Magnetism in cold subducting slabs at mantle transition zone depths.

The Earth's crust-mantle boundary, the Mohorovicic discontinuity, has been traditionally considered to be the interface between the magnetic crust and the non-magnetic mantle1. However, this assumption has been questioned by geophysical observations2,3 and by the identification of magnetic remanence in mantle xenoliths4, which suggest mantle magnetic sources. Owing to their high critical temperatures, iron oxides are the only potential sources of magnetic anomalies at mantle depths5. Haematite (α-Fe2O3) is the dominant iron oxide in subducted lithologies at depths of 300 to 600 kilometres, delineated by the thermal decomposition of magnetite and the crystallization of a high-pressure magnetite phase deeper than about 600 kilometres6. The lack of data on the magnetic properties of haematite at relevant pressure-temperature conditions, however, hinders the identification of magnetic boundaries within the mantle and their contribution to observed magnetic anomalies. Here we apply synchrotron Mössbauer source spectroscopy in laser-heated diamond anvil cells to investigate the magnetic transitions and critical temperatures in Fe2O3 polymorphs7 at pressures and temperatures of up to 90 gigapascals and 1,300 kelvin, respectively. Our results show that haematite remains magnetic at the depth of the transition zone in the Earth's mantle in cold or very cold subduction geotherms, forming a frame of deep magnetized rocks in the West Pacific region. The deep magnetic sources spatially correlate with preferred paths of the Earth's virtual geomagnetic poles during reversals8 that might not reflect the geometry of the transitional field. Rather, the paths might be an artefact caused by magnetized haematite-bearing rocks in cold subducting slabs at mid-transition zone depths. Such deep sources should be taken into account when carrying out inversions of the Earth's geomagnetic data9, and especially in studies of planetary bodies that no longer have a dynamo10, such as Mars.

A laser heating facility for energy-dispersive X-ray absorption spectroscopy.

A double-sided laser heating setup for diamond anvil cells installed on the ID24 beamline of the ESRF is presented here. The setup geometry is specially adopted for the needs of energy-dispersive X-ray absorption spectroscopic (XAS) studies of materials under extreme pressure and temperature conditions. We illustrate the performance of the facility with a study on metallic nickel at 60 GPa. The XAS data provide the temperature of the melting onset and quantitative information on the structural parameters of the first coordination shell in the hot solid up to melting.

Behaviour of niobium during early Earth's differentiation: insights from its local structure and oxidation state in silicate melts at high pressure.

Niobium (Nb) is one of the key trace elements used to understand Earth's formation and differentiation, and is remarkable for its deficiency relative to tantalum in terrestrial rocks compared to the building chondritic blocks. In this context, the local environment of Nb in silica-rich melts and glasses is studied by in situ x-ray absorption spectroscopy (XAS) at high pressure (P) up to 9.3 GPa and 1350 K using resistive-heating diamond-anvil cells. Nb is slightly less oxidized in the melt (intermediate valence between +4 and +5) than in the glass (+5), an effect evidenced from the shift of the Nb-edge towards lower energies. Changes in the pre-edge features are also observed between melt and glass states, consistently with the observed changes in oxidation state although likely enhanced by temperature (T) effects. The oxidation state of Nb is not affected by pressure neither in the molten nor glassy states, and remains constant in the investigated P-range. The Nb-O coordination number is constant and equal to [Formula: see text] below 5 GPa, and only progressively increases up to [Formula: see text] at 9.3 GPa, the maximum P investigated. If these findings were to similarly apply to basaltic melts, that would rule out the hypothesis of Nb/Ta fractionation during early silicate Earth's differentiation, thus reinforcing the alternative hypothesis of fractionation during core formation on reduced pre-planetary bodies.

Thermal decomposition of ammonium hexachloroosmate.

Structural changes of (NH4)2[OsCl6] occurring during thermal decomposition in a reduction atmosphere have been studied in situ using combined energy-dispersive X-ray absorption spectroscopy (ED-XAFS) and powder X-ray diffraction (PXRD). According to PXRD, (NH4)2[OsCl6] transforms directly to metallic Os without the formation of any crystalline intermediates but through a plateau where no reactions occur. XANES and EXAFS data by means of Multivariate Curve Resolution (MCR) analysis show that thermal decomposition occurs with the formation of an amorphous intermediate {OsCl4}x with a possible polymeric structure. Being revealed for the first time the intermediate was subjected to determine the local atomic structure around osmium. The thermal decomposition of hexachloroosmate is much more complex and occurs within a minimum two-step process, which has never been observed before.

Nb K-edge x-ray absorption investigation of the pressure induced amorphization in A-site deficient double perovskite La1/3NbO3.

Nb K-edge x-ray absorption spectroscopy is utilized to investigate the changes in the local structure of the A-site deficient double perovskite La1/3NbO3 which undergoes a pressure induced irreversible amorphization. EXAFS results show that with increasing pressure up to 7.5 GPa, the average Nb-O bond distance decreases in agreement with the expected compression and tilting of the NbO6 octahedra. On the contrary, above 7.5 GPa, the average Nb-O bond distance show a tendency to increase. Significant changes in the Nb K-edge XANES spectrum with evident low energy shift of the pre-peak and the absorption edge is found to happen in La1/3NbO3 above 6.3 GPa. These changes evidence a gradual reduction of the Nb cations from Nb(5+) towards Nb(4+) above 6.3 GPa. Such a valence change accompanied by the elongation of the average Nb-O bond distances in the octahedra, introduces repulsion forces between non-bonding adjacent oxygen anions in the unoccupied A-sites. Above a critical pressure, the Nb reduction mechanism can no longer be sustained by the changing local structure and amorphization occurs, apparently due to the build-up of local strain. EXAFS and XANES results indicate two distinct pressure regimes having different local and electronic response in the La1/3NbO3 system before the occurence of the pressure induced amorphization at ∼14.5 GPa.

The Time-resolved and Extreme-conditions XAS (TEXAS) facility at the European Synchrotron Radiation Facility: the energy-dispersive X-ray absorption spectroscopy beamline ID24.

The European Synchrotron Radiation Facility has recently made available to the user community a facility totally dedicated to Time-resolved and Extreme-conditions X-ray Absorption Spectroscopy--TEXAS. Based on an upgrade of the former energy-dispersive XAS beamline ID24, it provides a unique experimental tool combining unprecedented brilliance (up to 10(14) photons s(-1) on a 4 µm × 4 µm FWHM spot) and detection speed for a full EXAFS spectrum (100 ps per spectrum). The science mission includes studies of processes down to the nanosecond timescale, and investigations of matter at extreme pressure (500 GPa), temperature (10000 K) and magnetic field (30 T). The core activities of the beamline are centered on new experiments dedicated to the investigation of extreme states of matter that can be maintained only for very short periods of time. Here the infrastructure, optical scheme, detection systems and sample environments used to enable the mission-critical performance are described, and examples of first results on the investigation of the electronic and local structure in melts at pressure and temperature conditions relevant to the Earth's interior and in laser-shocked matter are given.

The time-resolved and extreme conditions XAS (TEXAS) facility at the European Synchrotron Radiation Facility: the general-purpose EXAFS bending-magnet beamline BM23.

BM23 is the general-purpose EXAFS bending-magnet beamline at the ESRF, replacing the former BM29 beamline in the framework of the ESRF upgrade. Its mission is to serve the whole XAS user community by providing access to a basic service in addition to the many specialized instruments available at the ESRF. BM23 offers high signal-to-noise ratio EXAFS in a large energy range (5-75 keV), continuous energy scanning for quick-EXAFS on the second timescale and a micro-XAS station delivering a spot size of 4 µm × 4 µm FWHM. It is a user-friendly facility featuring a high degree of automation, online EXAFS data reduction and a flexible sample environment.

Local structure of solid Rb at megabar pressures.

We have investigated the local and electronic structure of solid rubidium by means of x-ray absorption spectroscopy up to 101.0 GPa, thus doubling the maximum investigated experimental pressure. This study confirms the predicted stability of phase VI and was completed by the combination of two pivotal instrumental solutions. On one side, we made use of nanocrystalline diamond anvils, which, contrary to the more commonly used single crystal diamond anvils, do not generate sharp Bragg peaks (glitches) at specific energies that spoil the weak fine structure oscillations in the x-ray absorption cross section. Second, we exploited the performance of a state-of-the-art x-ray focussing device yielding a beam spot size of 5 × 5 µm(2), spatially stable over the entire energy scan. An advanced data analysis protocol was implemented to extract the pressure dependence of the structural parameters in phase VI of solid Rb from 51.2 GPa up to the highest pressure. A continuous reduction of the nearest neighbour distances was observed, reaching about 6% over the probed pressure range. We also discuss a phenomenological model based on the Einstein approximation to describe the pressure behaviour of the mean-square relative displacement. Within this simplified scheme, we estimate the Grüneisen parameter for this high pressure Rb phase to be in the 1.3-1.5 interval.

Synchrotron radiation Mössbauer spectra of a rotating absorber with implications for testing velocity and acceleration time dilation.

Many Mössbauer spectroscopy (MS) experiments have used a rotating absorber in order to measure the second-order transverse Doppler (TD) shift, and to test the validity of the Einstein time dilation theory. From these experiments, one may also test the clock hypothesis (CH) and the time dilation caused by acceleration. In such experiments the absorption curves must be obtained, since it cannot be assumed that there is no broadening of the curve during the rotation. For technical reasons, it is very complicated to keep the balance of a fast rotating disk if there are moving parts on it. Thus, the Mössbauer source on a transducer should be outside the disk. Friedman and Nowik have already predicted that the X-ray beam finite size dramatically affects the MS absorption line and causes its broadening. We provide here explicit formulas to evaluate this broadening for a synchrotron Mössbauer source (SMS) beam. The broadening is linearly proportional to the rotation frequency and to the SMS beam width at the rotation axis. In addition, it is shown that the TD shift and the MS line broadening are affected by an additional factor assigned as the alignment shift which is proportional to the frequency of rotation and to the distance between the X-ray beam center and the rotation axis. This new shift helps to align the disk's axis of rotation to the X-ray beam's center. To minimize the broadening, one must focus the X-ray on the axis of the rotating disk and/or to add a slit positioned at the center, to block the rays distant from the rotation axis of the disk. Our experiment, using the (57)Fe SMS, currently available at the Nuclear Resonance beamline (ID18) at the ESRF, with a rotating stainless steel foil, confirmed our predictions. With a slit installed at the rotation axis (reducing the effective beam width from 15.6 µm to 5.4 µm), one can measure a statistically meaningful absorption spectrum up to 300 Hz, while, without a slit, such spectra could be obtained up to 100 Hz only. Thus, both the broadening and the alignment shift are very significant and must be taken into consideration in any rotating absorber experiment. Here a method is offered to measure accurately the TD shift and to test the CH.

Hyperfine splitting and room-temperature ferromagnetism of Ni at multimegabar pressure.

Magnetic and elastic properties of Ni metal have been studied up to 260 GPa by nuclear forward scattering of synchrotron radiation with the 67.4 keV Mössbauer transition of 61Ni. The observed magnetic hyperfine splitting confirms the ferromagnetic state of Ni up to 260 GPa, the highest pressure where magnetism in any material has been observed so far. Ab initio calculations reveal that the pressure evolution of the hyperfine field, which features a maximum in the range of 100 to 225 GPa, is a relativistic effect. The Debye energy obtained from the Lamb-Mössbauer factor increases from 33 meV at ambient pressure to 60 meV at 100 GPa. The change of this energy over volume compression is well described by a Grüneisen parameter of 2.09.

BX90: a new diamond anvil cell design for X-ray diffraction and optical measurements.

We present a new design of a universal diamond anvil cell, suitable for different kinds of experimental studies under high pressures. Main features of the cell are an ultimate 90-degrees symmetrical axial opening and high stability, making the presented cell design suitable for a whole range of techniques from optical absorption to single-crystal X-ray diffraction studies, also in combination with external resistive or double-side laser heating. Three examples of the cell applications are provided: a Brillouin scattering of neon, single-crystal X-ray diffraction of α-Cr(2)O(3), and resistivity measurements on the (Mg(0.60)Fe(0.40))(Si(0.63)Al(0.37))O(3) silicate perovskite.

Human tuberculosis caused by Mycobacterium bovis in the United States, Latin America and the Caribbean.

Human tuberculosis (TB) caused by Mycobacterium bovis appears to be rare in most of the region of the Americas, although some localities have reported an unusually high prevalence of M. bovis among human TB cases (e.g., San Diego, CA, USA; parts of Mexico). As surveillance data are lacking in many countries, there is substantial uncertainty regarding actual incidence. M. bovis is most often not identified, as the diagnosis of TB is made by smear microscopy alone or using egg-containing culture media lacking pyruvate. Where human M. bovis cases have been studied in the region, they appear to be associated with ingestion of unpasteurized dairy products, or with airborne acquired infection in animal keepers and meat industry workers from countries where bovine TB remains a problem. Human-to-human transmission of M. bovis does occur, but appears to account for a very small proportion of cases. Efforts to eradicate M. bovis in humans in the Americas should therefore be directed at eradicating the disease in cattle, increasing pasteurization of dairy products and providing education about the dangers of consuming unpasteurized dairy products.

AIDS pacient's long-term battle with multiply recurrent tuberculosis: reinfection or reactivation?

The advent of Mycobacterium tuberculosis strain genotyping has allowed differentiation between disease relapse and exogenous re-infection. We report here a remarkable case of multiply recurrent tuberculosis in a patient living with HIV. Between 1995 and 2009, a young HIV-infected intravenous drug user, who was reluctant to comply with anti-retroviral treatment, underwent at least five tuberculosis episodes caused by three distinct M. tuberculosis strains sharply differentiated by drug susceptibility profile, genotype and infectious source. Eventually, the patient died during a relapse of tuberculosis due to a notorious multidrug-resistant outbreak-strain, which infected him during a prolonged hospitalization in the epicentre of such outbreak. Whether recurrent tuberculosis is due to a new infection or to reactivation of a previous one is a century-long controversial question. In our patient, both conditions alternated throughout his 15 years of living with HIV. Cases such as this might not be exceptional in certain underprivileged suburban areas of Argentina and should raise concern over three pending issues in tuberculosis control policies, namely secondary preventing therapy, institutional infection control and patient follow-up throughout the health network system.

High-pressure behavior of perovskite: FeTiO_{3} dissociation into (Fe_{1-delta},Ti_{delta})O and Fe_{1+delta}Ti_{2-delta}O_{5}.

The stability of perovskite-structured materials at high pressure and temperature is of fundamental interest in solid-state physics, chemistry, and the geosciences. As an alternative to decomposition into oxides or transformation of the CaIrO_{3} postperovskite structure, we observe in situ the breakdown of FeTiO_{3} perovskite into a (Fe_{1-delta},Ti_{delta})O + Fe_{1+delta}Ti_{2-delta}O_{5} assemblage beyond 53 GPa and 2000 K. The high-pressure high-temperature phase of Fe_{1+delta}Ti_{2-delta}O_{5} with a new structure (space group C2/c) could be preserved on decompression to 9 GPa, and amorphizes under further pressure release. Our study demonstrates that perovskite-structured materials can undergo chemical changes and form complex oxides with new structures, rather than only transform to denser polymorphs or decompose to simple oxides.

Superhard semiconducting optically transparent high pressure phase of boron.

An orthorhombic (space group Pnnm) boron phase was synthesized at pressures above 9 GPa and high temperature, and it was demonstrated to be stable at least up to 30 GPa. The structure, determined by single-crystal x-ray diffraction, consists of B12 icosahedra and B2 dumbbells. The charge density distribution obtained from experimental data and ab initio calculations suggests covalent chemical bonding in this phase. Strong covalent interatomic interactions explain the low compressibility value (bulk modulus is K300=227 GPa) and high hardness of high-pressure boron (Vickers hardness HV=58 GPa), after diamond the second hardest elemental material.

High-pressure studies of (Mg0.9Fe0.1)2SiO4 olivine using raman spectroscopy, X-ray diffraction, and mössbauer spectroscopy.

High-pressure studies of (Mg(0.9)Fe(0.1))2SiO4 olivine were performed at ambient temperature using X-ray diffraction, Raman spectroscopy, and Mössbauer spectroscopy. At approximately 40 GPa, a change of compressibility associated with saturation of the anisotropic compression mechanism was detected. This change is interpreted to result from the appearance of Si2O7 dimer defects, as deduced from Raman spectroscopy; the appearance of such defects also accounts for the previously reported pressure-induced amorphization observed for this material upon additional compression. Furthermore, this behavior is followed by a spin crossover of Fe(2+) that occurs over a wide pressure range, as revealed by Mössbauer spectroscopy.

Body-centered cubic iron-nickel alloy in Earth's core.

Cosmochemical, geochemical, and geophysical studies provide evidence that Earth's core contains iron with substantial (5 to 15%) amounts of nickel. The iron-nickel alloy Fe(0.9)Ni(0.1) has been studied in situ by means of angle-dispersive x-ray diffraction in internally heated diamond anvil cells (DACs), and its resistance has been measured as a function of pressure and temperature. At pressures above 225 gigapascals and temperatures over 3400 kelvin, Fe(0.9)Ni(0.1) adopts a body-centered cubic structure. Our experimental and theoretical results not only support the interpretation of shockwave data on pure iron as showing a solid-solid phase transition above about 200 gigapascals, but also suggest that iron alloys with geochemically reasonable compositions (that is, with substantial nickel, sulfur, or silicon content) adopt the bcc structure in Earth's inner core.

Beating the miscibility barrier between iron group elements and magnesium by high-pressure alloying.

Iron and magnesium are almost immiscible at ambient pressure. The low solubility of Mg in Fe is due to a very large size mismatch between the alloy components. However, the compressibility of Mg is much higher than that of Fe, and therefore the difference in atomic sizes between elements decreases dramatically with pressure. Based on the predictions of ab initio calculations, we demonstrate in a series of experiments in a multianvil apparatus and in electrically and laser-heated diamond anvil cells that high pressure promotes solubility of magnesium in iron. At the megabar pressure range, more than 10 at. % of Mg can dissolve in Fe and then the alloy can be quenched to ambient conditions. A generality of the concept of high-pressure alloying between immiscible elements is demonstrated by its application to two other Fe group elements, Co and Ni.