A high-magnetic field once existed in the early history of the Moon, suggesting the core once possessed a thermally driven dynamo. The thermal conductivity of core materials is a significant parameter of the dynamo. The lunar core composition is thought to be iron or iron-alloyed with some light elements (e.g., S, P, Si, and C), but its transport properties remain uncertain. We measured the electrical resistivity of iron and Fe-3 wt%P alloys at 5 GPa and high temperatures. Apart from the quasi four-point technique, the four-probe van der Pauw technique was also employed to measure the resistivity of pure iron. Adding similar to 3 wt% phosphorus to iron slightly increases the resistivity at 5 GPa and 1000-1500 K due to the impurity effect. The resistivity of Fe-3 wt%P alloys increases at the onset of melting. Via the Wiedemann-Franz law, the thermal conductivity at the lunar core-mantle boundary (CMB) is estimated to be 28.6-34.2 Wm(-1)K(-1) for a light-element free core and 31.5 +/- 1.9 Wm(-1)K(-1) for a phosphorus-bearing (similar to 3 wt% P) core. Therefore, small amounts of phosphorus in the lunar core slightly impact its thermal conductivity. The estimated conductive heat flow across the lunar CMB varies from 4.5 to 5.7 GW, and the adiabatic heat flux varies from 3.3 to 4.2 mW/m(2), depending on the core's composition (Fe or Fe-3 wt%P). Integrating our results with previous lunar core evolution models, we suggest that a thermally driven dynamo persisted until 3.63-3.88 Ga ago.

Using density functional perturbation theory, we computed the phonon frequencies and Raman and IR activities of hafnia polymorphs (P4(2)nmc, Pca2(1), Pmn2(1), Pbca OI, brookite, and baddeleyite) for phase identification. We investigated the evolution of Raman and IR activities with respect to epitaxial strain and provide plots of frequency differences as a function of strain for experimental calibration and identification of the strain state of the sample. We found Raman signatures of different hafnia polymorphs: omega ( A(1g) ) = 300 cm(-1) for P4(2)nmc, omega (A( 1)) = 343 cm(-1) for Pca2(1), omega ( B-2) = 693 cm(-1) for Pmn2(1), omega (A( g)) = 513 cm(-1) for Pbca (OI), omega (A(g)) = 384 cm(-1) for brookite, and omega (A(g)) = 496 cm(-1) for baddeleyite. We also identified the Raman B-1g mode, an anti-phase vibration of dipole moments [omega (B-1g) = 758 cm(-1) for OI and omega ( B-1g ) = 784 cm(-1) for brookite], as the Raman signature of antipolar Pbca structures. We calculated a large splitting between the longitudinal optical and transverse optical modes [delta omega(LO) - TO ( A(1)(z)) = 255 cm(-1) in Pca2(1) and delta omega( LO) (- TO) ( A 1 ) = 263 cm(-1) in Pmn2(1)] to the same order as those observed in perovskite ferroelectrics and related them to the anomalously large Born effective charges of Hf atoms [ Z * ( Hf ) = 5.54]. Published under an exclusive license by AIP Publishing.

We report a carbon-boron clathrate with composition 2 La@B6C6 (LaB3C3). Like recently reported SrB3C3,([1]) single-crystal X-ray diffraction and computational modelling indicate that the isostructural La member crystallizes in the cubic bipartite sodalite structure (Type-VII clathrate) with La atoms encapsulated within truncated octahedral cages composed of alternating carbon and boron atoms. The covalent nature of the B-C bonding results in a hard, incompressible framework, and owing to the balanced electron count, La3+[B3C3](3-) exhibits markedly improved pressure stability and is a semiconductor with an indirect band gap predicted near 1.3 eV. A variety of different guest atoms may potentially be substituted within Type-VII clathrate cages, presenting opportunities for a large family of boron-stabilized, carbon-based clathrates with ranging physical properties.

On the basis of the van der Pauw method, we developed a new technique for measuring the electrical resistivity of metals in a cubic multi-anvil high-pressure apparatus. Four electrode wires were introduced into the sample chamber and in contact with the pre-pressed metal disk on the periphery. The sample temperature was measured with a NiCr-NiSi (K-type) thermocouple, which was separated from the sample by a thin hexagonal boron nitride layer. The electrodes and thermocouple were electrically insulated from each other and from the heater by an alumina tube as well. Their leads were in connection with cables through the gap between the tungsten carbide anvils. We performed experiments to determine the temperature dependence of electrical resistivity of pure iron at 3 and 5 GPa. The experiments produce reproducible measurements and the results provide an independent check on electrical resistivity data produced by other methods. The new technique provides reliable electrical resistivity measurements of metallic alloys and compounds at high pressure and temperature.

A strong correlation exists between the average slip rate by short-term slow slip events (SSEs) and changes in the slab geometry in Cascadia and Nankai. The generation of short-term SSEs is generally assumed to be related to the presence of fluids and we investigate the hypothesis that fluids released by metamorphic dehydration reactions migrate in 3-D due to complex slab geometry. The associated along-arc focusing of fluid flux is likely to cause higher average slip rate in certain patches. To test this hypothesis, we investigate how fluid migration is modified by along-strike changes in slab geometry. We use a numerical model of two-phase flow in subduction zones. In this model fluids migrate subparallel to the slab surface due to the anisotropic permeability inside a serpentinite layer just above the slab. In 3-D, we find that fluids migrate in the maximum-dip direction of the slab, rather than subparallel to the plate motion. As a result fluid paths concentrate with increasing porosity where the slab has a convex shape (and diverge with decreasing porosity where it has a concave shape). These results suggest that regions with a high average slip rate by short-term SSEs in Cascadia and Nankai can be explained by 3-D focusing of fluid migration. We predict a defocusing of fluids below the Kii Channel, Nankai, which may be the reason for the observed small slip by short-term SSEs in this location.

We report the synthesis of bulk, highly oriented, crystalline 4H hexagonal silicon (4H-Si), through a metastable phase transformation upon heating the single-crystalline Si-24 allotrope. Remarkably, the resulting 4H-Si crystallites exhibit an orientation relationship with the Si-24 crystals, indicating a structural relationship between the two phases. Optical absorption measurements reveal that 4H-Si exhibits an indirect band gap near 1.2 eV, in agreement with first principles calculations. The metastable crystalline transition pathway provides a novel route to access bulk crystalline 4H-Si in contrast to previous transformation paths that yield only nanocrystalline-disordered materials.

Knowledge of the sound velocity of core materials is essential to explain the observed anomalously low shear wave velocity (V-S) and high Poisson's ratio (sigma) in the solid inner core. To date, neither V-S nor sigma of Fe and Fe-Si alloy have been measured under core conditions. Here, we present V-S and sigma derived from direct measurements of the compressional wave velocity, bulk sound velocity, and density of Fe and Fe-8.6 wt%Si up to similar to 230 GPa and similar to 5400 K. The new data show that neither the effect of temperature nor incorporation of Si would be sufficient to explain the observed low V-S and high sigma of the inner core. A possible solution would add carbon (C) into the solid inner core that could further decrease V-S and increase sigma. However, the physical property-based Fe-Si-C core models seemingly conflict with the partitioning behavior of Si and C between liquid and solid Fe.

Subduction of oceanic lithosphere transports surface H2O into the mantle. Recent studies show that dense SiO2 in the form of stishovite, an abundant mineral in subducted oceanic crust at depths greater than -270 km, has the potential to host and transport a considerable amount of H2O into the lower mantle, but the H2O storage capacity of SiO2 phases at high pressure and temperature remains uncertain. We investigate the hydration of stishovite and its higher-pressure polymorphs, beta-stishovite and seifertite, with in situ X-ray diffraction experiments at high pressures and temperatures. The H2O contents in SiO2 phases are quantified based on observed increases in unit cell volume relative to the anhydrous SiO2 system. Density functional theory (DFT) computations permit calibration of water content as a function of volume change based on interstitial substitution of H2O. Regression of our experimental data indicates an H2O storage capacity in stishovite of -3.5 wt% in the transition zone and shallow lower mantle, decreasing to about 0.8 wt% at the base of the mantle. We find that SiO2-bearing subducted oceanic crust can accommodate all the H2O in slab lithosphere that survives sub-arc dehydration. Hydration of silica phases in subducted oceanic crust and their unparalleled capacity to host significant amounts of H2O even at high mantle temperatures provides a unique mechanism for transport and storage of water in the deepest mantle. (c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

We introduce and describe a new software infrastructure TerraFERMA, the Transparent Finite Element Rapid Model Assembler, for the rapid and reproducible description and solution of coupled multiphysics problems. The design of TerraFERMA is driven by two computational needs in Earth sciences. The first is the need for increased flexibility in both problem description and solution strategies for coupled problems where small changes in model assumptions can lead to dramatic changes in physical behavior. The second is the need for software and models that are more transparent so that results can be verified, reproduced, and modified in a manner such that the best ideas in computation and Earth science can be more easily shared and reused. TerraFERMA leverages three advanced open-source libraries for scientific computation that provide high-level problem description (FEniCS), composable solvers for coupled multiphysics problems (PETSc), and an options handling system (SPuD) that allows the hierarchical management of all model options. TerraFERMA integrates these libraries into an interface that organizes the scientific and computational choices required in a model into a single options file from which a custom compiled application is generated and run. Because all models share the same infrastructure, models become more reusable and reproducible, while still permitting the individual researcher considerable latitude in model construction. TerraFERMA solves partial differential equations using the finite element method. It is particularly well suited for nonlinear problems with complex coupling between components. TerraFERMA is open-source and available at http://terraferma. github. io, which includes links to documentation and example input files.

The origin of major volatiles nitrogen, carbon, hydrogen, and sulfur in planets is critical for understanding planetary accretion, differentiation, and habitability. However, the detailed process for the origin of Earth's major volatiles remains unresolved. Nitrogen shows large isotopic fractionations among geochemical and cosmochemical reservoirs, which could be used to place tight constraints on Earth's volatile accretion process. Here we experimentally determine N-partitioning and -isotopic fractionation between planetary cores and silicate mantles. We show that the core/mantle N-isotopic fractionation factors, ranging from -4 to +10, are strongly controlled by oxygen fugacity, and the core/mantle N-partitioning is a multi-function of oxygen fugacity, temperature, pressure, and compositions of the core and mantle. After applying N-partitioning and -isotopic fractionation in a planetary accretion and core-mantle differentiation model, we find that the N-budget and -isotopic composition of Earth's crust plus atmosphere, silicate mantle, and the mantle source of oceanic island basalts are best explained by Earth's early accretion of enstatite chondrite-like impactors, followed by accretion of increasingly oxidized impactors and minimal CI chondrite-like materials before and during the Moon-forming giant impact. Such a heterogeneous accretion process can also explain the carbon-hydrogen-sulfur budget in the bulk silicate Earth. The Earth may thus have acquired its major volatile inventory heterogeneously during the main accretion phase.