679 research outputs found
An investigation of mineral dynamics in frozen seawater brines by direct measurement with synchrotron X-ray powder diffraction
Frozen seawater is a composite material with a sponge-like structure. The framework of the structure is composed of pure ice, and within the pores exists a concentrated seawater brine. When the temperature is reduced, the volume of this residual brine decreases, while its salinity increases. As a result of the paired changes to temperature and salinity, the brine eventually becomes supersaturated with respect to a mineral, resulting in the precipitation of microscopic crystals throughout the ice structure. Due to experimental constraints, the current understanding about the formation of these minerals relies on the analysis of the residual brine, rather than the mineral phase. Here synchrotron X-ray powder diffraction was used to assess the dynamics that occur between ice, brine, and mineral phases within frozen seawater brines that were subjected to cooling and warming at subzero temperatures. The method was able to detect crystalline phases of ice, mirabilite (Na2SO4Ă·10H2O), and hydrohalite (NaClĂ·2H2O). Results illustrate a highly dynamic geochemical environment where ice-brine-mineral interactions tend toward an equilibrium crystallization process, which supports the process of seawater freezing that is described by the Gitterman Pathway and FREZCHEM model. This study highlights the power of synchrotron techniques in observing the mineralogical dynamics of inaccessible environmental systems
Mirabilite solubility in equilibrium sea ice brines
The sea ice microstructure is permeated by brine channels and pockets that contain concentrated seawater-derived brine. Cooling the sea ice results in further formation of pure ice within these pockets as thermal equilibrium is attained, resulting in a smaller volume of increasingly concentrated residual brine. The coupled changes in temperature and ionic composition result in supersaturation of the brine with respect to mirabilite (Na2SO4·10H2O) at temperatures below â6.38 °C, which consequently precipitates within the sea ice microstructure. Here, mirabilite solubility in natural and synthetic seawater derived brines, representative of sea ice at thermal equilibrium, has been measured in laboratory experiments between 0.2 and â20.6 °C, and hence we present a detailed examination of mirabilite dynamics within the sea ice system. Below â6.38 °C mirabilite displays particularly large changes in solubility as the temperature decreases, and by â20.6 °C its precipitation results in 12.90% and 91.97% reductions in the total dissolved Na+ and SO42â concentrations respectively, compared to that of conservative seawater concentration. Such large non-conservative changes in brine composition could potentially impact upon the measurement of sea ice brine salinity and pH, whilst the altered osmotic conditions may create additional challenges for the sympagic organisms that inhabit the sea ice system. At temperatures above â6.38 °C, mirabilite again displays large changes in solubility that likely aid in impeding its identification in field samples of sea ice. Our solubility measurements display excellent agreement with that of the FREZCHEM model, which was therefore used to supplement our measurements to colder temperatures. Measured and modelled solubility data were incorporated into a 1D model for the growth of first-year Arctic sea ice. Model results ultimately suggest that mirabilite has a near ubiquitous presence in much of the sea ice on Earth, and illustrate the spatial and temporal evolution of mirabilite within sea ice as it grows throughout an Arctic winter, reaching maximum concentrations of 2.3 g kgâ1
Laboratory exploration of mineral precipitates from Europa's subsurface ocean
The precipitation of hydrated phases from a chondritic-like Na-Mg-Ca-SO4-Cl solution is studied using in situ synchrotron X-ray powder diffraction, under rapid (360
K hourâ1, T = 250-80 K, t = 3 hours) and ultra-slow (0.3 K dayâ1, T= 273-245K, t = 242 days) freezing conditions. The precipitation sequence under slow cooling initially follows the predictions of equilibrium thermodynamics models, however after ⌠50 days at 245 K, the formation of the highly hydrated sulphate phase Na2Mg(SO4)2·16H2O, a relatively recent discovery in the Na2Mg(SO4)2-H2O system, was observed. Rapid freezing, on the other hand, produced an assemblage of multiple phases which formed within a very short timescale (â€4 minutes, âT = 2 K) and, although remaining present throughout, varied in their relative proportions with
decreasing temperature. Mirabilite and meridianiite were the major phases, with pentahydrite, epsomite, hydrohalite, gypsum, blšodite, konyaite and loweite also observed. Na2Mg(SO4)2·16H2O was again found to be present and increased in proportion relative to other phases as the temperature decreased. Results are discussed in relation to possible implications for life on Europa and application to other icy ocean worlds
Segregation and charge-density-wave order in the spinless Falicov-Kimball model
The spinless Falicov-Kimball model is solved exactly in the limit of
infinite-dimensions on both the hypercubic and Bethe lattices. The competition
between segregation, which is present for large U, and charge-density-wave
order, which is prevalent at moderate U, is examined in detail. We find a rich
phase diagram which displays both of these phases. The model also shows
nonanalytic behavior in the charge-density-wave transition temperature when U
is large enough to generate a correlation-induced gap in the single-particle
density of states.Comment: 10 pages, 10 figure
Permeability of compacting porous lavas
The highly transient nature of outgassing commonly observed at volcanoes is in part controlled
by the permeability of lava domes and shallow conduits. Lava domes generally consist of a porous outer
carapace surrounding a denser lava core with internal shear zones of variable porosity. Here we examine
densification using uniaxial compression experiments on variably crystalline and porous rhyolitic dome lavas
from the Taupo Volcanic Zone. Experiments were conducted at 900°C and an applied stress of 3MPa to 60%
strain, while monitoring acoustic emissions to track cracking. The evolution of the porous network was
assessed via X-ray computed tomography, He-pycnometry, and relative gas permeability. High starting
connected porosities led to low apparent viscosities and high strain rates, initially accompanied by abundant
acoustic emissions. As compaction ensued, the lavas evolved; apparent viscosity increased and strain rate
decreased due to strain hardening of the suspensions. Permeability fluctuations resulted from the interplay
between viscous flow and brittle failure. Where phenocrysts were abundant, cracks had limited spatial
extent, and pore closure decreased axial and radial permeability proportionally, maintaining the initial
anisotropy. In crystal-poor lavas, axial cracks had a more profound effect, and permeability anisotropy
switched to favor axial flow. Irrespective of porosity, both crystalline samples compacted to a threshold
minimum porosity of 17â19%, whereas the crystal-poor sample did not achieve its compaction limit. This
indicates that unconfined loading of porous dome lavas does not necessarily form an impermeable plug
and may be hindered, in part by the presence of crystals
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Chemical and isotopic characteristics of the coso east flankhydrothermal fluids: implications for the location and nature of the heatsource
Fluids have been sampled from 9 wells and 2 fumaroles fromthe East Flank of the Coso hydrothermal system with a view toidentifying, if possible, the location and characteristics of the heatsource inflows into this portion of the geothermal field. Preliminaryresults show that there has been extensive vapor loss in the system, mostprobably in response to production. Wells 38A-9, 51-16 and 83A-16 showthe highest CO2-CO-CH4-H2 chemical equilibration temperatures, rangingbetween 300-340oC, and apart from 38A-9, the values are generally inaccordance with the measured temperatures in the wells. Calculatedtemperatures for the fractionation of 13C between CO2 and CH4 are inexcess of 400oC in fluids from wells 38A-9, 64-16-RD2 and 51A-16,obviously pointing to equilibrium conditions from deeper portions of thereservoir. Given that the predominant reservoir rock lithologies in theCoso system are relatively silicic (granitic to dioritic), the isotopicsignatures appear to reflect convective circulation and equilibrationwithin rocks close to the plastic-brittle transition. 3He/4He signatures,in conjunction with relative volatile abundances in the Coso fluids,point to a possibly altered mantle source for the heat sourcefluids
Grassmann Variables and the Jaynes-Cummings Model
This paper shows that phase space methods using a positive P type
distribution function involving both c-number variables (for the cavity mode)
and Grassmann variables (for the two level atom) can be used to treat the
Jaynes-Cummings model. Although it is a Grassmann function, the distribution
function is equivalent to six c-number functions of the two bosonic variables.
Experimental quantities are given as bosonic phase space integrals involving
the six functions. A Fokker-Planck equation involving both left and right
Grassmann differentiation can be obtained for the distribution function, and is
equivalent to six coupled equations for the six c-number functions.
The approach used involves choosing the canonical form of the (non-unique)
positive P distribution function, where the correspondence rules for bosonic
operators are non-standard and hence the Fokker-Planck equation is also
unusual. Initial conditions, such as for initially uncorrelated states, are
used to determine the initial distribution function. Transformations to new
bosonic variables rotating at the cavity frequency enables the six coupled
equations for the new c-number functions (also equivalent to the canonical
Grassmann distribution function) to be solved analytically, based on an ansatz
from a 1980 paper by Stenholm. It is then shown that the distribution function
is the same as that determined from the well-known solution based on coupled
equations for state vector amplitudes of atomic and n-photon product states.
The treatment of the simple two fermion mode Jaynes-Cummings model is a
useful test case for the future development of phase space Grassmann
distribution functional methods for multi-mode fermionic applications in
quantum-atom optics.Comment: 57 pages, 0 figures. Version
Thermodynamic studies of the two dimensional Falicov-Kimball model on a triangular lattice
Thermodynamic properties of the spinless Falicov-Kimball model are studied on
a triangular lattice using numerical diagonalization technique with Monte-Carlo
simulation algorithm. Discontinuous metal-insulator transition is observed at
finite temperature. Unlike the case of square lattice, here we observe that the
finite temperature effect is not able to smear out the discontinuous
metal-insulator transition seen in the ground state. Calculation of specific
heat (C_v) shows single and double peak structures for different values of
parameters like on-site correlation strength (U), f-electron energy (E_f) and
temperature.Comment: 6 pages, 7 figure
Trends in autoionization of Rydberg states converging to the 4s threshold in the Kr-Rbâș-SrÂČâș isoelectonic sequence: theory and experiment
We have measured the photoabsorption spectra of the Kr-like ions Rb+ and Sr2+ at photon energies corresponding to the excitation of 4s-np resonances using, the dual laser plasma photoabsorption technique. Dramatic changes in the line profiles, with increasing ionization and also proceeding along the Rydberg series of each ion, are observed and explained by the trends in 4s-transition amplitudes computed within a framework of configuration-interaction Pauli-Fock calculations. Total photoionization cross sections show very good agreement with relative absorption data extracted from the measured spectra
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