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This work reports on chemical isotopic fractionation of Te in a liquid-liquid extraction system using a crown ether of dicyclohexano-18-crown-6. Te isotopic composition was measured by multi-collector inductively coupled plasma spectrometer. We demonstrated the existence of a 125Te anomaly in two experiments at different molarities of HCl. The results of the present experiments fit the nuclear field shift theory of Bigeleisen. We finally showed the importance of the isotope of normalization in the representation of the mass-independent term.

Dauphas et al.'s model for the nucleosynthetic origin of Mo and Ru anomalies in meteorites leaves the case of Murchison (CM2) unexplained. We explore the possibility that such a discrepancy is due to mass-independent effects controlled by nuclear field shift with, in particular, "staggering" between odd and even masses. We first demonstrate the existence of such mass-independent fractionation of Mo and Ru isotopes by chemical exchange of Mo and Ru between DC18C6 crown ether and aqueous solutions. Our results fit the nuclear field shift theory of Bigeleisen. We then review the correlation between the mean-square charge radius (which controls the nuclear field shift) and the isotopic anomalies found in an Allende CAI and in Murchison. Although Mo and Ru in the Allende CAI show a clear indication of nucleosynthetic components, the mass-independent anomalies observed in Murchison show a strong correlation with the nuclear charge distribution. We therefore argue that some isotopic anomalies observed in meteorites may be due to nuclear field shift rather than nucleosynthetic processes. Such effects are temperature dependent and may represent either genuine nebular processes or analytical artifacts. This new interpretation may help assess the existence of anomalies due to the extinct isotopes 97Tc and 98Tc.

Preliminary results are given from an excimer 157 nm laser ablation multiple‐collector inductively coupled plasma‐mass spectrometer (LA‐MC‐ICP‐MS), used for the isotopic measurements of solid materials. Elements of geological interest with different volatilities such as Pb and U (e.g. zircon geochronology) and Cu and Zn (as examples of geochemical/biochemical tracers) were analysed. The range of ablation rates of 20‐150 nm s‐1enabled us to ablate the sample down to a depth of 45 μm for a 50 μm diameter pit. The Cu and Zn isotopic measurements gave values that were very stable with, on average, a 0.01 % standard error, comparable with that achieved in liquid mode measurements.

The Maladeta plutonic complex formed during the latest stages of the Variscan orogeny. It was emplaced into the Paleozoic sedimentary sequence of the Pyrenees. The eastern part, investigated in the present study, consists of an early intrusion of cumulate gabbronorites followed in order of emplacement by the main biotite-hornblende granodiorite, which was itself intruded by two small stocks of two-mica cordierite granite. An 87Rb-87Sr isochron dates the granodiorite at 277±7 m.y. with an initial (87Sr/86Sr)o ratio of 0.7117±3. Gabbroic rocks have lower strontium initial ratios, down to 0.7092, while those of granite range from that of the granodiorite up to about 0.715. The three rock types have distinctive d18O values: 8.7 to 9.6 for the gabbronorites, 9.4 to 10.4 for the granodiorites and 10.3 to 11.8 for the granites. Lead isotopic compositions of rocks and feldspars are all radiogenic. Feldspars give consistent Pb model ages around 280 m.y., with µ and ? values of about 9.7 and 4.05, respectively. No pristine, mantle-derived magma was found among the investigated samples and the rocks cannot be related to one another by any simple mechanism of fractional crystallization. Some type of mixing process involving two end members seems to be required: a high-18O, high-87Sr material that is clearly of crustal origin, and a lower-18O, lower-87Sr end member derived from the mantle. Examination of various mixing models does not support magma mixing nor assimilation of crustal rocks by a mafic magma. The most acceptable model involves melting at different levels of a vertically-zoned source in the continental crust; this source was formed by mixing between mantle-derived magmas and crustal metasediments. This material was apparently thickened, tectonically downwarped and partially melted. None of the Maladeta magma-types appear to have been derived at a consuming plate boundary.

Lead isotope compositions have been determined in many K-feldspars from granitic rocks emplaced during the Variscan (Hercynian) orogeny sensu lato (around 300 Myr) in various parts of Europe and the eastern US. The ratios show a regular geographical pattern with μ contours arched around southern France and parallel to the Atlantic coast of the US. This distribution parallels predriftpalaeogeographical zones during the Palaeozoic. The lead isotope pattern of Variscan granitic rocks is similar to that of some orogenic volcanic suites but is better explained by crustal melting than by an Hercynian mantle-crust material mixing. The continental crust from these areas must be older than 3,000 Myr. Large scale fluid circulation in the crust during orogenic processes apparently explains the observed geographical patterns of lead isotopes.

The isotope geochemistry (Sm-Nd, Pb-Pb and Rb-Sr) of mafic gneisses from the basement of the Carswell structure (Saskatchewan, Canada), rich both in Mg and incompatible elements (K, Rb, REE) has been investigated. A good Sm-Nd alignment gives a slope corresponding to an age of 3.7 Ga. However, comparison with major elements data strongly suggests that this alignment is a mixing line between Mg-rich, high CaO/Al2O3 magmas and the local felsic crust older than 2.9 Ga. The mafic magmas were probably of komatiitic affinity (MgO > 20 percent) but, nevertheless, were extracted from a source with nearly chondritic to slightly enriched light REE distribution. The age of the komatiite emplacement (1.9–2.9 Ga) is only loosely constrained by the oldest crustal residence age in the series and the subsequent metamorphic events. The granulite facies climax is dated at ca. 1.9 Ga by concordant whole rock Pb-Pb and Sm-Nd garnet-whole rock isochrons. The Rb-Sr systematics have been disturbed by later event(s) younger than 1.5–1.7 Ga, but do not permit a more precise assessment of the perturbation age.

Pb, Sr and Nd isotopic compositions have been analyzed in recent granites from Northern Africa, Northern Italy and Greece. Lead isotope compositions of K-feldspars are rather homogeneous, and cluster close to the modern lead of Stacey and Kramers (1975) but with slightly higher207Pb/204Pb and208Pb/204Pb ratios. The Cyclades samples, however, have higher206Pb/204Pb ratios. Addition of mantle-derived lead was probably very limited, which supports a quasi-closed system evolution of this element in the continental crust. The Sr, Nd data fall in the enriched part of the143Nd/144Nd vs.87Sr/86Sr diagram and define a smooth hyperbolic mixing curve. Over a wide area, straddling different orogens, most granites may be accounted for by a binary mixture between a recycled crustal component and a depleted mantle-like component. No correlation is observed between either Pb and Sr or Nd isotopic ratios, or any isotopic ratio and major element contents. Quantitative modelling suggests that two cases fit the Sr and Nd characteristics of these granites: they both require anatexis of the crust on a scale large enough to average the isotopic properties of heterogeneous terranes. In the first case, the mantle-derived component may be represented by differentiated Island Arc-type magmas, and the granites result from mixing these magmas with anatectic melts. In the second case, mantle-derived igneous rocks, such as obducted ophiolites, are part of the crustal source and their variable involvement in the anatectic process causes isotopic variations.

The time constants are calculated which rule the evolution of Nd isotopes in a model Earth composed of the continental crust, a depleted upper mantle and a pristine lower mantle and in which the upper mantle is not constrained to grow or not to grow at the expense of the lower mantle. The growth rate of the continental crust is calculated to be 1.8 ± 0.7 km3/ yr, with 2.5 ± 0.5 km 3/yr of new crust addition from the mantle and 0.8 ± 0.5 km 3/yr of sediment entrainment at subduction zones. The characteristic times for Nd evolution in the depleted mantle and continental crust imply that the Sm-Nd system evolves far from steady state.

O'Hara and co-workers1,2have recently summarized some of the evidence accumulated for open-system, cyclic magmatic systems and provided useful geochemical equations for the replenishment/eruption cycles of an ideal magma reservoir. I show here that O'Hara's theory fits the situation where the volcanic eruption taps the more differentiated magma before the mixing stage. An alternative ideal model is discussed, which is applicable to the more common process of hybrid magma eruption. In mid-ocean ridge basalts, this new model may explain why large variations in incompatible elements coexist with moderate depletion of compatible elements.

Although low-temperature alteration of the ocean floor traps large quantities of uranium from sea water1,2, the mass balance between the crust and sea water along the mid-ocean ridges and, consequently, the uranium content of subducted lithosphere, are uncertain. Intense scavenging from sea water by participates as a result of the dispersal of hydrothermal solutions in the water column3has prevented the assignment of deeper levels at ridge crests as a uranium source or as a sink4. Here we present uranium analyses for hydrothermal solutions from the East Pacific Rise (EPR) which support earlier data5–6showing that this element behaves coherently with magnesium in its removal from sea water during hydrothermal circulation. Our results indicate that oceanic crust is subducted with the U/Pb ratio increased relative to that of the ambient mantle, but this is thought to be due more to hydrothermal lead depletion than to uranium enrichment.