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.

We quantitatively explore element redistribution at subduction zones using numerical mass balance models to evaluate the roles of the subduction zone filter in the Earth's geochemical cycle. Our models of slab residues after arc magma genesis differ from previous ones by being internally consistent with geodynamic models of modern arcs that successfully explain arc magma genesis and include element fluxes from the dehydration/melting of each underlying slab component. We assume that the mantle potential temperature (T-p) was 1400-1650 degrees C at 3.5-1.7 Ga and gradually decreased to 1300-1350 degrees C today. Hot subduction zones with T-p approximate to 1650 degrees C have a thermal structure like modern SW Japan where high-Mg andesite is formed which is chemically like continental crust. After 2.5-1.7 Gyr of storage in the mantle, the residual igneous oceanic crust from hot subduction zones can evolve isotopically to the HIMU mantle component, the residual base of the mantle wedge to EMI, the residual sediment becomes an essential part of EMII, and the residual top of the mantle wedge can become the subcontinental lithosphere component. The Common or Focal Zone component is a stable mixture of the first three residues occasionally mixed with early depleted mantle. Slab residues that recycled earlier (approximate to 2.5 Ga) form the DUPAL anomaly in the southern hemisphere, whereas residues of more recent recycling (approximate to 1.7 Ga) underlie the northern hemisphere. These ages correspond to major continental crust forming events. The east-west heterogeneity of the depleted upper mantle involves subcontinental mantle except in the Pacific.

The Evje-Iveland pegmatite field in Norway contains pegmatites that are known for their rare scandium mineralization. The petrogenesis of these pegmatites has been debated in the literature for nearly a century. Hypotheses for the origin of the pegmatite-forming melt have included either anatexis of the host amphibolite in vapor-absent conditions, wherein scandium is scavenged from the host amphibolite; or magmatic differentiation, wherein scandium is concentrated through magmatic processes. In order to test the hypothesis that the pegmatite-forming melt was sourced from the host amphibolite, partial melting experiments on the host amphibolite have been performed. These experiments were performed at temperatures ranging from 700 to 1064. C and pressures between 400 and 550 MPa in a piston-cylinder apparatus. The solidus of the host amphibolite has been determined to be approximately 900 degrees C at 500 MPa and is significantly higher than the temperature of pegmatite formation. Partial melting of <40% can produce glasses that are broadly granitic in composition and are aluminum- and sodium-rich; however, they are less siliceous than the Evje-Iveland pegmatites. These glasses are also scandium- and REE-poor, and have REE patterns similar to leucosomes in vein-type migmatites, produced at low pressures, but dissimilar to the Evje-Iveland pegmatites. The results of these experiments are thus inconsistent with the hypothesis that the Evje-Iveland pegmatites or, by extension, other rare-element pegmatites, are the result of direct anatexis alone of common metamorphic rock such as amphibolites. It is proposed that the formation of the Evje-Iveland pegmatites is the result of partial melting of a scandium-rich ultramafic or mafic complex, differentiation of that partial melt, and emplacement of that melt into the host amphibolite. Thus, the pegmatite-forming melt may represent the final stages of magmatic differentiation, which is the preferred model for the formation of the Evje-Iveland pegmatites.

Recent studies of highly compressed organic molecules reveal the synthesis of one-dimensional (1D) nanothread structures, typically formed through addition reactions of unsaturated bonds. Although these nanostructures have been demonstrated from molecules such as benzene, pyridine, and thiophene, it remains unclear whether functionalized nanothreads can be produced from precursors with different substituent groups under high-pressure conditions. Here, we examine the controlled pressure-induced polymerization of several para-disubstituted benzene molecular crystals and cocrystals with different functional groups including dinitrobenzene, diethynylbenzene, and dicyanobenzene. X-ray diffraction and infrared spectroscopy provide evidence for the formation of ordered nanostructures that maintain their topological relationship with the starting molecular phase and preserve initial functionality. Although no clear correlation between specific functional groups and polymerization pressure was observed, the proximity toward sandwich-type pi-stacking within the starting molecular crystals influences reaction pathway selectivity and the formation of new saturated bonds under normal compression conditions. We propose a simple correlation related to pi-stacking, wherein the stacking distance between parallel planes of monomers and the slippage angle between pi-stacks are important aspects that influence polymerization pathway selectivity and the formation of ordered products under normal compression at room temperature. Functionalized nanothread structures are possible through careful precursor selection, and an improved understanding of pi-stacking polymerization may lead to the realization of well-defined organic nanostructures with designed functionality.