Deep mantle origin of large igneous provinces and komatiites.

Large igneous provinces (LIPs) resulted from intraplate magmatic events mobilizing volumes of magma up to several million cubic kilometers. LIPs and lavas with deep mantle sources have compositions ranging from komatiites found in Archean greenstone belts to basalts and picrites in Phanerozoic flood basalt and recent oceanic islands. In this study, we identify the mantle conditions appropriate to each type of lava based on an experimental study of the melting of pyrolite. The depth of the mantle source decreases from 600 to 700 km for the oldest komatiites to 100 to 300 km for picrites and basalts, and the extent of mantle melting ranges from 10 to 50%. We develop a geodynamical model that explains the origin of the hot mantle plumes capable of generating these melting P-T conditions. Within a superadiabatic temperature gradient persisting in the deep mantle, the ascent of hot mantle plumes creates excess temperatures up to 250 to 300 K by adiabatic decompression.

Stability and equation of state of face-centered cubic and hexagonal close packed phases of argon under pressure.

The compression of argon is measured between 10 K and 296 K up to 20 GPa and and up to 114 GPa at 296 K in diamond anvil cells. Three samples conditioning are used: (1) single crystal sample directly compressed between the anvils, (2) powder sample directly compressed between the anvils, (3) single crystal sample compressed in a pressure medium. A partial transformation of the face-centered cubic (fcc) phase to a hexagonal close-packed (hcp) structure is observed above 4.2-13 GPa. Hcp phase forms through stacking faults in fcc-Ar and its amount depends on pressurizing conditions and starting fcc-Ar microstructure. The quasi-hydrostatic equation of state of the fcc phase is well described by a quasi-harmonic Mie-Grüneisen-Debye formalism, with the following 0 K parameters for Rydberg-Vinet equation: [Formula: see text] = 38.0 Å[Formula: see text]/at, [Formula: see text] = 2.65 GPa, [Formula: see text] = 7.423. Under the current experimental conditions, non-hydrostaticity affects measured P-V points mostly at moderate pressure ([Formula: see text] 20 GPa).

In situ X-ray diffraction of silicate liquids and glasses under dynamic and static compression to megabar pressures.

Properties of liquid silicates under high-pressure and high-temperature conditions are critical for modeling the dynamics and solidification mechanisms of the magma ocean in the early Earth, as well as for constraining entrainment of melts in the mantle and in the present-day core-mantle boundary. Here we present in situ structural measurements by X-ray diffraction of selected amorphous silicates compressed statically in diamond anvil cells (up to 157 GPa at room temperature) or dynamically by laser-generated shock compression (up to 130 GPa and 6,000 K along the MgSiO3 glass Hugoniot). The X-ray diffraction patterns of silicate glasses and liquids reveal similar characteristics over a wide pressure and temperature range. Beyond the increase in Si coordination observed at 20 GPa, we find no evidence for major structural changes occurring in the silicate melts studied up to pressures and temperatures exceeding Earth's core mantle boundary conditions. This result is supported by molecular dynamics calculations. Our findings reinforce the widely used assumption that the silicate glasses studies are appropriate structural analogs for understanding the atomic arrangement of silicate liquids at these high pressures.

Formation of bridgmanite-enriched layer at the top lower-mantle during magma ocean solidification.

Thermochemical heterogeneities detected today in the Earth's mantle could arise from ongoing partial melting in different mantle regions. A major open question, however, is the level of chemical stratification inherited from an early magma-ocean (MO) solidification. Here we show that the MO crystallized homogeneously in the deep mantle, but with chemical fractionation at depths around 1000 km and in the upper mantle. Our arguments are based on accurate measurements of the viscosity of melts with forsterite, enstatite and diopside compositions up to ~30 GPa and more than 3000 K at synchrotron X-ray facilities. Fractional solidification would induce the formation of a bridgmanite-enriched layer at ~1000 km depth. This layer may have resisted to mantle mixing by convection and cause the reported viscosity peak and anomalous dynamic impedance. On the other hand, fractional solidification in the upper mantle would have favored the formation of the first crust.

Kinetics and detectability of the bridgmanite to post-perovskite transformation in the Earth's D″ layer.

Bridgmanite, the dominant mineral in the Earth's lower mantle, crystallizes in the perovskite structure and transforms into post-perovskite at conditions relevant for the D[Formula: see text] layer. This transformation affects the dynamics of the Earth's lowermost mantle and can explain a range of seismic observations. The thickness over which the two phases coexist, however, can extend over 100 km, casting doubt on the assignment of the observed seismic boundaries. Here, experiments show that the bridgmanite to post-perovskite transition in (Mg[Formula: see text],Fe[Formula: see text])SiO[Formula: see text] is fast on geological timescales. The transformation kinetics, however, affects reflection coefficients of [Formula: see text] and [Formula: see text] waves by more than one order of magnitude. Thick layers of coexisting bridgmanite and post-perovskite can hence be detected using seismic reflections. Morever, the detection and wave period dependence of D[Formula: see text] reflections can be used to constrain significant features of the Earth's lowermost mantle, such as the thickness of the coexistence layer, and obtain information on temperature and grain sizes.

Low hydrogen contents in the cores of terrestrial planets.

Hydrogen has been thought to be an important light element in Earth's core due to possible siderophile behavior during core-mantle segregation. We reproduced planetary differentiation conditions using hydrogen contents of 450 to 1500 parts per million (ppm) in the silicate phase, pressures of 5 to 20 GPa, oxygen fugacity varying within IW-3.7 and IW-0.2 (0.2 to 3.7 log units lower than iron-wüstite buffer), and Fe alloys typical of planetary cores. We report hydrogen metal-silicate partition coefficients of ~2 × 10-1, up to two orders of magnitude lower than reported previously, and indicative of lithophile behavior. Our results imply H contents of ~60 ppm in the Earth and Martian cores. A simple water budget suggests that 90% of the water initially present in planetary building blocks was lost during planetary accretion. The retained water segregated preferentially into planetary mantles.

Dehydration of chlorite explains anomalously high electrical conductivity in the mantle wedges.

Mantle wedge regions in subduction zone settings show anomalously high electrical conductivity (~1 S/m) that has often been attributed to the presence of aqueous fluids released by slab dehydration. Laboratory-based measurements of the electrical conductivity of hydrous phases and aqueous fluids are significantly lower and cannot readily explain the geophysically observed anomalously high electrical conductivity. The released aqueous fluid also rehydrates the mantle wedge and stabilizes a suite of hydrous phases, including serpentine and chlorite. In this present study, we have measured the electrical conductivity of a natural chlorite at pressures and temperatures relevant for the subduction zone setting. In our experiment, we observe two distinct conductivity enhancements when chlorite is heated to temperatures beyond its thermodynamic stability field. The initial increase in electrical conductivity to ~3 × 10(-3) S/m can be attributed to chlorite dehydration and the release of aqueous fluids. This is followed by a unique, subsequent enhancement of electrical conductivity of up to 7 × 10(-1) S/m. This is related to the growth of an interconnected network of a highly conductive and chemically impure magnetite mineral phase. Thus, the dehydration of chlorite and associated processes are likely to be crucial in explaining the anomalously high electrical conductivity observed in mantle wedges. Chlorite dehydration in the mantle wedge provides an additional source of aqueous fluid above the slab and could also be responsible for the fixed depth (120 ± 40 km) of melting at the top of the subducting slab beneath the subduction-related volcanic arc front.

Experimental evidence supports mantle partial melting in the asthenosphere.

The low-velocity zone (LVZ) is a persistent seismic feature in a broad range of geological contexts. It coincides in depth with the asthenosphere, a mantle region of lowered viscosity that may be essential to enabling plate motions. The LVZ has been proposed to originate from either partial melting or a change in the rheological properties of solid mantle minerals. The two scenarios imply drastically distinct physical and geochemical states, leading to fundamentally different conclusions on the dynamics of plate tectonics. We report in situ ultrasonic velocity measurements on a series of partially molten samples, composed of mixtures of olivine plus 0.1 to 4.0 volume % of basalt, under conditions relevant to the LVZ. Our measurements provide direct compressional (V P) and shear (V S) wave velocities and constrain attenuation as a function of melt fraction. Mantle partial melting appears to be a viable origin for the LVZ, for melt fractions as low as ~0.2%. In contrast, the presence of volatile elements appears necessary to explaining the extremely high V P/V S values observed in some local areas. The presence of melt in LVZ could play a major role in the dynamics of plate tectonics, favoring the decoupling of the plate relative to the asthenosphere.

Cosmochemical fractionation by collisional erosion during the Earth's accretion.

Early in the Solar System's history, energetic collisions of differentiated bodies affected the final composition of the terrestrial planets through partial destruction. Enstatite chondrites (EC) are the best candidates to represent the primordial terrestrial precursors as they present the most similar isotopic compositions to Earth. Here we report that collisional erosion of >15% of the early Earth's mass can reconcile the remaining compositional differences between EC and the Earth. We base our demonstration on experimental melting of an EC composition at pressures between 1 bar and 25 GPa. At low pressures, the first silicate melts are highly enriched in incompatible elements Si, Al and Na, and depleted in Mg. Loss of proto-crusts through impacts raises the Earth's Mg/Si ratio to its present value. To match all major element compositions, our model implies preferential loss of volatile lithophile elements and re-condensation of refractory lithophile elements after the impacts.

Confined H2O molecules as local probes of pressure-induced amorphisation in faujasite.

Confined H2O molecules act as local probes for depressurization phenomena during the pressure induced amorphisation of faujasite NaX at which the OH stretching frequency first decreases and then increases almost to its room pressure value upon further compression. Pair distribution function (PDF) analysis provides evidence that amorphisation corresponds to a collapse of the structure around hydrated sodium cations with strong distortion of the secondary building units (double six-membered rings, sodalite cages). Both the use of guest molecules as local probes in far- and mid-infrared spectroscopy, where we correlate intermolecular water H bonding vibrations and internal mode behaviour under confinement, and PDF analysis could be of great use to study the mechanical behaviour of other hydrated materials.

Melting of subducted basalt at the core-mantle boundary.

The geological materials in Earth's lowermost mantle control the characteristics and interpretation of seismic ultra-low velocity zones at the base of the core-mantle boundary. Partial melting of the bulk lower mantle is often advocated as the cause, but this does not explain the nonubiquitous character of these regional seismic features. We explored the melting properties of mid-oceanic ridge basalt (MORB), which can reach the lowermost mantle after subduction of oceanic crust. At a pressure representative of the core-mantle boundary (135 gigapascals), the onset of melting occurs at ~3800 kelvin, which is ~350 kelvin below the mantle solidus. The SiO2-rich liquid generated either remains trapped in the MORB material or solidifies after reacting with the surrounding MgO-rich mantle, remixing subducted MORB with the lowermost mantle.

Solid-liquid iron partitioning in Earth's deep mantle.

Melting processes in the deep mantle have important implications for the origin of the deep-derived plumes believed to feed hotspot volcanoes such as those in Hawaii. They also provide insight into how the mantle has evolved, geochemically and dynamically, since the formation of Earth. Melt production in the shallow mantle is quite well understood, but deeper melting near the core-mantle boundary remains controversial. Modelling the dynamic behaviour of deep, partially molten mantle requires knowledge of the density contrast between solid and melt fractions. Although both positive and negative melt buoyancies can produce major chemical segregation between different geochemical reservoirs, each type of buoyancy yields drastically different geodynamical models. Ascent or descent of liquids in a partially molten deep mantle should contribute to surface volcanism or production of a deep magma ocean, respectively. We investigated phase relations in a partially molten chondritic-type material under deep-mantle conditions. Here we show that the iron partition coefficient between aluminium-bearing (Mg,Fe)SiO(3) perovskite and liquid is between 0.45 and 0.6, so iron is not as incompatible with deep-mantle minerals as has been reported previously. Calculated solid and melt density contrasts suggest that melt generated at the core-mantle boundary should be buoyant, and hence should segregate upwards. In the framework of the magma oceans induced by large meteoritic impacts on early Earth, our results imply that the magma crystallization should push the liquids towards the surface and form a deep solid residue depleted in incompatible elements.

Topologically ordered amorphous silica obtained from the collapsed siliceous zeolite, silicalite-1-F: a step toward "perfect" glasses.

A dense amorphous form of silica was prepared at high pressure from the highly compressible, siliceous zeolite, silicalite-1-F. Reverse Monte Carlo modeling of total X-ray scattering data shows that the structure of this novel amorphous form of SiO(2) recovered under ambient conditions is distinct from vitreous SiO(2) and retains the basic framework topology (i.e., chemical bonds) of the starting crystalline zeolite. This material is, however, amorphous over the different length scales probed by Raman and X-ray scattering due to strong geometrical distortions. This is thus an example of new topologically ordered, amorphous material with a different intermediate-range structure, a lower entropy with respect to a standard glass, and distinct physical and mechanical properties, eventually approaching those of an "ordered" or "perfect" glass. The same process in more complex aluminosilicate zeolites will, in addition, lead to an amorphous material which conserves the framework topology and chemical order of the crystal. The large volume collapse in this material may also be of considerable interest for new applications in shock wave absorption.

Ultimate metastable solubility of boron in diamond: synthesis of superhard diamondlike BC5.

Here, we report the synthesis of cubic BC5 (c-BC5), the diamondlike B-C phase with the highest boron content ever achieved, at 24 GPa and about 2200 K, using both a laser-heated diamond anvil cell and large-volume multianvil apparatus. The synthesized phase is low compressible (bulk modulus of 335 GPa), conductive, and exhibits extreme Vickers hardness (71 GPa), unusually high for superhard materials fracture toughness (9.5 MPa m;{0.5}), and high thermal stability (up to 1900 K); this makes it an exceptional superabrasive and promising material for high-temperature electronics.

Oxygen K-edge fine structures of water by x-ray Raman scattering spectroscopy under pressure conditions.

Fine structure of the oxygen K edge was investigated for water at ambient pressure, 0.16, 0.21, 0.27, 0.47, and 0.60 GPa using x-ray Raman scattering spectroscopy (XRS). Similarity in near-edge structures at 0.16 and 0.60 GPa suggests little difference in the electronic state of oxygen in the low-pressure and high-pressure forms of water. Yet, we observed significant variation of preedge structure of the XRS spectra with compression. The intensity of the preedge peak at 535.7 eV has a minimal value at around 0.3 GPa, indicating that the number of hydrogen bonding increases first and then decreases as a function of pressure.