Paleomagnetism indicates that primary magnetite in zircon records a strong Hadean geodynamo.

Determining the age of the geomagnetic field is of paramount importance for understanding the evolution of the planet because the field shields the atmosphere from erosion by the solar wind. The absence or presence of the geomagnetic field also provides a unique gauge of early core conditions. Evidence for a geomagnetic field 4.2 billion-year (Gy) old, just a few hundred million years after the lunar-forming giant impact, has come from paleomagnetic analyses of zircons of the Jack Hills (Western Australia). Herein, we provide new paleomagnetic and electron microscope analyses that attest to the presence of a primary magnetic remanence carried by magnetite in these zircons and new geochemical data indicating that select Hadean zircons have escaped magnetic resetting since their formation. New paleointensity and Pb-Pb radiometric age data from additional zircons meeting robust selection criteria provide further evidence for the fidelity of the magnetic record and suggest a period of high geomagnetic field strength at 4.1 to 4.0 billion years ago (Ga) that may represent efficient convection related to chemical precipitation in Earth's Hadean liquid iron core.

A simplified silver phosphate extraction method for oxygen isotope analysis of bioapatite.

RATIONALE: Although phosphatic materials are chemically complex and are prone to exchange oxygen isotopes with their environments, the phosphate (PO43- ) component of these materials is robust and retains its original oxygen isotopic composition. As a result, there are currently several methods for the isolation of phosphate oxygen through the precipitation of silver phosphate (Ag3 PO4 ). However, some of these techniques produce Ag3 PO4 of questionable purity, while nearly all are lengthy and/or require relatively large sample sizes. METHODS: Five milligrams of bioapatite from modern cow teeth (enamel and cementum) were pre-treated for removal of organic material prior to digestion in 2 M HF. The digested samples were titrated with silver ammine solution at 50°C to precipitate Ag3 PO4 . Oxygen isotopic data were collected using a Thermal Combustion Elemental Analyzer (TC/EA) paired with a Delta VPlus isotope ratio mass spectrometer via a ConFlo III universal interface. RESULTS: The quality of Ag3 PO4 is dependent on effective removal of organic material and the volume of silver ammine solution used during titration. A two-step pre-treatment of 2.5% NaOCl, followed by a 0.125 M NaOH solution, is the most effective treatment for the removal of organic material from both enamel and cementum. Optimal yields of Ag3 PO4 were achieved using 1.8 mL of silver ammine solution. The reproducibility of the phosphate δ18 O compositions ranges from 0.3 to 0.4‰ (1σ) for modern cow teeth. CONCLUSIONS: We present a simplified method for phosphate extraction from organic-rich phosphatic material. Our method gave reproducible δ18 O values for enamel and cementum from cows' teeth.

Mass bias corrections for U-Pb isotopic analysis by secondary ion mass spectrometry: Implications for U-Pb dating of uraninite.

RATIONALE: Uranium (U)-lead (Pb) geochronology of uraninite is critical to the study of the genesis of U deposits throughout the world. Previous attempts at developing a technique to date uraninite using secondary ion mass spectrometry (SIMS) have, however, had limited success. Improper correction of mass bias results in incorrect reported U-Pb ratios. When these results are plotted on Concordia diagrams they produce erroneous ages, complicating the study of U deposits. METHODS: Uranium and Pb isotope ratios were measured in three uraninite reference materials (RMs) with varying Pb content and three samples with unknown U-Pb isotope compositions using a CAMECA 7f SIMS instrument. Measurements were made using a primary beam of O(-) accelerated at 12.5 kV. A mass resolving power of 1300 and a 50-V offset were used to minimize interferences. RESULTS: The study demonstrates that the mass bias for U-Pb isotope ratio measurements in uraninite by SIMS varies as a function of Pb content. A three-point calibration curve was developed using uraninite RMs with low-, intermediate- and high-Pb contents. Corrected ratios for both concordant and discordant uraninite were plotted on a Concordia diagram to demonstrate the effect that different correction techniques have on the resulting age. CONCLUSIONS: Accurate determination of U-Pb ratios in uraninite using a SIMS instrument requires a suite of RMs with varying Pb content and the construction of a calibration curve, or a uraninite RM with a Pb content similar to that of the unknowns. Failure to standardize correctly will result in erroneous ages being calculated using Concordia diagrams. Copyright © 2016 John Wiley & Sons, Ltd.

Uranium association with iron-bearing phases in mill tailings from Gunnar, Canada.

The speciation of uranium was studied in the mill tailings of the Gunnar uranium mine (Saskatchewan, Canada), which operated in the 1950s and 1960s. The nature, quantification, and spatial distribution of uranium-bearing phases were investigated by chemical and mineralogical analyses, fission track mapping, electron microscopy, and X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies at the U LIII-edge and Fe K-edge. In addition to uranium-containing phases from the ore, uranium is mostly associated with iron-bearing minerals in all tailing sites. XANES and EXAFS data and transmission electron microscopy analyses of the samples with the highest uranium concentrations (∼400-700 mg kg(-1) of U) demonstrate that uranium primarily occurs as monomeric uranyl ions (UO2(2+)), forming inner-sphere surface complexes bound to ferrihydrite (50-70% of the total U) and to a lesser extent to chlorite (30-40% of the total U). Thus, the stability and mobility of uranium at the Gunnar site are mainly influenced by sorption/desorption processes. In this context, acidic pH or alkaline pH with the presence of UO2(2+)- and/or Fe(3+)-complexing agents (e.g., carbonate) could potentially solubilize U in the tailings pore waters.

Theoretical study of the reduction of uranium(VI) aquo complexes on titania particles and by alcohols.

To provide insights into the adsorption and photoreduction of uranium(VI) on TiO(2), we have studied the structural and electronic properties of uranium(VI) aquo complexes adsorbed on stoichiometric and defected TiO(2) surfaces and nanoparticles. Plane wave calculations with the pure PBE density functional and the PBE+U approach were used to study U(VI) complexes on a periodic rutile (110) slab. In addition, a nanoparticulate Ti(38)O(76) cluster was used to simulate anatase nanoparticles. The electronic structures of the adsorbed U(VI) complexes indicate that the photoreduction process is a consequence of the photocatalytic properties of TiO(2). The reduction of the adsorbed complexes can only occur if the energy of the incident photon exceeds the semiconductor band gap. The gap states induced by single or neighboring hydrogen atoms and oxygen vacancies at the rutile (110) surface cannot reduce adsorbed U(VI) complexes as the unoccupied 5f  orbitals are found deeper in the conduction band. In the absence of a solid substrate, photoreduction proceeds by abstraction of a hydrogen atom from water or organic molecules present in solution. Photoreduction by chlorophenol results in lower product yield than reduction by aliphatic alcohols. This is because the triplet uranyl-chlorophenol complex is much more stable than similar complexes formed with methanol and ethanol. In the case of water, the hydroxyl photoproduct easily re-oxidizes the pentavalent species formed. In addition, it is easier for the triplet uranyl-water complex to decompose to the photoreactants.

Rapid Diffusion and Nanosegregation of Hydrogen in Magnesium Alloys from Exposure to Water.

Hydrogen gas is formed when Mg corrodes in water; however, the manner and extent to which the hydrogen may also enter the Mg metal is poorly understood. Such knowledge is critical as stress corrosion cracking (SCC)/embrittlement phenomena limit many otherwise promising structural and functional uses of Mg. Here, we report via D2O/D isotopic tracer and H2O exposures with characterization by secondary ion mass spectrometry, inelastic neutron scattering vibrational spectrometry, electron microscopy, and atom probe tomography techniques direct evidence that hydrogen rapidly penetrated tens of micrometers into Mg metal after only 4 h of exposure to water at room temperature. Further, technologically important microalloying additions of <1 wt % Zr and Nd used to improve the manufacturability and mechanical properties of Mg significantly increased the extent of hydrogen ingress, whereas Al additions in the 2-3 wt % range did not. Segregation of hydrogen species was observed at regions of high Mg/Zr/Nd nanoprecipitate density and at Mg(Zr) metastable solid solution microstructural features. We also report evidence that this ingressed hydrogen was unexpectedly present in the alloy as nanoconfined, molecular H2. These new insights provide a basis for strategies to design Mg alloys to resist SCC in aqueous environments as well as potentially impact functional uses such as hydrogen storage where increased hydrogen uptake is desired.