Velocity and density characteristics of subducted oceanic crust and the origin of lower-mantle heterogeneities

Nanosecond structural evolution in shocked coesite

The phase transitions in minerals under shock are crucial for understanding meteorite impact history. Recent time-resolved x-ray diffraction (XRD) studies on silica shocked to 65 GPa proposed the formation of different high-pressure phases between fused silica and quartz. Furthermore, the dynamics of silica behavior under higher pressure need to be investigated, particularly during nonequilibrium superheating before melting. This study examines the time-dependent response of coesite, using laser-driven shock coupled with fast XRD and molecular dynamics simulations with our recently developed machine learning interatomic potential. Our results reveal a transient dense supercooled liquid crystallizes into a semi-disordered d-NiAs-type silica, followed by transforming into either seifertite or stishovite, depending on the pressure. Instead of thermodynamically stable quartz, a back-transformation to coesite phase is identified after release. The complicated phase evolution pathways in shocked coesite provide deeper insights into the high-pressure silica phases observed in the meteorite bombardments on the early Moon, Mars, and Earth.

Unique composition and evolutionary histories of large low velocity provinces

The two "large low velocity provinces" (LLVPs) are broad, low seismic wave speed anomalies in Earth's lower mantle beneath Africa and the Pacific Ocean. Recent research suggests they contain relatively dense subducted oceanic crust (SOC), but the relative concentration of this recycled material within them is an open question. Using simulations of 3-D global mantle circulation over the past 1 Gyr, we find that two antipodal LLVPs develop naturally as a consequence of Earth's recent subduction history and the gravitational settling and stirring of SOC. Shear-wave velocity reductions in the two LLVPs are similar due to the dominating influence of temperature over composition. However, the formation histories are distinct. Circum-Pacific subduction of oceanic lithosphere has continuously replenished the Pacific LLVP with relatively young SOC since 300 Ma, while the African LLVP comprises older, well-mixed material. Our models suggest the Pacific LLVP stores up to 53% more SOC produced in the last 1.2 Gyr than the African LLVP, potentially making the Pacific domain denser and less buoyant.

Superionic iron hydride shapes ultralow-velocity zones at Earth's core-mantle boundary

Seismological studies have exposed numerous ultralow velocity zones (ULVZs) exhibiting extraordinary physical attributes at Earth's core-mantle boundary, yet their compositions and origins remain controversial. Water-iron reaction can generate unique phases under lowermost-mantle conditions and likely plays a crucial role in forming ULVZs. Through first-principles molecular dynamic simulations with machine learning techniques, we determine that iron hydride, the product of water-iron reaction, is stable as a superionic phase at the core-mantle boundary. This superionic iron hydride has much slower velocities and a higher density than the ambient mantle under lowermost-mantle conditions. Accumulation of iron hydride, created through either a chemical reaction between subducted water and iron or solidification of core material entrained in the lower mantle by convection, can explain the seismic observations of ULVZs particularly those associated with subduction. This work suggests that water may have a substantial role in creating seismic heterogeneities at the core-mantle boundary.

Highly oxidized intraplate basalts and deep carbon storage

Deep carbon cycle is crucial for mantle dynamics and maintaining Earth's habitability. Recycled carbonates are a strong oxidant in mantle carbon-iron redox reactions, leading to the formation of highly oxidized mantle domains and deep carbon storage. Here we report high Fe3+/∑Fe values in Cenozoic intraplate basalts from eastern China, which are correlated with geochemical and isotopic compositions that point to a common role of carbonated melt with recycled carbonate signatures. We propose that the source of these highly oxidized basalts has been oxidized by carbonated melts derived from the stagnant subducted slab in the mantle transition zone. Diamonds formed during the carbon-iron redox reaction were separated from the melt due to density differences. This would leave a large amount of carbon (about four times of preindustrial atmospheric carbon budget) stored in the deep mantle and isolated from global carbon cycle. As such, the amounts of subducted slabs stagnated at mantle transition zone can be an important factor regulating the climate.

Compositional and thermal state of the lower mantle from joint 3D inversion with seismic tomography and mineral elasticity

The compositional and thermal state of Earth's mantle provides critical constraints on the origin, evolution, and dynamics of Earth. However, the chemical composition and thermal structure of the lower mantle are still poorly understood. Particularly, the nature and origin of the two large low-shear-velocity provinces (LLSVPs) in the lowermost mantle observed from seismological studies are still debated. In this study, we inverted for the 3D chemical composition and thermal state of the lower mantle based on seismic tomography and mineral elasticity data by employing a Markov chain Monte Carlo framework. The results show a silica-enriched lower mantle with a Mg/Si ratio less than ~1.16, lower than that of the pyrolitic upper mantle (Mg/Si = 1.3). The lateral temperature distributions can be described by a Gaussian distribution with a standard deviation (SD) of 120 to 140 K at 800 to 1,600 km and the SD increases to 250 K at 2,200 km depth. However, the lateral distribution in the lowermost mantle does not follow the Gaussian distribution. We found that the velocity heterogeneities in the upper lower mantle mainly result from thermal anomalies, while those in the lowermost mantle mainly result from compositional or phase variations. The LLSVPs have higher density at the base and lower density above the depth of ~2,700 km than the ambient mantle, respectively. The LLSVPs are found to have ~500 K higher temperature, higher Bridgmanite and iron content than the ambient mantle, supporting the hypothesis that the LLSVPs may originate from an ancient basal magma ocean formed in Earth's early history.

Coupled deep-mantle carbon-water cycle: Evidence from lower-mantle diamonds

Diamonds form in a variety of environments between subducted crust, lithospheric and deep mantle. Recently, deep source diamonds with inclusions of the high-pressure H₂O-phase ice-VII were discovered. By correlating the pressures of ice-VII inclusions with those of other high-pressure inclusions, we assess quantitatively the pressures and temperatures of their entrapment. We show that the ice-VII-bearing diamonds formed at depths down to 800 ± 60 km but at temperatures 200–500 K below average mantle temperature that match the pressure-temperature conditions of decomposing dense hydrous mantle silicates. Our work presents strong evidence for coupled recycling of water and carbon in the deep mantle based on natural samples.

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A novel approach was developed to assess the pressure-temperature conditions of entrapment of inclusions in diamonds

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The viscoelastic relaxation of diamond has a significant effect on the evolution of pressure and temperature

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Ice-VII-bearing diamonds have formed in wet, cool environments at depths down to 800 km

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The coupled recycling of water and carbon is present in the deep mantle

Formation of large low shear velocity provinces through the decomposition of oxidized mantle

Large Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle are key to understanding the chemical composition and thermal structure of the deep Earth, but their origins have long been debated. Bridgmanite, the most abundant lower-mantle mineral, can incorporate extensive amounts of iron (Fe) with effects on various geophysical properties. Here our high-pressure experiments and ab initio calculations reveal that a ferric-iron-rich bridgmanite coexists with an Fe-poor bridgmanite in the 90 mol% MgSiO₃-10 mol% Fe₂O₃ system, rather than forming a homogeneous single phase. The Fe³⁺-rich bridgmanite has substantially lower velocities and a higher V_P/V_S ratio than MgSiO₃ bridgmanite under lowermost-mantle conditions. Our modeling shows that the enrichment of Fe³⁺-rich bridgmanite in a pyrolitic composition can explain the observed features of the LLSVPs. The presence of Fe³⁺-rich materials within LLSVPs may have profound effects on the deep reservoirs of redox-sensitive elements and their isotopes.