Enhancement in statistical and image analysis for in situ

Synchrotron-based X-ray microfluorescence (µSXRF) is an analytical method suitable for in situ investigation of the distribution of micronutrient and macronutrient elements in several-micrometres-thick unstained biological samples, e.g. single cells and tissues. Elements are mapped and quantified at sub-p.p.m. concentrations. In this study the quantity, distribution and grouping/co-localization of various elements have been identified in straight and twisted internodes of the stems of the monocotyledonous climber D. balcanica Košanin. Three different statistical methods were employed to analyse the macro­nutrient and micronutrient distributions and co-localization. Macronutrient elements (K, P, Ca, Cl) are distributed homogeneously in both straight and twisted internodes. Micronutrient elements are mostly grouped in the vasculature and in the sclerenchyma cell layer. In addition, co-localization of micronutrient elements is much more prominent in twisted than in straight internodes. These image analyses and statistical methods provided very similar outcomes and could be applied to various types of biological samples imaged by µSXRF

Strontium complexation in aqueous solutions and silicate glasses: Insights from high energy-resolution fluorescence detection X-ray spectroscopy and ab-initio modeling

Although fluid–melt partitioning of trace elements like Sr, Ba, La, and Y is known to be strongly influenced by the fluid and melt chemical composition, their speciation in silicate-saturated fluids is studied insufficiently at high temperatures and pressures. Here, high energy-resolution fluorescence detection–X-ray absorption spectroscopy (HERFD–XAS) has been applied to investigate the local environment of strontium in crystalline model compounds, silicate glasses, and aqueous solutions. Acquisition of Sr K-edge HERFD–XAS spectra of aqueous solutions of SrCl2 and Sr(OH)2, and three aqueous fluids with dissolved silicate components was done in situ at temperatures to 780 °C and pressures to ∼800 MPa using hydrothermal diamond-anvil cells.Experiments were complemented by theoretical spectroscopy calculations using the finite difference method near edge structure (FDMNES) code. This approach was validated for a number of crystalline model compounds. For the silicate glasses and aqueous solutions (SrCl2 and Sr(OH)2), small clusters were examined. Either symmetric or distorted SrO6 clusters were found to describe Sr complexation in peraluminous or peralkaline glasses. However, small ‘static’ clusters seem not to be fully suited to account for the dynamically changing atomic arrangements in aqueous solutions at high temperature. Therefore, ab-initio molecular dynamics simulations were performed and used as input for modeling of X-ray absorption spectra. Analyses of these simulations indicated [SrCl(H2O)6]+ and Sr(OH)2(H2O)4 as the most likely complexes in the chloride and hydroxide solutions, respectively.Analysis of the spectra of the silicate-rich fluids shows that both melt and fluid composition strongly influence Sr complexation. For the silicate-rich fluids, formation of Sr–Cl complexes occurs at low (Na + K)/Cl and (Si + Al)/(Na + K) ratios in the fluid, whereas Sr hydroxide and possibly silicate complexes (similar to those in the silicate glass) are favored at higher ratios. Our X-ray spectroscopic results offer an explanation for the dependence of fluid–melt partitioning of Sr on melt composition measured in previous ex situ studies, and highlight the importance of components other than chloride (silicate and aluminosilicate) in controlling metal speciation in fluid–melt systems at high temperatures and pressures

Enhancement in statistical and image analysis for in situ µµSXRF studies of elemental distribution and co-localization, using DioscoreabalcanicaDioscorea balcanica

Synchrotron-based X-ray microfluorescence (mu SXRF) is an analytical method suitable for in situ investigation of the distribution of micronutrient and macronutrient elements in several-micrometres-thick unstained biological samples, e. g. single cells and tissues. Elements are mapped and quantified at sub-p. p. m. concentrations. In this study the quantity, distribution and grouping/co-localization of various elements have been identified in straight and twisted internodes of the stems of the monocotyledonous climber D. balcanica Kosanin. Three different statistical methods were employed to analyse the macronutrient and micronutrient distributions and co-localization. Macronutrient elements (K, P, Ca, Cl) are distributed homogeneously in both straight and twisted internodes. Micronutrient elements are mostly grouped in the vasculature and in the sclerenchyma cell layer. In addition, co-localization of micronutrient elements is much more prominent in twisted than in straight internodes. These image analyses and statistical methods provided very similar outcomes and could be applied to various types of biological samples imaged by mSXRF

Corrigendum to “Zircon solubility and zirconium complexation in H2O+Na2O+SiO2±Al23H_{2}O+Na_{2}O+SiO_{2}±Al_{23} fluids at high pressure and temperature” [Earth Planet. Sci. Lett. 349–350 (2012) 15–25]

Corrigendu

The effect of alkalinity on Ni–O bond length in silicate glasses: Implications for Ni isotope geochemistry

Equilibrium mass-dependent (“stable”) isotopic fractionation of an element during magmatic processes is driven by a contrast in bonding environment between minerals and silicate melt, which is expressed as an isotopic fractionation factor. A quantitative understanding of such isotopic fractionation factors is vital to interpret observed isotopic variations in magmatic rocks. It is well known that the local environment and the bond strength of an element dictate the sign and magnitude of isotopic fractionation between minerals, but it is uncertain how the structure and chemical composition of a silicate melt can affect mineral-melt isotopic fractionation factors. To explore this, we studied the coordination environment of nickel (Ni) in different silicate glasses using extended X-ray absorption fine structure (EXAFS) measurements at the German synchrotron X-ray source (DESY).We determined –Ni–O bond lengths in a suite of synthetic but near-natural silicate glasses using EXAFS and found that the former vary systematically with melt alkalinity, which is best described by the parameter ln[1 + (Na + K)/Ca]. With increasing melt alkalinity, Ni occupies more IV-fold coordinated sites, which are associated with a shorter –Ni–O bond length. Next, we use the ionic model, which allows to predict isotopic fractionation factors based on the difference in bond length between two phases. We find that more alkaline melts have a stronger preference for the heavier isotopes of Ni than less alkaline melts. This implies that the magnitude of mineral-melt Ni isotope fractionation factors, for instance between olivine and melt, will depend on the alkalinity of the melt. At magmatic temperatures, however, the variation in fractionation factors caused by melt alkalinity will rarely exceed 0.05 ‰ and is thus mostly negligible, in particular in the realm of basaltic melt compositions. Nevertheless, the relationship between melt alkalinity and fractionation factor reported here can be used to extrapolate empirical data for mineral-melt Ni isotope fractionation factors, once such data become available, to the full range of magma compositions on Earth and other Solar System bodies

New Perspectives for Lower Mantle Reaction Mechanism Research using Modern X-Ray Sources

Chemical composition of the Earth's mantle phases have a major effect on their stabilities and consequently on their reactions. Especially concerning light elements and trace elements, compositions of phases in the lower mantle remain basically unknown because the phases are not accessible and cannot be studied directly. In order to betterlearn about reactions of mantle phases, related redistribution processes of elements and the resulting properties of the mantle, mantle phases have been studied by in-situ methods.Within the past years, laser-heated diamond anvil cells have been combined with synchrotron radiation induced methods to study chemical reactions by in-situ x-ray fluorescence analysis and combined x-ray diffraction (XRD) (Petitgirard et al., 2012). Synchrotron radiation provides sufficiently small spot sizes, hard x-rays and a high sensitivity.Chemical and structural information were obtained at temperatures of up to 4200 K and pressures up to 130 GPa. First time-resolved measurements have been made with the objective to follow reaction mechanisms. For XRD, a time-resolution of msec could be achieved by single shot pulsed laser heating (Goncharov et al., 2011), while for insituXRF studies, the time-resolution is currently limited to the sec regime. It is either limited by detector speed or sensitivity or both.An alternative method to study reactions of lower mantle phases are laser-driven shock and ramp compression experiments. The samples are generally pumped with an long-pulse optical laser and then probed with an x-ray source at a delayed period. Combined with x-ray free electron lasers as a probe beam, these experiments offer the uniquepossibility to study reactions at a rate of up to MHz due to the x-ray timing structure and the increased number of photons. The High Energy Density science instrument at the European XFEL (HED) will provide unique possibilities for research at extreme states of matter. The instrument is one of the six baseline instruments at the European XFEL andwill start user operation in the second half 2017.In this presentation, we will show results from in-situ experiments at conditions in the lower mantle at currently available sources and discuss the persectives to study reaction mechanisms at such conditions at the upcoming HED instrument

Insights from X-ray absorption/fluorescence spectroscopy and ab-initio molecular dynamics on concentration and complexation of Zr and Hf in aqueous fluids at high pressure and temperature

High-field-strength elements such as Zr and Hf are important geochemical indicators for processes in the deep Earth. Aqueous fluids play a significant role in the transport of heat and matter in these systems. However, concentrations and complexation of Zr and Hf in these fluids at high pressure (P) and temperature (T) conditions are largely unknown. Zr/Hf contents were determined in-situ at P and T in aqueous fluids containing NaOH, HCl or Na-Al silicate components in equilibrium with zircon or baddeleyite. Concentrations are strongly enhanced in comparison to pure H2O. The variation of Zr/Hf contents with fluid composition and their respective variation with P and T point to differences in the Zr/Hf complexation. High-resolution Zr K-edge XANES spectra were collected in-situ at P and T as well as Hf L3-edge XANES spectra. Analysis of spectra evidences considerable differences in the coordination of Zr and Hf as a function of fluid composition. [7]Zr is found in NaOH solution. [7]Zr is also found in HCl solution, however, coordinated by both chlorine and oxygen. In fluids with Na-silicate components, Zr and Hf are 6-fold coordinated in zircono-silicate complexes, likely complexed with Na and Si. Further insight into the speciation was obtained by ab-initio molecular dynamics simulations of Zr monomers in H2O-NaOH and H2O-HCl. In basic solutions, Zr is five-fold coordinated by oxygen, whereas in acidic solutions, mixed oxychloride complexes are formed. The oxychloride complexes also seem to be present in experimental fluids, whereas the Zr coordination in basic fluids is underestimated by the simulations. The most likely explanation for this discrepancy is the formation of Zr oligomers in the fluid

A con-focal setup for micro-XRF and XAFS experiments using diamond anvil cells

A confocal set-up is presented that improves micro-XRF and XAFS experiments with high-pressure diamond-anvil cells (DACs). In this experiment a probing volume is defined by the focus of the incoming synchrotron radiation beam and that of a polycapillary X-ray half-lens with a very long working distance, which is placed in front of the fluorescence detector. This set-up enhances the quality of the fluorescence and XAFS spectra, and thus the sensitivity for detecting elements at low concentrations. It efficiently suppresses signal from outside the sample chamber, which stems from elastic and inelastic scattering of the incoming beam by the diamond anvils as well as from excitation of fluorescence from the body of the DAC

Complexation of Zr and Hf in fluoride-rich hydrothermal aqueous fluids and its significance for high field strength element fractionation

International audienceZirconium and hafnium behave nearly identically in most geological processes due to their identical nominal ionic charge and similar radius. Some of the most pronounced exceptions from this rule are observed in fluoriderich aqueous systems, suggesting that aqueous fluoride complexation may be involved in Zr/Hf fractionation. To understand the mechanisms causing this phenomenon, we investigated complexation of Zr 4+ and Hf 4+ in fluoride-rich (1.0 mol/kg HF) aqueous solutions at 40 MPa and 100-400 • C, using synchrotron X-ray absorption spectroscopy (X-ray absorption near edge structure and extended X-ray absorption fine structure) combined with classical and ab initio molecular dynamics simulations. The dominant experimentally observed complexes are [Zr (F,OH) 4 ⋅2H 2 O] 0 and [Hf(F,OH) 4 ⋅2H 2 O] 0 , respectively. The first coordination shell comprises a distorted octahedron, with fluoride and hydroxide ligands at a similar mean radial distance (1.9-2.0 Å) from the central cation, and H 2 O ligands at a slightly greater distance (>2.1 Å). With increasing temperature, the H 2 O ligands move further out, causing first an increasing distortion of the octahedron and subsequently a partial transition to less hydrated complexes as a certain fraction of the H 2 O molecules move to the second shell at > 3 Å. As a consequence, the radial distance of the F-and OH-anions from the central cation, as well as the overall average radial distance of the first shell decreases due to decreased steric repulsion from the H 2 O ligands. Both experiments and simulations agree in that Hf forms slightly shorter bonds to its nearest neighbors than Zr. The results suggest two hypotheses for the mechanism of Zr/Hf fractionation during precipitation of minerals from fluoride-rich hydrothermal solutions: 1) The heavy twin (Hf) prefers the lower coordination (shorter bonds) and is thus less likely to enter into the higher coordination found in the solids. This mechanism would be analogous to equilibrium isotope fractionation. 2) The change of Hf into a higher coordination environment (e.g., from solution to solid) is slower because it forms stronger ligand-bonds than Zr. This would be analogous to reactive kinetic isotope fractionation. In either case mass dependent fractionation qualitatively matches the observations but mass independent effects on bond strength may also be significant. Quantitative investigations of these effects are needed and may also shed light on the currently still somewhat enigmatic fractionation behavior of Zr isotopes

A hydrothermal apparatus for x-ray absorption spectroscopy of hydrothermal fluids at DESY

We present a new autoclave that enables in situ characterization of hydrothermal fluids at high pressures and high temperatures at synchrotron x-ray radiation sources. The autoclave has been specifically designed to enable x-ray absorption spectroscopy in fluids with applications to mineral solubility and element speciation analysis in hydrothermal fluids in complex compositions. However, other applications, such as Raman spectroscopy, in high-pressure fluids are also possible with the autoclave. First experiments were run at pressures between 100 and 600 bars and at temperatures between 25 °C and 550 °C, and preliminary results on scheelite dissolution in fluids of different compositions show that the autoclave is well suited to study the behavior of ore-forming metals at P–T conditions relevant to the Earth’s crust