Numerous geophysical studies reported low seismic velocity and high electrical conductivity anomalies in the shallow, oceanic upper mantle of the Earth, at a level sometimes referred to as the Gutenberg discontinuity. This discontinuity is commonly interpreted to mark the onset of partial melting in the mantle, and also to correspond to the lithosphere-asthenosphere boundary (LAB), where the decoupling of the cold and rigid plates from the hot and convecting mantle drives plate tectonics. From petrological experiments and modelling, we determined the melting temperature of the mantle with relevant CO2 and H2O contents, which can be as low as similar to 1000 degrees C. We evidence that Earth's mantle does start melting at pressures and temperatures corresponding to the Gutenberg discontinuity. However, this discontinuity is clearly located within the lithosphere, at about two thirds of its thickness, and thus does not represent the LAB. The low temperatures (down to similar to 1000 degrees C) and the low fractions of melts at the Gutenberg discontinuity do not lower the viscosity of the mantle significantly. The LAB lies deeper at much higher temperatures (similar to 1300-1400 degrees C) where melting has progressed. This indicates it is not the onset of melting but the increase in melt fraction at the transition from volatile- to silicate-controlled melting that reduces mantle viscosity so as to drive plate tectonics at the LAB.

The effect of pressure on hydrogen diffusivity in San Carlos olivine (Fo90) single crystals was determined from hydrogenation experiments in a multi-anvil press at 6, 9 and 12 GPa, and at high temperature (900-1300 degrees C), for various durations. Crystallographically oriented prisms of gem-quality San Carlos olivine were hydrogenated under controlled oxygen fugacity (Ni-NiO) and silica activity (10% Opx). Polarized Fourier-transform infrared spectroscopy was used to quantify the hydroxyl distribution in the samples parallel to the crystallographic axes. The diffusivity of hydrogen is consistent with a single diffusion mechanism, proton-vacancy coupled diffusion, which is dominated by the presence of trivalent ions (infrared band doublet at 3357-3329 cm-1). The inferred chemical diffusion coefficients are slower than in olivine hydrogenated at lower pressure (<= 3 GPa) for the same diffusion mechanism and temperatures. Under the given experimental conditions, diffusion along the [001] axis is slightly faster than along [100] or [010]. A global fit to the data provides a master Arrhenius law of the metal vacancy diffusion (which rate-limits the hydrogen diffusion) along [001]: D[001] VMe = D[001] 0,VMe exp[ -(Q + PV)/RT], with the pre-exponential factorD[001]0,VMe= 10-4.00(+/- 0.46)m2s-1, the activation energy Q=183,000(+/- 12,000) J. mol-1, and the activation volume V=3.38(+/- 0.37)& times;10-6 m3.mol-1, and where pressure (P) is in Pa and the temperature (T) is in K. Despite the pressure effect, hydrogen diffusion coefficients in the entire upper mantle remain fast enough to alter hydrogen concentrations at the grain scale, although too slow to enable km-scale re-equilibration of hydrogen-rich or hydrogen-poor anomalies. Melt migration is thus required to enable large-scale re-equilibration of H heterogeneities.

We measured the thermal conductivity of composites consisting of mid-ocean ridge basalt (MORB) and polycrystalline olivine, as an analog for partially molten systems, to investigate the influence of low degree melting on heat transport. Experiments were conducted at 1 GPa and temperatures up to 1,600 K, with MORB fractions ranging from 0.1 to 10 vol.%. Adding MORB to the olivine matrix significantly altered the composite's thermal conductivity. Prior to melting, composites containing 0.1 and 10 vol.% MORB showed the most pronounced increase in conductivity relative to pure olivine, while intermediate fractions (similar to 1-5 vol.%) exhibited a decrease, followed by a rise at higher MORB contents. We attribute this non-monotonic behavior to impurity-lattice interactions within the MORB-bearing olivine, which reduce lattice disorder and enhance heat transport. Upon melting MORB, the thermal conductivity of the composites decreased, with the largest reductions (similar to 35%) observed in the 0.1 and 10 vol.% MORB samples, indicating that the melt acts as a thermal insulator. Applied to planetary interiors, these results suggest that lateral variations in melt fraction within thermal boundary layers could generate heterogeneities in heat flow, potentially affecting mantle convection patterns and the formation or evolution of thermal plumes.

Water in the mantle controls mantle convection and chemical transport processes within Earth's interior. However, whether the water in the lower mantle is primordial or recycled remains an open question. Here we report an electrical structure consisting of two stepped high‐conductivity anomalies crossing the 660‐km interface beneath northeastern Asia. The sheet‐like first‐step conductor locates in the mantle transition zone that can be best interpreted as hydrated stagnant Pacific slab, whereas the funnel‐like second‐step anomaly occurs in the uppermost lower mantle that is inferred to be a water reservoir replenished by the collapsed stagnant slab. The pivotal mechanism of this water transport process could be the dehydration reaction of dense hydrous magnesium silicates and the rehydration of stishovite and liebermannite in the transition zone. Our findings provide evidence for recycled lower‐mantle water reservoirs, and together with seismological and mineralogical studies, illustrate the deep‐mantle water plumbing system operated by cold slab subduction.

The earliest form of continental crust was produced by tonalite‐trondhjemite‐granodiorite (TTG) magmas. Molten albite (NaAlSi 3 O 8 ) is representative of TTGs and also a major component of modern crust‐forming magma. The viscosity of the melt controls the magma ascent rate and hence influences the production of new continental crust. It is well known that the viscosity ( η ) of albitic melt exhibits an anomalous pressure ( P ) dependence. However, prior results on the melt η at high‐ P differ significantly which limits our ability to predict the movement of crust‐forming magma at depth. In this study, we more tightly constrained the P ‐effect on η in anhydrous albitic melt via high‐ P and high‐temperature ( T ) falling sphere experiments. We limited undesirable drag effects by using small sphere‐to‐capsule diameter ratios ( d / D ) such that d / D ≤ 0.12, and evaluated uncertainties due to such drag using a Monte Carlo approach. Our results show that melt η first decreases with P (i.e., ∂ η /∂ P < 0) and then increases with continued compression (∂ η /∂ P > 0) with a well‐defined η minimum ( η min ) at ∼6 GPa along a ∼2,000 K isotherm. We find that the viscosity of the melt can be described by an Arrhenius formalism with an activation volume that varies with P and T . The results indicate that η of aluminosilicate magmas decrease with depth and temperature in the crust, thereby mobilizing the magmas to promote rapid volcanic eruptions. The results also suggest that TTG magmas relevant for the early Earth could pond during ascent due to the anomalous P ‐effect on η .

The continental crust is rich in aluminosilicates and formed by the crystallization of arc magmas. However, the magma produced at sub-arc depths is often silica-poor. The chemical evolution of sub-arc magma from silica-poor to aluminosilicate-rich is perplexing. Magnetotelluric (MT) observations in subduction zones and complementary laboratory-based constraints of electrical conductivity (a) are crucial to understanding this chemical evolution. The a of a magma is sensitive to pressure (P), temperature (T), and chemistry (X). To date, laboratory-based measurements on the a of silicate melts have helped to interpret MT observations at P < 2 GPa. Yet, the melting in subduction zones could occur deeper, at P < 6-7 GPa. The a of melt at such pressures is poorly constrained. To address this, we performed experiments at P < 6 GPa to examine the a of basaltic to andesitic melts, which are common in subduction zones. We constrained the effects of silica, alumina, alkali, alkaline, and water (H2O) contents on the a of melt. The activation volume of a increases with silica contents. Hence, the a of basaltic melt is overall greater than that of an andesitic counterpart. The a of basaltic magma is also less sensitive to P than andesitic magma. Water lowers the activation energy and enhances a for all melt compositions. Our results help constrain how the electrical properties of a magma change with an evolving composition in a subduction zone.

Dataset for the manuscript : Growing Diamonds in the Laboratory to investigate Growth, Dissolution, and Inclusions Formation processes after Hélène Bureau, Imène Estève, Caroline Raepsaet, Geeth Manthilake It comprises one excel file containing raw SEM EDX data and 10 SEM images of the samples

Comme la plupart des régions de l'intérieur de la Terre restent inaccessibles, la conductivité électrique des phases constitutives de la Terre profonde intéresse un large éventail de spécialistes, notamment les minéralogistes et les géophysiciens, pour comprendre la géodynamique actuelle. Ce chapitre rappelle les principes de base de la conductivité électrique et les caractéristiques des minéraux qui composent notre planète.

Included are central data and codes for the manuscript "Viscosity Measurements at High Pressures: A Critical Appraisal of Corrections to Stokes' Law", submitted for review.

Electrical conductivity is perhaps the physical property of rocks that is most sensitive to the presence of hydrogen. Hydrogen enhances conductivity via proton conduction in minerals or by stabilizing highly conductive phases, such as hydrous silicate melts or aqueous fluids. Hydrogen might also be stored in the metallic core. Electrical conductivity measurements in the laboratory can be used to interpret magnetotelluric maps of the mantle in terms of hydrogen content and distribution. In active tectonic settings like subduction zones, anomalously high conductivities have revealed the distribution and migration pathways of H-bearing melts and fluids, illuminating the transport of hydrogen in our planet’s interior.

A global analysis of seismic waves has identified a widespread sharp velocity anomaly at the base of the low seismic velocity zone that is consistent with partial melting, closing a decades-long debate about the origin of this zone.

AbstractDespite growing evidence suggesting chemically distinct regions and partial melting at the core-mantle boundary (CMB) throughout Earth’s history, current heat-flow models assume a homogeneous thermal boundary layer. To understand probable thermal response of bridgmanite to subducted slab, we measured thermal diffusivity of mid-ocean ridge basalt (MORB)-bearing olivine polycrystalline as an analogy. Our results show a sharp increase of thermal conductivity with an addition of 0.1 vol. % MORB, followed by a systematic decrease with increasing MORB. When the infection point of 1.2–5 vol.% is exceeded, thermal conductivity jump again with 10 vol.% MORB. If it were the case at the CMB, MORB introduced by subducted slab and scattered by mantle flow may have led to lateral variation of heat flux. It results in plume clusters with varying scales, which either grows into superplume with mobile plume root or vanishes when MORB is drained to the infection point.

Germanium nitride, having cubic spinel structure, γ-Ge3N4, is a wide band-gap semiconductor with a large exciton binding energy that exhibits high hardness, elastic moduli and elevated thermal stability up to approximately 700°C. Experimental data on its bulk and shear moduli (B0 and G0, respectively) are strongly limited, inconsistent and, thus, require verification. Moreover, earlier first-principles density functional calculations provided significantly scattering B0 values but consistently predicted G0 much higher than the so far available experimental value. Here, we examined the elasticity of polycrystalline γ-Ge3N4, densified applying high pressures and temperatures, using the techniques of laser ultrasonics (LU) and Brillouin light scattering (BLS) and compared with our extended first-principles calculations. From the LU measurements, we obtained its longitudinal- and Rayleigh wave sound velocities and, taking into account the sample porosity, derived B0 = 322(44) GPa and G0 = 188(7) GPa for the dense polycrystalline γ-Ge3N4. While our calculations underestimated B0 by approximately 17%, most of the predicted G0 matched well with our experimental value. Combining the LU- and BLS data and taking into account the elastic anisotropy, we determined the refractive index of γ-Ge3N4 in the visible range of light to be n = 2.4, similarly high as that of diamond or GaN, and matching our calculated value. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.

The accretion of planets from primordial materials and their subsequent differentiation to form a core and a mantle are fundamental questions in terrestrial and solar system. Many of the questions about the processes are still open and much debated. For example, could the presence of water during the metallic phase segregation affect the planet-accretion models? The existing studies on the elemental metal-silicate partitioning under hydrous conditions were extended recently to a range of P, T, f O2 and water content (5 - 20 GPa, 2000 - 2500 K, from 1 to 5 log units below the iron-wüstite buffer

This study aims to experimentally constrain the water storage capacities of olivine and wadsleyite at a depth near 410 km (12–14 GPa) under water-saturated conditions, as a function of temperature, oxygen fugacity, and the presence of carbon (molar H / C of 2). Experiments have been conducted in the multi-anvil press, with sealed double capsules to preserve fluids, at 1200 to 1400 ∘C and three different oxygen fugacities fixed at the rhenium–rhenium oxide buffer (RRO), nickel–nickel oxide buffer (NNO), and iron-wüstite (IW) for oxidizing, intermediate, and reducing conditions, respectively. The water contents of minerals were measured by Raman spectroscopy that allows a very small beam size to be used and were cross-checked on a few samples with NanoSIMS analyses. We observe an effect, although slight, of fO2 on the water storage capacity of both wadsleyite and olivine and also on their solidus temperatures. At 1200 ∘C, the storage capacity of the nominally anhydrous minerals (NAMS) increases with increasing oxygen fugacity (from the IW to the RRO buffer) from 1 wt % to 1.5 wt % H2O in wadsleyite and from 0.1 wt % to 0.2 wt % in olivine, owing to the increase in H2O / H2 speciation in the fluid, whereas at 1400 ∘C the storage capacity decreases from 1 wt % to 0.75 wt % H2O in wadsleyite and down to 0.03 wt % for olivine. At high temperature, the water storage capacity is lowered due to melting, and the more oxidized the conditions are the more the solidus is depressed. Still, at 1400 ∘C and IW, wadsleyite can store substantial amounts of water: 0.8 wt % to 1 wt % H2O. The effect of carbon is to decrease water storage capacity in both wadsleyite and olivine by an average factor 2 at 1300–1400 ∘C. The trends in water storage as a function of fO2 and C presence are confirmed by NanoSIMS measurements. The solidus at IW without C is located between 1300 and 1400 ∘C in the wadsleyite stability field and drops to temperatures below 1300 ∘C in the olivine stability field. With the addition of C, the solidus is found between 1200 and 1300 ∘C in both olivine and wadsleyite stability fields.