Using first-principles calculations, we predict a lightweight room-temperature ferroelectric carbonboron framework in a host-guest clathrate structure. This ferroelectric clathrate, with composition ScB3C3, exhibits high polarization density and low mass density compared with widely used commercial ferroelectrics. Molecular dynamics simulations show spontaneous polarization with a moderate above-room-temperature T-c of similar to 370 K, which implies large susceptibility and possibly large electrocaloric and piezoelectric constants at room temperature. Our findings open the possibility for a new class of ferroelectric materials with potential across a broad range of applications.

Nitrogen is essential to life, and yet is also the most depleted element in the Earth relative to gas-rich chondrites. A key expression of Earth's N depletion is its elevated sulfur-nitrogen (S/N) ratio. Primordial stratification into a core, mantle, and atmosphere is the largest mass transfer process that terrestrial planets experience, but the data required to evaluate how S/N ratios respond to primordial stratification of Earth-sized planets do not exist. We report new metal-silicate partitioning experiments on N up to 26 GPa and 3437 K. Our data indicate that nitrogen becomes more siderophile with increasing pressure and less siderophile with increasing carbon and nickel in the metal phase. We apply our new experiments with literature data for S partitioning to a core formation-primordial atmosphere degassing model. Our model demonstrates that the S/N ratio of the observable Earth can be set during primordial stratification under the same extreme P-T conditions that satisfy refractory siderophile element budgets while also yielding a bulk planet with S contents near that estimated from Earth's volatility trend.

We report new Os and Hf isotopic data on mafic lavas from several key portions of the East African Rift System (EARS) with the goal of determining how contributions from various source domains influence volcanism in the evolving rift system. Our study uses picrites and basalts associated with the Afar plume in NW Ethiopia and with prolonged extension in Turkana, N Kenya, as well as mafic lavas from Kivu and Rungwe in the Western Branch of the EARS. Basalts from NW Ethiopia and Turkana have low Os concentrations (9-22 ppt) and display a range of Os-187/Os-188 (0.1239-0.4366). The 30 Ma high-TiO2 picrites from NW Ethiopia and 20-23 Ma picrites from Turkana have higher Os concentrations (579-1120 ppt) than associated basalts. Picrites from NW Ethiopia have initial Os-187/Os-188 = 0.1239-0.1311 and epsilon(Hf) = 12.0-13.4, consistent with derivation from a mantle source common to global OIB (i.e. "C"). In contrast, 20-23 Ma Turkana picrites have more radiogenic initial Os-187/Os-188 (0.1450-0.1483). None of the picrites display convincing evidence for crustal or subcontinental lithospheric mantle input. Instead, the data are consistent with geochemical and geophysical models that demonstrate early evolution of the EARS was supported dynamically by geochemically distinct regions of mantle upwelling. Specifically, NW Ethiopian lavas are chemically analogous to the "C"-like Afar plume while Miocene Turkana lavas display HIMU-like geochemical features. The HIMU component in Turkana lavas can be generated by mixing similar to 30% ancient (1.7-2 Ga) hydrothermally altered subducted oceanic crust with similar to 70% "C"-like mantle material (i.e. < 1 Ga recycled hydrothermally altered oceanic crust). In contrast, Kivu and Rungwe lavas have low Os concentrations (3-87 ppt) and more radiogenic Os-187/Os-188 (0.1615-0.3610) that appear to be dominated by contributions from metasomatized lithospheric mantle. Seismological observations indicate that there are thermochemical heterogeneities within the deep-seated African super-plume; these heterogeneities are a plausible source for the ancient recycled oceanic crust contributing to Miocene volcanism in Turkana. We propose that mafic magmatism in both the Afar region and northern Kenya are derived from different portions of this long-lived thermochemical feature. (C) 2012 Elsevier B.V. All rights reserved.

The discovery of more than 4500 extrasolar planets has created a need for modeling their interior structure and dynamics. Given the prominence of iron in planetary interiors, we require accurate and precise physical properties at extreme pressure and temperature. A first-order property of iron is its melting point, which is still debated for the conditions of Earth's interior. We used high-energy lasers at the National Ignition Facility and in situ x-ray diffraction to determine the melting point of iron up to 1000 gigapascals, three times the pressure of Earth's inner core. We used this melting curve to determine the length of dynamo action during core solidification to the hexagonal close-packed (hcp) structure. We find that terrestrial exoplanets with four to six times Earth's mass have the longest dynamos, which provide important shielding against cosmic radiation.

Isothermal equations of state were determined for the open-framework silicon allotrope Si-24 and its sodium-filled precursor (Na4Si24) using different pressure media including hydrogen and argon, and with no pressure medium. Si-24 does not transform into diamond-cubic silicon under compression, and the low-density phase possesses a bulk modulus of 91(2) GPa. The sodium-filled precursor exhibits a comparable volumetric compressibility with different axial trends that are explained by the crystallographic structure. Above 11 GPa, Si-24 transforms to the beta-tin structure, followed by other high-pressure silicon allotropes similar to diamond-cubic silicon, driven by a large increase in density. Small molecules such as H-2 do not enter the channels of Si-24 during compression at room temperature, however, hydrostaticity strongly influences the transformation pressure and range of coexistence with other phases including beta-Sn, Imma, and simple-hexagonal Si.

We report results from multi-anvil (MA) and laser-heated diamond anvil cell (LH-DAC) experiments that synthesize high-pressure phases, including bridgmanite, ferropericlase, stishovite, and ultramafic liquid, in the presence of an argon-rich fluid. The goal of the experiments is to constrain the equilibrium distribution of argon in magma ocean environments. Argon concentrations in LH-DAC experiments were quantified by electron microprobe analysis, while argon concentrations in MA experiments were quantified by laser-ablation mass spectrometry and electron microprobe analysis. Our LH-DAC experiments demonstrate that argon solubility in ultramafic liquid is near or above 1.5 wt.% at conditions between 13-101 GPa and 2300-6300 K. Argon concentrations in bridgmanite and ferropericlase synthesized in LH-DAC experiments range from below detection to 0.58 wt.%. Argon concentrations in bridgmanite and ferropericlase synthesized in MA experiments range from below detection to 2.16 wt.% for electron microprobe measurements and laser-ablation measurements. We interpret this wide range of argon concentrations in minerals to reflect the variable presence of argon-rich fluid inclusions in analytical volumes. Our analyses therefore provide upper limit constraints for argon solubility in high-pressure minerals (<0.015 wt.%) across all mantle pressures and temperatures. The combination of relatively high argon solubility in ultramafic liquid (similar to 1.5 wt.%) and low argon solubility in minerals implies argon incompatibility (D-bridgmanite-melt (Ar) < 0.01, D-ferropericlase-melt(Ar) < 0.01) during magma ocean crystallization and that the initial distribution of argon, and likely other neutral species, may be controlled by liquids trapped in a crystallizing magma ocean. We thus predict a basal magma ocean would be enriched in noble gases relative to other regions of the mantle. Moreover, we predict that the noble gas parent-daughter ratio of magma ocean cumulates pile will increase with crystallization, assuming refractory and incompatible behavior for parent elements. (C) 2020 Elsevier B.V. All rights reserved.

We report results from a wide-angle controlled source seismic experiment across the Juan de Fuca plate designed to investigate the evolution of the plate from accretion at the Juan de Fuca ridge to subduction at the Cascadia margin. A two-dimensional velocity model of the crust and upper mantle is derived from a joint reflection-refraction traveltime inversion. To interpret our tomography results, we first generate a plausible baseline velocity model, assuming a plate cooling model and realistic oceanic lithologies. We then use an effective medium theory to infer from our tomography results the extent of porosity, alteration, and water content that would be required to explain the departure from the baseline model. In crust of ages >1Ma and away from propagator wakes and regions of faulting due to plate bending, we obtain estimates of upper crustal hydration of 0.5-2.1wt % and find mostly dry lower crust and upper mantle. In sections of the crust affected by propagator wakes we find upper estimates of upper crustal, lower crustal, and upper mantle hydration of 3.1, 0.8, and 1.8wt %, respectively. At the Cascadia deformation front, we find that the amount of water stored at uppermost mantle levels in the downgoing JdF plate is very limited (<0.3wt %), with most of the water carried into the subduction zone being stored in the oceanic crust.

The depth of melting beneath mid-ocean ridges (MORs) controls the melt composition as well as its rheology. Since mantle melting below MORs is the main mechanism of mantle degassing and CO2 emission into the atmosphere and oceans, there is an increasing interest in understanding the sub-ridge mantle conditions leading to its melting. Here we study the effect of oxygen fugacity on melting of carbonate-bearing peridotite at 3 GPa. Two metal-metal-oxide buffers (RRO and IW) were used to influence the fO(2) of the experimental charge. Using Ir-Fe alloy sliding redox sensors, the fO(2) of the two sets of experiments was measured. The solidus at IW + 4.5 was found to be at 950 degrees C, while at IW + 2.5 melting initiated at 1150 degrees C. In both sets of experiments, near-solidus carbonatitic melts evolved to carbon-bearing silicate melts with increasing temperature. This study together with previous studies suggest that increasing fO(2) of a carbonate-bearing peridotite results in lowering of its melting temperature. Extrapolating these solidi to higher pressures results in initiation of melting of a relatively oxidizing mantle at similar to 430 km while melting of a more reduced mantle will initiate at depth of similar to 320 km. Very low velocity anomalies in the sub-ridge mantle at depth may reflect the initiation of melting, triggered by the presence of carbonate in the mantle at 1-2 log units below QFM.

Mg2GeO4 is important as an analog for the ultrahigh-pressure behavior of Mg2SiO4, a major component of planetary interiors. In this study, we have investigated magnesium germanate to 275 GPa and over 2,000 K using a laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction and density functional theory (DFT) computations. The experimental results are consistent with the formation of a phase with disordered Mg and Ge, in which germanium adopts eightfold coordination with oxygen: the cubic, Th3P4-type structure. DFT computations suggest partial Mg-Ge order, resulting in a tetragonal I (4) over bar 2d structure indistinguishable from I (4) over bar 2d Th3P4 in our experiments. If applicable to silicates, the formation of this highly coordinated and intrinsically disordered phase may have important implications for the interior mineralogy of large, rocky extrasolar planets.

Relative to the rich library of small-molecule organics, few examples of ordered extended (i.e., nonmolecular) hydrocarbon networks are known. In particular, sp(3) bonded, diamond-like materials represent appealing targets because of their desirable mechanical, thermal, and optical properties. While many covalent organic frameworks (COFs)-extended, covalently bonded, and porous structures-have been realized through molecular architecture with exceptional control, the design and synthesis of dense, covalent extended solids has been a longstanding challenge. Here we report the preparation of a sp(3)-bonded, low-dimensional hydrocarbon synthesized via high-pressure, solid-state diradical polymerization of cubane (C8H8), which is a saturated, but immensely strained, cage-like molecule. Experimental measurements show that the obtained product is crystalline with three-dimensional order that appears to largely preserve the basic structural topology of the cubane molecular precursor and exhibits high hardness (comparable to fused quartz) and thermal stability up to 300 degrees C. Among the plausible theoretical candidate structures, one-dimensional carbon scaffolds comprising six- and four-membered rings that pack within a pseudosquare lattice provide the best agreement with experimental data. These diamond-like molecular rods with extraordinarily small thickness are among the smallest members in the carbon nanothread family, and calculations indicate one of the stiffest one-dimensional systems known. These results present opportunities for the synthesis of purely sp3-bonded extended solids formed through the strain release of saturated molecules, as opposed to only unsaturated precursors.