Argon Diffusion Research Articles

40Ar/ 39Ar-ages of phlogopite in mantle xenoliths from South African kimberlites: Evidence for metasomatic mantle impregnation during the Kibaran orogenic cycle

We applied the 40Ar/ 39Ar dating method to an extensive suite of phlogopites from kimberlite-hosted mantle xenoliths (dominantly garnet bearing) from the mines of Bultfontein (South Africa), Letseng-la-Terae and Liqhobong (Lesotho). Argon extraction was performed by conventional high resolution stepwise heating technique, laser incremental heating technique and laser spot analysis. All age spectra obtained by conventional analysis indicate various degrees of 40Ar loss during kimberlite emplacement, but never resulted in a total reset of the argon system. Most intriguingly, the sample-specific maximum apparent ages cluster between 1.0 and 1.22 Ga for the phlogopites with the least disturbed age spectra. A maximum apparent age of 1.02 Ga was observed during laser heating analysis. Individual grains tend to yield older ages in their cores, with successively younger ages at their rims. The range in age obtained via the laser fusion technique and with conventional stepwise heating technique agrees with each other, as well as with literature data. The often inferred presence of excess 40Ar in those phlogopites cannot explain the coherent age pattern in the large suite of samples. Hence, the age constraint of 1.0–1.25 Ga is regarded as geologically meaningful and assigned to metasomatism of the local cratonic mantle during the advent of Kibaran orogenesis (1.00–1.25 Ga). The major consequences of our findings are: (i) The argon system of phlogopite can remain closed for long time scales, even at ambient temperatures of 800–1200 °C within the mantle, most likely because the solid/solid partitioning behaviour of Ar between phlogopite and other major phases in the mantle strongly favours phlogopite, or because conventionally inferred diffusivity of argon in phlogopite is seriously overestimated. Thus, the 40Ar/ 39Ar phlogopite system appears to be a valuable tool for deciphering ancient metasomatic events affecting the lithospheric mantle. (ii) The cratonic lithospheric mantle below southern Africa may have been frequently influenced by different episodes of fluid or melt migration during subduction of oceanic crust at active continental margins.

Provenance studies from 40Ar/ 39Ar dating of mineral inclusions in diamonds: Methodological tests on the Orapa kimberlite, Botswana

Previous studies indicate that clinopyroxene inclusions extracted from their host diamonds give 40Ar/ 39Ar ages approaching the time of source kimberlite eruption. This behaviour is attributed to diffusion of argon to the diamond/inclusion interface region during mantle residence, with subsequent loss of this component on cleaving of the diamond to release the inclusion(s). In this study, we investigate the potential of the 40Ar/ 39Ar inclusion dating method as a niche provenance tool. As diamondiferous kimberlites are uncommon igneous rocks, largely restricted to Archaean cratons and have limited age ranges, provenance studies of detrital diamond occurrences should provide unique information on craton erosional histories, palaeo-drainage evolution and related basin development. As a methodological test, 40Ar/ 39Ar step-heating and in vacuo crushing experiments were conducted on 50 eclogitic clinopyroxene inclusions extracted from diamonds from the ∼ 93 Ma Orapa kimberlite in Botswana. Low temperature steps and crushing experiments produced anomalously old ages (up to 2.98 ± 0.10 Ga), consistent with partial retention of pre-eruption (inherited) argon in sub-micron(?) defect sites. High temperature (fusion) steps yielded significantly younger ages, although only 35% are within error of the time of Orapa kimberlite eruption. However, 77% of results are within 50 Ma, and 92% within 100 Ma, of the Orapa eruption age. These results mandate that individual fusion ages be treated as maximum estimates of source kimberlite eruption ages, with analyses of several (> 6) inclusions required to date single-age detrital populations. For multi-source diamond deposits, only the youngest detrital population, or populations separated by > 100 Ma, will be resolvable. Despite these limitations, we conclude that the 40Ar/ 39Ar inclusion dating method is still capable of resolving key provenance questions, such as the source of detrital diamond deposits in southern Africa, where controversy exists as to whether the diamonds originated from proximal Permo-Carboniferous glacial deposits or distal Cretaceous/Jurassic kimberlites. Finally, a model is presented to account for the variable retention of pre-eruption argon in clinopyroxene inclusions extracted from their host diamonds. The proportion of this component retained by inclusions is considered dependent on interface porosity volume, partitioning of argon into defect structures, and the timing of stress fracture development around inclusions during entrainment to surface.

Molecular dynamics simulation of helium–argon gas mixture under various wall conditions

As computational capabilities increase, molecular dynamics (MD) simulations become important tools of simulating reality. These simulations are especially useful for compressible gas mixture problems. In this study, binary diffusion of helium and argon was examined using a hard-sphere MD simulation method. For the sake of computational speed, low spacing ratios were chosen. Binary mass diffusion of gases in two equally sized halves of a box was simulated for identical initial kinetic energies and number densities. It has been noted that a purely mass diffusion mechanism of different gases is not physically possible. The resultant gas mixtures of several diffusion simulations were used as initial conditions for combined heat transfer – Couette flow, and heating and cooling experiments. The results showed the interesting behaviour of the mixture, which was subjected to various wall conditions. Energy of heavier molecules is found to be more sensitive to the wall velocities and less sensitive to the wall temperatures than lighter molecules. Diffusion, heat transfer, viscosity and heat capacity coefficients are deduced as well.

Fabrication of transparent yttria by HIP and the glass-encapsulation method

This method consists of a combination of vacuum sintering at 1600 °C followed by hot isostatic pressing (HIP) at 1500 °C of a highly agglomerated commercial powder. The use of evacuated glass capsules to perform HIP treatment allowed samples that showed open porosity after vacuum sintering to be sintered to transparency. The sintering response of the investigated powder was studied by careful microstructural observations using scanning electron microscopy and optical microscopy both in reflection and transmission. The successful key of this method was to keep porosity intergranular during pre-sintering, so that it can be removed subsequently by HIP treatment. It was found that agglomerates of closely packed particles are helpful to reach that purpose, since they densify fully and leave only intergranular porosity. However, performing HIP treatment at 1625 °C was found to result in opaque samples. This was attributed to the diffusion of argon inside the capsule. Contamination at different steps of processing was also investigated by inductively coupled plasma mass spectrometry (ICP-MS).

Lattice diffusion and solubility of argon in forsterite, enstatite, quartz and corundum

Argon diffusivities and solubilities in single, gem-quality crystals of forsterite, enstatite, quartz and corundum were determined from experiments conducted between 0.5 and ~ 6000 bars Ar pressure and temperatures from 425 °C to 1200 °C. Polished single-crystal slabs and specimens with natural facets were placed in open containers and exposed to an argon atmosphere either in a pressure vessel or in a gas-flow tube furnace at near-atmospheric pressure. Argon atoms from the pressure medium diffused into the crystals to produce near-surface concentration gradients, which were directly profiled using Rutherford backscattering spectrometry. The following Arrhenius relations were obtained for Ar diffusion: F o r s t e r i t e : D A r = 7.2 × 10 - 20 exp ( - 42 k J m o l - 1 / R T ) m 2 s - 1 E n s t a t i t e : D A r = 1.3 × 10 - 20 exp ( - 32 k J m o l - 1 / R T ) m 2 s - 1 Q u a r t z : D A r = 3.1 × 10 - 19 exp ( - 43 k J m o l - 1 / R T ) m 2 s - 1 C o r u n d u m : D A r = 1.2 × 10 - 20 exp ( - 33 k J m o l - 1 / R T ) m 2 s - 1 The reported activation energies probably represent apparent activation energies that are a combination of the effects of lattice diffusion of Ar atoms with trapping in point-defect vacancies. There are no discernible differences in Ar diffusion in different crystallographic directions, and diffusivities do not vary as a function of the intrinsic oxygen fugacity of the experimental vessels. In addition to diffusivities, the uptake gradients also yielded lattice solubilities of argon, as represented by the concentrations at the mineral surfaces. There are small differences between the solubilities of forsterite, quartz and corundum, but all solubilities are high at ~ 2000 ppm (by mass). The Ar solubilities for the iron-bearing minerals enstatite and San Carlos olivine are much higher than for the other minerals. The mean value for enstatite is 30–60% higher than the iron-free minerals, owing, perhaps, to incorporation of Ar in point defects that are more abundant in enstatite due to oxidation of minor FeO. Argon solubilities are independent of Ar pressure at P Ar > ~ 1 bar, but slightly lower at P Ar ≈ 0.5–1 bar. This behavior suggests that all sites capable of accommodating Ar atoms are filled at Ar pressures of ~ 1 bar. In all cases, Ar appears to be compatible in the minerals investigated, although the degree of compatibility will depend upon assumptions concerning the minimum Ar fugacity required to fully populate the available vacancies. A series of validation experiments and surface-sensitive analyses were conducted to confirm that the near-surface regions of crystals used in the experiments were crystalline, unreacted and free of structural damage that could potentially influence diffusion and solubility results.

Alpine burial and heterogeneous exhumation of Variscan crust in the West Carpathians: insight from thermodynamic and argon diffusion modelling

Phase equilibrium modelling of metagranitoids and metapelites in the MnO–Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O system was used to characterize Variscan and Alpine metamorphism in the crystalline basement of the Vepor Unit, West Carpathians. The calculated P – T conditions range between 570–670 °C and 6–8.5 kbar for the Variscan and 430–600 °C and 5–11 kbar for the Alpine event. These two events show contrasting metamorphic field gradients and P – T evolutions, indicating 22–27 °C km −1 during the retrograde Variscan metamorphism and 15–18 °C km −1 during the prograde Alpine metamorphism. The prograde Alpine metamorphism is associated with the Early Cretaceous overthrusting of the southern Gemer Unit, which resulted in apparently contemporaneous burial and horizontal ductile spreading of the underlying Vepor basement. The argon diffusion modelling was used to interpret the existing Variscan, Alpine and mixed 40 Ar/ 39 Ar cooling ages, and to constrain the T – t evolution and Alpine thermal overprint. Constructed Alpine metamorphic isograds and isotherms show horizontal P – T gradients resulting from heterogeneous exhumation of the deeper parts of the Vepor basement along two narrow belts during later folding. Here the structurally deeper metapelites form large-scale anticlinal cusp-like structures that separate structurally higher metagranitoids.

Rapid exhumation and cooling of the Liaonan metamorphic core complex: Inferences from 40Ar/39Ar thermochronology and implications for Late Mesozoic extension in the eastern North China Craton

The Liaonan metamorphic core complex formed during crustal extension in the Liaodong Peninsula, eastern North China craton, and consists of the Jinzhou detachment fault, Proterozoic–Paleozoic sedimentary rocks in its upper plate, and exhumed high-grade Archean metamorphic rocks and Early Cretaceous granitic plutons in the lower plate. Exhumation of its footwall from mid-crustal levels is evidenced in the detachment fault zone by the temporal transition from amphibolite facies mylonitization at depth, through retrograde chloritic shearing and brecciation, to brittle faulting during final uplift. The footwall mylonite zone is 2.5–3.5 km thick and includes Early Cretaceous (128–118 Ma) granitic rocks, together with older metamorphic rocks. The 40Ar/39Ar ages of muscovite, hornblende, biotite, and K-feldspar from the mylonitic rocks record that the core complex cooled between ca. 120 and 107 Ma, from the time of initial crystallization of zircons (122–118 Ma) at 700–800 °C in syntectonic leucocratic dikes and granitic rocks, to closure of argon diffusion in hornblende, micas, and K-feldspar at ∼500 to ∼200 °C. Throughout the eastern North China craton, the synchroneity of cooling and exhumation of metamorphic core complexes, the formation of pull-apart basins, and regional alkaline igneous activity, reflects regional extensional tectonics in the Early Cretaceous. This accompanied lithospheric thinning, possibly resulting from the rollback of the subducted Pacific plate along the eastern Asian margin during the Early Cretaceous.

Synthesis and optical properties of Yb0.6Y1.4O3 transparent ceramics

Transparent yttrium oxide ceramics doped with 30 at.% ytterbium were successfully produced by a combination of pre-sintering and hot isostatic pressing. The influence on the final transparency of different densification states and porosity configurations obtained by varying pre-sintering conditions were investigated by optical and electron microscopy. Our results show that densification prior to the final stage of sintering is necessary to limit final porosity. Samples showing open porosity were found to lead to pore entrapment because of the diffusion of argon inside the glass capsule during hot isostatic pressing. Transmittance measurements showed that the valence charge of the ytterbium ions was 3+ at the end of the process, indicating no effect of reduction when pre-sintering in vacuum was employed.

Earth holds its breath

Some inert-gas isotopes in Earth's atmosphere can only have come from deep inside the planet. We thought we knew how much gas Earth gives up, and how it does it — but a challenge has emerged to the prevailing model. The noble gases are widely used as tracers of mantle degassing on Earth and the other 'terrestrial' planets, but new work raises doubts over the practice. Specifically it is thought that the abundance of argon-40 in the atmosphere represents the total loss of gases from the interior over time via partial melting in the mantle followed by melt ascent to the surface and gas exsolution. New data on argon solubility in the olivine and orthopyroxene minerals of the upper mantle reveal two major difficulties with this scenario. First, the tacit assumption that all noble gases behave as incompatible elements during mantle melting is undermined by the finding that argon seems to be more compatible with the major phases of the terrestrial planets than was thought. And second, argon diffusion in these phases is slow. This challenges the common belief that the upper mantle is almost completely degassed of argon-40 and questions its value as a monitor of planetary degassing.

40Ar retention in the terrestrial planets

The solid Earth is widely believed to have lost its original gases through a combination of early catastrophic release and regulated output over geologic time. In principle, the abundance of 40Ar in the atmosphere represents the time-integrated loss of gases from the interior, thought to occur through partial melting in the mantle followed by melt ascent to the surface and gas exsolution. Here we present data that reveal two major difficulties with this simple magmatic degassing scenario--argon seems to be compatible in the major phases of the terrestrial planets, and argon diffusion in these phases is slow at upper-mantle conditions. These results challenge the common belief that the upper mantle is nearly degassed of 40Ar, and they call into question the suitability of 40Ar as a monitor of planetary degassing. An alternative to magmatism is needed to release argon to the atmosphere, with one possibility being hydration of oceanic lithosphere consisting of relatively argon-rich olivine and orthopyroxene.

Understanding K-feldspar 40 Ar/ 39 Ar data: reconciling models, methods and microtextures

Several descriptions of argon diffusion in K-feldspar have been developed, including the quantitative multiple-diffusion-domain (MDD) and multipath models and the qualitative ‘recrystallization’ model. These models remain contentious and the natural range in behaviour remains poorly understood. Feldspars from the Klokken syenite intrusion show a systematic relationship between 40 Ar/ 39 Ar age and microtexture suggesting that the loss of argon from different feldspars operates via different mechanisms such that the existing models represent only end-member scenarios. The MDD model can yield reliable thermal history information only for feldspars with pristine microtexture, with this model best viewed as a cipher for the real (but unknown) domain structure in such samples.

Meters-Scale Large-Area Plasma Sources with Multiple Low-Inductance Antenna Units for Next-Generation Flat-Panel Display Processing

Profiles of argon plasma sustained with multiple low-inductance antenna (LIA) units in a 300-cm rectangular plasma source have been investigated by a three-dimensional plasma simulation using fluid model. Profile of 4s excited Ar (Ar*) density was investigated as well as plasma density profile. It is found that FWHM of the profile for Ar* density is larger than 2 factors compared to that for density of Ar plasma sustained with two LIA units were installed at the center of top plate. This difference is caused by the different diffusion for particles; i.e., Ar ions diffuse along the electric field gradient while 4s excited Ar particles follow the diffusion of neutral argon. With multiple LIA units, the nonuniformity of the plasma density was 5.5% while that of Ar* density was 4.9% in 230-cm rectangular area, which is equivalent to the substrate size required for next generation flat-panel display processes.

Mass transport on composite dense PDMS membranes with palladium nanoclusters

Sorption, diffusion and permeability coefficients of argon, hydrogen, propane, propylene and propyne were determined for unfilled poly(dimethylsiloxane) (PDMS) membranes at 308 K and for PDMS membrane filled with palladium nanoclusters (5 wt.%). Time-lag and sorption methods were used to obtain kinetic and equilibrium relevant parameters for characterizing the mass transport. Mass transport in a PDMS composite membrane differs from the corresponding unfilled membrane mainly during the transient regime, when a concentration profile is being settling down across the membrane thickness. The presence of the palladium nanoclusters modifies the mass transport during the transient regime not only because the nanoclusters can act as adsorption wells (mainly in the case of hydrogen), but also because they lead to a tortuosity increase. At steady-state condition the relevant parameters that characterize the transport are the sorption and diffusion coefficients in the polymeric matrix, the filler volume content and the tortuosity. All experiments were successfully modelled using parameters obtained from independent experiments.

Modeling study on the characteristics of laminar and turbulent argon plasma jets impinging normally upon a flat plate in ambient air

Modeling study is performed to compare the flow and heat transfer characteristics of laminar and turbulent argon thermal-plasma jets impinging normally upon a flat plate in ambient air. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of argon in the argon–air mixture for the laminar and the turbulent cases, respectively. Modeling results presented include the flow, temperature and argon concentration fields, the air mass flow-rates entrained into the impinging plasma jets, and the distributions of the heat flux density on the plate surface. It is found that the formation of a radial wall jet on the plate surface appreciably enhances the mass flow rate of the ambient air entrained into the laminar or turbulent plasma impinging-jet. When the plate standoff distance is comparatively small, there exists a significant difference between the laminar and turbulent plasma impinging-jets in their flow fields due to the occurrence of a large closed recirculation vortex in the turbulent plasma impinging-jet, and no appreciable difference is found between the two types of jets in their maximum values and distributions of the heat flux density at the plate surface. At larger plate standoff distances, the effect of the plate on the jet flow fields only appears in the region near the plate, and the axial decaying-rates of the plasma temperature, axial velocity and argon mass fraction along the axis of the laminar plasma impinging-jet become appreciably less than their turbulent counterparts.

Crystallization rates of shock melts in three martian basalts: Experimental simulation with implications for meteoroid dimensions

Dynamic crystallization experiments have been performed on synthetic glasses representative of shock-generated melts observed in Los Angeles, Sayh al Uhaymir 150 and Dar al Gani 476 martian basalts. On the basis of qualitative (texture) and quantitative (fractal analysis) results, we show that melt pockets in Los Angeles cooled at a rate of 1040–1560 °C/h. Sayh al Uhaymir 150 and Dar al Gani 476 melt pockets cooled at 780 °C/h. Conductive cooling models, for a range of meteoroid diameters (10–50 cm), indicate that the minimum meteoroid diameter was small, on the order of 10–15 cm and that melt pockets cooled from post-shock temperatures within minutes. Our results also have bearing on shock implanted martian atmospheric components because it is during cooling that the melt pockets have the potential to lose gases. Modeling of argon diffusion in a spherical melt pocket indicates that during cooling and quench crystallization ∼4–60% of trapped martian atmospheric argon may be lost from the melt pocket through diffusive transport.

Measurement of diffusivities of helium and argon in silicalite by static single crystal membrane technique

Measurement of diffusivities of helium and argon in silicalite by static single crystal membrane technique

Measurement and correlation of sorption and transport properties of ethylene-propylene-diene monomer (EPDM) elastomers

The diffusivity and solubility of argon, methane, oxygen, and carbon dioxide in two EPDM-based polymers, one formulated with 34 wt% carbon black and the other formulated without carbon black, have been measured at 20 °C and at a range of pressures up to 266.448 × 10 2 Pa (2000 Torr). A linear relationship was observed between the amount sorbed and pressure for all gases, indicative of Henry's law behavior. The uptake kinetic measurements were well represented by a Fickian diffusion model and the resulting diffusivity was found to be independent of pressure. Analysis of solubility data revealed that the carbon black filler appeared to uptake negligible quantities of gas. The observed effect of non-porous filler on diffusivity and permeability was compared with Maxwell theory prediction and fell within experimental error. The experimental results were used to assess the applicability of a corresponding-states based correlation for predicting gas diffusivity and solubility in polymers. This correlation was found to adequately represent an order of magnitude estimate of the data for all gases, allowing estimates to be made for other gases not considered in this investigation.

Compositional controls on 40Ar/39Ar ages of zoned mica from a rare-element pegmatite

A high spatial resolution Ar–Ar dating study of compositionally zoned micas using UV laser extraction has been used to investigate the effect of composition and compositional boundaries upon argon diffusion in mica. The crystals are characterised by muscovite cores and Li–F–Rb–Cs-rich lepidolite rims produced by the interaction of a residual melt-vapour phase with the original muscovite during the late stages of pegmatite crystallisation. Single crystals exhibit dramatic intra-grain age variations, with a maximum range of 2,880–2,117 Ma. Backscattered electron images combined with UV-laser analyses reveal that apparent ages younger than 2,600 Ma are directly associated with the compositionally distinct rims, suggesting that Ar diffusion was faster in the lepidolite than in muscovite. Although it is not possible to be fully quantitative using the present data, it seems that argon diffusion rates in lepidolite are similar to those in biotite. Major rearrangements of the crystal structure needed to accommodate high concentrations of lithium, fluorine and large interlayer cations (Rb+, Cs+) in muscovite account for lower argon retentivity of the crystal rims. In most cases the age profiles show no disturbance at the compositional boundary, despite the transition from a di-octahedral to tri-octahedral mica, indicating that in general the boundaries do not act as fast diffusion pathways. However, in some cases there is a clear drop in ages at the compositional boundary, which acts as an effective sub-grain boundary. The difference between the two types may be related to the internal structure of the compositional boundaries or their degree of contact with the grain boundary network of the rock.

Time scales of deformation and exhumation in extensional detachment systems determined by high-spatial resolution in situ UV-laser 40Ar/ 39Ar dating

Once dated with adequate precision, intra grain recrystallization microstructures in deformed minerals provide valuable information about the tectonic and kinematic history of shear zones and entire orogens. We present combined in situ UV-laser 40Ar/ 39Ar geochronology and stable isotope data to decipher the relationships between intra grain recrystallization microstructures and 40Ar/ 39Ar ages in muscovite fish from extensional mylonite of the Porsgrunn–Kristiansand Shear Zone, Southern Norway. Texturally controlled UV-laser 40Ar/ 39Ar dating of isotopically and compositionally distinct cores, rims, and tips of muscovite fish resolves intra grain age variations on the micrometer scale. Twenty-eight in situ UV-laser 40Ar/ 39Ar analyses from two muscovite cores yield a weighted mean age of 891.2 ± 2.9 Ma, interpreted to date synkinematic neocrystallization of muscovite. The integrated 880.2 ± 2.8 Ma ( n = 30 analyses) weighted mean age of rims and tips is in good agreement with observed recrystallization relationships which suggest that rims and tips of the muscovite fish record the final increments of the ductile deformation history. Diffusion considerations based on published argon diffusion rates and peak temperature estimates from oxygen isotope thermometry show that the 40Ar/ 39Ar ages are unlikely to be disturbed during cooling. The 11.0 ± 4.0 my core-to-rim age difference together with the stable isotope data are consistent with protracted ductile deformation, extensional detachment faulting, and meteoric fluid infiltration occurring over this time interval. This approach combining X-ray element mapping, stable isotope geochemistry, and in situ UV-laser 40Ar/ 39Ar dating allows well-characterized microstructures to be analyzed with high precision and permits direct determination of the time scales of deformation in crustal-scale shear zones.

Annealing behavior of oxygen in-diffusion from SiO2 film to silicon substrate

Diffusion behavior of oxygen at (near) the Si∕SiO2 interface was investigated. We first oxidized the floating-zone-grown silicon substrates, and then annealed the SiO2-covered substrates in an argon ambient. We examined two different conditions for oxidation: wet and dry oxidation. By the secondary-ion-mass spectrometry, we measured the depth profiles of the oxygen in-diffusion of these heat-treated silicon substrates: We found that the energy of dissolution (in-diffusion) of an oxygen atom that dominates the oxygen concentration at the Si∕SiO2 interface depends on the oxidation condition: 2.0 and 1.7eV for wet and dry oxidation, respectively. We also found that the barrier heights for the oxygen diffusion in argon anneal were significantly different for different ambients adopted for the SiO2 formation: 3.3 and 1.8eV for wet and dry oxidation, respectively. These findings suggest that the microscopic behavior of the oxygen atoms at the Si∕SiO2 interface during the argon anneal depends on the ambient adopted for the SiO2 formation.