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Journal of synchrotron radiation 30(4), 671 - 685 (2023). doi:10.1107/S1600577523003910, An experimental platform for dynamic diamond anvil cell (dDAC) research hasbeen developed at the High Energy Density (HED) Instrument at the EuropeanX-ray Free Electron Laser (European XFEL). Advantage was taken of the highrepetition rate of the European XFEL (up to 4.5 MHz) to collect pulse-resolvedMHz X-ray diffraction data from samples as they are dynamically compressed atintermediate strain rates ( 10 3 s1), where up to 352 diffraction images can becollected from a single pulse train. The set-up employs piezo-driven dDACscapable of compressing samples in 340 ms, compatible with the maximumlength of the pulse train (550 ms). Results from rapid compression experimentson a wide range of sample systems with different X-ray scattering powers arepresented. A maximum compression rate of 87 TPa s1 was observed duringthe fast compression of Au, while a strain rate of 1100 s1 was achievedduring the rapid compression of N2 at 23 TPa s1., Published by Wiley-Blackwell, [Erscheinungsort nicht ermittelbar]

Physical review / B 107(22), 224108 (2023). doi:10.1103/PhysRevB.107.224108, Understanding the high-pressure behavior of ammonia hydrates is relevant for modeling the interior of solarand extrasolar icy bodies. We present here the results of high-pressure x-ray diffraction studies on ammonia-dihydrate (ADH) at room temperature (298 K) up to 112 GPa, employing both static and dynamic compressionexperiments performed in diamond anvil cells. The derived pressure-volume (P-V) compression curves arein excellent agreement regardless of the compression technique. In contrast to early theoretical predictions,our results indicate the stability of the disordered molecular alloy (DMA) phase, a body centered cubic (bcc)structure, and the absence of self-ionization in the investigated pressure range. By combining the P-V data fromseven different compression runs, we derive the first equation of state for ADH-DMA based on the third-orderBirch-Murnaghan formalism with best-fit parameters: V$_0$ = 23.96 ± 0.03 (Å$^3$ /molecule), B$_0$ = 9.95 ± 0.14 GPa,and B′$_0$ = 6.59 ± 0.03. The instantaneous bulk modulus directly derived from the quasicontinuous compressioncurves displays a smooth increase upon compression that further supports the absence of structural transitions., Published by Inst., Woodbury, NY

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International audience; Many nonmetals and metal dioxides MO2, including the dense form of SiO2 stishovite, crystalize in a rutile structure at low pressure and transform to a denser CaCl2 structure under high pressure. Structures and transformations in MO2 dioxides hence serve as an archetype for applications in materials science and inside the Earth and terrestrial planets. Despite its significance, however, the deformation behavior of MO2 compounds in the CaCl2 structure is very poorly constrained. Here we use radial x-ray diffraction in a diamond-anvil cell and study MnO2 as a representative system of the MO2 family. We identify the dominant slip systems and constrain texture evolution in CaCl2-structured phases. After phase transition to a CaCl2 structure above 3.5 GPa, the dominant (010)[100] and secondary {110}[001] and {011}[0-11] slip systems induce a 121 texture in compression. Further compression increases the activity of the {011}⟨0−11⟩ slip system, with an enhanced 001 texture at ∼50GPa. During pressure release, the 001 texture becomes dominant over the original 121 texture. This clearly demonstrates the effect of pressure on the deformation behavior and slip systems of CaCl2-structured dioxides. Finally, MnO2 transforms back to a rutile structure upon pressure release, with a significant orientation memory, highlighting the martensitic nature of the CaCl2 to rutile structural transformation. These findings provide key guidance regarding the plasticity of CaCl2-structured dioxides, with implications in materials and Earth and planetary science.

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The cores of terrestrial planets are comprised of Fe-Ni alloys, with around 5-10 weight % of the light elements. They account for the observed core density deficit compared to the Fe-Ni alloy at high-pressure high-temperature conditions. Silicon has long been considered as a major light element of the cores due to its high cosmic abundance and ability to incorporate into the Fe-Ni alloy during core formation. Thus, Fe-Si alloys and compounds are likely to be present in the cores of terrestrial planets such as Mercury. The seismological observations indicate that compressional waves travel slower through the equatorial plane of the Earth’s core compared to the waves traveling along the polar axis. This anisotropy is likely related to the plastic deformation of the inner core material. Yet, the effect of light elements on the plasticity of iron is poorly known. Here we investigate lattice strains and textures of ε-FeSi up to 50 GPa at 300 K and 1100 K. We employed the radial x-ray diffraction technique using diamond anvil cells as deformation apparatus. We used our data to determine yield strength of ε-FeSi and to assess the sound-velocity anisotropy of the polycrystalline ε-FeSi as a function of pressure and temperature. Upon compression, FeSi aggregate does not develop strong textures and, as a result, shows weak sound-velocity anisotropy. The strength of the material is nearly temperature-independent. Thus, we infer the constraints on the silicon budget in planetary deepest interior by demonstrating that strong sound-velocity anisotropy is incompatible with the high silicon content.

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Iron oxides are important materials for Geoscience but also for basic science and applied technologies (Kupenko et al., 2019). Iron monoxide is likely to be the final constituent of the evolution of subducted banded iron formations and might be a source of the low-velocity zones at the Earth’s core-mantle boundary (Dobson and Brodholt, 2005). The stability and high-pressure properties of FexO could, thus, determine the fate of banded iron formations and their potential role in processes in Earth and planetary interiors, including controls on redox cycles. FexO has the NaCl (B1) structure at ambient pressure and temperature and transforms into rhombohedral distorted-B1 phase (rB1) at about 16 GPa. A further transition from a rhombohedral rB1 phase to a hexagonal B8 phase has been proposed at 74 GPa and 900 K (Fei and Mao, 1994). Although numerous studies were focused on the investigation of the electronic and magnetic properties and phase diagram of FexO, information on the Fe2+/Fe3+ interplay, magnetic and structural coupling of FexO at high pressures and high temperature are very limited. We will present our investigation on the pressure dependence of the electronic, magnetic and structural properties of FexO by means of Synchrotron Mössbauer Source spectroscopy and Single-Crystal X-ray diffraction with diamond anvil cells up to 94 GPa and 1200 K. We will discuss the interplay between Fe2+ and Fe3+ and its effects on the electronic and magnetic properties of FexO and their potential role in the mineralogy, chemistry, and physics of the Earth’s deep interior.

Niobium is a critical metal used in various superconducting materials and alloys. It is predominantly mined from ores rich in pyrochlore-group minerals with the general formula A2B2O6Y (B ≥ 50% Nb). Fluorcalciopyrochlore, (Ca,Na)2Nb2O6F, is considered as primary magmatic pyrochlore. However, metasomatism by hydrothermal fluids, amongst other processes, can lead to a wide range of chemical compositions (Mitchell 2015). Different stages of pyrochlore alteration were assigned to specific p-T-ranges using associated minerals in the host rock ores. Enrichment in Ba for example is documented in low-temperature hydrothermal environments (Lumpkin & Ewing 1995). However, it is difficult to unravel the role of temperature, pressure, fluid composition or time in alteration processes of primary fluorcalciopyrochlore in natural carbonatite systems. Therefore, alteration experiments using an autoclave are necessary to identify and quantify alteration mechanisms. To our knowledge, such experiments have only been conducted for microlite and betafite (Geisler et al. 2005), but not for pyrochlore sensu stricto. Hence, we studied the interaction of natural fluorcalciopyrochlore with Cl-rich, acidic hydrothermal fluids of varying composition in teflon-lined hydrothermal reactor vessels at 200 °C. Here we will present preliminary results of our experiments and discuss consequences of hydrothermal alteration of pyrochlores, with application to Nb-rich carbonatite ores. R. H. Mitchell, Ore Geology Reviews, 64, 2015; G. R. Lumpkin & R. C. Ewing, American Mineralogist, 80, 1995; T. Geisler et al., American Mineralogist, 90, 2005

The inner core is comprised of iron and nickel as the main components. However, the composition of the Earth’s core remains enigmatic to date. The comparison of densities and seismic velocities between pure Fe-Ni alloys and seismological models reveals a density deficit and reduced velocities in the core. These observations can be explained by the addition of 3-7 wt% light elements to Fe-Ni alloys. Possible candidates are H, C, O, Si, and S. To understand the formation, evolution, and dynamics of the core of the Earth and other terrestrial planets, constraining the light element budget is crucial. Silicon could be the dominant light element because of its partitioning behaviour, isotope fractionation, and chemical properties. The low shear velocity (Vs) and high Poisson ratio in the inner core could be explained by the addition of carbon. Indeed, recent ab initio calculations proposed that simultaneous incorporation of Si and C can provide a good match both for density and sound velocities. In search of the light elements and their amounts, we carried out nuclear inelastic scattering (NIS) and x-ray diffraction experiments in diamond anvil cells at pressures up to 160 GPa. We determined the sound velocities of the Fe-2wt%Si-0.4wt%C-alloy at high pressures and used thermoelastic parameters extracted from the NIS data to extrapolate them to core conditions. Here, we will present the results and discuss the availability of silicon and carbon in the inner core, as well as the possible amount of each element to match seismic observations.

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International audience; It has long been known that phase transitions have contributed to seismic reflections in the mantle. These transitions are caused by pressure and temperature variations. More specifically, at the 660 km discontinuity, phase transitions in pyrolitic composition samples, where ringwoodite and majoritic garnet decompose to form bridgmanite and ferropericlase, have been observed. Transformations at the 660 km discontinuity are not martensitic due to chemical variations within the material. Questions remain on the microstructures obtained after the phase transition. Are there preferred orientation within individual phases after the synthesis? Do grain sizes change during this transition? Better understanding the microstructures induced by phase transformations will give better insight to seismic anisotropy shortly after the discontinuity, the observed seismic reflection and its overall effect on mantle dynamics. Determining transformation microstructures at 660 km conditions will hence lead to a better understanding of the Earth’s mantle. By implementing synchroton multigrain x-ray diffraction in a laser heated diamond anvil cell, we are able to track individual grains, their crystal structure, and orientations, while being compressed in-situ at pressures ranging from 18-55 GPa and temperatures of ~1800K. Using this experimental approach, we can then use obtained transformation textures to refine current seismic models to better understand observable anisotropies and reflections at conditions relevant to the 660 km discontinuity.

International audience; It has long been known that phase transitions have contributed to seismic reflections in the mantle. These transitions are caused by pressure and temperature variations. More specifically, at the 660 km discontinuity, phase transitions in pyrolitic composition samples, where ringwoodite and majoritic garnet decompose to form bridgmanite and ferropericlase, have been observed. Transformations at the 660 km discontinuity are not martensitic due to chemical variations within the material. Questions remain on the microstructures obtained after the phase transition. Are there preferred orientation within individual phases after the synthesis? Do grain sizes change during this transition? Better understanding the microstructures induced by phase transformations will give better insight to seismic anisotropy shortly after the discontinuity, the observed seismic reflection and its overall effect on mantle dynamics. Determining transformation microstructures at 660 km conditions will hence lead to a better understanding of the Earth’s mantle.By implementing synchroton multigrain x-ray diffraction in a laser heated diamond anvil cell, we are able to track individual grains, their crystal structure, and orientations, while being compressed in-situ at pressures ranging from 18-55 GPa and temperatures of ~1800K. Using this experimental approach, we can then use obtained transformation textures to refine current seismic models to better understand observable anisotropies and reflections at conditions relevant to the 660 km discontinuity.

International audience; Seismic anisotropy is a critical material property which allow us to identify structures, deformation behaviour and flow in the Earth’s mantle. Interpreting seismic signals, however, requires extensive knowledge of the underlying physical properties of minerals.Large scale seismic anisotropy can arise from the anisotropy of individual crystals and their alignment due to specific mechanisms. In the upper mantle, strong seismic anistotropy is explained by lattice preferred orientation (LPO) of olivine due to mantle flow. In the mantle transition zone, seismic anisotropy is weaker and there is no consensus on the source of this anisotropy. The 410 km depth discontinuity, which separates these two regions, is attributed to the transformation of olivine into its high pressure polymorph wadsleyite. Experiments show that this transformation can happen either by a martensitic-like mechanism or by a non-martensitic-like mechanism. In the first case, the LPO of the parent olivine will be inherited by the daughter wadsleyite whereas in the second case, the daughter wadsleyite will form with a random orientation. Hence, if this transformation is martensitic-like, it can be a source for seismic anisotropy in the mantle transition zone.Here we present a project in which an experimental study of the olivine-wadsleyite transformation is used to make inferences about the prevalent transformation mechanism at mantle conditions. We rely on laboratory experiments to produce the transformation at the conditions of the 410 km depth discontinuity by compressing either polycrystals or single crystals of olivine in diamond anvils cells at pressures up to 20 GPa and temperatures of 1600-1800 K. Using multigrain crystallography we evaluate the orientation and position of individual grains in the sample. We then use the experimental LPO to model the seismic anisotropy resulting from the transformation. Finally, we simulate different observables as anisotropy or seismic reflections from the Earth’s transition zone and compare our predictions with real seismic data.

Understanding the behavior of halogens (Cl, Br, and I) in subduction zones is critical to constrain the geochemical cycle of these volatiles and associated trace metals, as well as to quantify the halogen fluxes to the atmosphere via volcanic degassing. Here, the partitioning of bromine between coexisting aqueous fluids and hydrous granitic melts and its speciation in slab-derived fluids have been investigated in situ up to 840 ∘C and 2.2 GPa by synchrotron x-ray fluorescence (SXRF) and x-ray absorption spectroscopy (XAS) in diamond anvil cells. The partition coefficients range from ∼2 to ∼15, with an average value of 6.7±3.6 (1σ) over the whole pressure–temperature (P–T) range, indicating a moderate Br enrichment in aqueous fluids, in agreement with previous work. Extended x-ray-absorption fine-structure (EXAFS) analysis further evidences a gradual evolution of Br speciation from hydrated Br ions [Br(H2O)6]− in slab dehydration fluids to more complex structures involving both Na ions and water molecules, [BrNax(H2O)y], in hydrous silicate melts and supercritical fluids released at greater depth (> 200 km). In denser fluids (ρ > 1.5 g cm−3) containing 60 wt % dissolved alkali–silicates and in hydrous Na2Si2O5 melts (10 wt % H2O), Br is found to be in a “salt-like” structure involving the six nearest Na ions and several next-nearest O neighbors that are either from water molecules and/or the silicate network. Bromine (and likely chlorine and iodine) complexing with alkalis is thus an efficient mechanism for the mobilization and transport of halogens by hydrous silicate melts and silica-rich supercritical fluids. Our results suggest that both shallow dehydration fluids and deeper silicate-bearing fluids efficiently remove halogens from the slab in the sub-arc region, thus favoring an efficient transfer of halogens across subduction zones., Finanziert durch den Open-Access-Publikationsfonds der Westfälischen Wilhelms-Universität Münster (WWU Münster).