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Many arc lavas are more oxidized than mid-ocean-ridge basalts and subduction introduces oxidized components into the mantle (1-4). As a consequence, the sub-arc mantle wedge is widely believed to be oxidized (3,5). The Fe oxidation state of sub-arc mantle is, however, difficult to determine directly, and debate persists as to whether this oxidation is intrinsic to the mantle source (6,7). Here we show that Zn/[Fe.sub.T] (where [Fe.sub.T] = [Fe.sup.2+] + [Fe.sup.3+]) is redox-sensitive and retains a memory of the valence state of Fe in primary arc basalts and their mantle sources. During melting of mantle peridotite, [Fe.sup.2+] and Zn behave similarly, but because [Fe.sup.3+] is more incompatible than [Fe.sup.2+], melts generated in oxidized environments have low Zn/[Fe.sub.T]. Primitive arc magmas have identical Zn/[Fe.sub.T] to mid-ocean-ridge basalts, suggesting that primary mantle melts in arcs and ridges have similar Fe oxidation states. The constancy of Zn/[Fe.sub.T] during early differentiation involving olivine requires that [Fe.sup.3+]/[Fe.sub.T] remains low in the magma. Only after progressive fractionation does [Fe.sub.3+]/[Fe.sub.T] increase and stabilize magnetite as a fractionating phase. These results suggest that subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. Thus, the higher oxidation states of arc lavas must be in part a consequence of shallow-level differentiation processes, though such processes remain poorly understood., Oxygen fugacity [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is an intensive parameter describing the chemical activity of [O.sub.2] in a given system (fugacity represents a corrected partial pressure due to [...]

Accretion left the terrestrial planets depleted in volatile components. Here I examine evidence for the hypothesis that the Moon and the Earth were essentially dry immediately after the formation of the Moon--by a giant impact on the proto-Earth--and only much later gained volatiles through accretion of wet material delivered from beyond the asteroid belt. This view is supported by U-Pb and I-Xe chronologies, which show that water delivery peaked ~100 million years after the isolation of the Solar System. Introduction of water into the terrestrial mantle triggered plate tectonics, which may have been crucial for the emergence of life. This mechanism may also have worked for the young Venus, but seems to have failed for Mars., Earth, Mars and Venus are three planets with very different histories. Active plate tectonics, the presence of a liquid ocean, and teeming life characterize the Earth. Mars has a tenuous [...]

Author(s): Christine M. Meyzen (corresponding author) [1]; Janne Blichert-Toft [1]; John N. Ludden [2]; Eric Humler [3]; Catherine Mével [4]; Francis Albarède [1] Radiogenic isotope studies of Sr, Nd and [...]

The zoning of clinopyroxene and garnet from the Munchberg eclogites in Northeastern Bohemia was examined using electron probe to determine their exhumation history. Zoning patterns of eclogitic minerals were interpreted by numerical modelling of Fe-Mg exchange in both concentric and aggregate arrangements. Findings show both growth zoning in the biggest grains and a 30 micron diffusion boundary layer in garnets. A multi-stage history with initial exhumation rates of more than 5 mm yr(super -1) rapidly declining in a few million years is required in the simultaneous presence of these two zoning patterns.

Author(s): Francis Albarède (corresponding author) [1]; Janne Blichert-Toft [1]; Jeffrey D. Vervoort [2]; James D. Gleason [1]; Minik Rosing [3] Modern basalts have seemingly lost all 'memory' of the primitive [...]

The chemical isotope fractionation of Fe(III) by a solvent extraction method with a crown ether of dicyclohexano-18-crown-6 is described. It is found that the isotope fractionation between the Fe(III) species is mass dependent, which is confirmed by calculating the reduced partition function ratios.

Extraction of potassium into magmas and outgassing of argon during melting constrain the relative amounts of potassium in the crust with respect to those of argon in the atmosphere. No more than 30% of the modern mass of the continents was subducted back into the mantle during Earth's history. It is estimated that 50 to 70% of the subducted sediments are reincorporated into the deep continental crust. A consequence of the limited exchange between the continental crust and the upper mantle is that the chemistry of the upper mantle is driven by exchange of material with the deep mantle., New continental crust is extracted from the mantle by magmatic processes, whereas old crust is recycled into the mantle at subduction zones. The history of these transfers is not sufficiently [...]

Scientists have long looked at the constituent elements of meteorites to find out how old the Sun and its planets are. The most perfect example of the oldest meteorites--those that [...]

As a young postdoc at the California Institute of Technology (Caltech) in Pasadena I remember glancing through the 1952 logbook of a gas mass spectrometer while the machine readied my [...]

Author(s): Francis Albarède [1] It is thought that Earth formed, along with the rest of the Solar System, from the gravitational collapse of a huge cloud of gas. But what [...]

It was shown that the temperature determined from the rim compositions of coexisting garnet and clinopyroxene grains through Fe-MG thermometry differ significantly from that off the metamorphic climate in the eclogite facies because of retrogressive exchange by volume diffusion. The extent of the bias correlated with a number of factors. These include the peak temperature, the cooling rate, the clinopyroxene/garnet volume ratio, the median grain size, and the grain size distribution oft he minerals.

No 60Ni or 61Ni anomalies have been detected in troilite inclusions from Muonionalusta, a 4565.3 4 ± 0.1 Ma old IVA iron meteorite, to the level of the analytical precision of 10 ppm. Because Muonionalusta troilite is very old, has a high Fe/Ni ratio, and is free of 61Ni anomalies, it is the ideal material in which to search for potential excesses of 60Ni produced by the decay of 60Fe (t1/2 = 2.62 Ma). The 60Ni/58Ni and Fe/Ni ratios (up to 1680) measured here imply an upper limit for the initial 60Fe/56Fe for the Muonionalusta troilite of 3 x 10-9. Assuming that 60Fe was homogeneously distributed across the nebula, this result suggests that the solar system initial 60Fe/56Fe ratio was less than 5 x 10-9, which is lower than previously measured by at least a factor of 50. [ABSTRACT FROM AUTHOR]

We report high-precision multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) analyses of Hf isotopic compositions for 39 Kerguelen Archipelago volcanic rocks that add a new perspective to the temporal evolution of the Kerguelen mantle plume. Samples cover the entire range of chemical and Sr, Nd and Pb isotopic variations reported for the archipelago, and vary in age from 29 to 0·1 Ma. Their Hf isotopic compositions show the largest variation for an individual oceanic island and εHf correlates negatively with alkalinity. The Hf–Nd isotopic compositions of the pre-Miocene flood basalt group (>25 Ma) are distinct from those of the <10 Ma group of Upper Miocene and Quaternary Mt. Ross volcanic rocks and plot along separate en-echelon εHf–εNd arrays. The Lower Miocene Series mildly alkaline basalts from the Southeast Province are transitional between the two groups and indicate a major replacement of plume source components over a relatively short period of time. The geochemical systematics of the >25 Ma group of flood basalts require interaction between a shallow reservoir of asthenospheric mantle or oceanic lithosphere and the Kerguelen plume core, best represented by the Mt. Crozier basalts on the archipelago. This scenario is consistent with the relative proximity of the Kerguelen Archipelago to the Southeast Indian Ridge (∼300–400 km) when the >25 Ma flood basalts erupted. The younger alkaline volcanic rocks were generated by low-degree melting in the plume envelope, an area of enriched mantle material, different from the plume core, and presumably formed by different combinations of old recycled oceanic crust and plateaux. The enriched geochemical character of the Kerguelen plume core supports the hypothesis that the EM I-type plume source includes recycled oceanic plateaux. [ABSTRACT FROM PUBLISHER]

Focuses on a study which examined the potassium-argon constraints on recycling continental crust into the Earth's mantle. Model for outgassing and crustal growth; Consequence of the limited exchange between the continental crust and the upper mantle; Conclusion.