Argon Diffusion Research Articles

Numerical study of detonation propagation in a semi-confined nonuniform layer bound by an inert gas

This study investigates detonation propagation through a stoichiometric hydrogen-oxygen layer stratified above an inert gas, i.e., argon, with a diffuse interface at atmospheric pressure. A simple model capable of replicating the detonation inert gas diffuse layer interaction was developed that simulates previously conducted experiments. Detonation propagation is modeled by solving the two-dimensional reactive Euler equations for a calorically perfect gas with single-step Arrhenius chemistry. The acoustic impedance gradient associated with the argon diffusion into the hydrogen-oxygen results in a nonuniform initial density. Because of the perfect gas assumption, for a uniform initial pressure, as in the experiment, results in an initial temperature that is artificially nonuniform. The activation energy is adjusted for the imposed temperature nonuniformity in order to yield the correct ZND reaction zone length for the local argon dilution. Numerical soot foils are compared to experiments. Detonation propagation is possible due to the generation of triple points along the diffuse layer interface. These triple points, that generate new detonation cells, form from inflections in the reaction zones that develop because of the local increases in detonation strength.

Molecular Simulation of Argon Adsorption and Diffusion in a Microporous Carbon with Poroelastic Couplings.

Neglected for a long time in molecular simulations of fluid adsorption and transport in microporous carbons, adsorption-induced deformations of the matrix have recently been shown to have important effects on both sorption isotherms and diffusion coefficients. Here we investigate in detail the behavior of a recently proposed 3D-connected mature kerogen model, as a generic model of aromatic microporous carbon with atomic H/C ∼ 0.5, in both chemical and mechanical equilibrium with argon at 243 K over an extended pressure range. We show that under these conditions the material exhibits some viscoelasticity, and simulations of hundreds of nanoseconds are required to accurately determine the equilibrium volumes and sorption loadings. We also show that neglecting matrix internal deformations and swelling can lead to underestimations of the loading by up to 19% (swelling only) and 28% (swelling and internal deformations). The volume of the matrix is shown to increase up to about 8% at the largest pressure considered (210 MPa), which induces an increase of about 33% of both pore volume and specific surface area via the creation of additional pores, yet does not significantly change the normalized pore size distribution. Volume swelling is also rationalized by using a well-known linearized microporomechanical model. Finally, we show that self-diffusivity decreases with applied pressure, following an almost perfectly linear evolution with the free volume. Quantitatively, neglecting swelling and internal deformations tends to reduce the computed self-diffusivities.

High nitrogen and argon diffusion in cyclic olefin copolymer foil versus temperature

Measurements of nitrogen and argon diffusion coefficient in cyclic olefin copolymer (COC) have been performed at different temperatures ranging between 21 °C and 101 °C. Optical transparent COC foils have been prepared with 10 μm thickness and total surface of about 25 cm2. The foils have exhibited high purity in composition. The morphology of the COC surface has been investigated by Scanning Electron and Atomic Force Microscopies. The room temperature measured diffusion coefficients, of about 6.56 × 10−3 cm2/s and 6.30 × 10−3 cm2/s for N2 and Ar, respectively, have been obtained with a home-made experimental set-up by measuring the gas pressure gradient reduction applied to the two faces of COC thin foils versus the time. The foils show a significant increase of the N2 and Ar diffusion coefficients with the temperature. The native amorphous nature, the obtained nano-porous structure, the damage suffered by the thin foils of COC under high applied pressure are responsible of the evaluated high diffusion coefficients for nitrogen and argon gases. The experimental apparatus to measure the diffusion coefficients vs. temperature, the obtained results, the COC sheets microstructure, the measurements comparison with the literature data and possible COC applications in biomedicine are presented and discussed.

Modeling of argon–steam thermal plasma flow for abatement of fluorinated compounds

This study presents a numerical model of the hybrid-stabilized argon–steam thermal DC plasma torch of a new design for generating an argon–steam plasma suitable for efficient abatement of persistent perfluorinated compounds. The model includes the discharge region and the plasma jet flowing to the surrounding steam atmosphere contained in a plasma-chemical chamber. Compared to previous studies, the torch had a smaller nozzle diameter (5.3 mm) and a reduced input power (20–40 kW) and arc current (120–220 A). The outlet region for the plasma jet extends to 20 cm downstream of the exit nozzle. Fluid dynamic and thermal characteristics together with diffusion of argon, hydrogen and oxygen species, and distribution of plasma species in the discharge and the plasma jet are obtained for currents from 120 to 220 A. The results of the calculations show that the plasma jet exhibits high spatiotemporal fluctuations in the shear layer between the plasma jet and colder steam atmosphere. The most abundant species in the plasma jet are hydrogen and oxygen atoms near the jet center, and molecules of H2, O2 and OH in colder surrounding regions. Satisfactory agreement is obtained with measurements of the radial temperature and electron number density profiles near the jet center close to the nozzle exit.

Argon diffusion in graphene oxide and reduced graphene oxide foils

Measurements of argon diffusion coefficient vs. temperature in graphene oxide (GO) and reduced graphene oxide (rGO) have been performed. GO and rGO foils have been employed with a thickness of 15 μm. GO has been reduced in vacuum at a temperature of 130 °C for 30 min. Measurements have been obtained by controlling the gas pressure gradient applied to two faces of the thin foils versus the time and the temperature. The measured Ar diffusion coefficients have been compared to those obtained for N2 in the same foils. In particular, the calculated room temperature Ar and N2 diffusion coefficients in GO are 2.95 × 10−3 cm2/s and 0.168 × 10−3 cm2/s respectively. At 100 °C in rGO the Ar and N2 diffusion coefficients are 2.15 × 10−3 cm2/s and 0.4 × 10−3 cm2/s respectively. Consequently, the Ar diffusion coefficients in GO and rGO appear greater than the nitrogen ones. The high diffusion coefficient of Ar increases with the temperature and decreases going from GO to rGO, due to the highest compactness and density of the reduced GO foil. The smaller diffusion coefficient of N2 increases both with the temperature and going from GO to rGO, due to the partial water molecules and oxygen functional groups removal. The obtained results, their correlation with the inner structure of the graphene sheets and the comparison with the literature data are presented.

A novel adsorption differential volumetric apparatus to measure mass transfer in nanoporous materials

A novel adsorption differential volumetric apparatus was developed for the determination of diffusional time constants in nanoporous materials and applied to diffusion of nitrogen and argon in commercial pellets of 4A zeolite. The system is designed for high rates of data acquisition allowing to determine mass transfer time constants of seconds over the pressure range from vacuum to 130 kPa. Diffusion of N2 and Ar on a single pellet and fragments obtained from the pellet are studied between –10 °C and 35 °C and 0.8 to 55 kPa. These systems are chosen as representing weak adsorption to demonstrate the sensitivity of the apparatus that gives a good signal-to-noise ratio even with a single pellet in the entire pressure range. The systems studied confirm micropore diffusion control and an isothermal diffusion model was shown to reproduce accurately the observed kinetics using reduced pressure plots. As the crystal size in the pellet is not known accurately, the resulting activation energies and the ratio of diffusional time constants of N2 and Ar were used to validate the results against known literature values.

Molecular diffusion in gases and liquids

The diffusion coefficients in gases and liquids calculated by the molecular dynamics method with the use of the hard absolutely rough elastic spheres model are compared with those calculated using the Lennard-Jones potential. It is shown that dependences of reduced diffusion coefficients on density are similar, but differ numerically for different intermolecular interaction models. The simulation results have been compared with the experimental data on the diffusion in gaseous and liquid argon and in liquid benzene.

Mesoproterozoic thermal evolution of the northern Gawler Craton from 40Ar/39Ar geochronology

Previous thermochronology studies of the northern Gawler Craton have interpreted cooling from mid-crustal temperatures in two phases; either cooling subsequent to a c. 1610–1580 Ma event or over the interval c. 1460–1440 Ma, however the extent and the significance of these ages has been uncertain. We have obtained nine new 40Ar/39Ar thermochronological dates on igneous and metamorphic rocks from drill hole samples across the region. The new data show that different fault-bound blocks have different thermal histories, and that cooling to below biotite closure to argon diffusion over the interval c. 1460–1415 Ma was widespread across the region. Arrhenius data from step heating experiments in this study yield closure temperature estimates for biotite samples that range from 436 to 310 °C. Cooling rates ranging from 3 to 5 °C/Myr are widespread across the region, with one block having slightly faster cooling, up to 14 °C/Myr. These rates suggest a combination of cooling related to thermal relaxation and modest erosion rates, with faster cooled zones likely resulting from moderate degrees of active exhumation along shear zone systems. This cooling may be linked to early stages of break-up of the Nuna supercontinent as c. 1460–1415 Ma tectonic activity is also documented across many terranes of the Nuna supercontinent including the Terre Adelie Craton in Antarctica and terranes of Laurentia.

40Ar/39Ar ages of Northwest Africa 7034 and Northwest Africa 7533

AbstractThe Martian breccias NWA 7034, NWA 7533, and paired meteorites record events ranging in age from 4.47 Ga to <200 Ma. Published ages indicate a period of major disturbance at ~1.4 Ga, examined in detail here through 40Ar/39Ar dating of handpicked grains and two small chips. Argon diffusion parameters were obtained for six samples. Also presented are He, Ne, Ar, Kr, and Xe contents of two small (<100 µg), handpicked mineral separates, a felsic “Light” sample and a mafic/pyroxene‐rich “Dark” sample. The 40Ar/39Ar ages of five samples, four containing >1 wt% K and thought to be rich in feldspar and one containing <~1 wt% K, cluster near 1.4 Ga. The 40Ar/39Ar ages of nine grains with low K contents have a wide range of apparent ages from 0.3 ± 0.1 Ga to 2.9 ± 0.1 Ga for individual temperature steps, and from 0.74 ± 0.06 Ga to ~2.1 Ga for plateau ages. Isochron ages are less precise, but generally agree with plateau ages. Only two isochrons have the significantly positive intercepts expected in the presence of terrestrial or Martian atmospheric argon. At higher release temperatures, activation energies for diffusion obtained from 39Ar data for six samples are generally 160–200 kJ mol−1, consistent with published values for feldspathic minerals. For three of these samples, lower temperature data on Arrhenius plots are best fit with a much lower activation energy of <100 kJ mol−1. We attribute the low values to the effects of varying degrees of shock on feldspathic minerals and/or the presence of phases in vitrophyric spherules produced by hydrothermal alteration. The low activation energies place an upper limit of ~14 ka on the terrestrial age of NWA 7034. Much lower concentrations of cosmogenic (c) 3He and 21Ne in the Light than in the Dark separate indicate substantial losses concurrent with or postdating cosmic ray irradiation. A one‐stage, cosmic ray exposure (CRE) age for the Dark separate from NWA 7034 is estimated to be between 7 and 10 Ma from the concentrations of 3Hec and 38Arc, and of close to 15 Ma from the concentration of 21Nec. Most of the 40Ar/39Ar and noble gas data are compatible with (1) a heating and alteration event ~1.40 Ga caused by contact metamorphism, an impact, and/or the infiltration of hydrothermal fluids; and (2) at least one later event at lower temperatures that led to either loss of He and Ar from phases with low activation energies, or to gain of K. Most of the 40Ar/39Ar ages are consistent with the assembly of NWA 7034 1.4 Ga ago or perhaps earlier followed more recently by selective alteration. A more recent time of assembly is also consistent with these ages provided that the temperature stayed low. The five most precise 40Ar/39Ar ages of the samples analyzed are all ~1.4 Ga, a value seen frequently in other NWA 7034 chronometers and very similar to crystallization ages of nakhlites and chassignites (NC). Some CRE ages based on noble gases in NWA 7034 agree within their considerable uncertainties with those of NC. These two chronometric coincidences suggest that the NWA 7034 clan and the NC share a launch date on Mars. We propose that K‐rich fluids derived from the nakhlite source area interacted with proto‐NWA 7034 and modified the K/Ar ratios and ages of previously shocked feldspar grains, with the degree of modification depending on the degree of shock. The NWA 7034 clan may therefore be considered components from a metamorphic aureole around a nakhlite massif.

Modelling and measurements of gas tungsten arc welding in argon–helium mixtures with metal vapour

Argon–helium mixtures in gas tungsten arc welding of an iron workpiece are investigated using an axisymmetric computational model that includes the cathode, workpiece, and arc plasma in the computational domain. The three-gas combined diffusion coefficient method is used to treat diffusion of helium, argon, and iron vapour. Calculations for argon–helium mixtures without metal vapour are performed; good agreement with previous numerical results is found. A transition from a helium-like to an argon-like arc occurs when the argon mole fraction increases above about 0.3. Calculations for a wide range of argon–helium mixtures including iron vapour are then performed. Adding helium to argon alters the arc properties and affects the weld geometry. Iron vapour cools the arc for all argon–helium mixtures. Iron vapour is present above the workpiece, near the cathode and in the arc fringes for very low argon mole fractions. As the argon mole fraction increases, the iron vapour becomes increasingly confined to the region above the workpiece, with small amounts near the cathode tip. Emission spectroscopy measurements of arcs in argon–helium mixtures with water-cooled copper and uncooled iron workpieces were performed. The measured distributions of atomic helium and iron emission show good agreement with the predictions of the model.

Transport coefficients of isotopic mixtures of noble gases based on ab initio potentials.

The transport coefficients such as viscosity, thermal conductivity, diffusion and thermal diffusion of neon, argon, krypton, and xenon are computed for a wide range of temperatures taking into consideration their real isotopic compositions. A new concept of isotopic thermal diffusion factor is introduced and calculated. The Chapman-Enskog method based on the 10th order approximation with respect to the Sonine polynomial expansion is applied. Ab initio potentials of interatomic interactions are employed to compute the transport cross-sections as they are part of the coefficient expressions. To study the influence of the isotopic composition, the same transport coefficients have been calculated for the single gases having an average atomic mass. The estimated numerical error of the present results is a function of the temperature and is different for each coefficient. At the room temperature, the relative numerical error of viscosity, thermal conductivity and diffusion coefficient is on the order of 10-6. The numerical error of the thermal diffusion factor affects the fifth decimal digit. The influence of the isotopic composition on viscosity and thermal conductivity depends on the gas species. It is negligible for argon and significant (about 0.02%) for xenon, while neon and krypton are weakly affected by the isotopic composition. The diffusion coefficient for each pair of isotopes differs from the corresponding self-diffusion coefficient by about 3%. The thermal diffusion factor of each isotope differs from the thermal self-diffusion factor in the third decimal digit.

Consistent lattice Boltzmann model for multicomponent mixtures

Abstract

Numerical simulation of an atmospheric pressure plasma jet with coaxial shielding gas

A two-dimensional (r, z) numerical simulation of the discharge characteristics of an atmospheric pressure plasma jet (APPJ), with coaxial shielding gas, was performed. The helium working gas flowed in a central capillary tube, engulfing a needle electrode powered by 13.7 MHz radio frequency sinusoidal voltage. The N2 shielding gas flowed in the annular space of a coaxial tube. These gases emerged, in laminar flow, in a 78%N2-21%O2-1%Ar dry air ambient. The characteristics of the APPJ with shielding gas were compared to those of the APPJ without shielding gas. The nitrogen shielding gas hindered the diffusion of oxygen and argon from the ambient air into the helium jet. With the shielding gas present, more nitrogen penetrated into the helium core, causing a shorter plasma ‘plume’. The flow rates of the working and shielding gas, critically affected the gas temperature, and in turn the discharge characteristics. For a He flow of 2 standard liters per minute (slm), switching on the nitrogen shielding gas flow (at 4.5 slm) reduced the on-axis O2 and Ar mole fractions from to and from to , respectively, at an axial distance of 3 mm downstream of the nozzle. The radial profiles of the mole fractions of the ambient gases were monotonically and strongly decreasing towards the system axis, for short axial distances from the nozzle (∼1 mm), but became progressively flatter at longer distances from the nozzle (3 mm and 5 mm). Simulation predictions captured the salient features of experimental data of ambient species mole fractions in the plasma jet, and the 706 nm optical emission intensity profiles of the He 33S excited state.

Mechanisms and kinetics of argon diffusion in hypogene and supergene jarosites: Implications for geochronology and surficial geochemistry on Earth and Mars

Jarosite [KFe3(SO4)2(OH)6] occurs both as a hydrothermal mineral and as the product of weathering and chemical sedimentation. It has been used in 40Ar/39Ar geochronology to date water–rock interaction and weathering processes on the surfaces of Earth and Mars, but the lack of information about Ar diffusivity parameters relevant to specific types of jarosites makes the interpretation of geochronological results tentative. We have filled this gap by investigating Ar diffusion parameters in representative supergene and hypogene jarosites. Detailed diffusion studies were carried out on a hypogene jarosite sample from Gilbert, Nevada, and two supergene jarosite samples from Baiyin, China. The diffusion studies were accompanied by in-situ heating investigations in a transmission electron microscope to directly determine the thermal stability and the phase transformations that jarosite undergoes under progressive heating under ultra-high vacuum. The TEM results suggest that jarosite is stable under vacuum up to ∼400 °C, when it undergoes phase transition to yavapaiite [KFe(SO4)2] and hematite (Fe2O3). Incremental-heating experiments reveal average diffusion parameters of Ea = 138.6 ± 4.2 kJ/mol and ln(Do/a2) = 9.9 ± 0.9 ln(s−1) for hypogene jarosite; Ea = 110.3 ± 3.2 kJ/mol and ln(Do/a2) = 5.7 ± 0.7 ln(s−1) for one supergene jarosites; and Ea = 141.2 ± 7.9 kJ/mol and ln(Do/a2) = 11.3 ± 1.7 ln(s−1) for the other supergene jarosite sample from the same weathering profile. Jarosite closure temperatures depend strongly on sieve size. For samples between 500–200 µm (grain size usually used for samples in Ar geochronology), at a cooling rate 100 °C·Ma−1, the closure temperatures are 143 ± 18 °C for the hypogene jarosite, and 105 ± 8 and 113 ± 14 °C, respectively, for the two supergene jarosites. Forward modelling of incremental-heating results predicts that coarse-grained hypogene jarosite is retentive of Ar below 50 °C for 100 Ma and below 25 °C for 4 Ga. Densely packed supergene jarosite grains larger than 200 µm are suitable for 40Ar/39Ar geochronology at the timescales suitable for investigating water–rock interaction at the surface of Earth and Mars. Fine-grained, porous jarosites require detailed diffusion analyses prior to geochronology due to possible high Ar losses over Ma timescales. The absence of jarosites older than ∼40 Ma on Earth suggests that jarosite may require continuous exposure to acid oxidizing conditions, and that it does not survive burial and exhumation. Therefore, the occurrence of jarosite on Earth and Mars may identify segments of the planets’ surfaces continuously exposed to acid-oxidizing conditions since jarosite precipitation.

A test of the interlayer ionic porosity model as a measure of argon diffusivity in trioctahedral micas

Micas are among the most commonly-dated minerals by the 40Ar/39Ar technique to determine the crystallization or cooling ages of rocks. Interpretation of the geological significance of these ages is largely dependent on understanding how 40Ar* (radiogenic argon) can be retained or released from the mineral during the thermal history of a terrane. The interlayer ionic porosity (Zi) model, serving as a measure of a mineral’s atomic packing density, has been proposed in an attempt to understand and explain the diffusive behaviour of argon (Ar) and its retention in micas. We tested this model by dating micas from several outcrops using the 40Ar/39Ar and Rb/Sr techniques along a ∼5.2 km transect in the Frontenac terrane, Grenville Province, Ontario. This transect is ideal because: (a) the rocks in this small area experienced the same geologic history, and (b) there are a variety of lithologies containing trioctahedral and dioctahedral micas with different chemical compositions. Our results show that trioctahedral and dioctahedral micas yielded 40Ar/39Ar plateau ages that differ by ∼150 Ma. Significantly, although both Mg# and F# correlate with Zi, as predicted by the model, there is no correlation between Mg#, F#, and Zi and the age of the micas in similar lithologies. This suggests that the interlayer ionic porosity model, in itself, may not be suitable for explaining mechanisms of Ar mobility in micas alone. Instead, both internal (chemical composition, bulk rock properties) and external (fluid composition, open/closed-system behaviour) factors that may influence Ar retentivity in micas should be more broadly considered in order to elucidate the significance of their 40Ar/39Ar ages.

Geochronological and petrological constraints from the evolution in the Saxon Granulite Massif, Germany, on the Variscan continental collision orogeny

AbstractControversy over the plate tectonic affinity and evolution of the Saxon granulites in a two‐ or multi‐plate setting during inter‐ or intracontinental collision makes the Saxon Granulite Massif a key area for the understanding of the Palaeozoic Variscan orogeny. The massif is a large dome structure in which tectonic slivers of metapelite and metaophiolite units occur along a shear zone separating a diapir‐like body of high‐P granulite below from low‐P metasedimentary rocks above. Each of the upper structural units records a different metamorphic evolution until its assembly with the exhuming granulite body. New age and petrologic data suggest that the metaophiolites developed from early Cambrian protoliths during high‐P amphibolite facies metamorphism in the mid‐ to late‐Devonian and thermal overprinting by the exhuming hot granulite body in the early Carboniferous. A correlation of new Ar–Ar biotite ages with published P–T–t data for the granulites implies that exhumation and cooling of the granulite body occurred at average rates of ~8 mm/year and ~80°C/Ma, with a drop in exhumation rate from ~20 to ~2.5 mm/year and a slight rise in cooling rate between early and late stages of exhumation. A time lag of c. 2 Ma between cooling through the closure temperatures for argon diffusion in hornblende and biotite indicates a cooling rate of 90°C/Ma when all units had assembled into the massif. A two‐plate model of the Variscan orogeny in which the above evolution is related to a short‐lived intra‐Gondwana subduction zone conflicts with the oceanic affinity of the metaophiolites and the timescale of c. 50 Ma for the metamorphism. Alternative models focusing on the internal Variscan belt assume distinctly different material paths through the lower or upper crust for strikingly similar granulite massifs. An earlier proposed model of bilateral subduction below the internal Variscan belt may solve this problem.

Modelling of inhomogeneous mixing of plasma species in argon–steam arc discharge for broad range of operating conditions

Numerical simulation of mixing of argon- and water-plasma species in argon-steam arc discharge has been investigated in thermal plasma generator with the combined stabilization of arc by axial gas flow (argon) and water vortex. Mixing process is described by the combined diffusion coefficients method in which the coefficients describe the diffusion of argon “gas”, with respect to steam “gas”. Calculations for currents 150–600 A with 15–40 standard liters per minute (slm) of argon reveal inhomogeneous mixing of argon and oxygen-hydrogen species with the argon species prevailing near the arc axis. However, calculations for currents higher than 400 A were not straightforward and a phenomenon of premixing of argon and steam species in the upstream discharge region was predicted from modelling to obtain reasonable agreement with experimental data. Premixed argon-steam plasma has a global impact on the plasma jet parameters near the exit nozzle as well as on the overall arc performance. The results of thermo-fluid fields, species mole fractions and radiation losses from the discharge are presented and discussed. Our former calculations based on the homogeneous mixing assumption differ from the present model in temperature, enthalpy, radiation losses, and flow field. Comparison with available experiments exhibits very good qualitative and quantitative agreements for the radial temperature profiles and satisfactory agreement for the velocity profiles 2 mm downstream of the exit nozzle.

Argon diffusion in hypogene and supergene alunites: Implications to geochronology and thermochronometry on Earth and Mars

Argon release mechanisms and diffusivity were quantified for coarsely crystalline hypogene alunite [KAl 3 (SO 4 ) 2 (OH) 6 ] from Marysvale, Utah, and microcrystalline supergene alunite aggregates from Coober Pedy, South Australia. Prior to diffusivity studies, all alunite samples, sorted in the 500–200, 200–100, 100–50, 50–10, and <10 µm sieve-size ranges, were vacuum encapsulated to quantify 39 Ar recoil losses. Incremental-heating of single capsules inserted into Ta-crucibles and heated by a projector-lamp in a specially designed diffusion cell permits quantifying 39 Ar released at precisely measured temperature steps (±1–3 °C). Incremental-heating 40 Ar/ 39 Ar analyses of the various sieve-size ranges yield reproducible ages that are indistinguishable from single grain (1–2 mm) laser-heating 40 Ar/ 39 Ar dating of the same samples. The results, cast in Arrhenius plots, yield activation energies (E a ) ranging from 248.0 ± 18.5 to 281.7 ± 15.2 kJ/mol and ln(D o /a 2 ) from 26.2 ± 2.0 to 27.8 ± 3.2 ln(s −1 ) for hypogene alunite; supergene alunite yield E a ranging between 233.3 ± 5.4 and 293.8 ± 13.7 kJ/mol and ln(D o /a 2 ) between 27.3 ± 1.0 and 36.8 ± 2.6 ln(s −1 ). These diffusion parameters correspond to closure temperatures of 264 ± 22 °C and 246 ± 19 °C for hypogene and supergene alunite, respectively, assuming a cooling rate 100 °C·Ma −1 . In-situ TEM experiments on aliquots of alunite crystals from the same samples indicate that alunite single crystals undergo transformation to nanocrystalline aggregates at 430–460 °C, showing that alunite releases Ar by volume diffusion below ∼430 °C, retains a significant amount of Ar during phase transformation, and proceeds to release Ar by volume or multipath diffusion from a modified polycrystalline structure at T > 460 °C. Isothermal holding time and AGESME modelling using our calculated diffusion parameters indicate that alunite should preserve Ar quantitatively for long periods (4.0 Ga) at Earth and Mars surface conditions, and both hypogene and supergene alunite should preserve original formation ages, independently of precipitation mechanism.

Argon storage and diffusion in Earth’s upper mantle

In this study, fine-grained polycrystalline olivine was doped with argon at static high pressure (0.30 ± 0.01 GPa) and high temperature (1050 ± 25 °C) conditions during 24 h in a Paterson press, and analysed using a step-heating extraction protocol coupled with noble gas mass spectrometry to investigate argon storage and diffusivity in Earth’s upper mantle. Our results show that a single diffusion mechanism controlled argon diffusion in our samples during the step heating experiments. Effective Ar diffusion in olivine has a low activation energy, implying that argon diffusivity is governed by both grain boundary and lattice diffusion. Mean values of lattice diffusion parameters obtained from our results and by reprocessing literature data are Ea = 166 ± 44 kJ mol−1 and D0 = 10−7.04 ± 1.13 m2·s−1, and grain boundary diffusion parameters determined from our data are Ea = 22 ± 5 kJ·mol−1 and D0 = 10−12.87 ± 0.3 m2·s−1. Isotopic diffusivity ratios were constant and close to the values determined by Graham’s law in the C-regime (i.e., bulk diffusion dominated by grain boundary diffusion) and A-regime (i.e., bulk diffusion controlled by grain boundary and lattice diffusion in proportion to the segregation of Ar between those sites), but varied in the B-regime (i.e., bulk diffusion controlled by both grain boundary and lattice diffusion in a complex manner), implying a higher isotopic fractionation in the kinetic B-regime. Extrapolation to typical mantle grain sizes implies that around 22% of the argon in the upper mantle can be stored at grain boundaries and that effective diffusion is mostly in the A-regime, suggesting a low isotopic fractionation and diffusivities faster than lattice diffusivities alone. The consideration of grain boundaries as a potential Ar storage site can modify equilibrium during partial melting and significantly enrich a liquid in Ar during fluid percolation. The grain size dependence of Ar storage and diffusivity highlights the underestimated role of grain boundaries in the upper mantle, especially in zones of reduced grain size (via dynamic recrystallization) possibly followed by fluid percolation and/or partial melting, such as in subduction zones or below oceanic ridges

Diffusion of Ar atoms implanted in a TiO2 matrix studied with Temperature Programmed Out-Diffusion✰

The diffusion of noble gas argon in the near-surface region of rutile TiO2 has been explored with the Temperature Programmed Out-Diffusion (TPOD) method. The Ar atoms were deposited in a several-nanometer-deep layer of the single-crystal TiO2(110) surface by bombardment with 1–5 keV energy Ar ions. Subsequently, in the TPOD experiments this sample was heated at a linear rate and the out-diffusion of argon was monitored with a mass-spectrometer. Surface conditions were probed with Auger Electron Spectroscopy (AES) and Low-Energy Electron Diffraction (LEED). The experimental results were analyzed with the aid of numerical simulations. The measurements showed a dependence of the Ar diffusion rates on the concentration of buried argon and composition of the surface layer. The kinetic parameters of Ar diffusion in pristine rutile TiO2 were estimated as Ea = 104 kJ/mol and D0 = 6 ⋅ 10−9 m2/s. A distinctive diffusion regime, related to the rock salt TiO phase formation, were identified.