Yasuhiro Kuwayama

Experimental determination of the electrical resistivity of iron at Earth’s core conditions

Earth continuously generates a dipole magnetic field in its convecting liquid outer core by a self-sustained dynamo action. Metallic iron is a dominant component of the outer core, so its electrical and thermal conductivity controls the dynamics and thermal evolution of Earths core. However, in spite of extensive research, the transport properties of iron under core conditions are still controversial. Since free electrons are a primary carrier of both electric current and heat, the electron scattering mechanism in iron under high pressure and temperature holds the key to understanding the transport properties of planetary cores. Here we measure the electrical resistivity (the reciprocal of electrical conductivity) of iron at the high temperatures (up to 4,500 kelvin) and pressures (megabars) of Earths core in a laser-heated diamond-anvil cell. The value measured for the resistivity of iron is even lower than the value extrapolated from high-pressure, low-temperature data using the Bloch-Grüneisen law, which considers only the electron-phonon scattering. This shows that the iron resistivity is strongly suppressed by the resistivity saturation effect at high temperatures. The low electrical resistivity of iron indicates the high thermal conductivity of Earths core, suggesting rapid core cooling and a young inner core less than 0.7 billion years old. Therefore, an abrupt increase in palaeomagnetic field intensity around 1.3 billion years ago may not be related to the birth of the inner core.

Phase relations in the system Fe-FeSi at 21 GPa

Abstract High-pressure experiments were conducted to investigate the phase relations in the binary system Fe-FeSi at a pressure of 21 GPa and temperatures between 1350 and 2200 °C by using a multi-anvil apparatus. The system has a eutectic point at 26 wt% Si and 1820 °C, which is 400 °C lower than the melting point of pure Fe. On the Fe-rich side of the eutectic point, the temperature difference between the solidus and liquidus curves is less than 50 °C and the compositional difference is small between the coexisting solid and liquid. At subsolidus temperatures, homogeneous Fe-Si alloys were quenched with compositions up to 21.7 wt% Si. The more Si-enriched composition, Fe(+25.1 wt% Si), was found to coexist to with CsCl-type FeSi. These results suggest that a large amount of Si could have dissolved into liquid Fe at high pressures during core formation if it occurred under reducing conditions. However, the difference in Si content between the outer core and the inner core would be very small, if the solubility of Si in solid Fe remains large at inner core pressures.

Carbon-depleted outer core revealed by sound velocity measurements of liquid iron–carbon alloy

The relative abundance of light elements in the Earths core has long been controversial. Recently, the presence of carbon in the core has been emphasized, because the density and sound velocities of the inner core may be consistent with solid Fe7C3. Here we report the longitudinal wave velocity of liquid Fe84C16 up to 70 GPa based on inelastic X-ray scattering measurements. We find the velocity to be substantially slower than that of solid iron and Fe3C and to be faster than that of liquid iron. The thermodynamic equation of state for liquid Fe84C16 is also obtained from the velocity data combined with previous density measurements at 1 bar. The longitudinal velocity of the outer core, about 4% faster than that of liquid iron, is consistent with the presence of 4–5 at.% carbon. However, that amount of carbon is too small to account for the outer core density deficit, suggesting that carbon cannot be a predominant light element in the core.

The pyrite-type high-pressure form of FeOOH

Water transported into Earth’s interior by subduction strongly influences dynamics such as volcanism and plate tectonics. Several recent studies have reported hydrous minerals to be stable at pressure and temperature conditions representative of Earth’s deep interior, implying that surface water may be transported as far as the core–mantle boundary. However, the hydrous mineral goethite, α-FeOOH, was recently reported to decompose under the conditions of the middle region of the lower mantle to form FeO2 and release H2, suggesting the upward migration of hydrogen and large fluctuations in the oxygen distribution within the Earth system. Here we report the stability of FeOOH phases at the pressure and temperature conditions of the deep lower mantle, based on first-principles calculations and in situ X-ray diffraction experiments. In contrast to previous work suggesting the dehydrogenation of FeOOH into FeO2 in the middle of the lower mantle, we report the formation of a new FeOOH phase with the pyrite-type framework of FeO6 octahedra, which is much denser than the surrounding mantle and is stable at the conditions of the base of the mantle. Pyrite-type FeOOH may stabilize as a solid solution with other hydrous minerals in deeply subducted slabs, and could form in subducted banded iron formations. Deep-seated pyrite-type FeOOH eventually dissociates into Fe2O3 and releases H2O when subducted slabs are heated at the base of the mantle. This process may cause the incorporation of hydrogen into the outer core by the formation of iron hydride, FeHx, in the reducing environment of the core–mantle boundary.

Highly conductive iron-rich (Mg,Fe)O magnesiowüstite and its stability in the Earth's lower mantle

It has been repeatedly suggested that an iron-rich oxide might accumulate on the Earths core-mantle boundary by various processes. Recent studies showed that FeO with a rock salt (B1)-type structure undergoes pressure- and temperature-induced metallization at the Earths lower mantle conditions. This implies similar metallization or decomposition of the lower mantle phase, (Mg,Fe)O, under high pressure-temperature conditions. We performed simultaneous X-ray diffraction and electrical conductivity measurements on (Mg0.20Fe0.80)O and (Mg0.05Fe0.95)O magnesiowustites up to 140 GPa and 2100 K, and we examined recovered samples by means of an analytical transmission electron microscope. The experiments revealed very high electrical conductivity of the magnesiowustite samples and their minimal temperature dependence above 85 GPa and 1300 K, yet the samples remained insulators. We also found decomposition of (Mg0.05Fe0.95)O into almost pure FeO and iron-rich (Mg,Fe)O due to metallization of the FeO component, while such a reaction was not observed in (Mg0.20Fe0.80)O. The observed high electrical conductivity and decomposition of iron-rich (Mg,Fe)O magnesiowustite could enhance the heterogeneities in the electrical and thermal conductivity at the Earths core-mantle boundary region.

Melting temperatures of H2O up to 72 GPa measured in a diamond anvil cell using CO2 laser heating technique

The melting curve of H2O from 49 to 72 GPa was determined by using a laser-heated diamond anvil cell. Double-sided CO2 laser heating technique was employed in order to heat the sample directly. Discontinuous changes of the heating efficiency attributed to the H2O melting were observed between 49 and 72 GPa. The obtained melting temperatures at 49 and 72 GPa are 1200 and 1410 K, respectively. We found that the slope of the melting curve significantly decreases with increasing pressure, only 5 K/GPa at 72 GPa while 44 K/GPa at 49 GPa. Our results suggest that the melting curve does not intersect with the isentropes of Uranus and Neptune, and hence, H2O should remain in the liquid state even at the pressure and temperature conditions found deep within Uranus and Neptune.

Sound velocity of liquid Fe–Ni–S at high pressure

The sound velocity of liquid Fe47Ni28S25 and Fe63Ni12S25 was measured up to 52 GPa/2480 K in externally-resistance-heated and laser-heated diamond-anvil cells (DACs) using high-resolution inelastic X-ray scattering. From these experimental data, we obtained the elastic parameters of liquid Fe47Ni28S25, KS0 = 96.1 ± 2.7 GPa and KS0’ = 4.00 ± 0.13, where KS0 and KS0’ are the adiabatic bulk modulus and its pressure derivative at 1 bar, when the density is fixed at ρ0 = 5.62 ± 0.09 g/cm3 for 1 bar and 2000 K. With these parameters, the sound velocity and density of liquid Fe47Ni28S25 were calculated to be 8.41 ± 0.17 km/s and 8.93 ± 0.19 to 9.10 ± 0.18 g/cm3, respectively, at the core mantle boundary (CMB) conditions of 135 GPa and 3600 − 4300 K. These values are 9.4 % higher and 17–18 % lower than those of pure Fe respectively. Extrapolation of measurements and comparison with seismological models suggest the presence of 5.8–7.5 wt.% sulfur in the Earths outer core if it is the only light element.