We combine nanosecond laser shock compression with \emph{in-situ} picosecond X-ray diffraction to provide structural data on iron up to 275 GPa. We constrain the extent of hcp-liquid coexistence, the onset of total melt, and the structure within the liquid phase. Our results indicate that iron, under shock compression, melts completely by 258(8) GPa. A coordination number analysis indicates that iron is a simple liquid at these pressure-temperature conditions. We also perform texture analysis between the ambient body-centered-cubic (bcc) $\alpha$, and the hexagonal-closed-packed (hcp) high-pressure $\epsilon-$phase. We rule out the Rong-Dunlop orientation relationship (OR) between the $\alpha$ and $\epsilon-$phases. However, we cannot distinguish between three other closely related ORs: Burger's, Mao-Bassett-Takahashi, and Potter's OR. The solid-liquid coexistence region is constrained from a melt onset pressure of 225(3) GPa from previously published sound speed measurements and full melt (246.5(1.8)-258(8) GPa) from X-ray diffraction measurements, with an associated maximum latent heat of melting of 623 J/g. This value is lower than recently reported theoretical estimates and suggests that the contribution to the earth's geodynamo energy budget from heat release due to freezing of the inner core is smaller than previously thought. Melt pressures for these nanosecond shock experiments are consistent with gas gun shock experiments that last for microseconds, indicating that the melt transition occurs rapidly.
We present measurements of the pressure dependence of thermal conductivity for high pressure phases of KCl and NaCl using the laser-heated diamond anvil cell (LHDAC) and a 3D finite element model for heat flow. Temperature measurements are made in the LHDAC of KCl in the B2 phase, from 15 GPa to 24 GPa, and of NaCl from 14 GPa to 43 GPa, across the B1-B2 phase transition. The measurements are forward modeled, using the geometry and material properties of the cell as inputs, solving for the change in thermal conductivity between pressure steps. The results for B2 KCl indicate increasing thermal conductivity over the experimental pressure range and give dlnκdlnρ=3.75 ± 0.9. For NaCl, thermal conductivity increases in the B1 and B2 phases, dlnκdlnρ=1.6 ± 0.5 and dlnκdlnρ=2.9 ± 0.8, respectively. Our results constrain the reduction in thermal conductivity across the NaCl B1-B2 transition to 37% ± 7%.
Abstract Despite making up 5–20 wt.% of Earth's predominantly iron core, the melting properties of elemental nickel at core conditions remain poorly understood, due largely to a dearth of experimental data. We present an in situ X-ray diffraction study performed on laser shock-compressed samples of bulk nickel, reaching pressures up to ~500 GPa. Hugoniot states of nickel were targeted using a flat-top laser drive, with in situ X-ray diffraction data collected using the Linac Coherent Light Source. Rietveld methods were used to determine the densities of the shocked states from the measured diffraction data, while peak pressures were determined using a combination of measured particle velocities, shock transit times, hydrodynamic simulations, and laser intensity calibrations. We observed solid compressed face-centered cubic (fcc) Ni up to at least 332(30) GPa along the Hugoniot---significantly higher than expected from the majority of melt lines that have been proposed for nickel. We also bracket the partial melting onset to between 377(38) GPa and 486(35) GPa.
Abstract Compression and decompression experiments on face-centered cubic (fcc) γ′-Fe4N to 77 GPa at room temperature were conducted in a diamond-anvil cell with in situ X-ray diffraction (XRD) to examine its stability under high pressure. In the investigated pressure range, γ′-Fe4N did not show any structural transitions. However, a peak broadening was observed in the XRD patterns above 60 GPa. The obtained pressure-volume data to 60 GPa were fitted to the third-order Birch-Murnaghan equation of state (EoS), which yielded the following elastic parameters: K0 = 169 (6) GPa, K′ = 4.1 (4), with a fixed V0 = 54.95 Å at 1 bar. A quantitative Schreinemakers' web was obtained at 15–60 GPa and 300–1600 K by combining the EoS for γ′-Fe4N with reported phase stability data at low pressures. The web indicates the existence of an invariant point at 41 GPa and 1000 K where γ′-Fe4N, hexagonal closed-packed (hcp) ε-Fe7N3, double hexagonal closed-packed β-Fe7N3, and hcp Fe phases are stable. From the invariant point, a reaction γ′-Fe4N = β-Fe7N3 + hcp Fe originates toward the high-pressure side, which determines the high-pressure stability of γ′-Fe4N at 56 GPa and 300 K. Therefore, the γ′-Fe4N phase observed in the experiments beyond this pressure must be metastable. The obtained results support the existing idea that β-Fe7N3 would be the most nitrogen-rich iron compound under core conditions. An iron carbonitride Fe7(C,N)3 found as a mantle-derived diamond inclusion implies that β-Fe7N3 and Fe7C3 may form a continuous solid solution in the mantle deeper than 1000 km depth. Diamond formation may be related to the presence of fluids in the mantle, and dehydration reactions of high-pressure hydrous phase D might have supplied free fluids in the mantle at depths greater than 1000 km. As such, the existence of Fe7(C,N)3 in diamond can be an indicator of water transportation to the deep mantle.
Abstract We use high‐resolution lidar microtopographic data and luminescence dating to constrain incremental Holocene–latest Pleistocene slip rates for the Wairau fault, a major dextral strike‐slip fault in the Marlborough Fault System, South Island, New Zealand. Our data come from two closely spaced study areas along the structurally simple, central portion of the fault: The well‐known Branch River terrace flight, and a previously undated series of offset risers and channel features several km to the east that we refer to as the Dunbeath site. Field work and mapping using lidar‐derived topography yields revised or novel measurements of nine fault offsets. We date those features using a post‐IR 50 ‐IRSL 225 infrared stimulated luminescence dating method, and a stratigraphically informed Bayesian age model. The dated slip history of the Wairau fault is further constrained using newly cataloged offset measurements collected along a ∼35 km stretch of the fault, and available paleoseismic data. Incremental slip rates are precisely computed using a Monte Carlo resampling scheme. Our results provide a nearly earthquake‐by‐earthquake record of incremental slip, with pronounced variations in incremental slip rate spanning multiple millennia and tens of m of slip. These extreme, multi‐millennial variations in fault slip rate have basic implications for earthquake occurrence, plate boundary lithosphere behavior, and probabilistic seismic hazard assessment.
X-ray free electron laser (XFEL) sources coupled to high-power laser systems offer an avenue to study the structural dynamics of materials at extreme pressures and temperatures. The recent commissioning of the DiPOLE 100-X laser on the high energy density (HED) instrument at the European XFEL represents the state-of-the-art in combining x-ray diffraction with laser compression, allowing for compressed materials to be probed in unprecedented detail. Here, we report quantitative structural measurements of molten Sn compressed to 85(5) GPa and ∼3500 K. The capabilities of the HED instrument enable liquid density measurements with an uncertainty of ∼1% at conditions which are extremely challenging to reach via static compression methods. We discuss best practices for conducting liquid diffraction dynamic compression experiments and the necessary intensity corrections which allow for accurate quantitative analysis. We also provide a polyimide ablation pressure vs input laser energy for the DiPOLE 100-X drive laser which will serve future users of the HED instrument.
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