Factors Controlling Chemistry of Magmatic Spinel: an Empirical Study of Associated Olivine, Cr-spinel and Melt Inclusions from Primitive Rocks

Compositions of     

Computer simulation of electron transfer at hematite surfaces

Molecular dynamics simulations in combination with ab initio calculations were carried out to determine the rate of electron transfer at room temperature in bulk hematite (α-Fe₂O₃) and at two low-index surfaces, namely the (012) and (001) surfaces. The electron transfer reactions considered here involve the II/III valence interchange between nearest-neighbor iron atoms. Two electron transfer directions were investigated, namely the basal plane and c direction electron transfers. Electron transfer rates obtained in bulk hematite were in good agreement with ab initio electronic structure calculations thus validating the potential model. The surfaces were considered both in vacuum and in contact with an equilibrated aqueous solution. The reorganization energy is found to increase significantly at the first surface layer and this value is little affected by the presence of water. In addition, in the case of the (012) surface, the electronic coupling matrix element for the topmost basal plane transfer was calculated at the Hartree-Fock level and was found to be weak compared to the corresponding electron transfer in the bulk. Therefore, most surfaces show a decrease in the rate of electron transfer at the surface. However, where iron atoms involved in the electron transfer reaction are directly coordinated to water molecules, water lowers the free energy of activation to a great extent and provides a large driving force for electrons to diffuse toward the bulk thus opposing the intrinsic surface effect. The surfaces considered in this work show different electron transfer properties. Hematite has been shown to exhibit anisotropic conductivity and thus different surfaces will show different intra- and inter-layer rates depending on their orientation. Moreover, the calculations of electron transfers at the hydroxyl- and iron-terminated (001) surfaces revealed that surface termination has a significant effect on the electron transfer parameters in the vicinity of the surface. Finally, our findings indicate that undercoordinated terminal iron atoms could act as electron traps at the surface.     

Biological and Temporal Variations of Trace Element Concentrations in the Moss Species Scleropodium purum (Hedw.) Limpr.

Over a period of one year, the moss Scleropodium purum was sampled every two weeks in a French rural area to determine the levels of Li, Na, Al, Si, P, Ca, V, Mn, Fe, Zn, Ba, Hg, and Pb. The element distribution in the moss shoot was studied throughout the year. An apical bioconcentration was discovered for Na, P, Ca, Mn, and Zn, whereas higher levels were found in the basal fraction for Li, Al, Si, V, Fe, Ba, Hg, and Pb. A significant variation of element concentrations was observed during the sampling period. In the apical part Li, Al, Si, V, Fe, Ba, and Hg show maximum levels in the summer and minimum in the autumn. The same pattern was found with Ca and Mn in the whole plant, whereas Na showed opposite fluctuations.     

Ozone abundance on Mars from infrared heterodyne spectra: II. Validating photochemical models

Ozone is an important observable tracer of martian photochemistry, including odd hydrogen (HO_x) species important to the chemistry and stability of the martian atmosphere. Infrared heterodyne spectroscopy with spectral resolution ?¹⁰⁶ provides the only ground-based direct access to ozone absorption features in the martian atmosphere. Ozone abundances were measured with the Goddard Infrared Heterodyne Spectrometer and the Heterodyne Instrument for Planetary Wind and Composition at the NASA Infrared Telescope Facility on Mauna Kea, Hawai'i. Retrieved total ozone column abundances from various latitudes and orbital positions (LS=40°, 74°, 102°, 115°, 202°, 208°, 291°) are compared to those predicted by the first three-dimensional gas phase photochemical model of the martian atmosphere [Lefèvre, F., Lebonnois, S., Montmessin, F., Forget, F., 2004. J. Geophys. Res. 109, doi:10.1029/2004JE002268. E07004]. Observed and modeled ozone abundances show good agreement at all latitudes at perihelion orbital positions (LS=202°, 208°, 291°). Observed low-latitude ozone abundances are significantly higher than those predicted by the model at aphelion orbital positions (LS=40°, 74°, 115°). Heterogeneous loss of odd hydrogen onto water ice cloud particles would explain the discrepancy, as clouds are observed at low latitudes around aphelion on Mars.     

In-flight calibration of the Cassini imaging science sub-system cameras

We describe in-flight calibration of the Cassini Imaging Science Sub-system narrow- and wide-angle cameras using data from 2004 to 2009. We report on the photometric performance of the cameras including the use of polarization filters, point spread functions over a dynamic range greater than 10⁷, gain and loss of hot pixels, changes in flat fields, and an analysis of charge transfer efficiency. Hot pixel behavior is more complicated than can be understood by a process of activation by cosmic ray damage and deactivation by annealing. Point spread function (PSF) analysis revealed a ghost feature associated with the narrow-angle camera Green filter. More generally, the observed PSFs do not fall off with distance as rapidly as expected if diffraction were the primary contributor. Stray light produces significant signal far from the center of the PSF. Our photometric analysis made use of calibrated spectra from eighteen stars and the spectral shape of the satellite Enceladus. The analysis revealed a shutter offset that differed from pre-launch calibration. It affects the shortest exposures. Star photometry results are reproducible to a few percent in most filters. No degradation in charge transfer efficiency has been detected although uncertainties are large. The results of this work have been digitally archived and incorporated into our calibration software CISSCAL available online.     

Central Mediterranean tephrochronology between 313 and 366 ka: New insights from the Fucino palaeolake sediment succession

Thirty-two tephra layers were identified in the time-interval 313–366 ka (Marine Isotope Stages 9–10) of the Quaternary lacustrine succession of the Fucino Basin, central Italy. Twenty-seven of these tephra layers yielded suitable geochemical material to explore their volcanic origins. Investigations also included the acquisition of geochemical data of some relevant, chronologically compatible proximal units from Italian volcanoes. The record contains tephra from some well-known eruptions and eruptive sequences of Roman and Roccamonfina volcanoes, such as the Magliano Romano Plinian Fall, the Orvieto–Bagnoregio Ignimbrite, the Lower White Trachytic Tuff and the Brown Leucitic Tuff. In addition, the record documents eruptions currently undescribed in proximal (i.e. near-vent) sections, suggesting a more complex history of the major eruptions of the Colli Albani, Sabatini, Vulsini and Roccamonfina volcanoes between 313 and 366 ka. Six of the investigated tephra layers were directly dated by single-crystal-fusion ⁴⁰Ar/³⁹Ar dating, providing the basis for a Bayesian age–depth model and a reassessment of the chronologies for both already known and dated eruptive units and for so far undated eruptions. The results provide a significant contribution for improving knowledge on the peri-Tyrrhenian explosive activity as well as for extending the Mediterranean tephrostratigraphical framework, which was previously based on limited proximal and distal archives for that time interval.     

The key role of mica during igneous concentration of tantalum

Igneous rocks with high Ta concentrations share a number of similarities such as high Ta/Nb, low Ti, LREE and Zr concentrations and granitic compositions. These features can be traced through fractionated granitic series. Formation of Ta-rich melts begins with anatexis in the presence of residual biotite, followed by magmatic crystallization of biotite and muscovite. Crystallization of biotite and muscovite increases Ta/Nb and reduces the Ti content of the melt. Titanium-bearing oxides such as rutile and titanite are enriched in Ta and have the potential to deplete Ta at early stages of fractionation. However, mica crystallization suppresses their saturation and allows Ta to increase in the melt. Saturation with respect to Ta and Nb minerals occurs at the latest stages of magmatic crystallization, and columbite can originate from recrystallization of mica. We propose a model for prediction of intrusion fertility for Ta.     

Structure, charge distribution, and electron hopping dynamics in magnetite (Fe3O4) (1 0 0) surfaces from first principles

For the purpose of improving fundamental understanding of the redox reactivity of magnetite, quantum-mechanical calculations were applied to predict Fe²⁺ availability and electron hopping rates at magnetite (1 0 0) surfaces, with and without the presence of adsorbed water. Using a low free energy surface reconstruction (½-Fe_tet layer relaxed into the Fe_oct (1 0 0) plane), the relaxed outermost layer of both the hydrated and vacuum-terminated surfaces were found to be predominantly enriched in Fe²⁺ within the octahedral sublattice, irrespective of the presence of adsorbed water. At room temperature, mobile electrons move through the octahedral sublattice by Fe²⁺-Fe³⁺ valence interchange small polaron hopping, calculated at 10¹⁰-10¹² hops/s for bulk and bulk-like (i.e., near-surface) environments. This process is envisioned to control sustainable overall rates of interfacial redox reactions. These rates decrease by up to three orders of magnitude (10⁹ hops/s) at the (1 0 0) surface, and no significant difference is observed for vacuum-terminated versus hydrated cases. Slower hopping rates at the surface appear to arise primarily from larger reorganization energies associated with octahedral Fe²⁺-Fe³⁺ valence interchange in relaxed surface configurations, and secondarily on local charge distribution patterns surrounding Fe²⁺-Fe³⁺ valence interchange pairs. These results suggest that, with respect to the possibility that the rate and extent of surface redox reactions depend on Fe²⁺ availability and its replenishment rate, bulk electron hopping mobility is an upper-limit for magnetite and slower surface rates may need to be considered as potentially rate-limiting. They also suggest that slower hopping mobilities calculated for surface environments may be amenable to Fe²⁺-Fe³⁺ site discrimination by conventional spectroscopic probes.     

The COVID-19 pandemic and global environmental change: Emerging research needs

The outbreak of COVID-19 raised numerous questions on the interactions between the occurrence of new infections, the environment, climate and health. The European Union requested the H2020 HERA project which aims at setting priorities in research on environment, climate and health, to identify relevant research needs regarding Covid-19. The emergence and spread of SARS-CoV-2 appears to be related to urbanization, habitat destruction, live animal trade, intensive livestock farming and global travel. The contribution of climate and air pollution requires additional studies. Importantly, the severity of COVID-19 depends on the interactions between the viral infection, ageing and chronic diseases such as metabolic, respiratory and cardiovascular diseases and obesity which are themselves influenced by environmental stressors. The mechanisms of these interactions deserve additional scrutiny. Both the pandemic and the social response to the disease have elicited an array of behavioural and societal changes that may remain long after the pandemic and that may have long term health effects including on mental health. Recovery plans are currently being discussed or implemented and the environmental and health impacts of those plans are not clearly foreseen. Clearly, COVID-19 will have a long-lasting impact on the environmental health field and will open new research perspectives and policy needs.