The volume diffusion coefficient of water in ice VI was determined in the pressure-temperature range of 1.3–1.9 GPa and 300–320 K by in situ isotope tracer diffusion experiments. We determined the ...activation energy of the volume diffusion to be 61.9 ± 9.5 kJ/mol. The viscosity of polycrystalline ice VI under diffusion creep was estimated from the diffusion coefficients based on the theory of the diffusion creep. From a compilation of viscosity values in the current diffusion creep regime and the viscosity previously determined by plastic deformation experiments of ice VI in a high stress regime (Durham et al. 1996), here we provide the relationships between viscosity, stress, and the average grain size of polycrystalline ice VI. The most plausible deformation mechanism of the layers of ice VI underneath the internal oceans in large icy bodies was inferred from the viscosity-stress-average grain size relationship. We also discuss the critical thickness of the ice VI layer, which determines the onset of thermal convection.
•The volume diffusion coefficient of water in ice VI was determined by tracer diffusion experiments.•The viscosity of polycrystalline ice VI was estimated from the diffusion coefficients.•The most plausible deformation mechanism of the ice VI layers in the large icy bodies is diffusion creep.•Critical thickness of the ice VI layers is discussed according to Newtonian rheology.•Thermal convection is actively occurring in the ice VI layers in the current Titan and Ganymede.
Abstract
High-pressure polymorphism of olivine (α-phase of Mg
2
SiO
4
) is of particular interest for geophysicists aiming to understand the structure and dynamics of the Earth’s interior because of ...olivine’s prominent abundance in the upper mantle. Therefore, natural and synthetic olivine polymorphs have been actively studied in the past half century. Here, we report a new high-pressure polymorph, the ε*-phase, which was discovered in a heavily shocked meteorite. It occurs as nanoscale lamellae and has a topotaxial relationship with the host ringwoodite (γ-phase of Mg
2
SiO
4
). Olivine in the host rock entrapped in a shock-induced melt vein initially transformed into polycrystalline ringwoodite through a nucleation and growth mechanism. The ringwoodite grains then coherently converted into the ε*-phase by shear transformation during subsequent pressure release. This intermediate metastable phase can be formed by all Mg
2
SiO
4
polymorphs via a shear transformation mechanism. Here, we propose high-pressure transformations of olivine that are enhanced by diffusionless processes, not only in shocked meteorites but also in thick and cold lithosphere subducting into the deep Earth.
Iron may critically influence the physical properties and thermochemical structures of Earth’s lower mantle. Its effects on thermal conductivity, with possible consequences on heat transfer and ...mantle dynamics, however, remain largely unknown. We measured the lattice thermal conductivity of lower-mantle ferropericlase to 120 GPa using the ultrafast optical pump-probe technique in a diamond anvil cell. The thermal conductivity of ferropericlase with 56% iron significantly drops by a factor of 1.8 across the spin transition around 53 GPa, while that with 8–10% iron increases monotonically with pressure, causing an enhanced iron substitution effect in the low-spin state. Combined with bridgmanite data, modeling of our results provides a self-consistent radial profile of lower-mantle thermal conductivity, which is dominated by pressure, temperature, and iron effects, and shows a twofold increase from top to bottom of the lower mantle. Such increase in thermal conductivity may delay the cooling of the core, while its decrease with iron content may enhance the dynamics of large low shear-wave velocity provinces. Our findings further show that, if hot and strongly enriched in iron, the seismic ultralow velocity zones have exceptionally low conductivity, thus delaying their cooling.
During the decompression of plastically deformed glasses at room temperature, some aspects of irreversible densification may be preserved. This densification has been primarily attributed to ...topological changes in glass networks. The changes in short-range structures like cation coordination numbers are often assumed to be relaxed upon decompression. Here the NMR results for aluminosilicate glass upon permanent densification up to 24 GPa reveal noticeable changes in the Al coordination number under pressure conditions as low as ∼6 GPa. A drastic increase in the highly coordinated Al fraction is evident over only a relatively narrow pressure range of up to ∼12 GPa, above which the coordination change becomes negligible up to 24 GPa. In contrast, Si coordination environments do not change, highlighting preferential coordination transformation during deformation. The observed trend in the coordination environment shows a remarkable similarity to the pressure-induced changes in the residual glass density, yielding a predictive relationship between the irreversible densification and the detailed structures under extreme compression. The results open a way to access the nature of plastic deformation in complex glasses at room temperature.
High‐quality single‐crystals of (Al,Fe)‐bearing bridgmanite, Mg0.88 Fe3+0.065Fe2+0.035Al0.14Si0.90O3, of hundreds of micrometer size were synthesized at 24 GPa and 1800 °C in a Kawai‐type apparatus ...from the starting hydrous melt containing ~6.7 wt% water. Analyses of synthesized bridgmanite using petrographic microscopy, scanning electron microscopy, and transmission electron microscopy show that the crystals are chemically homogeneous and inclusion free in micrometer‐ to nanometer‐spatial resolutions. Nanosecondary ion mass spectrometry (NanoSIMS) analyses on selected platelets show ~1,020(±70) ppm wt water (hydrogen). The high water concentration in the structure of bridgmanite was further confirmed using polarized and unpolarized Fourier‐transform infrared spectroscopy (FTIR) analyses with two pronounced OH‐stretching bands at ~3,230 and ~3,460 cm−1. Our results indicate that lower‐mantle bridgmanite can accommodate relatively high amount of water. Therefore, dehydration melting at the topmost lower mantle by downward flow of transition zone materials would require water content exceeding ~0.1 wt%.
Plain Language Summary
Water cycle between surface oceans and Earth's deep interior is a key to understanding the evolution and physical/chemical states of the planet. Early studies show that major transition zone minerals, wadsleyite, and ringwoodite, could accommodate abundant water (1–3 wt%), in the form of lattice‐bonded hydrogen atoms, in their crystal structures. However, water solubility in lower‐mantle bridgmanite, the most abundant mineral in the most volumetric layer of the planet, has remained poorly understood. The scientific challenge here was largely due to difficulties in making large‐sized high‐quality single‐crystals of bridgmanite for reliable characterizations of its water concentration. Here we synthesized single‐crystal bridgmanite of a few hundred micrometers in diameter, which are examined to be inclusion and precipitate free and thus can be used for reliable water concentration measurements using NanoSIMS analyses. Unpolarized and polarized FTIR analyses are used to identify characteristic OH‐stretching bands. Our results show that (Al,Fe)‐bearing bridgmanite could contain as high as 1,020(±70) ppm wt water. This high water concentration in bridgmanite has implications for our understanding of how melting can occur deep in the mantle below the transition zone.
Key Points
High‐quality, inclusion‐free bridgmanite single crystals (Mg0.88Fe3+0.065Fe2+0.035Al0.14Si0.90O3) were synthesized and characterized
The crystals contain ~1,020(±70) ppm wt water using NanoSIMS and show pronounced OH‐stretching bands at ~3230 and ~3460 cm‐1 in FTIR spectra
Dehydration melting at the topmost lower mantle can occur when water content exceeds ~0.1 wt% solubility limit
Preferential iron partitioning into melt during melting and crystallization of lower mantle minerals – bridgmanite and ferropericlase – can play a critical role in our understanding of the origin of ...the early Earth and its evolution to form chemically and seismically distinct regions in the present lowermost mantle. Of particular interest is the consequence of iron spin crossover in ferropericlase on the physical and chemical properties of the molten materials under relevant pressure–temperature (P–T) conditions of the lowermost mantle. However, the spin crossover in liquid (Mg, Fe)O and its effects on melting curves, iron partitioning, melt density – and thus the evolution of an early basal magma ocean – remain poorly studied. Here we conducted high P–T melting experiments on ferropericlase with a starting composition of (Mg0.86Fe0.14)O using synchrotron X-ray diffraction up to ∼120 GPa and ∼5400 K in laser-heated diamond anvil cells, together with chemical analyses on quenched samples using focused ion beam and energy dispersive spectroscopy technique. An ideal solid solution model could be satisfactorily used to fit the experimental data of the liquidus and solidus of (Mg, Fe)O for pure high-spin (HS, below ∼83 GPa), and low-spin (LS, above ∼120 GPa) states, respectively. The experimental solidus and liquidus at 99 GPa and ∼4000–5200 K strongly deviate from ideal solid solution behavior for pure HS and LS states alone, but can be qualitatively explained using a thermodynamics model for a mixture of HS and LS states across the spin crossover. We found that LS (Mg, Fe)O exhibits ∼6–8% lower solidus and liquidus temperature than its HS counterpart. Furthermore, our results show that iron preferentially partitions into melt within the spin crossover to generate iron-rich LS melt. Such iron-rich LS (Mg, Fe)O is ∼27(±5)% denser than materials expected for lowermost mantle and could potentially persist as residual melt in the lowermost mantle at the late stage of magma ocean crystallization. Modeled results indicate that the existence of the dense, iron-rich LS (Mg, Fe)O melt in the lowermost mantle could provide plausible explanations for characteristic seismological signatures of ultra-low velocity zones (ULVZs).
•We constructed temperature–composition phase diagram of (Mg, Fe)O up to ∼120 GPa.•We observed and modeled non-ideal melting of ferropericlase across the spin crossover.•The spin crossover in liquid (Mg, Fe)O was experimentally constrained the first time.•Significant iron partitions into the melt phase during the melting of ferropericlase.•The dense LS iron-rich (Mg, Fe)O melt in the lowermost mantle could explain ULVZs.
The origin and evolution of solar system bodies, including water on the Earth, have been discussed based on the assumption that the relevant ingredients were simply silicates and ices. However, large ...amounts of organic matter have been found in cometary and interplanetary dust, which are recognized as remnants of interstellar/precometary grains. Precometary organic matter may therefore be a potential source of water; however, to date, there have been no experimental investigations into this possibility. Here, we experimentally demonstrate that abundant water and oil are formed via the heating of a precometary-organic-matter analog under conditions appropriate for the parent bodies of meteorites inside the snow line. This implies that H
O ice is not required as the sole source of water on planetary bodies inside the snow line. Further, we can explain the change in the oxidation state of the Earth from an initially reduced state to a final oxidized state. Our study also suggests that petroleum was present in the asteroids and is present in icy satellites and dwarf planets.
•Stishovite has an exceptionally high thermal conductivity.•Presence of stishovite in subducted crust rapidly warms a sinking slab.•High temperature anomaly by stishovite could alter stability zone ...of slab minerals.•It also promotes stagnation and detachment of crust in the shallow lower mantle.
Thermal transport in subducted slabs and mantle critically influences their thermo-chemical evolution and dynamics, where the thermal conductivity controls the magnitude of conductive heat transfer. Here we investigate high-pressure thermal conductivities of stishovite and new-hexagonal-aluminous (NAL) phase, two major slab minerals in the shallow lower mantle, and their impacts on slab dynamics. Pure and Al-bearing stishovite exhibit conductivities of 60–70 and 30–40 Wm−1K−1, respectively, at 25–60 GPa and 300 K, much higher than the Fe-bearing NAL, 10–33 Wm−1K−1, and assemblage of a pyrolitic mantle or subducted basaltic crust. Numerical simulations indicate that subducted crustal materials particularly with local silica-enrichment would have efficient thermal conduction which promotes faster warming of a sinking slab, altering dynamic stability of slab materials and leading to slab stagnation and crust detachment in the shallow lower mantle. Conductive heat transfer in silica-enriched regions along subducted slabs in the shallow lower mantle can be more influential on mantle dynamics than previously thought.
Knowing the elasticity of ferropericlase across the spin transition can help explain seismic and mineralogical models of the lower-mantle including the origin of seismic heterogeneities in the middle ...to lowermost parts of the lower mantle. However, the effects of spin transition on full elastic constants of ferropericlase remain experimentally controversial due to technical challenges in directly measuring sound velocities under lower-mantle conditions. Here we have reliably measured both VP and VS of a single-crystal ferropericlase ((Mg0.92,Fe0.08)O) using complementary Brillouin Light Scattering and Impulsive Stimulated Light Scattering coupled with a diamond anvil cell up to 96 GPa. The derived elastic constants show drastically softened C11 and C12 within the spin transition at 40-60 GPa while C44 is not affected. The spin transition is associated with a significant reduction of the aggregate VP/VS via the aggregate VP softening because VS softening does not visibly occur within the transition. Based on thermoelastic modelling along an expected geotherm, the spin crossover in ferropericlase can contribute to 2% reduction in VP/VS in a pyrolite mineralogical model in mid lower-mantle. Our results imply that the middle to lowermost parts of the lower-mantle would exhibit enhanced seismic heterogeneities due to the occurrence of the mixed-spin and low-spin ferropericlase.
Abstract
Meteorites from interplanetary space often include high-pressure polymorphs of their constituent minerals, which provide records of past hypervelocity collisions. These collisions were ...expected to occur between kilometre-sized asteroids, generating transient high-pressure states lasting for several seconds to facilitate mineral transformations across the relevant phase boundaries. However, their mechanisms in such a short timescale were never experimentally evaluated and remained speculative. Here, we show a nanosecond transformation mechanism yielding ringwoodite, which is the most typical high-pressure mineral in meteorites. An olivine crystal was shock-compressed by a focused high-power laser pulse, and the transformation was time-resolved by femtosecond diffractometry using an X-ray free electron laser. Our results show the formation of ringwoodite through a faster, diffusionless process, suggesting that ringwoodite can form from collisions between much smaller bodies, such as metre to submetre-sized asteroids, at common relative velocities. Even nominally unshocked meteorites could therefore contain signatures of high-pressure states from past collisions.
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