Yan Yang

Two seismic anomalies, known as large low velocity provinces (LLVPs), are detected seismically at the bottom of the Earth’s lower mantle. Acting as the source of mantle plumes and as barriers to mantle convection, LLVPs have stayed in the lower mantle over a long duration and significantly influence the dynamics of the Earth. However, it is unclear how the LLVPs could be preserved in the lower mantle because the elevated temperature and water content within LLVPs should reduce their rheological strength, leading to a rapid mixing by mantle convection. Here, through high-pressure experiments, we demonstrate that the grain growth of bridgmanite, the dominant mineral in the lower mantle, is significantly enhanced by water incorporation. The relatively water-rich conditions of LLVPs and consequent larger grain size may cause their viscosity 1.4 orders of magnitude higher than the surrounding mantle despite the negatively thermo-hydrous effect on mineral strength, providing a reliable mechanism for their longevity.

The fate of sedimentary nitrogen during subduction is essential for understanding the origin of nitrogen in the deep Earth. Here we study the behavior of nitrogen in slab sediments during the phengite to K-hollandite transition at 10–12 GPa and 800–1100 °C. Phengite stability is extended by 1–3 GPa in the nitrogen (NH₄⁺)-bearing system. The phengite-fluid partition coefficient of nitrogen is 0.031 at 10 GPa, and K-hollandite-fluid partition coefficients of nitrogen range from 0.008 to 0.064, showing a positive dependence on pressure but a negative dependence on temperature. The nitrogen partitioning data suggest that K-hollandite can only preserve ~43% and ~26% of the nitrogen from phengite during the phengite to K-hollandite transition along the cold and warm slab geotherms, respectively. Combined with the slab sedimentary nitrogen influx, we find that a maximum of ~1.5 × 10⁸kg/y of nitrogen, representing ~20% of the initial sedimentary nitrogen influx, could be transported by K-hollandite to the lower mantle. We conclude that slab sediments may have contributed less than 15% of the lower mantle nitrogen, most of which is probably of primordial origin.

The transition zone is dominated by polymorphs of olivine, wadsleyite, and ringwoodite, which are to date considered the main water carriers in the Earth’s mantle. Despite considerable studies on water solubility and its impact on physical properties of the two minerals, knowledge of their hydrogen defects and framework behavior at high temperature and high pressure is still lacking. Here, we systematically assess this issue, by in situ high-temperature (20–800 °C) infrared spectroscopic studies, in situ high-temperature (20–800 °C) and high temperature-pressure (14.27 and 18.84 GPa, 20–400 °C) Raman spectroscopic studies on the iron-bearing wadsleyite and ringwoodite. The results show that dehydrogenation in wadsleyite happens at a higher temperature than in ringwoodite. The infrared absorption patterns of hydrogen defects in wadsleyite and ringwoodite are temperature sensitive, resulting from hydrogen defects transfer and site-specific stabilities. As for the framework, it is more sensitive to temperature and pressure for ringwoodite than wadsleyite. These results provide new knowledge about hydrogen defects and framework of wadsleyite and ringwoodite at high temperature and high pressure, which is indispensable for understanding water solubility and its impacts on physical properties of these two minerals.

Nitrogen is considered to be transported from Earth′s surface to the top of the lower mantle through subduction. However, little is known on the transportation and fate of subducted nitrogen to the Earth′s interior during slab‐mantle interactions. In this study, the stability of subducted sedimentary nitrogen in the reduced mantle was investigated to 35 GPa and 1600 K by laser‐heated diamond anvil cell experiments and first‐principles calculations. Our results showed that subducted nitrogen‐bearing silicates and fluids could not coexist with the metallic iron or iron‐rich alloys, and reacted with them to form different products at high pressure‐temperature conditions. Combining our results with previous data, we re‐determined the relative stability of iron‐light element binary compounds to 35 GPa and 1600 K to be Fe‐O > Fe‐N > Fe‐S > Fe‐C. This stability sequence contributes to explaining the observation that iron nitrides are trapped as inclusions in sulfur‐depleted lower‐mantle diamonds and are absent in sulfur‐rich ones. The recycling efficiency of subducted sedimentary nitrogen is strongly related to the availability of the metallic iron of the reduced mantle. Hydration of the metallic iron limits the storage of nitrogen in it and contributes to recycling nitrogen to Earth′s surface. Therefore, unlike subducted continental sediments, subducted marine sediments are unlikely to transport a large amount of surficial nitrogen to the metallic iron of the reduced mantle in which nitrogen could reside over long geologic periods.

Clay minerals are distributed in Earth's crust and troposphere and in Martian crust where temperature varies. Understanding the changes of chemical bonding and crystalline swelling-shrinkage of montmorillonite (Mnt) upon temperature changes is fundamental for studying its surface reactivity and interaction in specific surroundings. However, such an issue remains poorly understood. Here, in situ high- and low-temperature Fourier transform infrared (HT- and LT-FTIR) spectroscopy and X-ray diffraction (HT- and LT-XRD) were performed to study the evolution of chemical bonding and crystalline swelling-shrinkage of sodium-montmorillonite (NaMnt) upon temperature changes. The FTIR results show that the hydroxyl content in NaMnt decreased when the temperature increased from 20 to 700 °C, while it is independent of temperature from 0 to -150 °C. The formation of hydroxyls at the "broken" layer edges of NaMnt is related to adsorbed water molecules on the surfaces, and its content increased when the particle size of the NaMnt decreased. The water molecules in the interlayer space of NaMnt could bond to the tetrahedral sheet of NaMnt through Si₂O-H₂O bonds. HT- and LT-XRD results reveal that all of those water molecules in NaMnt were removed after heating to 100 °C in the heating chamber. The NaMnt was transformed from a state of monolayer interlayer water molecules at 20 °C to a dehydrated state at 100 °C, and then to a dehydroxylated state at 700 °C. Accordingly, the basal spacings of NaMnt were changed from 1.27 to 0.97 nm and then to 0.96 nm, respectively. When NaMnt was cooled from 20 to -268 °C, a crystalline swelling of NaMnt occurred with an increase of 0.03 nm of basal spacing. This work demonstrates that high/low temperature has a remarkable effect on the hydroxyls and the water molecules in NaMnt, which in turn affects its swelling-shrinkage performance. These findings provide some in-depth understanding of the changes of chemical bonding and crystalline swelling-shrinkage of montmorillonite upon temperature changes and the reasons behind these, which might be helpful for the design of engineering Mnt in high-/low-temperature applications.

Nitrogen recycling between Earth's interior and exterior determines the evolution of the atmosphere and habitability of the planet. However, the contribution of subduction to the deep nitrogen recycling is still highly debated. Ammonium‐bearing silicate minerals are the main nitrogen carriers in subduction zones. Nitrogen retention in subduction‐zone minerals determines the subducted nitrogen fluxes to the deep Earth. Here, we reveal ammonium behavior in a natural ammonium‐bearing feldspar at elevated temperatures to 1000°C and pressures to 19.99 GPa. With increasing temperature and pressure, ammonium interacts with the silicate framework and forms hydrogen bonding. The ammonium loss rates are on the order of 10⁻¹⁵, 10⁻¹⁴, and 10⁻¹³ at 800, 900, and 1000°C, respectively, with activation energy of 271 ± 13 kJ/mol. The activation energy of ammonium loss in feldspar is higher than that in phengite. Moreover, the ammonium diffusivities are similar to that of hydrogen in feldspar, but much slower than that of molecular water and faster than those of K and Ar. The results imply that most of nitrogen can be potentially retain in the feldspar structure before feldspar breakdown. Considering that feldspar could be stable to a great depth along cold subduction, it could be a potential carrier for nitrogen transport to the deep Earth.

Spectral features of hydrogen defects in natural mantle minerals derive from physico-chemical conditions of the lithosphere. Although hydrogen defects in synthetic orthopyroxene have been well investigated, their complex spectral features in natural orthopyroxenes are still difficult to decipher. To clarify this issue, it is indispensable to reveal what happens to hydrogen defects during high-temperature processes, thereby fingerprinting the origins of hydrogen defects observed in natural orthopyroxene. Here, we carry out Fourier transform infrared spectroscopic studies on hydrogen defects of three natural orthopyroxenes at elevated temperatures to 1000 °C. Hydrogen defects display reversible disordering at temperatures above 700 °C, which is different from those at ambient conditions. Moreover, hydrogen diffusivities are significantly different between the orthopyroxene samples from different tectonic settings despite their similar iron contents. Even for the same crystal, different hydrogen defects display different diffusion behaviors. Hydrogen defects corresponding to the 3420 cm−1 band have the fastest diffusivity relative to the other hydrogen defects. Most importantly, hydrogen defects can redistribute in the crystal, with new hydrogen defects produced at the cost of the initial hydrogen defects rather than involving a reaction with an external hydrogen source. Combining these findings with previously reported hydrogen defects in natural olivine and clinopyroxene at high temperatures, we propose that: (1) to correctly relate hydrogen defects features to geological processes, it is imperative to understand their behavior and origin, and (2) hydrogen disordering should be taken into account when predicting and extrapolating data on physical properties of the mantle from room-temperature measurements.

Nitrogen cycling between the Earth’s surface and interior influences atmosphere evolution, climate and habitability of our planet. Nitrogen transport process in the Earth’s interior is a key part of the cycling, which remains enigmatic. Silicate minerals are the main carriers of nitrogen mainly in the form of ammonium. Thus, untangling interactions of ammonium and lattice of the host minerals at the deep Earth’s conditions, is essential for understanding nitrogen transport process.Feldspar, the most abundant mineral in the Earth’s crust, is a carrier of nitrogen to the deep Earth. Nitrogen is incorporated as ammonium in the M site of feldspar framework. To investigate interactions of ammonium and lattice at high temperatures, we conducted FTIR, Raman and XRD spectra measurements at high temperatures to 1000 oC on an ammonium-bearing feldspar, and revealed ammonium diffusivities and impacts on the lattice. The results show that diffusivities of ammonium at 800, 900 and 1000 oC are comparable to those of hydroxyl in feldspar, but much slower than structural molecular water. Importantly, ammonium in the M site of feldspar seems more stable than that in the layered site of phengite previously reported. Moreover, ammonium-bearing feldspar has smaller temperature-induced Raman mode wavenumber shifts and thermal expansion coefficients, as compared with ammonium-free feldspar.The above results suggest interactions of ammonium and lattice of the silicate minerals at high temperatures. Thermal stabilities of ammonium depend on structure of the host silicates, and thermal stabilities of the host silicates are in turn affected by ammonium incorporated.