Major Phase Transitions Research Articles

A Strong He ii λ1640 Emitter with an Extremely Blue UV Spectral Slope at z = 8.16: Presence of Population III Stars?

Cosmic hydrogen reionization and cosmic production of the first metals are major phase transitions of the Universe occurring during the first billion years after the Big Bang; however, these are still underexplored observationally. Using the JWST/NIRSpec prism spectroscopy, we report the discovery of a sub-L * galaxy at z spec = 8.1623 ± 0.0007, dubbed RX J2129–z8He II, via the detection of a series of strong rest-frame UV/optical nebular emission lines and the clear Lyman break. RX J2129–z8He II shows a pronounced UV continuum with an extremely steep (i.e., blue) spectral slope of β=−2.53−0.07+0.06 , the steepest among all spectroscopically confirmed galaxies at z spec ≳ 7, in support of its very hard ionizing spectrum that could lead to a significant leakage of its ionizing flux. Therefore, RX J2129–z8He II is representative of the key galaxy population driving the cosmic reionization. More importantly, we detect a strong He II λ1640 emission line in its spectrum, one of the highest redshifts at which such a line is robustly detected. Its high rest-frame equivalent width (EW = 21 ± 4 Å) and extreme flux ratios with respect to UV metal and Balmer lines raise the possibility that part of RX J2129–z8He II’s stellar population could be Pop III (Pop III)-like. Through careful photoionization modeling, we show that the physically calibrated phenomenological models of the ionizing spectra of Pop III stars with strong mass loss can successfully reproduce the emission line flux ratios observed in RX J2129–z8He II. Assuming the Eddington limit, the total mass of the Pop III stars within this system is estimated to be 7.8 ± 1.4 × 105 M ⊙. To date, this galaxy presents the most compelling case in the early Universe where trace Pop III stars might coexist with metal-enriched populations.

Most of the photons that reionized the Universe came from dwarf galaxies.

The identification of sources driving cosmic reionization, a major phase transition from neutral hydrogen to ionized plasma around 600-800 Myr after the Big Bang1-3, has been a matter of debate4. Some models suggest that high ionizing emissivity and escape fractions (fesc) from quasars support their role in driving cosmic reionization5,6. Others propose that the high fesc values from bright galaxies generate sufficient ionizing radiation to drive this process7. Finally, a few studies suggest that the number density of faint galaxies, when combined with a stellar-mass-dependent model of ionizing efficiency and fesc, can effectively dominate cosmic reionization8,9. However, so far, comprehensive spectroscopic studies of low-mass galaxies have not been done because of their extreme faintness. Here we report an analysis of eight ultra-faint galaxies (in a very small field) during the epoch of reionization with absolute magnitudes between MUV ≈ -17 mag and -15 mag (down to 0.005L⋆ (refs. 10,11)). We find that faint galaxies during the first thousand million years of the Universe produce ionizing photons with log[ξion (Hz erg-1)] = 25.80 ± 0.14, a factor of 4 higher than commonly assumed values12. If this field is representative of the large-scale distribution of faint galaxies, the rate of ionizing photons exceeds that needed for reionization, even for escape fractions of the order of 5%.

(Nearly) Model-independent Constraints on the Neutral Hydrogen Fraction in the Intergalactic Medium at z ∼ 5–7 Using Dark Pixel Fractions in Lyα and Lyβ Forests

Cosmic reionization was the last major phase transition of hydrogen from neutral to highly ionized in the intergalactic medium (IGM). Current observations show that the IGM is significantly neutral at z > 7 and largely ionized by z ∼ 5.5. However, most methods to measure the IGM neutral fraction are highly model dependent and are limited to when the volume-averaged neutral fraction of the IGM is either relatively low () or close to unity (). In particular, the neutral fraction evolution of the IGM at the critical redshift range of z = 6–7 is poorly constrained. We present new constraints on at z ∼ 5.1–6.8 by analyzing deep optical spectra of 53 quasars at 5.73 < z < 7.09. We derive model-independent upper limits on the neutral hydrogen fraction based on the fraction of “dark” pixels identified in the Lyα and Lyβ forests, without any assumptions on the IGM model or the intrinsic shape of the quasar continuum. They are the first model-independent constraints on the IGM neutral hydrogen fraction at z ∼ 6.2–6.8 using quasar absorption measurements. Our results give upper limits of (1σ), (1σ), and (1σ). The dark pixel fractions at z > 6.1 are consistent with the redshift evolution of the neutral fraction of the IGM derived from Planck 2018.

Definitive upper bound on the negligible contribution of quasars to cosmic reionization

Cosmic (hydrogen) reionization marks one of the major phase transitions of the universe at redshift z >= 6. During this epoch, hydrogen atoms in the intergalactic medium (IGM) were ionized by Lyman continuum (LyC) photons. However, it remains challenging to identify the major sources of the LyC photons responsible for reionization. In particular, individual contributions of quasars (or active galactic nuclei, AGNs) and galaxies are still under debate. Here we construct the far-ultraviolet (far-UV) luminosity function for type 1 quasars at z >= 6 that spans 10 magnitudes (-19 < M_UV < -29), conclusively showing that quasars made a negligible contribution to reionization. We mainly search for quasars in the low-luminosity range of M_UV > -23 mag that is critical to determine quasars' total LyC photon production but has been barely explored previously. We find that the quasar population can only provide less than 7% (95% confidence level) of the total photons needed to keep the universe ionized at z = 6.0 - 6.6. Our result suggests that galaxies, presumably low-luminosity star-forming systems, are the major sources of hydrogen reionization.

The impact of the first galaxies on cosmic dawn and reionization

ABSTRACT The formation of the first galaxies during cosmic dawn and reionization (at redshifts z = 5–30), triggered the last major phase transition of our universe, as hydrogen evolved from cold and neutral to hot and ionized. The 21-cm line of neutral hydrogen will soon allow us to map these cosmic milestones and study the galaxies that drove them. To aid in interpreting these observations, we upgrade the publicly available code 21cmFAST. We introduce a new, flexible parametrization of the additive feedback from: an inhomogeneous, H2-dissociating (Lyman–Werner; LW) background; and dark matter – baryon relative velocities; which recovers results from recent, small-scale hydrodynamical simulations with both effects. We perform a large, ‘best-guess’ simulation as the 2021 installment of the Evolution of 21-cm Structure (EOS) project. This improves the previous release with a galaxy model that reproduces the observed UV luminosity functions (UVLFs), and by including a population of molecular-cooling galaxies. The resulting 21-cm global signal and power spectrum are significantly weaker, primarily due to a more rapid evolution of the star formation rate density required to match the UVLFs. Nevertheless, we forecast high signal-to-noise detections for both HERA and the SKA. We demonstrate how the stellar-to-halo mass relation of the unseen, first galaxies can be inferred from the 21-cm evolution. Finally, we show that the spatial modulation of X-ray heating due to relative velocities provides a unique acoustic signature that is detectable at z ≈ 10–15 in our fiducial model. Ours are the first public simulations with joint inhomogeneous LW and relative-velocity feedback across the entire cosmic dawn and reionization, and we make them available at this link https://scholar.harvard.edu/julianbmunoz/eos-21.

The evolutionarily conserved kinase SnRK1 orchestrates resource mobilization during Arabidopsis seedling establishment.

The onset of plant life is characterized by a major phase transition. During early heterotrophic seedling establishment, seed storage reserves fuel metabolic demands, allowing the plant to switch to autotrophic metabolism. Although metabolic pathways leading to storage compound mobilization are well-described, the regulatory circuits remain largely unresolved. Using an inducible knockdown approach of the evolutionarily conserved energy master regulator Snf1-RELATED-PROTEIN-KINASE1 (SnRK1), phenotypic studies reveal its crucial function in Arabidopsis thaliana seedling establishment. Importantly, glucose feeding largely restores growth defects of the kinase mutant, supporting its major impact in resource mobilization. Detailed metabolite studies reveal sucrose as a primary resource early in seedling establishment, in a SnRK1-independent manner. Later, SnRK1 orchestrates catabolism of triacylglycerols and amino acids. Concurrent transcriptomic studies highlight SnRK1 functions in controlling metabolic hubs fuelling gluconeogenesis, as exemplified by cytosolic PYRUVATE ORTHOPHOSPHATE DIKINASE (cyPPDK). Here, SnRK1 establishes its function via phosphorylation of the transcription factor BASIC LEUCINE ZIPPER63 (bZIP63), which directly targets and activates the cyPPDK promoter. Taken together, our results disclose developmental and catabolic functions of SnRK1 in seed storage mobilization and describe a prototypic gene regulatory mechanism. As seedling establishment is important for plant vigor and crop yield, our findings are of agronomical importance.

On the Role of Arkypallidal and Prototypical Neurons for Phase Transitions in the External Pallidum.

The external pallidum (globus pallidus pars externa [GPe]) plays a central role for basal ganglia functions and dynamics and, consequently, has been included in most computational studies of the basal ganglia. These studies considered the GPe as a homogeneous neural population. However, experimental studies have shown that the GPe contains at least two distinct cell types (prototypical and arkypallidal cells). In this work, we provide in silico insight into how pallidal heterogeneity modulates dynamic regimes inside the GPe and how they affect the GPe response to oscillatory input. We derive a mean-field model of the GPe system from a microscopic spiking neural network of recurrently coupled prototypical and arkypallidal neurons. Using bifurcation analysis, we examine the influence of dopamine-dependent changes of intrapallidal connectivity on the GPe dynamics. We find that increased self-inhibition of prototypical cells can induce oscillations, whereas increased inhibition of prototypical cells by arkypallidal cells leads to the emergence of a bistable regime. Furthermore, we show that oscillatory input to the GPe, arriving from striatum, leads to characteristic patterns of cross-frequency coupling observed at the GPe. Based on these findings, we propose two different hypotheses of how dopamine depletion at the GPe may lead to phase-amplitude coupling between the parkinsonian beta rhythm and a GPe-intrinsic γ rhythm. Finally, we show that these findings generalize to realistic spiking neural networks of sparsely coupled Type I excitable GPe neurons.SIGNIFICANCE STATEMENT Our work provides (1) insight into the theoretical implications of a dichotomous globus pallidus pars externa (GPe) organization, and (2) an exact mean-field model that allows for future investigations of the relationship between GPe spiking activity and local field potential fluctuations. We identify the major phase transitions that the GPe can undergo when subject to static or periodic input and link these phase transitions to the emergence of synchronized oscillations and cross-frequency coupling in the basal ganglia. Because of the close links between our model and experimental findings on the structure and dynamics of prototypical and arkypallidal cells, our results can be used to guide both experimental and computational studies on the role of the GPe for basal ganglia dynamics in health and disease.

Tuning the structural and luminescence properties of LiYF4:Yb, Er upconversion phosphor: Understanding the role of K ion concentration

Influence of K+ ion concentrations on the upconversion emissions of LiYF4: Yb, Er phosphor has been investigated. Li1−xKxY0.77F4:Yb0.2, Er0.03 phosphors are synthesized by solid-state reaction method under reduced atmosphere. X-ray diffraction analysis revealed a major phase transition from tetragonal LiYF4 to hexagonal KYF4 as the concentration of K+ ion increases. Blue, green and red upconversion emissions are observed when the samples are excited at 980 nm. Green and red luminescence intensities are affected by the inclusion of K+ ion concentrations in the host lattice. Downshifting photoluminescence and the corresponding decay analysis are carried out for the Li1−xKxY0.77F4:Yb0.2, Er0.03 phosphors under the excitation wavelength of 378 nm. Significant changes in the emission intensity of the green to red ratio suggest the tuning of the emission color with the modification of the K+ ion concentrations in Li1−xKxY0.77F4:Yb0.2, Er0.03 host and such successful tuning in the color of emission can find potential applications in the field of display devices.

The Variation Mechanism of Thermal Properties of Loess with Different Water Contents during Freezing

The thermal properties of soils are affected by many factors, such as temperature, water content, and structure. Based on the transient plane source method of thermal physics, the thermal properties of loess with different water content during the freezing process were tested. We analyzed the variation mechanism of thermal properties from the perspective of phase change. Based on the Pore/Particle and Crack Analysis System (PCAS) and theory of heat transfer, we then analyzed the microstructure and heat conduction process of loess. And a calculation model of volumetric heat capacity of frozen soil was presented. The results show that, in the major phase transition zone, the variation of the thermal properties of loess with temperature is the most significant. And the thermal diffusivity increases sharply with the significant increase of thermal conductivity and the rapid decrease of volumetric heat capacity. Moisture content not only increases the thermal conductivity and volume heat capacity of loess but also makes the influence of temperature on the thermophysical parameters more significant. The effect of temperature on thermal properties is mainly due to the change of heat transfer media caused by phase transition of water-ice, followed by the change of thermal properties of heat transfer media such as soil particles, water, ice, and air with temperature. Increasing the water content reduces the contact thermal resistance between soil particles because of the increase in the thickness of the water film on the surface of soil particles and the thermal conductivity of the heat transfer medium between particles, thus changing the thermal properties of soils.

Mitigating the optical depth degeneracy using the kinematic Sunyaev-Zel’dovich effect with CMB-S4 data

The epoch of reionization is one of the major phase transitions in the history of the universe, and is a focus of ongoing and upcoming cosmic microwave background (CMB) experiments with improved sensitivity to small-scale fluctuations. Reionization also represents a significant contaminant to CMB-derived cosmological parameter constraints, due to the degeneracy between the Thomson-scattering optical depth, $\ensuremath{\tau}$, and the amplitude of scalar perturbations, ${A}_{s}$. This degeneracy subsequently hinders the ability of large-scale structure data to constrain the sum of the neutrino masses, a major target for cosmology in the 2020s. In this work, we explore the kinematic Sunyaev-Zel'dovich (kSZ) effect as a probe of reionization, and show that it can be used to mitigate the optical depth degeneracy with high-sensitivity, high-resolution data from the upcoming CMB-S4 experiment. We discuss the dependence of the kSZ power spectrum on physical reionization model parameters, as well as on empirical reionization parameters, namely $\ensuremath{\tau}$ and the duration of reionization, $\mathrm{\ensuremath{\Delta}}z$. We show that by combining the kSZ two-point function and the reconstructed kSZ four-point function, degeneracies between $\ensuremath{\tau}$ and $\mathrm{\ensuremath{\Delta}}z$ can be strongly broken, yielding tight constraints on both parameters. We forecast $\ensuremath{\sigma}(\ensuremath{\tau})=0.003$ and $\ensuremath{\sigma}(\mathrm{\ensuremath{\Delta}}z)=0.25$ for a combination of CMB-S4 and Planck data, including detailed treatment of foregrounds and atmospheric noise. The constraint on $\ensuremath{\tau}$ is nearly identical to the cosmic-variance limit that can be achieved from large-angle CMB polarization data. The kSZ effect thus promises to yield not only detailed information about the reionization epoch, but also to enable high-precision cosmological constraints on the neutrino mass.

Feedbacks between subduction dynamics and slab deformation: Combined effects of nonlinear rheology of a weak decoupling layer and phase transitions

Seismic tomography reveals a wide variety of slab deformation in the mantle transition zone and shallow lower mantle. Numerical modeling of subduction has identified several factors that control slab deformation, among them the properties of the mechanical decoupling between subducting and overriding plates was shown to have a major influence on slab velocity, rollback and deformation in the transition zone. Models with weak crust generally yield fast rollback and slab stagnation while stronger crust results in slab penetration into the lower mantle. Here we perform a detailed analysis of the effects of this weak crustal layer. First, in models with constant crustal viscosity, we quantify combined effects of crustal viscosity and thickness. Further we apply nonlinear crustal rheology that combines dislocation creep and pseudoplastic deformation. We test the effects of varying parameterisations of the nonlinear rheological mechanisms and evaluate their effects on slab deformation. We conclude that the variations of subducting slab velocity (controlled by the buoyancy effects of the major phase transitions) induce strong time variations of the crustal viscosity that in turn enforce further acceleration or deceleration of the slab. This feedback between slab velocity and crustal viscosity strenghtens transient behavior of the subducting slabs and enforces slab penetration after a transient period of stagnation.

Crystal structure, magnetic phase transitions and magnetocaloric effect (MCE) in layer-like RE11Ni4In9 (RE = Gd, Dy and Ho) compounds

The layer-like RE11Ni4In9 (RE = Gd, Dy and Ho) intermetallic compounds were successfully fabricated and systematically investigated concerning crystal structure, magnetic phase transitions and magnetocaloric effect (MCE). The RE11Ni4In9 compounds crystallize in layer-like orthorhombic crystal structure (Cmmm space group) which is composed of CsCl-type and AlB2-type units with the ratio of 9 : 2. The RE11Ni4In9 compounds present multiple magnetic phase transitions at low temperature. Except for the phase transition at low temperature region, the major phase transitions of the present series compounds are of second-ordered phase transitions (SOPTs). Around the magnetic phase transition temperatures at 91, 18 and 13.5 K for Gd11Ni4In9, Dy11Ni4In9 and Ho11Ni4In9 compounds, the values of peak magnetic entropy change (−ΔSMmax) with magnetic flux density change ΔH = 0–7 T and the temperature averaged magnetic entropy change with 3 K lift are 3.58 and 3.55 J/kg-K, 6.02 and 5.94 J/kg-K, as well as 12.44 and 12.25 J/kg-K for Gd11Ni4In9, Dy11Ni4In9 and Ho11Ni4In9 compounds, respectively. The corresponding relative cooling power (refrigerant capacity) values are 269.0 (206.4), 194.9 (144.7), and 353.0 (269.2) J/kg, respectively.

Geodynamic subduction models constrained by deep earthquakes beneath the Japan Sea and eastern China

Details of Pacific plate subduction under the Japan Sea and associated current seismicity remain challenging. Seismic tomography reveals a continuous slab dipping at ~30° down to ~600 km, and earthquake mechanisms point to down-dip compression. Further, the slab is lying at the 660-km discontinuity, and this zone is aseismic. We suggest that this pattern results from the slab’s negative thermal buoyancy, resistance of the viscous lower mantle, and buoyancy forces associated with the phase transitions at 410 km and 660 km. Our model comprises an ageing subducting plate, nonlinear rheology and major phase transitions. The model explains the observed low dip angle of the slab and predicts a detailed stress pattern related to bending down to 450 km, followed by unbending as the slab is laid flat upon the 660 km boundary. Remarkably, in the bending/unbending regions, down-dip compression occurs close to the slab top/bottom, respectively. As only down-dip compression is observed, we argue that the earthquakes are mapping the top and bottom of the slab. The absence of seismicity in the flat-lying slab is explained by significantly lower stresses and higher temperatures. With this new knowledge, increasingly accurate seismic locations will considerably improve images of finite-extent slab geometry.

Extrinsic Elastic Anisotropy in a Compositionally Heterogeneous Earth's Mantle.

Several theoretical studies indicate that a substantial fraction of the measured seismic anisotropy could be interpreted as extrinsic anisotropy associated with compositional layering in rocks, reducing the significance of strain‐induced intrinsic anisotropy. Here we quantify the potential contribution of grain‐scale and rock‐scale compositional anisotropy to the observations by (i) combining effective medium theories with realistic estimates of mineral isotropic elastic properties and (ii) measuring velocities of synthetic seismic waves propagating through modeled strain‐induced microstructures. It is shown that for typical mantle and oceanic crust subsolidus compositions, rock‐scale compositional layering does not generate any substantial extrinsic anisotropy (<1%) because of the limited contrast in isotropic elastic moduli among different rocks. Quasi‐laminated structures observed in subducting slabs using P and S wave scattering are often invoked as a source of extrinsic anisotropy, but our calculations show that they only generate minor seismic anisotropy (<0.1–0.2% of Vp and Vs radial anisotropy). More generally, rock‐scale compositional layering, when present, cannot be detected with seismic anisotropy studies but mainly with wave scattering. In contrast, when grain‐scale layering is present, significant extrinsic anisotropy could exist in vertically limited levels of the mantle such as in a mid‐ocean ridge basalt‐rich lower transition zone or in the uppermost lower mantle where foliated basalts and pyrolites display up to 2–3% Vp and 3–6% Vs radial anisotropy. Thus, seismic anisotropy observed around the 660‐km discontinuity could be possibly related to grain‐scale shape‐preferred orientation. Extrinsic anisotropy can form also in a compositionally homogeneous mantle, where velocity variations associated with major phase transitions can generate up to 1% of positive radial anisotropy.

Phase Transitions in Isotactic Polybutene-1

The possibilities to influence the 4 major phase transitions of isotactic polybutene – 1 (iPB-1) and their transformation rate are described. The phase transformations result in new polymer structures with new physical and, in some cases, practically important properties. The transformation of phase II to I is spontaneous contrary to the phase transformation of III to I or II, which depends on the temperature (96 °C or higher). For the practically most important phase transition, II → I, there are 3 major transformation types (plus (P), neutral (N) and minus (M)) depending on the phase transition rate.

A homologous series of apoptosis-inducing N‑acylserinols: Thermotropic phase behavior, interaction with cholesterol and characterization of cationic N‑myristoylserinol-cholesterol-CTAB niosomes

N‑Acylserinols (NASOHs) exhibit anti-cancer activity by elevating ceramide levels, and/or by activating proapoptotic effectors. In the present work we investigated the thermotropic phase behavior and supramolecular organization of a homologous series of NASOHs (number of C-atoms in the acyl chain, n = 8–18), and the interaction of N-myristoylserinol (NMSOH) with cholesterol, and characterized cationic niosomes made up of NMSOH, cholesterol and cetyltrimethylammonium bromide (CTAB). Differential scanning calorimetric studies revealed that NASOHs exhibit a major chain-melting phase transition in both dry and hydrated states. The thermodynamic parameters, transition enthalpy and entropy show linear dependence on the acyl chain length in the dry state, but exhibit odd-even alternation in the hydrated state. Powder X-ray diffraction studies revealed that NASOHs adopt a tilted bilayer structure, wherein the bilayer repeat distances (d-spacings) also showed odd-even alteration, with even-chainlength compounds exhibiting slightly higher d-spacings. Studies on the interaction between NMSOH and cholesterol revealed that both lipids mix well with up to 55 mol% cholesterol, whereas phase separation was observed at higher cholesterol content. The transition enthalpy corresponding to the NMSOH-cholesterol complex increases up to 55 mol% cholesterol and decreases at higher cholesterol content. Presence of the cationic surfactant CTAB affects the phase behavior, fluidity and size of the NMSOH-cholesterol (45,55, mol/mol) niosomes, with unilamellar vesicles of about 85 (±20) nm in diameter being obtained at 10 mol% CTAB. These results provide a thermodynamic and structural basis for further investigations on these cationic niosomes towards their use in drug delivery applications, especially for anticancer drugs.

Extending the Solidus for a Model Iron‐Rich Martian Mantle Composition to 25 GPa

AbstractThe solidus for the mantle of Mars is an important geophysical parameter in modeling the thermal history and evolution of the planet. This study provides solidus data for a simplified model Martian mantle composition from the midmantle (8 GPa) to the core–mantle boundary (25 GPa) using multianvil experiments. Combining this work with previous studies, the solidus for the entire mantle of Mars is constrained to within 70 °C. The major mineral assemblages and phase transitions observed are consistent with those predicted for an iron‐rich Martian mantle. The solidus for the Martian composition falls above the solidus of MORB and below that of terrestrial peridotite, providing direct melting data for geophysical modeling of the Martian interior. Our solidus also predicts that a Martian mantle plume should produce melt at depths over a range of 200–350 km in the mantle.

Detecting Cosmic Reionization Using the Bispectrum Phase.

Detecting neutral hydrogen (Hi) via the 21cm line emission from the intergalactic medium at z≳6 has been identified as one of the most promising probes of the epoch of cosmic reionization-a major phase transition of the Universe. However, these studies face severe challenges imposed by the bright foreground emission from cosmic objects. Current techniques require precise instrumental calibration to separate the weak Hi line signal from the foreground continuum emission. We propose to mitigate this calibration requirement by using measurements of the interferometric bispectrum phase. The bispectrum phase is unaffected by antenna-based direction-independent calibration errors and hence for a compact array it depends on the sky brightness distribution only (subject to the usual thermal-like noise). We show that the bispectrum phase of the foreground synchrotron continuum has a characteristically smooth spectrum relative to the cosmological line signal. The two can be separated effectively by exploiting this spectral difference using Fourier techniques, while eliminating the need for precise antenna-based calibration of phases introduced by the instrument, and the ionosphere, inherent in existing approaches. Using fiducial models for continuum foregrounds, and for the cosmological Hi signal, we show the latter should be detectable in bispectrum phase spectra, with reasonable significance at |k_{∥}|≳0.5h Mpc^{-1}, using existing instruments. Our approach will also benefit other Hi intensity mapping experiments that face similar challenges, such as those measuring baryon acoustic oscillations (BAO).

The combined effects of post-spinel and post-garnet phase transitions on mantle plume dynamics

Mineralogical studies indicate that two major phase transitions occur near the depth of 660 km in the Earth's pyrolitic mantle: the ringwoodite (Rw) to perovskite (Pv) + magnesiowüstite (Mw) and the majorite (Mj) to perovskite (Pv) phase transitions. Seismological results also show a complicated phase boundary structure at this depth in plume regions. However, previous geodynamical modeling has mainly focused on the effects of the Rw–Pv+Mw phase transition on plume dynamics and has largely neglected the effects of the Mj–Pv phase transition. Here, we develop a 3-D regional spherical geodynamic model to study the combined influence of these two phase transitions on plume dynamics. Our results show the following: (1) A double phase boundary occurs in the high-temperature center of the plume, corresponding to the double reflections in seismic observations. Other plume regions feature a single, flat uplifted phase boundary, causing a gap of high seismic velocity anomalies. (2) Large amounts of relatively low-temperature plume materials can be trapped in the transition zone due to the combined effects of phase transitions, forming a complex truncated cone shape. (3) The Mj–Pv phase transition greatly enhances the plume penetration capability through 660-km phase boundary, which has a significant influence on the plume dynamics. Our results provide new insights which can be used to better constrain the 660-km discontinuity variations, seismic wave velocity structure and plume dynamics in the mantle transition zone. The model can also help to estimate the mantle temperature and Clapeyron slopes at the 660 km phase boundary.

Variation of the thermal conductivity of a silty clay during a freezing-thawing process

The thermal conductivity of soils is a key factor in calculating soil heat transfer and analyzing temperature fields of geotechnical engineering in cold regions. We measured the thermal conductivity of a silty clay by a QuickLine-30 Thermal Properties Analyzer during a freezing-thawing process, and analyzed the variation of the thermal conductivity. We then calculated the thermal conductivity under the same experimental conditions using the three general models, i.e. weighted arithmetic mean model, weighted harmonic mean model, and weighted geometric mean model. The results show that for the thawed or frozen soils with little variation of unfrozen water content, the thermal conductivity is slightly influenced by temperature; however, for the soils in the major phase transition zone, the variation of the thermal conductivity with temperature is significant. After a freezing-thawing process, the thermal conductivities of the soils with higher initial dry densities become smaller, while those with lower initial dry densities become larger. We also found that the variation of porosity and hysteresis effect of unfrozen water content cause the difference of the thermal conductivity of soils between freezing and thawing processes, however the variation of porosity acts as the primary role. Furthermore, the three general models can all be used to calculate the thermal conductivity of soils; however, the weighted geometric mean model agrees best with the experimental data.