Argon Diffusion Research Articles

04/02412 Characterisation of direct epitaxial silicon thin film solar cells on a low-cost substrate: Liang, Z. C. et al. Solar Energy Materials and Solar Cells, 2003, 80, (2), 181–193

From electron microprobe, petrographic and image analyses of samples from the northern tip of the Luzon Arc, the effects of various alteration reactions, phenocryst and groundmass assemblages on the whole-rock 40Ar39Ar step-heating spectra have been delineated. The alteration of primary hornblende to an assemblage of zoisite+chlorite+sphene, the palagonitization of glass and the crystallization of phillipsite tends to cause discordant 40Ar39Ar dates in the low-temperature steps of the age spectra. Discordant 40Ar39Ar dates in the high-temperature steps appear to result from degassing of inherited or trapped 40Ar from coarse-grained phenocrysts. Samples in which most of the K resides in groundmass plagioclase tend to yield meaningful plateau dates, whereas samples in which most of the K resides primarily in palagonitized glass tend to yield meaningless plateau and isochron dates. Substantial loss of 39Ar (13–40%) occurred from irradiated samples prior to 40Ar39Ar step-heating analysis, which appears to be correlated with the abundance of groundmass and the degree of alteration. This argon loss is attributed to thermally-induced argon diffusion from the fine-grained groundmass plagioclase, glass and alteration phases during the pre-step-heating bakeout.

40Ar/ 39Ar analyses of clinopyroxene inclusions in African diamonds: implications for source ages of detrital diamonds

Clinopyroxene inclusions in diamond contain elevated potassium contents and can potentially be dated by 40Ar/ 39Ar techniques. Previous 40Ar/ 39Ar studies of clinopyroxene inclusions contained in cleaved diamonds have suggested that argon, produced from the decay of potassium prior to eruption of the host kimberlite magma, diffuses to the diamond/clinopyroxene interface under mantle conditions. After intrusion and cooling below the closure temperature for argon diffusion, radiogenic argon is retained by the clinopyroxene inclusions. This behaviour complicates efforts to date diamond crystallisation events; however, extraction of inclusions from their host diamond should induce loss of all interface argon, thus raising the possibility of determining kimberlite emplacement ages. This possibility has important implications for constraining the source localities of detrital diamond deposits worldwide, with concomitant benefits to diamond exploration. To investigate this premise, 40Ar/ 39Ar laser probe results are presented for single clinopyroxene inclusions extracted from a total of fifteen gem-quality diamonds from the Mbuji-Mayi kimberlite in the Democratic Republic of Congo, and the Jwaneng and Orapa kimberlites in Botswana. Initial fusion analyses of clinopyroxene inclusions from Mbuji-Mayi diamonds yielded ages older than the time of host kimberlite intrusion, indicating partial retention of extraneous argon by the clinopyroxene inclusions themselves. Step-heating analyses of clinopyroxene inclusions from Orapa and Jwaneng diamonds produced older apparent ages from lower temperature steps and the ‘rim’ fragment of one Orapa inclusion. High temperature (fusion) analyses yielded younger apparent ages, commonly approaching the times of host kimberlite eruption. Total-gas integrated 40Ar/ 39Ar ages are mostly intermediate between the times of inferred diamond crystallisation and kimberlite eruption. Ca/K ratios for each sample are uniform across step-heating increments, indicating that age variations are not due to compositional, mineralogical or alteration effects. The favoured explanation for these results is partial retention of extraneous argon in primary and/or secondary fluid inclusions. This component is then preferentially outgassed in lower temperature heating steps, yielding older apparent ages. The partial retention of extraneous argon by clinopyroxene inclusions clearly restricts efforts to determine source ages for detrital diamond deposits. Results from individual samples must necessarily be interpreted as maximum source emplacement ages. Nonetheless, step-heating analyses of several clinopyroxene inclusions from a detrital diamond deposit may provide reasonable constraints on the ages of source kimberlites/lamproites; however minor age populations as well as those closely spaced in time, may be difficult to resolve. It is argued that the majority of older 40Ar/ 39Ar ages can be explained in terms of the partial retention of inherited argon, produced between the times of diamond crystallisation and kimberlite eruption. Although the presence of excess argon in some clinopyroxene inclusions cannot be excluded, available evidence (e.g. no excess argon in Premier eclogitic inclusions or potassium-poor inclusions) suggests that this is not a factor for most samples. Three possible mechanistic models are forwarded to account for the uptake of inherited (± excess) argon in fluid inclusions. The first envisages negligible interface porosity and diffusion of extraneous argon exclusively to primary fluid inclusions, which subsequently partially decrepitated during eruption, causing accumulation of argon at the diamond/clinopyroxene interface. The second model permits diffusive loss of extraneous argon to both the interface region and primary fluid inclusions. The third involves diffusion of extraneous argon to the interface region, with later entrapment of some interface argon in secondary fluid inclusions, produced by fracture/annealing processes active during eruption. The first model can account for all 40Ar/ 39Ar results, whereas the latter two mechanisms require the presence of an excess argon component to explain older integrated ages (up to 2.9 Ga) from two Jwaneng samples. Excess argon contamination would compromise efforts to determine diamond genesis ages using the 40Ar/ 39Ar dating technique. However, if the first model is valid, then the older 40Ar/ 39Ar integrated ages support previous Re-Os age results for the crystallisation of Jwaneng diamonds.

Cooling and exhumation of the Shuswap Metamorphic Core Complex constrained by 40Ar/39Ar thermochronology

The exhumation history of the Shuswap Metamorphic Core Complex (Shuswap MCC) is deciphered from structural analysis and 4 0 Ar/ 3 9 Ar thermochronology. With the exception of a few samples contaminated by excess argon, the analytical results indicate a consistent range of early Tertiary 4 0 Ar/ 3 9 Ar ages throughout the area. Hornblende yields ages ranging from ∼59-54 Ma; muscovite and biotite ages are clustered between 49.5 and 48 Ma, and are consistent with the timing of closure of K-feldspars, typically ranging from 50 to 43 Ma, except for samples in the immediate footwall of the Columbia River fault that yield ages as young as 26 Ma. These 4 0 Ar/ 3 9 Ar ages, combined with existing U/Pb ages on zircons and monazites, constrain the cooling history at several localities in the Shuswap MCC between Trans Canada Highway 1 and the Thor-Odin dome. The migmatitic core of the complex is affected by a rapid cooling event (∼700 °C to ∼300 °C), from crystallization of zircons through closure of argon diffusion in hornblende and micas, between ∼56 and ∼48 Ma. Based on our previous structural analysis, we propose that this first period of rapid cooling follows an exhumation event associated with the formation of the Shuswap MCC by activation of a low-angle detachment and ductile thinning of a previously thickened and partially molten crust. Multidiffusion-domain thermal modeling of K-feldspar constrains the lower temperature history from ∼350 °C to ∼150 °C and indicates a period of thermal stability from ∼48 to ∼45 Ma, and a cooling event at ∼45 Ma, except for the K-feldspars sampled in the immediate footwall of the Columbia River fault, which indicate rapid cooling at ∼33 Ma. These cooling events are attributed to exhumation accommodated by local activation of high-angle normal faults. Therefore, we distinguish two periods of exhumation in the Canadian Cordillera: (1) Exhumation in Paleocene time related to crustal thinning and tectonic denudation by activation of detachment zones at ∼60-50 Ma, which corresponds to gravitational collapse of the thickened and partially molten Cordilleran crust; and (2) Exhumation related to Eocene regional extension, Basin-and-Range style, at ∼45 Ma, with reactivation of the eastern boundary fault of the core complex, the Columbia River fault, in Oligocene time at ∼33 Ma.

Stacking fault-enhanced argon diffusion in naturally deformed muscovite

Abstract Recent advances in microscale 40 Ar/ 39 Ar geochronology have revealed argon concentration gradients in naturally deformed muscovite that are incompatible with volume diffusion uniquely, and have been interpreted to result from intragranular defect-enhanced diffusion. Defects and heterogeneously spaced stacking faults observed by transmission electron microscopy in such muscovites are evaluated as potential fast pathways for argon diffusion. Two-dimensional defects, such as stacking faults, are of particular interest for noble gas diffusion because of the net dilatation effect that a stacking fault is able to generate in minerals. In micas, partial dislocations (and the area between them known as stacking faults) within the interlayer displace the potassium atoms from a stable hexagonally centred position between opposing tetrahedral layers to an unstable position relative to one of the tetrahedral layers such that repulsive forces lead to a localized net dilatation effect within the interlayer. Such a dilatation effect may have direct consequences for argon retention in micas. Numerical modelling of the effects stacking faults have on argon diffusion was performed on the basis of the calculated interlayer spacing, measured isotope data, and observed linear stacking fault density. These calculations result in effective diffusivity ratios defined by volume diffusion to defect-enhanced diffusion of 10 6 to 10 7 , which are comparable with diffusivity ratios in other materials (ceramics or metals). In the absence of defects causing physical grain size reduction (e.g. kink bands or subgrain boundaries), stacking faults are potentially the main defect in sheet silicates exerting a measurable influence on intragranular argon diffusion. Stacking-fault-enhanced argon diffusion differs from pipe diffusion, whose significance on bulk diffusion depends on high dislocation densities, by the small volume fraction of dislocations required to affect bulk diffusivities. In contrast to pipe diffusion, the linked occurrence of dislocations and stacking faults within mica interlayers represents a potentially significant volume fraction, even in samples that do not have high apparent dislocation densities.

40Ar/ 39Ar Single Grain Dating and Mineral Chemistry of Hornblendes South of the Red River Shear Zone (Vietnam): New Evidence for Early Proterozoic Tectonothermal Event

The pre-Mesozoic metamorphic belt runs parallel to the Day Nui Con Voi - Red River shear zone in Vietnam to the south. The belt is mainly composed of hornblende gneisses, amphibolite lenses and mica-schists. Five hornblendes from a gneiss and an amphibolite were analyzed chemically and chronologically by Electron Probe Micro Analysis (EPMA) and 40Ar/ 39Ar methods. EPMA analyses show that hornblendes in the gneiss and the amphibolite have significant amount of edenite component and similar average composition. However, the recalculated Fe 3+ content is significantly heterogeneous in a thin section while total Fe is nearly the same among the analyses. The rim of each crystal is higher in Fe 3+/(Fe 3+ + Fe 2+) than the core. These chemical and petrological features suggest that the hornblendes have suffered significant oxidation, in particular, largely in the gneiss. 40Ar/ 39Ar analyses showed that the gneiss has a significant variation of plateau ages (2089±14, 1977±19 and 1873±13 Ma) among three hornblende grains, whereas the amphibolite gives the same plateau ages (2056±14 and 2044±21 Ma) for two grains. All grains of both samples have excess ages in the first few fractions at low temperatures and partial-loss ages between the excess and plateau spectra. The Ca/K ratios indicate some disturbed phases for the lower temperature spectra but the partial-loss ages are also derived from hornblende phase. These facts suggest that hornblende in the gneiss has experienced partial argon loss by oxidation and/or thermally activated argon diffusion process. However, the gneiss and the associated amphibolite have preserved the early Proterozoic tectono-metamorphic event in the hornblende crystals except for their rims, giving new evidence for the early Proterozoic event within the pre-Mesozoic metamorphic belt (northern Vietnam) south of the Red River shear zone in Indochina.

Diffusional behaviour of simple sorbates in zeolites: effect of anisotropic frameworks and geometrical correlations

Molecular dynamics (MD) studies of helium and argon diffusion in all-silica analogues of ZSM-5, ZSM-11 and ferrierite are used to study diffusional anisotropy. All three zeolites have channels running parallel to only two of the three crystallographic axes. ZSM-5 and ZSM-11, however, allow for three-dimensional diffusional motion due to the presence of geometrical correlations. Motion in the correlated direction, which has no direct displacement channels, is distinctly subdiffusional on time scales of the order of 1 ns. The ballistic to diffusional crossover times are much greater for displacements in the direction showing correlated, as opposed to, direct displacements.

The Thermal Conductivity, Thermal Diffusivity, and Heat Capacity of Gaseous Argon

This paper presents new absolute measurements for the thermal conductivity and thermal diffusivity of gaseous argon obtained with a transient hot-wire instrument. Six isotherms were measured in the supercritical dense gas at temperatures between 296 and 423 K and pressures up to 61 MPa. A new analysis for the influence of temperature-dependent properties and residual bridge unbalance is used to obtain the thermal conductivity with an uncertainty of less than 1% and the thermal diffusivity with an uncertainty of less than 4%. Isobaric heat capacity results were derived from measured values of thermal conductivity and thermal diffusivity using a density calculated from an equation of state. The heat capacities presented here have a nominal uncertainty of 4% and demonstrate that this property can be obtained successfully with the transient hot wire technique over a wide range of fluid states. The technique will be useful when applied to fluids which lack specific heat data.

In-situ UV-laser 40Ar/39Ar geochronology of a micaceous mylonite : an example of defect-enhanced argon loss

Muscovite and biotite from a crustal-scale mylonite zone (Pogallo Shear Zone, southern Alps) were investigated using furnace step-heating and in-situ UV-laser ablation 40Ar/39Ar geochronology. Undeformed muscovite porphyroclasts yield 40Ar/39Ar plateau ages of 182.0±1.6 Ma, whereas in-situ UV-laser ablation 40Ar/39Ar dating and furnace step-heating of strongly deformed muscovite and biotite grains display a range of apparent ages that are systematically younger. The range of 40Ar/39Ar ages measured in the deformed muscovite and biotite is consistent with protracted cooling through argon closure in minerals that exhibit variably developed segmentation on the intra-grain scale. These microstructurally controlled segments are bound by either first-order lattice discontinuities, sub-microscopic structural defects and/or zones of high defect density, which create variable length-scales for intragranular argon diffusion. The observed deformational microstructures within muscovite and biotite acted as intra-grain fast diffusion pathways in the slowly cooled mylonitic rocks. Therefore, the high-spatial resolution 40Ar/39Ar data record the initial and final closure to argon diffusion over a time span of about 60 Ma.

Heterogeneous 40Ar* distributions in naturally deformed muscovite: in situ UV-laser ablation evidence for microstructurally controlled intragrain diffusion

Micas are commonly used in 40Ar/ 39Ar thermochronological studies of variably deformed rocks yet the physical basis by which deformation may affect radiogenic argon retention in mica is poorly constrained. This study examines the relationship between deformation and deformation-induced microstructures on radiogenic argon retention in muscovite. A combination of furnace step-heating and high-spatial resolution in situ UV-laser ablation 40Ar/ 39Ar analyses are reported for deformed muscovites sampled from a granitic pegmatite vein within the Siviez–Mischabel Nappe, western Swiss Alps (Penninic domain, Briançonnais unit). The pegmatite forms part of the Variscan (∼350 Ma) Alpine basement and exhibits a prominent Alpine S–C fabric including numerous mica ‘fish’ that developed under greenschist facies metamorphic conditions, during the dominant Tertiary Alpine tectonic phase of nappe emplacement. Furnace step-heating of milligram quantities of separated muscovite grains yields an 40Ar/ 39Ar age spectrum with two distinct staircase segments but without any statistical plateau, consistent with a previous study from the same area. A single (3×5 mm) muscovite porphyroclast (fish) was investigated by in situ UV-laser ablation. A histogram plot of 170 individual 40Ar/ 39Ar UV-laser ablation ages exhibit a range from 115 to 387 Ma with modes at approximately 340 and 260 Ma. A variogram statistical treatment of the 40Ar/ 39Ar results reveals ages correlated with two directions; a highly correlated direction at 310° and a lesser correlation at 0° relative to the sense of shearing. Using the highly correlated direction a statistically generated (Kriging method) age contour map of the 40Ar/ 39Ar data reveals a series of elongated contours subparallel to the C-surfaces which where formed during Tertiary nappe emplacement. Similar data distributions and slightly younger apparent ages are recognized in a smaller mica fish. The observed intragrain age variations are interpreted to reflect the partial loss of radiogenic argon during Alpine (∼35 Ma) greenschist facies metamorphism. One-dimensional diffusion modelling results are consistent with the idea that the zones of youngest apparent age represent incipient shear band development within the mica porphyroclasts, thus providing a network of fast diffusion pathways. During Alpine greenschist facies metamorphism the incipient shear bands enhanced the intragrain loss of radiogenic argon. The structurally controlled intragrain age variations observed in this investigation imply that deformation has a direct control on the effective length scale for argon diffusion, which is consistent with the heterogeneous nature of deformation.

Cretaceous evolution of a metamorphic core complex, the Veporic unit, Western Carpathians (Slovakia): P–T conditions and in situ40Ar/39Ar UV laser probe dating of metapelites

AbstractAlpine metamorphism, related to the development of a metamorphic core complex during Cretaceous orogenic events, has been recognized in the Veporic unit, Western Carpathians (Slovakia). Three metamorphic zones have been distinguished in the metapelites: 1, chloritoid + chlorite + garnet; 2, garnet + staurolite + chlorite; 3, staurolite + biotite + kyanite. The isograds separating the metamorphic zones have been modelled by discontinuous reactions in the system K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH). The isograds are roughly parallel to the north‐east‐dipping foliation related to extensional updoming along low‐angle normal faults. Thermobarometric data document increasing P–T conditions from c. 500 °C and 7–8 kbar to c. 620 °C and 9–10 kbar, reflecting a coherent metamorphic field gradient from greenschist to middle amphibolite facies. 40Ar/39Ar data obtained by high spatial resolution in situ ultraviolet (UV) laser ablation of white micas from the rock slabs constrain the timing of cooling and exhumation in the Late Cretaceous. Mean dates are between 77 and 72 Ma; however, individual white mica grains record a range of apparent 40Ar/39Ar ages indicating that cooling below the blocking temperature for argon diffusion was not instantaneous. The reconstructed metamorphic P–T–t path is ‘clockwise’, reflecting post‐burial decompression and cooling during a single Alpine orogenic cycle. The presented data suggest that the Veporic unit evolved as a metamorphic core complex during the Cretaceous growth of the Western Carpathian orogenic wedge. Metamorphism was related to collisional crustal shortening and stacking, following closure of the Meliata Ocean. Exhumation was accomplished by synorogenic (orogen‐parallel) extension and unroofing in an overall compressive regime.

Ionization of sputtered material in a planar magnetron discharge

Emission spectra in the visible and near ultraviolet have been recorded for the magnetron sputtering of titanium by argon at pressures between 0.5 and 100 Pa. Intense emission lines from both atomic and ionic material were detected, and comparison of line intensities yielded an ionization fraction of over 10% and an electron temperature of the order of 1 eV for the sputtered material. The ionization fraction decreased with increasing magnetron power, and an optimal operating pressure was found for maximum ion content of the sputtered flux. Scaling laws of emission line intensity with magnetron power indicated the incomplete thermalization of sputtered metal with the sputtering plasma, and spatially resolved measurements of ionization profiles were used to investigate the transport and collisional ionization of sputtered material. Penning ionization of sputtered titanium by metastable argon was found to be the dominant ionization process, and the effect of diffusion of metastable argon was noted. Control of ion content in sputtered fluxes is of interest for the deposition of thin films and for metallization of semiconductors.

Transport diffusion of argon in AlPO4-5 from equilibrium molecular dynamics simulations

Transport diffusion of argon in the unidirectional channels of the molecular sieve AlPO4-5 has been studied using molecular dynamics simulations. Using the Green–Kubo formalism, this nonequilibrium property is, for the first time, extracted from just one equilibrium simulation. Apart from the computational advantages above nonequilibrium simulations, the new method also provides a way to check the validity of the assumption of linear response, which is at the basis of both methods. The transport diffusion coefficient for argon at 87 K and half the maximum loading is found to be equal to Dt=(1.4±0.1)×10−5 cm2/s, of which approximately 20% can be attributed to correlated, collective motion.

Brecciation-related argon redistribution in alkali feldspars: an in naturo crushing study

Alkali feldspar thermochronologic modeling with the 40Ar/ 39Ar method has generated marked advances in knowledge of the mechanisms for argon diffusion in feldspars. While the goal in many cases has been to extrapolate the observed and modeled argon behavior in feldspars to natural geological settings, scientific debate surrounding the true feasibility of such extrapolations and indeed, the validity of thermochronologic modeling in itself, have provided much impetus to improve laboratory techniques to test, and increase basic understanding of, argon diffusion. Two cornerstones for the debate over the feasibility of alkali feldspar thermochronology for modeling natural, geologic processes have been: 1. is volume diffusion the main mechanism for argon movement in feldspars? and 2. if volume diffusion is a viable mechanism, does argon then reside in discrete ‘domains’ within the feldspar lattice? We describe a study of alkali feldspars from a profile through a well-controlled brittle fault zone in western Norway; the feldspars document argon loss during deformation and strongly suggest the existence of argon ‘domains’ within the feldspars, at least during laboratory step heating. The progressive change in the character of argon diffusion is recognizable in the logr/r o diffusion data from the feldspars and is mimicked by physical changes observed optically in the feldspars through progressive degrees of brittle deformation. Modeling results indicate a reduction in size of the biggest domains and the appearance of smaller domains during the strongest stages of deformation. Whether or not this reveals the existence and the transformation of the domain structure in naturo is difficult to prove from our data alone, but interestingly, this behaviour corresponds directly to the physical (optical) appearance of more intense crack networks and subgrains in progressively more brecciated feldspars. Because the thermochronologic histories derived from modeling the feldspar data conform very well to the known tectonic history of the area, the feldspars appear to have successfully retained physical (optical and isotopic) records of episodic tectonic processes operating from ductile through low-temperature brittle regimes in rocks with a Caledonian history overprinted by several later (younger) geologic events. However, because the ‘cold’ brecciation is the last tectonothermal event recorded by these rocks, it is impossible to truly test for the existence of diffusion domains in naturo. Argon loss appears to have been effective only in the most highly brecciated (deformed) samples where the combination of the connected crack network, increased fluid flow and higher temperatures enhanced diffusion via fast diffusion pathways and thus, volume diffusion from the lattice. Only minor argon loss occurred in zones of lower brittle strain, although some development of cracks and brittle features is evident. Independent of the existence of diffusion domains, this study highlights the possible pitfalls when cooling histories are deduced from brecciated feldspars in which age and diffusion charateristics have been decoupled: while the geochronological memory has survived and is identical to that of nonbrecciated feldspars (suggesting no loss and minor effects of deformation), the diffusion characteristics have been completely transposed by brecciation and the appearance of new domains. Modeling feldspars with these latter characteristics would effectively utilise a new feldspar diffusion structure with an ‘old’ (relict) age memory.

Empirical constraints on closure temperatures from a single diffusion coefficient

The elucidation of thermal histories by geochronological and isotopic means is based fundamentally on solid-state diffusion and the concept of closure temperatures. Because diffusion is thermally activated, an analytical solution of the closure temperature (Tc*) can only be obtained if the diffusion coefficient D of the diffusion process is measured at two or more different temperatures. If the diffusion coefficient is known at only one temperature, however, the true closure temperature (Tc*) cannot be calculated analytically because there exist an infinite number of possible (apparent) closure temperatures (T¯c) which can be generated by this single datum. By introducing further empirical constraints to limit the range of possible closure temperatures, however, mathematical analysis of a modified form of the closure temperature equation shows that it is possible to make both qualitative and quantitative estimates of Tc* given knowledge of only one diffusion coefficient DM measured at one temperature TM. Qualitative constraints of the true closure temperature Tc* are obtained from the shapes of curves on a graph of the apparent Tc (T¯c) vs. activation energy E, in which each curve is based on a single diffusion coefficient measurement DM at temperature TM. Using a realistic range of E, the concavity of the curve shows whether TM is less than, approximately equal to, or greater than Tc*. Quantitative estimates are obtained by considering two dimensionless parameters [lnERT^c vs. Tc*/TM] derived from these curves. When these parameters are plotted for known argon diffusion data and for a given diffusion size and cooling rate, it is found that the resultant curves are almost identical for all of the commonly dated K–Ar minerals – biotite, phlogopite, muscovite, hornblende and orthoclase – in spite of differences in their diffusion parameters. A common curve for Ar diffusion can be derived by least-squares fitting of all the Ar diffusion data and provides a way of predicting a “model” closure temperature Tcm from a single diffusion coefficient DM at temperature TM. Preliminary diffusion data for a labradorite lead to a Tcm of 507 ± 17 °C and a corresponding activation energy of about 65 kcal/mol, given a grain size of 200 μm and a cooling rate of 5 °C/Ma. Curves for He diffusion in silicates (augite, quartz and sanidine) also overlap to a significant degree, both among themselves and with the Ar model curve, suggesting that a single model curve may be a good representation of noble gas closure temperatures in silicates. An analogous model curve for a selection of 18O data can also be constructed, but this curve differs from the Ar model curve. A single model curve for cationic species does not appear to exist, however, suggesting that chemical bonding relationships between the ionic size/charge and crystal structure may influence the closure temperatures of diffusing cations. An indication of the degree of overlap among the various curves for Ar, He, 18O and cations is also obtained by considering the dimensionless parameter E/RTc*; for the noble gases and 18O, E/RTc* values for the respective minerals are very similar, whereas for cations, there is significant dispersion. Given these constraints, this may be a potential method of estimating closure temperatures for certain diffusing species when there are limited diffusion data.

Argon release mechanisms of biotite in vacuo and the role of short-circuit diffusion and recoil

Understanding argon release mechanisms in K-bearing minerals is essential in interpreting the 40 Ar/ 39 Ar data and their application to geological studies. The release mechanisms of argon in vacuo have been examined in a series of 40 Ar/ 39 Ar isothermal heating experiments on two biotite specimens with Fe/(Fe+Mg) (Fe #)=0.50 and 0.87 respectively. The crystal structure of the biotite was also monitored during in vacuo heating by an in–situ high temperature X-ray diffractometer (HTXRD), and also examined by scanning electron microscopy (SEM). At temperatures greater than 600°C, argon release is mainly controlled by the structural decomposition of the biotite crystal arising from oxidation and dehydroxylation, whereas at temperatures less than 600°C, argon release appears to be controlled by a multipath-diffusion mechanism, with effective D/ a 2 values about 2–4 orders of magnitude higher than those extrapolated from hydrothermal data. Both the argon diffusivity and Ar release patterns are strongly related to biotite composition, in which the Fe-rich biotite has a higher argon diffusivity and degasses at lower temperatures than the Mg-rich biotite. Unless contaminated by other phases, biotites will tend to yield flat age spectra for temperature steps higher than 600°C, regardless of the initial distribution of argon isotopes in the crystal structure, since the argon released at T>600°C is strongly correlated with the decomposition process. At temperature steps lower than 600°C, however, biotite age spectra can exhibit discordant dates since the gas release is controlled mainly by defect-enhanced (short-circuit) diffusion mechanisms. Consequently, models using such low- T steps with the intent of extracting information on the spatial distribution of Ar will not lead to accurate interpretations of geologic histories, unless the potential effects of short-circuit diffusion are well-constrained.

Nature's diffusion experiment: The cooling-rate cooling-age correlation

Correlation between the age of orogeny and the cooling rate of metamorphic rocks (through ∼500–180 °C), as determined through various radioisotopic dating methods, suggests that ancient orogens cooled at rates as much as two orders of magnitude slower than those of active orogens. I propose that the correlation has arisen because ancient orogens have undergone long postorogenic periods, as long as hundreds of millions of years, during which the rocks remained effectively isothermal. Laboratory experiments suggest that argon diffusion in biotite may be sufficiently rapid at temperatures of ∼220 °C to produce the correlation by slow loss of argon during extended isothermal periods. One interesting realization is that nature has provided a long-term argon diffusion experiment, the results of which have been inverted in this paper to give an independent estimate of the diffusivity of argon in biotite under natural conditions. This estimate compares favorably with a projection of the results of laboratory hydrothermal experiments down 10 orders of magnitude. The analysis confirms the theory that there is a critical thermal window below accepted closure temperatures in which long residence will result in significant partial resetting of thermochronometers.

Quantitative evaluation of vibrational excitation of SiO A 1Π–X 1Σ + flame emission from a silane/hydrogen/oxygen/argon diffusion flame

Spontaneous emission spectra in the wavelength range from 200 to 420 nm from a SiH 4/H 2/O 2/Ar diffusion flame (used for the synthesis of nanosized particles) were obtained. Si, SiH and SiO intermediates were identified from the spectral emission signature. Emission from the SiO A 1Π–X 1Σ + transition was analyzed in detail. Comparison of the experimental data with an emission model for SiO indicated significant vibrational non-equilibrium. The level of vibrational excitation was quantified in terms of a vibrational temperature independent of a translational temperature. The best-fit model results obtained by comparison with the experimental data, yielded a translational temperature of T trans=3250±500 K and a vibrational temperature of T vib =15 000±5000 K for high oxygen concentration systems. The vibrational temperature corresponds to an energy equivalent of ∼29 kcal/mol and changes in fractional population of over an order of magnitude from equilibrium distributions for some vibrational levels. For low oxygen concentration systems, the vibrational temperature was significantly higher T vib =40 000±15 000 K, which indicates a different SiO formation mechanism is active for these conditions.

Constraining absolute deformation ages: the relationship between deformation mechanisms and isotope systematics

Isotope diffusion in a mineral is strongly temperature dependent but is also a function of grain size. Deformation must, therefore, be an important consideration in the interpretation of isotopic data because it provides a means of modifying grain size and shape. We illustrate the range of different deformation mechanisms common in micas and use simple models to investigate the relationship between these and isotope diffusion. We consider three different thermal scenarios with deformation taking place during: (a) the prograde heating path, (b) at the closure temperature of the deforming mineral, and (c) at temperatures significantly below the closure temperature. We have modelled these simple systems using a finite difference algorithm that simulates argon diffusion profiles and bulk ages. This modelling illustrates that obtaining deformation ages is critically dependent on an understanding and recognition of the different deformation mechanisms that have affected the sample. In the cases where deformation causes a change in grain size, it is important to characterise both the temperature at which deformation takes place and the closure temperature of grains formed during the deformation. The development of grains with T c greater than the deformation temperature may record a deformation age. Examples of this condition include: (i) neocrystallisation; (ii) grain size reduction occurring at temperatures below T c (of the reduced grain size) where the deformation mechanism has reset the grains; and (iii) deformation-induced grain coarsening.

Direct measurement of Ar diffusion profiles in a gem-quality Madagascar K-feldspar using the ultra-violet laser ablation microprobe (UVLAMP)

Controversy surrounding the mechanisms and controls on argon diffusion in K-feldspar has led us to undertake direct diffusion measurements on a crystal with simple microtextures, over a range of temperatures. Measurements of argon diffusion profiles in a gem-quality iron-rich orthoclase heated in a cold seal apparatus, have been undertaken in situ using an ultra-violet laser ablation microprobe (UVLAMP) technique. The results agree very closely with the previously determined bulk values for Benson Mines orthoclase (activation energy ( E)=43.8±1 kcal mol −1) and vacuum furnace cycle-heating studies of K-feldspars ( E=46±6 kcal mol −1). However, instead of defining a single activation energy ( E) and diffusion coefficient ( D o), the data yield two sets of parameters: a low-temperature (550–720°C) array with an E of 47.2±2.5 kcal mol −1 (198.2±10.5 kJ mol −1) and a D o of 0.0374 +0.1123 −0.0281 cm 2 s −1, and a high-temperature (725–1019°C) array with an E of 63.8±3.4 kcal mol −1 (268.0±14.3 kJ mol −1) and a D o of 55.0 +225.5 −44.2 cm 2 s −1. The new results closely reproduce two sets of apparent activation energies previously measured in cycle-heating studies of Madagascar K-feldspar (40±3 and 57±3 kcal mol −1). Previous interpretations of the two arrays have included multiple domains with variable activation energies and fast track diffusion. However, the UV depth profile analyses indicate simple diffusion to the grain surface and importantly, diffusion radii calculated by combining the UVLAMP and cycle-heating data, are the same as the physical grain sizes used in the experiments, around 1 mm. Vacuum furnace stepped heating experiments on slowly cooled K-feldspars have been interpreted as showing diffusion radii of around 6 μm and indicate complex populations of sub-grains. This study indicates that Madagascar K-feldspar and thus probably all gem-quality K-feldspars act as single diffusion domains and that short-circuit (or pipe) diffusion was not an important loss mechanism. An apparent diffusion compensation relationship in the stepped heating data for Madagascar K-feldspar implies that similar relationships seen in other K-feldspars are a result of a range of diffusion mechanisms.

40Ar/39Ar dating of ribbon-textured veins and wall-rock material from Meguma lode gold deposits, Nova Scotia: implications for timing and duration of vein formation in slate-belt hosted vein gold deposits

Results of 15 40Ar/39Ar age spectra for whole-rock argillite samples collected from within and adjacent to veins from eight Meguma gold districts in the Meguma Terrane of southern Nova Scotia are presented. The samples give excellent plateau ages (i.e., concordance of plateaux, correlation, and integrated ages) that range from ca. 379 to ca. 403 Ma, and there is no apparent correlation of age with bulk rock composition (K2O, Al2O3) or sampling localities. In addition, apparent ages for samples from a district are similar within analytical error, except for one deposit, and there is no difference for samples from within ribbon-textured veins versus samples collected outside of the gold districts (i.e., in areas free of quartz veining). The results of this work compare well to previous whole-rock 40Ar/39Ar dating of Meguma Group samples and we concur with previous workers that the range in dates (i.e., 380-410 Ma) reflects diachronous cooling of the area through the intracrystalline retention temperature for argon in mica (i.e., approx. 300-350°C). However, with respect to the vein samples, there is a marked difference between the 40Ar/39Ar ages of vein-hosted whole-rock samples and hydrothermal minerals (amphibole, muscovite, biotite) from the same deposits previously dated, which indicates that whole-rock samples have retained their metamorphic ages and have not been reset by the later hydrothermal event responsible for vein formation, despite being incorporated within the high-temperature fluids (approx. 400-450°C). This discrepancy in ages indicates that the wall rocks and veins were in thermal disequilibrium, as the vein temperatures were well above that required to cause diffusion of argon out of mica phases within the whole-rock samples, and implies therefore that the fluids must have been derived from depth. The results have the following important implications for models of vein formation: (i) vein formation was rapid and is consistent with models of hydrofracturing due to fluid overpressure, and (ii) the vein-forming fluids were derived from depth and cannot have been produced by a lateral secretion processes whereby fluids and gold are derived from the Meguma Group.