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1. Crystallographic texture formation in Fe-9wt%Si alloy during deformation and phase transition at high pressure

   Vasin, R. N. ; Kunz, M. ; Wenk, H-R and ( March 2023 , Geophysical journal international)

   The seismic anisotropy of the Earth’s solid inner core has been the topic of much research. It could be explained by the crystallographic preferred orientation (CPO) developing during convection. The likely phase is hexagonal close-packed iron (hcp), alloyed with nickel and some lighter elements. Here we use high energy synchrotron X-rays to study CPO in Fe-9wt%Si, uniaxially compressed in a diamond anvil cell in radial geometry. The experiments reveal that strong preferred orientation forms in the low-pressure body-centred cubic (bcc) phase that appears to be softer than pure iron. CPO is attributed to dominant {110}<111>slip. The onset of the bcc→hcp transition occurs at a pressure of ≈15 GPa, and the alloy remains in a two phase bcc+hcp state up to 40 GPa. The hcp phase forms first with a distinct {11¯20} maximum perpendicular to compression. Modelling shows that this is a transformation texture, which can be described by Burgers orientation relationship with variant selection. Experimental results suggest that bcc grains oriented with <100> parallel to compression transform into hcp first. The CPO of the hcp changes only slowly during further pressure and deviatoric stress increase at ambient temperature. After heating to 1600 K, a change in the hcp CPO ismore »observed with alignment of (0001) planes perpendicular to compression that can be interpreted as dominant (0001)<11¯20> slip, combined with {10¯12}<¯1011> mechanical twinning, which is similar to the deformation modes suggested previously for pure hcp iron at inner core conditions.« less
2. Effect of Carbon on the Volume of Solid Iron at High Pressure: Implications for Carbon Substitution in Iron Structures and Carbon Content in the Earth’s Inner Core

   https://doi.org/10.3390/min9120720  

   Yang, Jing ; Fei, Yingwei ; Hu, Xiaojun ; Greenberg, Eran ; Prakapenka, Vitali B. ( December 2019 , Minerals)

   Understanding the effect of carbon on the density of hcp (hexagonal-close-packed) Fe-C alloys is essential for modeling the carbon content in the Earth’s inner core. Previous studies have focused on the equations of state of iron carbides that may not be applicable to the solid inner core that may incorporate carbon as dissolved carbon in metallic iron. Carbon substitution in hcp-Fe and its effect on the density have never been experimentally studied. We investigated the compression behavior of Fe-C alloys with 0.31 and 1.37 wt % carbon, along with pure iron as a reference, by in-situ X-ray diffraction measurements up to 135 GPa for pure Fe, and 87 GPa for Fe-0.31C and 109 GPa for Fe-1.37C. The results show that the incorporation of carbon in hcp-Fe leads to the expansion of the lattice, contrary to the known effect in body-centered cubic (bcc)-Fe, suggesting a change in the substitution mechanism or local environment. The data on axial compressibility suggest that increasing carbon content could enhance seismic anisotropy in the Earth’s inner core. The new thermoelastic parameters allow us to develop a thermoelastic model to estimate the carbon content in the inner core when carbon is incorporated as dissolved carbon hcp-Fe. Themore »required carbon contents to explain the density deficit of Earth’s inner core are 1.30 and 0.43 wt % at inner core boundary temperatures of 5000 K and 7000 K, respectively.« less
3. Ultrafast X-ray Diffraction Study of a Shock-Compressed Iron Meteorite above 100 GPa

   https://doi.org/10.3390/min11060567  

   Tecklenburg, Sabrina ; Colina-Ruiz, Roberto ; Hok, Sovanndara ; Bolme, Cynthia ; Galtier, Eric ; Granados, Eduardo ; Hashim, Akel ; Lee, Hae Ja ; Merkel, Sébastien ; Morrow, Benjamin ; et al ( June 2021 , Minerals)

   Natural kamacite samples (Fe92.5Ni7.5) from a fragment of the Gibeon meteorite were studied as a proxy material for terrestrial cores to examine phase transition kinetics under shock compression for a range of different pressures up to 140 GPa. In situ time-resolved X-ray diffraction (XRD) data were collected of a body-centered cubic (bcc) kamacite section that transforms to the high-pressure hexagonal close-packed (hcp) phase with sub-nanosecond temporal resolution. The coarse-grained crystal of kamacite rapidly transformed to highly oriented crystallites of the hcp phase at maximum compression. The hcp phase persisted for as long as 9.5 ns following shock release. Comparing the c/a ratio with previous static and dynamic work on Fe and Fe-rich Fe-Ni alloys, it was found that some shots exhibit a larger than ideal c/a ratio, up to nearly 1.65. This work represents the first time-resolved laser shock compression structural study of a natural iron meteorite, relevant for understanding the dynamic material properties of metallic planetary bodies during impact events and Earth’s core elasticity.
4. Hydrogen and Silicon Effects on Hexagonal Close Packed Fe Alloys at High Pressures: Implications for the Composition of Earth's Inner Core

   https://doi.org/10.1029/2022JB026016  

   Fu, Suyu ; Chariton, Stella ; Prakapenka, Vitali B. ; Shim, Sang‐Heon ( April 2023 , Journal of Geophysical Research: Solid Earth)

   Hexagonal close‐packed (hcp) structured Fe‐Ni alloy is believed to be the dominant phase in the Earth's inner core. This phase is expected to contain 4%–5% light elements, such as Si and H. While the effects of individual light element candidates on the equation of state (EoS) of the hcp Fe metal have been studied, their combined effects remain largely unexplored. In this study, we report the equations of state for two hcp‐structured Fe‐Si‐H alloys, namely Fe_0.83Si_0.17H_0.07and Fe_0.83Si_0.17H_0.46, using synchrotron X‐ray diffraction measurements up to 125 GPa at 300 K. These alloys were synthesized by cold compression of Fe‐9wt%Si in either pure H₂or Ar‐H₂mixture medium in diamond‐anvil cells. The volume increase caused by a H atom in hcp Fe‐Si‐H alloys is approximately eight times greater than that by a Si atom. We used the improved data set to develop a composition‐dependent EoS that covers a wide range of compositions. Our calculated density and bulk sound velocity of hcp Fe‐Si‐H alloys suggest a large trade‐off between Si and H contents in fitting the seismic properties of the inner core. Combining our new EoS with geophysical and geochemical constraints, we propose 1.6–3 wt% Si and 0.15–0.6 wt% H in the Earth's inner core.
5. Progressive Development of a Distributed Ductile Shear Zone beneath the Patagonian Retroarc Fold-Thrust Belt, Chile

   https://doi.org/10.2113/2022/3820115  

   Betka, Paul ; Mosher, Sharon ; Klepeis, Keith ; Sun, ed., Shengsi ( May 2022 , Lithosphere)

   The southern Patagonian Andes record Late Cretaceous–Paleogene compressional inversion of the Rocas Verdes backarc basin (RVB) and development of the Patagonian fold-thrust belt (FTB). A ductile décollement formed in the middle crust and accommodated underthrusting, thickening, and tectonic burial of the continental margin (Cordillera Darwin Metamorphic Complex (CDMC)) beneath the RVB. We present new geologic mapping, quartz microstructure, and crystallographic preferred orientation (CPO) fabric analyses to document the kinematic evolution and deformation conditions of the décollement. Within the CDMC, the décollement is defined by a quartz/chlorite composite schistose foliation (S1-2) that is progressively refolded by two generations of noncylindrical, tight, and isoclinal folds (F3–F4). Strain intensifies near the top of the CDMC, forming a >5 km thick shear zone that is defined by a penetrative L-S tectonite (S2/L2) and progressive noncylindrical folding (F3). Younger kink folds and steeply inclined tight folds (F4) with both north- and south-­dipping axial planes (S4) overprint D2 and D3 structures. Quartz textures from D2 fabrics show subgrain rotation and grain boundary migration recrystallization equivalent to regime 3, and quartz CPO patterns indicate mixed prism <a> and [c] slip systems with c-axis opening angles indicative of deformation temperatures between ~500° and >650°C. Approximately 40 km toward themore »foreland, the shear zone thins (~1 km thick) and is defined by the L-S tectonite (S2/L2) and tightening of recumbent isoclinal folds (F3). Quartz textures and CPO patterns indicate subgrain rotation recrystallization typical of regime 2 and dominantly basal <a> slip, and c-axis opening angles are consistent with deformation temperatures between ~375° and 575°C. Deformation occurred under greenschist and amphibolite facies conditions in the foreland and hinterland, respectively, indicating that the shear zone dipped shallowly toward the hinterland. The Magallanes décollement is an example of a regional ductile shear zone that accommodated distributed middle to lower crustal thickening below a retroarc FTB.

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