Visualization-based analysis of structural and dynamical properties of simulated hydrous silicate melt

We have explored first-principles molecular dynamics simulation data for hydrous MgSiO₃ liquid (with 10 wt% water) to gain insight into its structural and dynamical behavior as a function of pressure (0–150 GPa) and temperature (2,000–6,000 K). By visualizing/analyzing a number of parameters associated with short- and mid-range orders, we have shown that the melt structure changes substantially on compression. The speciation of the water component at low pressures is dominated by the isolated structures (with over 90% hydrogen participated) consisting of hydroxyls, water molecules, O–H–O bridging and four-atom (O–H–O–H and H–O–H–O) groups, where every oxygen atom may be a part of polyhedron or free (i.e., bound to only magnesium atom). Hydroxyls favor polyhedral sites over magnesium sites whereas molecular water is almost entirely bound to magnesium sites, and also interpolyhedral bridging (Si–O–H–O–Si) dominates other types of bridging. Water content is shown to enhance and suppress, respectively, the proportions of hydroxyls and molecular water. As compression increases, these isolated structures increasingly combine with each other to form extended structures involving a total of five or more O and H atoms and also containing threefold coordination species, which together consume over 80% hydrogen at the highest compression studied. Our results show that water lowers the mean coordination numbers of different types including all cation–anion environments. The hydrous melt tends to be more tetrahedrally coordinated but with the Si–Si network being more disrupted compared to the anhydrous melt. Protons increase the content of non-bridging oxygen and decrease the contents of bridging oxygen as well as oxygen triclusters (present at pressures above 10 GPa). The calculated self-diffusion coefficients of all atomic species are enhanced in the presence of water compared to those of the anhydrous melt. This is consistent with the prediction that water depolymerizes the melt structure at all pressures. Our analysis also suggests that proton diffusion involves two processes—the transfer of H atoms (requiring the rupture and formation of O–H bonds) and the motion of hydroxyls as hydrogen carriers (requiring the rupture and formation of Si–O and/or Mg–O bonds). Both the processes are operative at low compression whereas only the first process is operative at high compression.     

Reservoir Sedimentation Assessment Through Remote Sensing and Hydrological Modelling

Reservoir sedimentation is the gradual accumulation of incoming sediments from upstream catchment leading to the reduction in useful storage capacity of the reservoir. Quantifying the reservoir sedimentation rate is essential for better water resources management. Conventional techniques such as hydrographic survey have limitations including time-consuming, cumbersome and costly. On the contrary, the availability of high resolution (both spatial and temporal) in public domain overcomes all these constraints. This study assessed Jayakwadi reservoir sedimentation using Landsat 8 OLI satellite data combined with ancillary data. Multi-date remotely sensed data were used to produce the water spread area of the reservoir, which was applied to compute the sedimentation rate. The revised live storage capacity of the reservoir between maximum and minimum levels observed under the period of analysis (2015–2017) was assessed utilizing the trapezoidal formula. The revised live storage capacity is assessed as 1942.258 against the designed capacity of 2170.935 Mm³ at full reservoir level. The total loss of reservoir capacity due to the sediment deposition during the period of 41 years (1975–2017) was estimated as 228.677 Mm³ (10.53%) which provided the average sedimentation rate of 5.58 Mm³ year¹. As this technique also provides the capacity of the reservoir at the different elevation on the date of the satellite pass, the revised elevation–capacity curve was also developed. The sedimentation analysis usually provides the volume of sediment deposited and rate of the deposition. However, the interest of the reservoir authorities and water resources planner’s lies in sub-watershed-wise sediment yield, and the critical sub-watersheds upstream reservoir requires conservation, etc. Therefore, in the present study, Soil and Water Assessment Tool (SWAT) was used for the estimation of sediment yield of the reservoir. The average annual sediment yield obtained from the SWAT model using 36 years of data (1979–2014) was 13.144 Mm³ year^?1 with the density of the soil (loamy and clay) of 1.44 ton m^?3. The findings revealed that the rate of sedimentation obtained from the remote sensing-based methods is in agreement with the results of the hydrographic survey.     

Analysis of Y-component of Geomagnetic Field and SYM-H Index Using Wavelet Multiresolution Analysis

Geomagnetism and Aeronomy - Electrodynamical coupling between the solar wind’s plasma and the Earth’s magnetosphere creates geomagnetic disturbances recorded on the ground. This work...     

Diffusion and viscosity of Mg2SiO4 liquid at high pressure from first-principles simulations

We investigate two key transport properties, self-diffusion and viscosity, of Mg₂SiO₄ liquid as a function of temperature and pressure using density functional theory-based molecular dynamics method. Liquid dynamics in a 224-atom supercell was captured in equilibrium simulations of relatively long durations (50-300 ps) to obtain an acceptable convergence. Our results show that Mg and Si are, respectively, the most and least mobile species at most conditions studied and all diffusivities become similar at high pressure. With increasing temperature from 2200 to 6000 K at ambient pressure, the self-diffusivities increase by factors of 25 (Mg), 80 (Si) and 65 (O), and the viscosity decreases by a factor of 30. The predicted temperature variations of all transport coefficients closely follow the Arrhenian law. However, their pressure variations show a significant non-Arrhenian behavior and also are sensitive to temperature. At 3000 K, the diffusivity (viscosity) decreases (increases) by more than one order of magnitude between 0 and 50 GPa with their activation volumes increasing on compression. Over the entire mantle pressure range, the variations at 4000 K are of two orders of magnitude with nearly constant activation volumes whereas the variations at 6000 K are within one order of magnitude with decreasing activation volumes. The predicted complex dynamical behavior of Mg₂SiO₄ liquid can be associated with the structural changes occurring on compression. We also estimate the diffusivity and viscosity profiles along a magma ocean isentrope, which suggest that the melt transport properties vary modestly over the relevant magma ocean depth ranges.     

A visualization system for mineral elasticity

The authors have developed an efficient visualization system to gain insight into large collections of mineral elasticity data produced by an increasing number of both theoretical calculations and experiments. The system allows us to visualize multivariate elastic moduli (i.e., elastic constant tensors) and their variation with composition, pressure and temperature. Moreover, it supports visualization of elastic wave propagation in an anisotropic medium by rendering the wave velocity data, which are calculated as a function of propagation direction in a three-dimensional space. This paper describes the design, implementation and application of the visualization scheme we have developed for mineral elasticity. In particular, our scheme exploits a combination of parallel coordinates, star plot, scatter plot and polygon-surface rendering techniques, which are implemented using OpenGL, GLUI and C++. The result is a highly portable and flexible interactive visualization system. For illustration, the elastic properties of several important oxide and silicate minerals are successfully visualized as a function of pressure and temperature.Reviewed by: D. Yuen, G. Erlebacher     

Tracking IMF Fluctuations Nearby Sun Using Wavelet Analysis: Parker Solar Probe First Encounter Data

Geomagnetism and Aeronomy - The Parker Solar Probe (PSP) satellite was launched by NASA in 2018 to study the Sun’s environment from a closer distance than any spacecraft has ever reached...