Manuel Muñoz

Energy-dispersive absorption spectroscopy for hard-X-ray micro-XAS applications.

Originally developed for time-resolved X-ray absorption spectroscopy (XAS), energy-dispersive absorption spectroscopy offers new opportunities for applications such as fluorescence detection and microbeams for scanning probe spectroscopy, thanks to recent developments in both instrumentation and optics. In this context, this paper presents a first example of chemical mapping recorded at ID24, the energy-dispersive XAS beamline at the ESRF. Attributes of this geometry for microanalysis are addressed. Finally, present and future plans are discussed and developed in the light of the evolution of the focal spot on this instrument in the past ten years.

Occurrence, composition and growth of polyhedral serpentine

Occurrences, compositions and crystallization of polyhedral serpentine were investigated by SEM, AEM/TEM and μ-XANES analysis of samples from different ultramafic units. Polyhedral serpentines are identified in all of these contexts and form as an alteration product of orthopyroxene (enstatite) and as late veining events. They are always the last serpentine type to crystallize. Their formation requires a combination of three factors: 1) open space, 2) a relatively low temperature ( T < 200–300 °C), and 3) the presence of trivalent cations (Al3+ in this study, ⩾ 0.1 atoms per serpentine formula unit). μ-XANES data at the iron K -edge indicate that Fe is predominantly ferrous and octahedrally coordinated in our Al-rich samples. This microstructure therefore cannot be systematically used as a marker of oxidizing conditions. Textural and microstructural criteria suggest that polyhedral serpentine crystallizes via a “gel” precursor first reorganized into a poorly-crystallized proto-serpentine, in which onion nucleation takes place as nested, discontinuous sheets. Grains expand radially, inwards and outwards, by a layer-by-layer mechanism. Thick layers, made of tens of serpentine sheets, propagate laterally in the (001) plane and result in a pseudo-spherical “onion-like” morphology. By analogy with available clay synthesis experiments, the relatively low temperature conditions under which polyhedral serpentine form may favor a segregation of trivalent cations in the structure. This could create locally dioctahedral components in the structure that may explain the peculiar bending along 〈010〉 responsible for the faceted morphology of polyhedral serpentine.

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.

Hyperspectral μ-XANES mapping in the diamond-anvil cell: analytical procedure applied to the decomposition of (Mg,Fe)-ringwoodite at the upper/lower mantle boundary

We propose a new analytical approach to extract information from μ-X-ray Absorption Spectroscopy (XAS) based mapping techniques, where each pixel of a map contains full XANES (X-ray Absorption Near Edge Structure) information. Data reduction is performed thanks to specifically developed software, XASMAP, which allows automatic normalization, linear combination fits, map reconstruction and spectrum extraction. We illustrate an example of application on data acquired in a laser heated diamond-anvil cell, devoted to the investigation of the behavior of iron during the decomposition of (Mg,Fe)-ringwoodite into perovskite and ferropericlase at conditions relevant to Earth upper/lower mantle boundary. The analysis of 1600 Fe K-edge XANES spectra allows the reconstruction of maps based on iron-speciation, but it also drives to iron-concentration maps in a complex mixture of three crystalline phases. This analytical procedure opens the way to in situ studies at extreme conditions of pressure and temperature for the geosciences, physics and chemistry communities.

Development of micro-XANES mapping in the diamond anvil cell.

Energy-dispersive X-ray absorption spectroscopy is now a well established method that has been applied to a broad range of applications. At the energy-dispersive EXAFS beamline of the ESRF, ID24, the recently achieved 5 x 5 microm focal spot combined with fast acquisition has allowed complex and non-uniform samples to be mapped and images to be obtained where each pixel contains full XAS information. This method has been applied to a study under extreme conditions of pressure and temperature in a diamond anvil cell in transmission mode. The case study was the investigation of the Fe K-edge XANES of (Mg,Fe)SiO(3)-perovskite and (Mg,Fe)O-ferropericlase on decomposition of the spinel-structured olivine [gamma-(Mg,Fe)(2)SiO(4)] at 78 (3) GPa after laser heating at 2200 (100) K.

Earliest microbial trace fossils in Archaean pillow lavas under scrutiny: new micro-X-ray absorption near-edge spectroscopy, metamorphic and morphological constraints

Abstract Filament-shaped titanite microtextures in early Archaean (c. 3.47 Ga) metabasalts from the Barberton Greenstone Belt of South Africa have been argued to represent Earths oldest microbial trace fossils, but petrological data on the environmental conditions that led to the formation of these microtextures are sparse. We present here new metamorphic constraints on the equilibrium mineral assemblage containing the titanite microtextures, together with Fe speciation redox variations in the chlorite matrix surrounding the microtextures and morphospace analysis of the microtextures to test their biogenicity. Thermodynamic phase diagram modelling using a calculated rock microdomain composition indicated that the titanite mineral assemblage is stable at temperature conditions of T=240–360°C. The high-resolution quantitative mapping results combined with micro-X-ray absorption near-edge spectroscopy at the Fe K-edge in chlorite revealed that the titanite microtextures are located in low-temperature, high XFe3+ chlorite bands, veins and microdomains, supporting an origin as abiotic, retrograde mineral cooling textures. These new data, combined with a late c. 2.9 Ga U–Pb date for the titanite, are incompatible with the existing biological model, which invokes microbial micro-tunnel formation followed by titanite ‘infilling’ and preservation in the early Archaean sub-seafloor. The continuum of titanite morphologies reported herein indicates that titanite morphology cannot be used as reliable evidence in support of a biogenic origin in these metavolcanic rocks. It is proposed that the titanite microtextures of purported biogenic origin from other greenstone belts, such as the Pilbara Craton of Western Australia, also deserve to be scrutinized by high-resolution petrological investigations. Supplementary material: Representative electron microprobe analyses of retrograde minerals and µXANES spectra and corresponding fitted pre-edge peaks for all spot analyses are available at https://doi.org/10.6084/m9.figshare.c.3498465

Scientific Knowledge and Challenges Related to Hydrogen

Hydrogen is endowed with excellent physical and chemical properties, like its very high calorific value (three times higher than that of gasoline). With increasing gasoline and gas prices and diminishing fossil fuel reserves, hydrogen is emerging as a clean and renewable alternative energy source. The political and industrial worlds are beginning to see hydrogen as one of the fuels of the future. Several hydrogen production solutions exist, all based on artificial processes, which themselves require energy. Currently, 96 % of hydrogen is produced by thermochemical processes (48 % by natural gas reforming, 30 % by hydrocarbon reforming and 18 % by carbon gasification) and the remaining 4 % by water electrolysis. These processes require the use of fossil fuels (with CO2 production); their cost therefore directly depends on oil prices.

Dispersive XAS on a High Brilliance Source: Highlights and Future Opportunities

Energy Dispersive X‐ray Absorption Spectroscopy is a now a well‐established method which has been applied to a broad range of applications. Ten years from the construction of beamline ID24 at the ESRF, the first dispersive XAS spectrometer using undulator radiation on a third generation source, we report an overview of recent results in very diverse fields of research, ranging from automotive catalysts to magnetism at extreme conditions. We also illustrate how pushing the instrument to its limits has opened new opportunities, such as an enhanced sensitivity to detection of tiny atomic displacements and the potential for micro‐probe redox and speciation imaging.

Energy Dispersive X-Ray Absorption Spectroscopy: Beamline Results and Opportunities

ID24 is the energy dispersive beamline of the European Synchrotron Radiation Facility dedicated to X‐ray Absorption Spectroscopy (XAS). Since 2000, a complete refurbishment program has started, including source upgrade, mirrors replacement, polychromator optimization and detection improvements. These have made possible the development of new applications in a variety of different fields, ranging from the measurement of tiny atom displacements to time resolved techniques, from measurements under extreme conditions to micro‐XAS studies.