Density deficit of Earth's core revealed by a multimegabar primary pressure scale.

An accurate pressure scale is a fundamental requirement to understand planetary interiors. Here, we establish a primary pressure scale extending to the multimegabar pressures of Earth's core, by combined measurement of the acoustic velocities and the density from a rhenium sample in a diamond anvil cell using inelastic x-ray scattering and x-ray diffraction. Our scale agrees well with previous primary scales and shock Hugoniots in each experimental pressure range and reveals that previous scales have overestimated laboratory pressures by at least 20% at 230 gigapascals. It suggests that the light element content in Earth's inner core (the density deficit relative to iron) is likely to be double what was previously estimated, or Earth's inner core temperature is much higher than expected, or some combination thereof.

An experimental system for time-resolved x-ray diffraction of deforming silicate melt at high temperature.

Rheological behavior of silicate melts controls the dynamics of volcanic eruptions. Previous experimental studies have investigated melt viscosity and found non-Newtonian behavior of the melt under a high shear rate. However, the relationship between macroscopic rheology and atomic-scale behavior under shear remains unclear. We developed an experimental system for time-resolved x-ray diffraction (XRD) at high temperature to investigate the atomic-scale structural change in melts under shear. The manufactured deformation apparatus and heating furnace were set on the synchrotron radiation x-ray beamline (BL20XU) of SPring-8; the XRD pattern of the melt at high temperature could be observed using this system because the furnace mainly consists of a boron nitride cylinder with high x-ray transmittance. Here, we report results of fiber elongation experiments for a soda-lime melt. Melt fibers with ∼0.7 mm in diameter and ∼27 mm long were elongated at 100 µm sec-1 at temperatures of 595 °C and 620 °C, and the XRD pattern was obtained every 100 msec. Brittle failure of the melt occurred at 595 °C, whereas the melt viscously elongated at 620 °C. The XRD patterns obtained during elongation did not indicate any clear change immediately before brittle failure. The intensity of the XRD pattern decreased with the elongation at 620 °C, although there was no clear variation in its shape. These results indicate that the atomic-scale structure observed by XRD may not change during the elastic and viscous elongation of the soda-lime melt. This experimental system will be further developed and applied to more polymerized and natural silicate melts.

Hydrous magnesium-rich magma genesis at the top of the lower mantle.

Several igneous activities occur on the surface of the Earth, including island arcs, mid-ocean ridges and hot spots. Based on geophysical observations, melting phenomena in the interior also occur at the asthenosphere's top and the upper mantle's bottom. Additionally, a seismological low-velocity anomaly was observed at the top of the lower mantle that may result from mantle melting due to dehydration decomposition of ringwoodite to bridgmanite and ferropericlase with a downward flow. However, the corresponding high-pressure experimental data are too poor to understand the melting phenomena under the lower mantle condition. Herein, we conducted hydrous peridotite melting experiments at pressures from 23.5 to 26 GPa and at temperatures from 1300 to 1600 °C for demonstrating the melt composition and the gravitational stability of magma at the top of the lower mantle. The melt had a SiO2-poor and MgO-rich composition, which is completely different than that of dry peridotite melting experiments. Compared with the seismological lower mantle, the experimental melt is gravitationally lighter; thus, a similar melt could be observed as seismological low-velocity zone at the lower mantle's top. The generated magma plays as a filter of down-welling mantle and can contribute to a formation of a silicate perovskitic lower mantle.

Diamond formation in the deep lower mantle: a high-pressure reaction of MgCO3 and SiO2.

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.

Constraints on Earth's inner core composition inferred from measurements of the sound velocity of hcp-iron in extreme conditions.

Hexagonal close-packed iron (hcp-Fe) is a main component of Earth's inner core. The difference in density between hcp-Fe and the inner core in the Preliminary Reference Earth Model (PREM) shows a density deficit, which implies an existence of light elements in the core. Sound velocities then provide an important constraint on the amount and kind of light elements in the core. Although seismological observations provide density-sound velocity data of Earth's core, there are few measurements in controlled laboratory conditions for comparison. We report the compressional sound velocity (V P) of hcp-Fe up to 163 GPa and 3000 K using inelastic x-ray scattering from a laser-heated sample in a diamond anvil cell. We propose a new high-temperature Birch's law for hcp-Fe, which gives us the V P of pure hcp-Fe up to core conditions. We find that Earth's inner core has a 4 to 5% smaller density and a 4 to 10% smaller V P than hcp-Fe. Our results demonstrate that components other than Fe in Earth's core are required to explain Earth's core density and velocity deficits compared to hcp-Fe. Assuming that the temperature effects on iron alloys are the same as those on hcp-Fe, we narrow down light elements in the inner core in terms of the velocity deficit. Hydrogen is a good candidate; thus, Earth's core may be a hidden hydrogen reservoir. Silicon and sulfur are also possible candidates and could show good agreement with PREM if we consider the presence of some melt in the inner core, anelasticity, and/or a premelting effect.

Atomistic insight into viscosity and density of silicate melts under pressure.

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 ~3-5 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.

A compact system for generating extreme pressures and temperatures: an application of laser-heated diamond anvil cell to inelastic X-ray scattering.

A compact system for generating extreme pressures and temperatures was developed for versatile experiments based on laser-heated diamond anvil cell technique. This system has been used for inelastic X-ray scattering measurements for iron.

Simultaneous structure and elastic wave velocity measurement of SiO2 glass at high pressures and high temperatures in a Paris-Edinburgh cell.

An integration of multi-angle energy-dispersive x-ray diffraction and ultrasonic elastic wave velocity measurements in a Paris-Edinburgh cell enabled us to simultaneously investigate the structures and elastic wave velocities of amorphous materials at high pressure and high temperature conditions. We report the first simultaneous structure and elastic wave velocity measurement for SiO(2) glass at pressures up to 6.8 GPa at around 500°C. The first sharp diffraction peak (FSDP) in the structure factor S(Q) evidently shifted to higher Q with increasing pressure, reflecting the shrinking of intermediate-range order, while the Si-O bond distance was almost unchanged up to 6.8 GPa. In correlation with the shift of FSDP position, compressional wave velocity (Vp) and Poisson's ratio increased markedly with increasing pressure. In contrast, shear wave velocity (Vs) changed only at pressures below 4 GPa, and then remained unchanged at ~4.0-6.8 GPa. These observations indicate a strong correlation between the intermediate range order variations and Vp or Poisson's ratio, but a complicated behavior for Vs. The result demonstrates a new capability of simultaneous measurement of structures and elastic wave velocities at high pressure and high temperature conditions to provide direct link between microscopic structure and macroscopic elastic properties of amorphous materials.

Stability of hydrous melt at the base of the Earth's upper mantle.

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.