The boundary between Earth's rigid lithosphere and the underlying, ductile asthenosphere is marked by a distinct seismic discontinuity 1 . A decrease in seismic-wave velocity and increase in attenuation at this boundary is thought to be caused by partial melt 2 . The density and viscosity of basaltic magma, linked to the atomic structure 3,4 , control the process of melt separation from the surrounding mantle rocks 5-9 . Here we use high-pressure and high-temperature experiments and in situ X-ray analysis to assess the properties of basaltic magmas under pressures of up to 5.5 GPa. We find that the magmas rapidly become denser with increasing pressure and show a viscosity minimum near 4 GPa. Magma mobility-the ratio of the melt-solid density contrast to the magma viscosityexhibits a peak at pressures corresponding to depths of 120-150 km, within the asthenosphere, up to an order of magnitude greater than pressures corresponding to the deeper mantle and shallower lithosphere. Melts are therefore expected to rapidly migrate out of the asthenosphere. The diminishing mobility of magma in Earth's asthenosphere as the melts ascend could lead to excessive melt accumulation at depths of 80-100 km, at the lithosphere-asthenosphere boundary. We conclude that the observed seismic discontinuity at the lithosphereasthenosphere boundary records this accumulation of melt.Along the axial zone of mid-ocean ridges (MORs), asthenospheric mantle rises in response to the diverging motion of oceanic lithosphere and experiences decompression melting. Depending on the volatile content and temperature of the upper mantle, peridotite partial melting initiates at depths of about 80-130 km (ref. 10). The resulting basaltic magmas are buoyant and mobile, percolating upward to form the crust, and leaving a refractory residuum that forms the oceanic lithosphere. Along the more than 50,000-km-long global MOR system, roughly 60,000 tons of magma are processed per minute 11 , replenishing the entire ocean floor in ∼100 Myr. This process is the primary engine for present-day geochemical fractionation of our planet.Structural changes in basaltic magmas with pressure (or depth) play a central role in controlling magma mobility and melting. Pressure-dependent structural changes in silicate melts associated with transformations in the coordination of aluminium ions have been suggested from nuclear magnetic resonance spectroscopic studies of quenched glasses 3 . Such structural changes usually

Seismological observations have revealed the existence of low-velocity and high-attenuation zones above the discontinuity at 410 km depth, at the base of the Earth's upper mantle. It has been suggested that a small amount of melt could be responsible for such anomalies. The density of silicate melt under dry conditions has been measured at high pressure and found to be denser than the surrounding solid, thereby allowing the melt to remain at depth. But no experimental investigation of the density of hydrous melt has yet been carried out. Here we present data constraining the density of hydrous basaltic melt under pressure to examine the stability of melt above the 410-km discontinuity. We infer that hydrous magma formed by partial melting above the 410-km discontinuity may indeed be gravitationally stable, thereby supporting the idea that low-velocity or high-attentuation regions just above the mantle transition zone may result from the presence of melt.

A compositional variety of planetary cores provides insight into their core/mantle evolution and chemistry in the early solar system. To infer core composition from geophysical data, a precise knowledge of elastic properties of core‐forming materials is of prime importance. Here, we measure the sound velocity and density of liquid Fe‐Ni‐S (17 and 30 at% S) and Fe‐Ni‐Si (29 and 38 at% Si) at high pressures and report the effects of pressure and composition on these properties. Our data show that the addition of sulfur to iron substantially reduces the sound velocity of the alloy and the bulk modulus in the conditions of this study, while adding silicon to iron increases its sound velocity but has almost no effect on the bulk modulus. Based on the obtained elastic properties combined with geodesy data, S or Si content in the core is estimated to 4.6 wt% S or 10.5 wt% Si for Mercury, 9.8 wt% S or 18.3 wt% Si for the Moon, and 32.4 wt% S or 30.3 wt% Si for Mars. In these core compositions, differences in sound velocity profiles between an Fe‐Ni‐S and Fe‐Ni‐Si core in Mercury are small, whereas for Mars and the Moon, the differences are substantially larger and could be detected by upcoming seismic sounding missions to those bodies.

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A defining characteristic of silicate melts is the degree of polymerization (tetrahedral connectivity), which dictates viscosity and affects compressibility. While viscosity of depolymerized silicate melts increases with pressure consistent with the free-volume theory, isothermal viscosity of polymerized melts decreases with pressure up to B3-5 GPa, above which it turns over to normal (positive) pressure dependence. Here we show that the viscosity turnover in polymerized liquids corresponds to the tetrahedral packing limit, below which the structure is compressed through tightening of the inter-tetrahedral bond angle, resulting in high compressibility, continual breakup of tetrahedral connectivity and viscosity decrease with increasing pressure. Above the turnover pressure, silicon and aluminium coordination increases to allow further packing, with increasing viscosity and density. These structural responses prescribe the distribution of melt viscosity and density with depth and play an important role in magma transport in terrestrial planetary interiors.

Effect of alkyl chain spacer length between the charged groups (CSL) in zwitterionic poly(sulfobetaine) (PSB) brushes on the hydration state was investigated. PSB brushes with ethyl (PMAES), propyl (PMAPS), or butyl (PMABS) CSL were prepared by surface-initiated atom transfer radical polymerization on silicon wafers. Hydration states of the PSB brushes in aqueous solutions and/or humid vapor were investigated by contact angle measurement, infrared spectroscopy, AFM observation, and neutron reflectivity. The PSB brushes are swollen in humid air and deionized water due to the hydration of the charged groups leading to the reduction of hydrated PSB brushes/water interfacial free energy. The hydrated PSB brushes exhibit clear interface with low interfacial roughness due to networking of the PSB brush chains through association of the SBs. The hydrated PSB brushes produce diffusive swollen layer in the presence of NaCl because of the charge screening followed by SB dissociation by the bound ions. The ionic strength sensitivity in the hydration got more significant with increasing the CSL in SBs because of the augmentation in partial charge by charged group separation.

Diamond is an evidence for carbon existing in the deep Earth. Some diamonds are considered to have originated at various depth ranges from the mantle transition zone to the lower mantle. These diamonds are expected to carry significant information about the deep Earth. Here, we determined the phase relations in the MgCO3-SiO2 system up to 152 GPa and 3,100 K using a double sided laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction. MgCO3 transforms from magnesite to the high-pressure polymorph of MgCO3, phase II, above 80 GPa. A reaction between MgCO3 phase II and SiO2 (CaCl2-type SiO2 or seifertite) to form diamond and MgSiO3 (bridgmanite or post-perovsktite) was identified in the deep lower mantle conditions. These observations suggested that the reaction of the MgCO3 phase II with SiO2 causes formation of super-deep diamond in cold slabs descending into the deep lower mantle.

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