Iron oxidation state in the Fe‐rich layer and silica matrix of Libyan Desert Glass: A high‐resolution XANES study

Abstract— Libyan Desert Glass (LDG) is an enigmatic type of glass that occurs in western Egypt in the Libyan Desert. Fairly convincing evidence exists to show that it formed by impact, although the source crater is currently unknown. Some rare samples present dark‐colored streaks with variable amounts of Fe, and they are supposed to contain a meteoritic component. We have studied the iron local environment in an LDG sample by means of Fe K‐edge highresolution X‐ray absorption near edge structure (XANES) spectroscopy to obtain quantitative data on the Fe oxidation state and coordination number in both the Fe‐poor matrix and Fe‐rich layers. The pre‐edge peak of the high‐resolution XANES spectra of the sample studied displays small but reproducible variations between Fe‐poor matrix and Fe‐rich layers, which is indicative of significant changes in the Fe oxidation state and coordination number. Comparison with previously obtained data for a very low‐Fe sample shows that, while iron is virtually all trivalent and in tetrahedral coordination (^[4]Fe³⁺) in the low‐Fe sample, the sample containing the Fe‐rich layers display a mixture of tetra‐coordinated trivalent iron (^[4]Fe³⁺) and penta‐coordinated divalent iron (^[5]Fe²⁺), with the Fe in the Fe‐rich layer being more reduced than the matrix. From these data, we conclude the following: a) the significant differences in the Fe oxidation state between LDG and tektites, together with the wide intra‐sample variations in the Fe‐oxidation state, confirm that LDG is an impact glass and not a tektite‐like glass; b) the higher Fe content, coupled with the more reduced state of the Fe, in the Fe‐rich layers suggests that some or most of the Fe in these layers may be directly derived from the meteoritic projectile and that it is not of terrestrial origin.     

Mantle peridotites from the Bouvet Triple Junction Region, South Atlantic

ABSTRACT The Bouvet Triple Junction (BTJ) region in the South Atlantic, where the African, South American and Antarctica plates meet, is affected by several topographic/melting anomalies. Causes of these anomalies were investigated through a study of mantle-derived serpentinized peridotites sampled from three sites in the BTJ region: (1) the Inner Corner High at the intersection of the America Antarctic Ridge (AAR) with the Conrad transform; (2) the south wall of the Bouvet transform (South West Indian Ridge, SWIR); and (3) the eastern Bouvet SWIR Transform Intersection. The degree of melting undergone by these rocks was estimated from relic mineral major- and trace-element composition. Geochemical profiles from residual peridotites and associated basalts show a > 1000-km-wide melting anomaly centred on the Bouvet and Spiess topographic anomalies.     

The amphipod Talitrus saltator as a bioindicator of human trampling on sandy beaches

The present paper assesses the use of the supralittoral amphipod Talitrus saltator as a bioindicator of the effects of human trampling on the supralittoral sandy band. Samplings in delimited areas were carried out at sites subjected to different human impact. The results showed a strong negative correlation between the number of swimmers and the sandhopper population density, while there was no clear relationship between sandhopper abundance and the other factors considered: granulometry, compactness and organic carbon content of the sand, and trace metal contents in the sand and sandhoppers. A field test of trampling conducted in a confined space showed its direct negative effect on sandhopper survival. However, trace metal analysis confirmed the ability of T. saltator to bioaccumulate some elements (Hg, Zn, Cu, Cd). Our study demonstrates that T. saltator is a good bioindicator of human impact in the supralittoral zone of sandy shores.     

Mn distribution in sphalerite: an EPR study

A study on Zn replacement in sphalerite by divalent Mn has been performed by using EPR spectroscopy. Twelve natural and synthetic samples with different Mn-contents were selected for the spectroscopic investigation. The EPR spectra vary from an hyperfine sextet to a single broad line according to their Mn-content. Three components, attributed to Mn(II) in different chemical coordination (namely isolated ions, interacting ions, clusters) have been determined by means of spectral simulation. The variations of the EPR signals among the different spectra are due only to the mixing of these components, without any variation of each spectral parameter. Differing distribution of Mn(II) was, therefore, observed at a local scale. Consequently, the specific interactions driving the coalescence of Mn(II) ions to form clusters may be responsible for the banded pattern in sphalerite crystals.     

Middle and late Pleistocene glaciations in the Campo Felice Basin (central Apennines, Italy)

The present paper refers to research conducted in the tectonic-karst depression of Campo Felice in the central Apennines (Italy), in which glacial, alluvial and lacustrine sediments have been preserved. Stratigraphic interpretations of sediments underlying the Campo Felice Plain are based on evidence obtained from nine continuous-core boreholes. The boreholes reach a depth of 120 m and provide evidence of five sedimentation cycles separated by erosion surfaces. Each cycle is interpreted as an initial response to a mainly warm stage, characterized by low-energy alluvial and colluvial deposition, pedogenesis, and limited episodes of marsh formation. In turn, a mainly cold stage follows during which a lake formed, and glaciers developed and expanded, leading to deposition of glacial and fluvioglacial deposits. The chronological framework is established by eleven accelerator mass spectrometer ¹⁴C ages and three ³⁹Ar-⁴⁰Ar ages on leucites from interbedded tephra layers. These age determinations indicate five glacial advances that respectively occurred during marine oxygen isotope stages 2, 3-4, 6, 10 and 14.     

Flexural uplift of a lithospheric slab near the Vema transform (Central Atlantic): Timing and mechanisms

The Vema Transverse Ridge (VTR) is a prominent, long and narrow topographic anomaly that runs for over 300 km along a sea floor spreading flow line south of the Vema transform at 11° N in the Atlantic. It rises abruptly about 140 km from the axis of the Mid-Atlantic Ridge (MAR) in 10 Myr old crust and runs continuously up to 25 Myr old crust. It reaches over 3 km above the predicted lithospheric thermal contraction level. It is absent in crust younger than 10 Myr; thus, the uplift of the VTR must have ended roughly 10 Ma. The VTR is interpreted as the exposed edge of a flexured and uplifted slab of oceanic lithosphere that was generated at an 80 km long MAR segment. Based on satellite gravimetry imagery this MAR segment was born roughly 50 Ma and increased its length at an average rate of 1.6 mm/yr. Multibeam data show that the MAR-parallel sea floor fabric south of the VTR shifts its orientation by 5° to 10° clockwise in 11–12 Myr old crust, indicating a change at that time of the orientation of the MAR axis and of the position of the Euler rotation pole. This change caused extension normal to the transform, followed between 12 and 10 Ma by flexure of the edge of the lithospheric slab, uplift of the VTR at a rate of 2 to 4 mm/yr, and exposure of a lithospheric section (Vema Lithospheric Section or VLS) at the northern edge of the slab, parallel to the Vema transform. Ages of pelagic carbonates encrusting ultramafic rocks sampled at the base of the VLS at different distances from the MAR axis suggest that the entire VTR rose vertically as a single block within the active transform offset. A 50 km long portion of the crest of the VTR rose above sea level, subsided, was truncated at sea level and covered by a carbonate platform. Subaerial and submarine erosion has gradually removed material from the top of the VTR and has modified its slopes. Spreading half rate of the crust south of the transform decreased from 17.2 mm/yr between 26 and 19 Ma to 16.9 mm/yr between 19 and 10 Ma, to 13.6 mm/yr from 10 Ma to present. The slowing down of spreading occurred close in time to the change in ridge/transform geometry, suggesting that the two events are related. A numerical model relates lithospheric flexure to extension normal to the transform, suggesting that the extent of the uplift depends on the thickness of the brittle layer, consistent with the observed greater uplift of the older lithosphere along the VTR.     

Electron paramagnetic resonance and electron spin echo spectroscopy study of natural bornite

Summary A minero-chemical and spectroscopic characterisation has been performed on a natural bornite sample from the Natural History Museum of the University of Florence, through the use of EPMA, XRD, continuous-wave (cw) and pulsed EPR spectroscopy. The use of different EPR techniques allowed to fully investigate the distribution and valence states of Cu and Fe and to study the early oxidation products of bornite. The microanalytical and XRD characterisation proved bornite to be homogeneous and stoichiometric. Cw-EPR measurements confirmed the presence of Fe(III) as fundamental valence state in bornite; moreover, no evidence of Cu(II) as a bulk transient species, due to a charge transfer process, has been revealed. The broadening of the Fe(III) EPR spectrum is ascribed to a partial transfer of spin density from Fe(III) to the surrounding cationic sublattice. In contrast, the pulsed EPR experiments, registered at 4.2 K, revealed the presence of both Fe(III) and Cu(II). These species were supposed to be present at the early stage of the surface alteration. The data provided by the electron spin-echo decay and modulation suggest both the oxidised Cu(II) and Fe(III) species to belong to premature surface water adducts (evidenced by proton nuclear modulation), which may evolve to sulphate species.     

Natural Fe-oxide and -oxyhydroxide nanoparticles: an EPR and SQUID investigation

Summary Magnetic hematite (α-Fe₂O₃) and goethite (α-FeOOH) nanoparticles, formed in the abandoned Libiola Mine (Liguria, Italy) as a result of Acid Mine Drainage (AMD) processes that involved sulfide-bearing rocks and sulfide-rich waste material, were investigated through the study of their magnetic properties in combination with electron paramagnetic resonance (EPR) spectroscopy. Both techniques revealed that the behaviour of the nanominerals involved was predominantly superparamagnetic. In contrast, the bulk antiferromagnetic behaviour of both hematite and goethite was substantially reduced. The observed magnetic properties reflected a great variability in the particle dimensions and confirmed that both nanominerals were the transformation products of a metastable amorphous species. Moreover, two peculiar phenomena were hypothesised: the transformation to hematite may occur before the coarsening of the amorphous transient and hematite and goethite can interconvert without intermediate crystallisation of metastable precursors. Goethite, in fact, appears to be formed as an alteration product of hematite, after a drastic change of the physico-chemical boundary conditions.     

On the clustering phase transition in self-gravitating N-body systems

The thermodynamic behaviour of self-gravitating N -body systems has been worked out by borrowing a standard method from molecular dynamics. The link between dynamics and thermodynamics is made in the microcanonical ensemble of statistical mechanics. Through the computation of basic thermodynamic observables and of the equation of state in the     plane, the clustering phase transition appears to be of the second-order type. The dynamical–microcanonical averages are compared with their corresponding canonical ensemble averages, obtained through standard Monte Carlo computations. The latter seem to have completely lost any information about the phase transition. Finally, our results – obtained in a 'microscopic' framework – are compared with some existing theoretical predictions – obtained in a 'macroscopic' (thermodynamic) framework: qualitative and quantitative agreement is found, with an interesting exception.