flux is necessary to sustain hydrous melting in the sub-arc mantle during the complete melting range. The calculated fluid composition matches well previous estimates based on the arc basalt chemistry ([McCulloch, M.T., Gamble, J.A., 1991. Geochemical and geodynamical constraints on subduction zone magmatism. Earth Planet. Sci. Lett. 102, 358–374; Stolper, E., Newman, S., 1994. The role of water in the petrogenesis of Mariana trough magmas. Earth Planet. Sci. Lett. 121, 293–325.]) and experiments on fluids ([Ayers, J.C., Dittmer, S.K., Graham, D.L., 1997. Partitioning of elements between peridotite and H2O at 2.0–3.0 GPa and 900–1100°C, and application to models of subduction zone processes. Earth Planet. Sci. Lett. 150, 381–398; Ayers, J.C., 1998. Trace element modeling of aqueous fluid–peridotite interaction in the mantle wedge of subduction zones. Contrib. Mineral. Petrol. 132, 390–404.]). The resulting melts from our model closely resemble melts from an arc environment (boninites) further arguing for the plausibility of this model.

Domains of depleted mantle: New evidence from hafnium and neodymium isotopes

Isotope systematics of basalts provide information on the distribution of mantle components and the length scale of mantle heterogeneity. To obtain this information, high data and sampling density are crucial. We present hafnium and neodymium isotope data on more than 400 oceanic volcanics. Over length scales of several hundred to over one thousand kilometers hafnium and neodymium isotopes of mid-ocean ridge basalts are correlated and form an array of parallel trends on a global scale. On a larger scale these domains differ in the amount of highly depleted mantle material with radiogenic hafnium and neodymium isotope ratios. Compared to the Atlantic and Indian Ocean basins the asthenosphere of the Pacific basin seems to have a more uniform and a less radiogenic Hf isotopic composition for a given Nd isotopic composition. The parallel arrays of mid-ocean ridge basalts provide strong constraints on the makeup of the MORB mantle and are evidence for the presence of a highly depleted and highly radiogenic neodymium and hafnium component. This component, because of its highly depleted character, is unrecognized in the strontium-neodymium-lead isotope systems alone. Alternatively, the parallel arrays can have an ancient origin of systematic variations in the degree of depletion. Each array then represents the variations in this fossil melting regime. Individual ocean island basalt suites display different slopes in hafnium-neodymium isotope space, which are also best explained by varying amounts of highly residual mantle rather than isotopic differences in enriched mantle components as previously invoked. The ocean island basalt arrays diverge at the depleted end and project to radiogenic compositions that are similar to those of the asthenosphere through which they travel. This is strong evidence that the plume material interacts with its surrounding mantle as it ascends. The isotopic compositions of the ocean island and ridge basalts suggest that their systematics are influenced by a heretofore unrecognized depleted component.

Hf^Nd isotope decoupling in the oceanic lithosphere: constraints from spinel peridotites from Oahu, Hawaii §

Abstract 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.

THE HF ISOTOPIC COMPOSITION OF FERROMANGANESE NODULES AND CRUSTS AND HYDROTHERMAL MANGANESE DEPOSITS : IMPLICATIONS FOR SEAWATER HF

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 zirconium and hafnium distributions across a shelf break in the northeastern Atlantic Ocean

Abstract 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.

Disequilibrium effects in metal speciation by capillary electrophoresis inductively coupled plasma mass spectrometry (CE-ICP-MS); theory, simulations and experiments

A theoretical-experimental approach to evaluate disequilibrium effects in capillary electrophoresis inductively coupled plasma mass spectrometry (CE-ICP-MS) is presented. Electrophoresis requires metal ligand (ML) complexes to be stable on the time scale of separation and detection. By expressing ML complex stability in terms of half-life during a CE separation, an evaluation of separation artifacts can be made. Kinetically slow metals like Cr, Al or Fe form complexes that are stable on the time scale of electrophoretic separations. Kinetically fast metals, like Pb, Hg, Cu, Cd and REE, however tend to form labile complexes which unless complexed by strong chelators will dissociate during CE separations. A reactive transport simulation model of CE separations involving ML complexes allows a more detailed prediction of disequilibrium bias and identifies kinetically limited from mobility-limited types of dissociation. Complementary experimental results are given for kinetic and equilibrium binding experiments of Sm with humic acid. The equilibrium logK for Sm-Leonardite humic acid (HA) binding at pH 7 and 0.01 mol L(-1) ionic strength was determined to be 13.04. Kinetic rates of formation and dissociation for SmHA were 5.9 10(8) and 5.3 10(-5) mol s(-1).