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Alex N Halliday

Publications and source records attributed to Alex N Halliday.

10 recordsLinked to original sources
PubMed2006

Iron isotope fractionation during proton-promoted, ligand-controlled, and reductive dissolution of Goethite.

Iron isotope fractionation during dissolution of goethite (alpha-FeOOH) was studied in laboratory batch experiments. Proton-promoted (HCl), ligand-controlled (oxalate dark), and reductive (oxalate light) dissolution mechanisms were compared in order to understand the behavior of iron isotopes during natural weathering reactions. Multicollector ICP-MS was used to measure iron isotope ratios of dissolved iron in solution. The influence of kinetic and equilibrium isotope fractionation during different time scales of dissolution was investigated. Proton-promoted dissolution did not cause iron isotope fractionation, concurrently demonstrating the isotopic homogeneity of the goethite substrate. In contrast, both ligand-controlled and reductive dissolution of goethite resulted in significant iron isotope fractionation. The kinetic isotope effect, which caused an enrichment of light isotopes in the early dissolved fractions, was modeled with an enrichment factor for the 57Fe/ 54Fe ratio of -2.6 per thousandth between reactive surface sites and solution. Later dissolved fractions of the ligand-controlled experiments exhibit a reverse trend with a depletion of light isotopes of approximately 0.5 per thousandth in solution. We interpret this as an equilibrium isotope effect between Fe(III)-oxalate complexes in solution and the goethite surface. In conclusion, different dissolution mechanisms cause diverse iron isotope fractionation effects and likely influence the iron isotope signature of natural soil and weathering environments.

Chemical Fractionation↗
PubMed2006

Thallium isotopic evidence for ferromanganese sediments in the mantle source of Hawaiian basalts.

Ocean island basalts are generally thought to be the surface expression of mantle plumes, but the nature of the components in the source regions of such mantle plumes is a subject of long-standing debate. The lavas erupted at Hawaii have attracted particular attention, as it has been proposed that coupled 186Os and 187Os anomalies reflect interaction with the Earth's metallic core. It has recently been suggested, however, that such variations could also result from addition of oceanic ferromanganese sediments to the mantle source of these lavas. Here we show that Hawaiian picrites with osmium isotope anomalies also exhibit pronounced thallium isotope variations, which are coupled with caesium/thallium ratios that extend to values much lower than commonly observed for mantle-derived rocks. This correlation cannot be created by admixing of core material, and is best explained by the addition of ferromanganese sediments into the Hawaii mantle source region. However, the lack of correlation between thallium and osmium isotopes and the high thallium/osmium ratios of ferromanganese sediments preclude a sedimentary origin for the osmium isotope anomalies, and leaves core-mantle interaction as a viable explanation for the osmium isotope variations of the Hawaiian picrites.

Journal Article↗
PubMed2005

Hf-W chronometry of lunar metals and the age and early differentiation of the Moon.

The use of hafnium-tungsten chronometry to date the Moon is hampered by cosmogenic tungsten-182 production mainly by neutron capture of tantalum-181 at the lunar surface. We report tungsten isotope data for lunar metals, which contain no 181Ta-derived cosmogenic 182W. The data reveal differences in indigenous 182W/184W of lunar mantle reservoirs, indicating crystallization of the lunar magma ocean 4.527 +/- 0.010 billion years ago. This age is consistent with the giant impact hypothesis and defines the completion of the major stage of Earth's accretion.

Journal Article↗
PubMed2005

Cooling of the Earth and core formation after the giant impact.

Kelvin calculated the age of the Earth to be about 24 million years by assuming conductive cooling from being fully molten to its current state. Although simplistic, his result is interesting in the context of the dramatic cooling that took place after the putative Moon-forming giant impact, which contributed the final approximately 10 per cent of the Earth's mass. The rate of accretion and core segregation on Earth as deduced from the U-Pb system is much slower than that obtained from Hf-W systematics, and implies substantial accretion after the Moon-forming impact, which occurred 45 +/- 5 Myr after the beginning of the Solar System. Here we propose an explanation for the two timescales. We suggest that the Hf-W timescale reflects the principal phase of core-formation before the giant impact. Crystallization of silicate perovskite in the lower mantle during this phase produced Fe(3+), which was released during the giant impact, and this oxidation resulted in late segregation of sulphur-rich metal into which Pb dissolved readily, setting the younger U-Pb age of the Earth. Separation of the latter metal then occurred 30 +/- 10 Myr after the Moon-forming impact. Over this time span, in surprising agreement with Kelvin's result, the Earth cooled by about 4,000 K in returning from a fully molten to a partially crystalline state.

Journal Article↗
PubMed2004

Fast delivery of meteorites to Earth after a major asteroid collision.

Very large collisions in the asteroid belt could lead temporarily to a substantial increase in the rate of impacts of meteorites on Earth. Orbital simulations predict that fragments from such events may arrive considerably faster than the typical transit times of meteorites falling today, because in some large impacts part of the debris is transferred directly into a resonant orbit with Jupiter. Such an efficient meteorite delivery track, however, has not been verified. Here we report high-sensitivity measurements of noble gases produced by cosmic rays in chromite grains from a unique suite of fossil meteorites preserved in approximately 480 million year old sediments. The transfer times deduced from the noble gases are as short as approximately 10(5) years, and they increase with stratigraphic height in agreement with the estimated duration of sedimentation. These data provide powerful evidence that this unusual meteorite occurrence was the result of a long-lasting rain of meteorites following the destruction of an asteroid, and show that at least one strong resonance in the main asteroid belt can deliver material into the inner Solar System within the short timescales suggested by dynamical models.

Journal Article↗
PubMed2004

Iron isotope fractionation and the oxygen fugacity of the mantle.

The oxygen fugacity of the mantle exerts a fundamental influence on mantle melting, volatile speciation, and the development of the atmosphere. However, its evolution through time is poorly understood. Changes in mantle oxidation state should be reflected in the Fe3+/Fe2+ of mantle minerals, and hence in stable iron isotope fractionation. Here it is shown that there are substantial (1.7 per mil) systematic variations in the iron isotope compositions (delta57/54Fe) of mantle spinels. Spinel delta57/54Fe values correlate with relative oxygen fugacity, Fe3+/sigmaFe, and chromium number, and provide a proxy of changes in mantle oxidation state, melting, and volatile recycling.

Journal Article↗
PubMed2004

Mixing, volatile loss and compositional change during impact-driven accretion of the Earth.

The degree to which efficient mixing of new material or losses of earlier accreted material to space characterize the growth of Earth-like planets is poorly constrained and probably changed with time. These processes can be studied by parallel modelling of data from different radiogenic isotope systems. The tungsten isotope composition of the silicate Earth yields a model timescale for accretion that is faster than current estimates based on terrestrial lead and xenon isotope data and strontium, tungsten and lead data for lunar samples. A probable explanation for this is that impacting core material did not always mix efficiently with the silicate portions of the Earth before being added to the Earth's core. Furthermore, tungsten and strontium isotope compositions of lunar samples provide evidence that the Moon-forming impacting protoplanet Theia was probably more like Mars, with a volatile-rich, oxidized mantle. Impact-driven erosion was probably a significant contributor to the variations in moderately volatile element abundance and oxidation found among the terrestrial planets.

Journal Article↗
PubMed2003

Origin and migration of the Alpine Iceman.

The Alpine Iceman provides a unique window into the Neolithic-Copper Age of Europe. We compared the radiogenic (strontium and lead) and stable (oxygen and carbon) isotope composition of the Iceman's teeth and bones, as well as 40Ar/39Ar mica ages from his intestine, to local geology and hydrology, and we inferred his habitat and range from childhood to adult life. The Iceman's origin can be restricted to a few valleys within approximately 60 kilometers south(east) of the discovery site. His migration during adulthood is indicated by contrasting isotopic compositions of enamel, bones, and intestinal content. This demonstrates that the Alpine valleys of central Europe were permanently inhabited during the terminal Neolithic.

Aluminum Silicates↗
PubMed2002

Niobium-zirconium chronometry and early solar system development.

Niobium-92 (92Nb) decays to zirconium-92 (92Zr) with a half-life of 36 million years and can be used to place constraints on the site of p-process nucleosynthesis and the timing of early solar system processes. Recent results have suggested that the initial 92Nb/93Nb of the solar system was high (>10(-3)). We report Nb-Zr internal isochrons for the ordinary chondrite Estacado (H6) and a clast of the mesosiderite Vaca Muerta, both of which define an initial 92Nb/93Nb ratio of approximately 10(-5). Therefore, the solar system appears to have started with a ratio of <3 x 10(-5), which implies that Earth's initial differentiation need not have been as protracted as recently suggested.

Journal Article↗