Ar Diffusion Research Articles

The effects of composition on retentivity of argon and oxygen in hornblende and related amphiboles: A field-tested empirical model

The ionic porosity model of Fortier and Giletti (1989), which parameterizes variations in diffusion coefficients among different mineral structures, has been extended to estimate the variation in diffusivities in a single mineral structure as a function of composition ( X). Applied to Ar and O diffusion in hornblende and related K-rich amphiboles, the extended model predicts that diffusivity, D, increases one-hundredfold as ionic porosity ( Z, a monitor of atomic packing density) increases from 36.5% (edenite) to 39.7% (ferro-actinolite). Partitioning this trend into its separate activation energy ( E) and frequency factor ( D 0) components leads to new E- Z and D 0- Z expressions that predict antipathetic trends of closure temperature ( T c) vs. Z in the clinoamphiboles. The Ar model yields a ΔT C/ ΔZ of −38 ±3°C/ Z %, which translates into: (1) a ∼120°C increase in T C from synthetic ferro-actinolite to edenite and (2) a ∼70°C range in T c for natural hornblendes amenable to 40Ar/ 39Ar dating. Model T C- X effects isolated from available amphibole data include increases in T C of up to: (1) 60 ± 10°C as Mg# ranges from 0–100; (2) 40 ± 15°C as A-sites progress from empty to full; (3) ∼38°C as Al-Tschermak's substitution progresses from zero to full; and (4) ∼17°C as Fe 3+/(Fe 2+ + Fe 3+) varies from 0.2–0.5. Comparable results are obtained for O. The Ar diffusion model also predicts a 38 ± 3 Ma difference in 40 Ar 39 Ar cooling age between adjacent, slowly-cooled (1°C/Ma) hornblendes differing in Z by 1% (absolute)—all other age-determining factors equal. At a cooling rate of 10°C/Ma, however, the model age discordance reduces to 3.8 ± 0.3 Ma, such that any compositional effects become lost within analytical uncertainty. As calibrated, the T C- Z- X relationship is more applicable for determining relative (rather than absolute) T C values among hornblendes. Preliminary evidence supportive of the model (in its relative form) is provided by antipathetic age- Z trends preserved in two lower crustal terranes. Finally, the model D- T C- Z- X results promote physical understanding of the diffusional closure process, as shown at both unit-cell and grain scales.

Diffarg: A program for simulating argon diffusion profiles in minerals

An understanding of argon transport is essential for the interpretation of K Ar , 40Ar 39Ar stepped heating and laser probe apparent ages of metamorphic rocks. The DIFFARG program, written in the Matlab 4.1 interpreted language, uses a finite-difference algorithm to simulate argon diffusion profiles and bulk ages of individual mineral grains for any thermal history and grain boundary argon history. Volume diffusion through the mineral lattice is assumed to be the dominant process, so the Ar diffusion coefficient is spatially homogeneous (though temperature dependent). The 40Ar concentration at the edge of the grain can be varied with time to model the diffusion of “excess” Ar from the surroundings. Plane sheet, cylindrical, or spherical grain shapes may be selected. The temperature history may be a linear decrease with time, to simulate slow cooling; a temperature pulse, to simulate a reheating event; or any temperature history composed of linear T- t segments. The grain boundary 40Ar history is defined indirectly via the apparent age at the grain edge, itself dependent on time. Nonlinear temperature and grain boundary argon histories can be defined by the user if required. Numerical models are compared to analytical predictions which concern simple scenarios and real data. The program is useful particularly for interpreting 40Ar profiles obtained by laser microprobe 40Ar 39Ar dating.

Excess 40Ar transport scale and mechanism in high-pressure phengites: A case study from an eclogitized metabasite of the Dora-Maira nappe, western Alps

Application of the bulk-separate furnace heating, laser step-heating, and in situ laser agemapping 40Ar/ 39Ar techniques reveals that high-pressure phengites from an Alpine eclogite of the Dora-Maira basement nappe, western Alps, were pervasively contaminated with excess 40Ar to yield ages > 130 Ma. Spot-fusion dating of individual grains with the 40Ar/ 39Ar laserprobe reveals concentric radial patterns with core ages as high as 320 Ma, showing that this excess component was trapped at an early stage and subsequently remobilized by volume diffusion during a late greenschist thermal pulse. The single grains and population from this hand-sample retain major differences in apparent ages indicating that the extent of the excess 40Ar contamination varies from grain to grain. Attempt to model these differences in terms of grain-size-dependent fractional loss falls short and demonstrates that strong cm-scale argon isotopic gradients locally prevailed during syn-kinematic high-pressure metamorphism. On petrostructural grounds, a model of internal isotopic buffering is outlined, whereby the excess 40Ar is argued to have been internally derived from within the eclogite protolith itself. Low intergranular Ar diffusion rates due to reduced water activity (buffered below unity by the phengite assemblage) resulted in the local segregation and partitioning of the excess 40Ar along grain boundaries during ongoing ductile deformation. Supporting evidence for this model comes from the recent literature on fluid and stable isotope behaviour in Alpine eclogites and very-high-pressure metasediments, which shows that volatile recycling during high-pressure metamorphism may occur on a very limited scale.

SmNd garnet ages from the Uluguru granulite complex of Eastern Tanzania: further evidence for post-metamorphic slow cooling in the Mozambique belt

Two samples from the Uluguru granulite complex yield garnet SmNd ages of 633 ± 7 and 618 ± 16 Ma, similar to previously published hornblende 40Ar 39Ar and KAr ages. The similarity of the SmNd to the KAr age suggests that the closure temperature of garnet to Nd diffusion is similar to that of hornblende to Ar diffusion. Assuming that published zircon UPb ages of about 700 Ma date peak granulite-facies metamorphism, a mean post-metamorphic cooling rate of 2–3°C/Ma can be calculated for the time interval 700 to 630 Ma. Such slow cooling rates imply thermal relaxation with a thickness length-scale greater than the thickness of average continental crust. This, in turn, implies that the thermal perturbation responsible for metamorphism was preceded by regional crustal thickening probably in a collisional orogen.

Multipath diffusion in geochronology

Recent developments in microanalytical tools such as the ion and laser microprobe have revealed spatial distributions of radiogenic isotopes in minerals which cannot be explained by a simple volume diffusion mechanism. Although it is known that diffusion of a substance along extended defects (such as dislocations, exsolution lamellae, micropores, microfractures, fission tracks, etc.), which may serve as high-diffusivity pathways in a crystal, can significantly influence the bulk diffusivity of a mineral, this has largely been ignored in the field of geochronology. A general numerical model has been developed, which solves coupled multipath diffusion equations that describe the simultaneous diffusion of a solute species through both the crystal lattice (via volume diffusion) and high-diffusivity pathways (via short-circuit diffusion) under non-steady state conditions. Addition of a radioactive source term to the appropriate equations further allows for the modelling of integrated cooling ages and closure temperatures, and has direct pertinence to geochronological and thermochronologial studies. Three key criteria can be used to distinguish multipath diffusion mechanisms from volume diffusion mechanisms: (a) non-Fickian concentration profiles, (b) enhanced solute diffusivity with increasing mineral grain size, and (c) a lack of any correlation between closure temperatures (and cooling ages) and larger grain sizes. With multipath diffusion, the effective diffusion dimension a for certain minerals appears to remain on the order of the grain size, and the model can adequately explain observed increases in the bulk diffusion coefficient D b with a in the hydrothermal bomb data of previous Ar diffusion studies. Arrhenius diagrams of a multipath diffusion D b vs 1/T will consist of curves that have a kink in them, reflecting a continuous change in the relative importance of the different diffusion mechanisms with temperature. The most important consequence of multipath diffusion is that the overall bulk diffusion coefficient D b of a diffusing species can be enhanced significantly above its volume diffusion coefficient D v . As a result, integrated ages and effective closure temperatures (T c ) can be much lower than those predicted assuming only a volume diffusion mechanism, to the extent that minerals normally characterized by low volume-diffusion T c may potentially have older integrated ages that minerals normally associated with higher volume-diffusion T c .

39Ar 40Ar age and petrology of Chico: Large-scale impact melting on the L chondrite parent body

Our studies of the 105 kg Chico L chondrite show that it contains ∼60% impact melt and the largest volume of impact melt recognized in stony meteorites. We suggest that it is part of a much larger dike complex that formed when chondritic impact melt was intruded into host chondrite during a large, if not catastrophic, impact on the L chondrite parent body at about 0.5 Ga. Petrologic and 39Ar 40Ar dating studies were made on several lithologies, including the massive melt zone, host chondrite, and melt-chondrite boundaries, for the purpose of studying the melting and thermal histories associated with impacts on small bodies and their effects on the KAr chronometer. The chondritic host is shocked to stage S6 and contains pockets and veins of melt. There are no unmelted clasts in the interior of the melt; coalesced metal-troilite nodules reach up to 2 cm in size. Melt near the contact with the host chondrite contains numerous clasts and quenched more rapidly. Metal-troilite textures suggest cooling rates of ∼0.1°C/s in the interior of the melt dike during crystallization. Secondary kamacite rims indicate cooling at 0.01–1°C/y over the range of 700-500°C, consistent with an impact-heated volume of up to a kilometer in thickness. Compositions of olivines and pyroxenes are generally similar in melt and chondritic host, reflecting rapid crystallization, not metamorphic equilibration. The interior melt shows an overall depletion in K, whereas the melt near the boundary is enriched in K. The 39Ar 40Ar release spectra during stepwise heating of both melt and chondrite samples can be divided into two parts, based on Ar diffusion properties and K/Ca ratios. The low-temperature, high K/Ca phase of both melt and host chondrite show ages of 0.54–0.78 Ga. Ages of the high-temperature, low K/Ca phase of the melt are comparable or higher, 0.61–1.35 Ga, whereas those of the host chondrite are even higher, 0.87–1.86 Ga, due to lesser degrees of degassing. Isochron plots for several melt samples suggest an age of ∼0.53 Ga and the presence of variable amounts of excess 40Ar not completely degassed by the impact. Even this age, however, is significantly higher than the previously reported RbSr isochron age of 0.467 ± .015 Ga. The apparent retention of radiogenic 40Ar in the Chico impact melt, in spite of its relatively large size, absence of clasts, and moderately slow cooling rate below 700°C, raises questions as to the reliability of using melts for 39Ar 40Ar dating of meteoritic impact events.

Clay minerals as records of temperature conditions and duration of thermal anomalies in the Paris Basin, France

AbstractUpper Triassic sandy horizons in the Paris Basin were sampled at depths ranging from outcrop in the northeast to 2,700 m in the centre of the basin. The smallest clay sub-fractions (<0.2 μm) from the deepest central samples consist mainly of illite and chlorite whose K-Ar age is ∼ 190 Ma. These minerals formed at a relatively high temperature of 220-250°C, as determined from the oxygen isotope fractionation between authigenic illite and associated quartz overgrowths, but at a burial depth of only 500 m. In a nearby drillhole that crosscuts a fault zone reactivated during the 190 Ma event, illite is less well-crystallized and has a higher δ18O value suggesting different physical and chemical conditions of formation during the same hydrothermal episode. Two other generations of illite-smectite mixed-layer clays formed in the same Triassic horizon: one at ∼150 Ma and the other at ∼80 Ma. These younger clays have higher δ18O values and thus may have formed at somewhat lower temperatures. The δ18O values of the fluids from which the different illite-smectite mixed-layer minerals form range from +9 to +13.5 per mil (SMOW).An Ar diffusion code was used to estimate, on the basis of the Ar loss of the clay-type material, the duration of these events. The results suggest that the duration of the hydrothermal events at 190 Ma and 150 Ma were rather short, <1 Ma, whereas the youngest event was protracted over a much longer period of ∼37 Ma. Comparison between K-Ar ages of the different mixed-layer minerals and sedimentation rates of the sediments since the Palaeozoic shows significant accelerations of the rates at ∼ 190-200 Ma and 150 Ma, and a less important one at ∼80 Ma. These observations provide additional evidence that the first two events were promoted by basement-related tectono-thermal activities. The third event is considered to be of diagenetic origin.

Real-time acquisition of laser-induced fluorescence decays

Picosecond time-resolved laser-induced fluorescence (PITLIF) has the potential to provide rapid measurements of minor-species concentrations by correction for local quenching conditions on the time scale of turbulence. Previous studies demonstrated that this technique could provide laser-induced fluorescence data and local quenching rates in flames but used equivalent-time sampling to obtain the required fluorescence decays. This precludes the use of PITLIF in turbulent systems. Fluorescence decays of sodium seeded into a laminar H(2)-O(2)-Ar diffusion flame are obtained from real-time data with an acquisition rate on the time scale of turbulence. The results obtained with this method are shown to be similar to those obtained from equivalent-time sampling.

Ion Exchange and Dehydration Effects on Potassium and Argon Contents of Clinoptilolite

AbstractZeolite-rich Miocene tuffs are an important part of the principal hydrochemical barrier to water-borne radionuclide transport from a potential high-level nuclear waste repository at Yucca Mountain, Nevada. The timing of zeolitization is an issue that relates to paleohydrology, permeability, zeolite stability, and unsaturated-zone geochemical processes. Exploratory K/Ar dating of clinoptilolite, the most abundant and widespread zeolite, shows a striking and consistent pattern of increasing apparent ages (2–13 Ma) with depth. Only the isotopic ages from the saturated zone are compatible with geologic evidence suggesting an age >10 Ma for most of the zeolites.Factors that may be responsible for the young apparent ages in the unsaturated zone were investigated. Cation exchange with recharge water and Ar diffusion under unsaturated conditions (processes that may be characteristic of the unsaturated zone) were evaluated experimentally for their effects on K/Ar systematics. Cation exchanging a natural clinoptilolite with Ca-, Cs-, K-, and Na-chloride solutions showed minimal effects on radiogenic Ar content. However, clinoptilolite heated at 2007deg;C for 16 hours in air lost a significant amount of its radiogenic Ar compared with minimal losses from clinoptilolite heated in water at 100°C for over 5 months. The preliminary results indicate that Ar loss from incompletely hydrated clinoptilolite may be a major factor contributing to the young apparent ages of clinoptilolite in the unsaturated zone at Yucca Mountain.

Influence of gas composition on convective and diffusive intrapulmonary gas transport.

The influence of gas composition on convective and diffusive gas transport in the lungs was assessed by studying the dispersion of combined particle and argon (Ar) boluses induced by the passage through the lungs filled with three different gas mixtures. Particles, as a "nondiffusing gas," served as a tracer for convective gas transport, while the significance of diffusive gas transport was inferred from the difference in the behavior of Ar and particles. The lungs of six anesthetized and mechanically ventilated beagle dogs were equilibrated with air or either of the test atmospheres, He-O2 or SF6-O2, where nitrogen was replaced by helium (He) or sulfur hexafluoride (SF6). Due to differences in gas density and gas viscosity Reynolds numbers varied by a factor of twelve and Ar diffusivity by a factor of four between He-O2 and SF6-O2, suggesting that both kinds of intrapulmonary gas transport, convection and diffusion, should be affected. Combined particle and Ar boluses were inhaled into various lung depths and the extent of gas transport was inferred from changes in bolus shape induced by the passage through the lungs. In air, convective bulk gas transport generally followed the symmetric first-in, last-out principle and acted at all tested lung depths. Within the conducting airways, gas transport to the lung periphery was primarily due to convection but beyond these airways diffusion became rapidly significant. Breathing test atmospheres affects intrapulmonary gas transport only slightly. The extent of convective mixing was increased by 4% in SF6-O2 (p < .01) and reduced by 5% in He-O2 (p < .01) as compared to air. The symmetry of convective lung filling and emptying was slightly disturbed. In SF6-O2 the mean of the exhaled bolus was shifted by 8% toward the lung periphery. In He-O2 it was shifted by 4% toward the airway opening. In both test atmospheres exhaled Ar boluses were similar, suggesting that diffusive gas transport overwhelms the small changes in convective gas transport. Hence, factors other than gas composition-related flow characteristics, e.g., nonreversibility of in- and expiratory flow profiles or features of lung geometry, are the major determinants of gas transport in the lungs.

The cooling history of the late Pliocene Eldzhurtinskiy granite (Caucasus, Russia) and the thermochronological potential of grain-size/age relationships

Isotopic age investigations have been carried out on samples from the late Pliocene Eldzhurtinskiy granite (Great Caucasus, Russia) in order to elucidate the cooling history of this kilometere sized plutonic body. The samples were taken from a horizontal profile (from the edge to the centre of the granite) and from a vertical profile (from the top to a depth of 4570 m). The 40Ar/ 39Ar results on biotites ranging from 2.5 to 1.2 Ma indicate a vertical age zoning of the granite caused by regional uplift. An uplift rate of 4 mm/a is calculated from the age differences. Together with the results of fission track dating on apatite and zircon and the present-day rock temperatures the biotite ages can be used to evaluate cooling rates below 400°C. The cooling rates of about 180°C/Ma are relatively high and spatially and temporarily fairly uniform throughout the granite. A geothermal gradient of 45°C/km is derived from uplift and cooling rates; this is identical to the present-day borehole gradient. The 40Ar/ 39Ar ages of distinct biotite grain fractions from one and the same sample specimen are discordant. Generally, they show clear age/grain-size correlation, as expected from Ar diffusion theory for cooling systems. Thus, the biotite results support the additional detailed thermochronological information. The cooling rates in the temperature range of biotite closure, based on the grain-size/age correlations of the biotites, are close to 180°C/Ma. They provide evidence that within a certain temperature interval the time-temperature path of a rock may be determined exclusively by isotopic dating of different grain-size fractions of only one type of mineral.

A structural analysis of the Main Central Thrust zone, Langtang National Park, central Nepal Himalaya

The Main Central Thrust (MCT) is one of the most tectonically significant structures in the Himalayan orogen. Detailed geologic mapping and structural analysis of the MCT in the Langtang National Park region of central Nepal reveals that this segment of the fault zone experienced multiple episodes of south-directed movement, under both brittle and ductile conditions, during the Tertiary period. Early (mid-Miocene) movement resulted in the development of mylonitic fabrics synchronous with amphibolite-facies metamorphism. The mean orientation of the dominant mylonitic foliation is N28°W, 38°NE. An associated mineral/stretching lineation plunges 40° to N40°E. Kinematic indicators suggest hanging-wall movement to the southwest relative to the footwall along the north-dipping fault. It is not possible to constrain the magnitude of high-temperature displacement on the MCT at the longitude of Langtang. Late-stage structures in the MCT zone at Langtang include a series of imbricate, brittle thrust faults that separate different lithostratigraphic units and correspond to metamorphic discontinuities. We interpret this fault system as a duplex structure. Muscovite 40 Ar/ 39 Ar cooling ages from the MCT zone range from 8.9-6.9 Ma. Because the nominal closure temperature of Ar diffusion in muscovite (approximately 625 K) is higher than the apparent temperature conditions under which late brittle deformation occurred, we suggest that brittle deformation was a latest Miocene-Pliocene phenomenon. Another major Himalayan fault, the Main Boundary Thrust (MBT), was developing to the south of Langtang at approximately the same time. We speculate that brittle faulting within the MCT zone may have initiated as the MCT zone was transported over a ramp in the MBT.

LASER step-heating [formula omitted] age spectra from early Archean (~3.5 Ga) Barberton greenstone belt sediments: A technique for detecting cryptic tectono-thermal events

Samples from sediments of the Fig Tree Group located in the central part of the circa 3.2 to 3.5 Ga Barberton greenstone belt (BGB) have been analyzed by the 40Ar 39Ar laser step-heating technique. This technique has enabled previously cryptic thermal overprints to be detected in various sedimentary units which include: reworked chemical sediments (barite), clastic sediments (sandstone/shale), and one stromatolite. In most cases the apparent age plateaux can be identified by corresponding Ca K and Cl K ratio plots as belonging to separate mineral phases. Most of the samples exhibit simple Ar diffusion loss during the low temperature part of the experiments while occasionally showing excess Ar, or possible 39Ar recoil, effects. The various sediments analyzed show evidence for distinct overprinting between 2025 and 2090 Ma. One barite sample gave T 0 (original blocking age) = 2673 ± 3 Ma (1σ) which is close to a calculated model Ar diffusion-loss age for the same sample of 2688 ± 3 Ma (1σ). This age is interpreted as representing final granitoid activity adjacent to the BGB, and/or craton-scale tectonism associated with the Limpopo Orogeny. Two samples gave T 0 ages of ∼2350–2400 Ma which may reflect increased thermal gradients associated with the formation of the thick Transvaal sedimentary basin that may once have covered the BGB. The dominant apparent age plateaux together with modelled diffusive Ar loss ages of 2025–2090 Ma, are thought to represent regional thermal anomalies related to large-scale tectono-thermal activity in the Kaapvaal craton, of which the Bushveld Complex (which covers a surface area of ~ 60,000 km 2) and formation of the Vredefort Structure are obvious manifestations. The strong thermal overprinting recorded by the sediments has effectively removed any ancient atmosphere (i.e., 40Ar 36Ar ratios < 295.5) signatures.

Diffusion of Ar in rhyolite, orthoclase and albite composition glasses

The diffusivity of Ar has been determined in a natural rhyolite obsidian (Rhy) and in glasses of NaAlSi 3O 8 (Ab) and KAlSi 3O 8 (Or) composition at temperatures between 400 and 900°C, at pressures from 1150 to 3700 bar. The observed variations in the Ar diffusion coefficient ( D) with temperature can be described by the Arrhenius relationship D = D 0exp(−E/RT) with the following results (for pressures from 1100 to 2500 bar: D in cm 2/s; R in cal/mole): (Ab) D = 1.62 × 10 −3 ·exp(−32780RT) ; (Or) D = 6.64 × 10 −3 ·exp(−29000RT) ; (Rhy) D = 6.86 × 10 −3 ·exp(−34490RT) . The data, although of limited extent, also indicate a slight decrease in D with increasing pressure and the calculated activation volume of 36 (±8) cm 3/mole for Ar in Rhy at 800°C agrees with the idea that the volume change associated with a diffusive jump is similar to the size of the diffusing species. Comparison of the Ar results with cation tracer diffusivities indicates that Ar diffusivity is similar to that of rubidium in Rhy and Ab. Argon diffusivity is several orders of magnitude slower than H 2O at T<900°C and exsolution of H 2O to form bubbles might leave a residual rhyolitic melt enriched in Ar (and Kr, Xe) due to kinetic effects. Noble gases show lower activation energies for diffusion than are observed for tracer diffusion of cations with comparable radii. A systematic increase in activation energy for diffusion with increasing size of the noble gas atoms, and similarities in activation energies in rhyolitic through basaltic melts suggests that in basaltic melts, He, Ne, and probably Ar can diffuse faster than CO 2, the major volatile component in basalts. In contrast, Xe and possibly Kr, due to their (estimated) high activation energies may diffuse more slowly than CO 2 at temperatures below 1200°C. This implies that kinetic factors may influence noble gas abundance patterns during separation of a CO 2-rich vapor phase from degasing basaltic magma during ascent.

Laser microprobe measurement of chlorine and argon zonation in biotite

Neutron activation of chlorine yields 38Ar which can be liberated by the laser microprobe (LMP) in sufficient quantities to be measured by gas mass spectrometry. When used in conjunction with electron microprobe (EMP) analyses, this technique can precisely measure low concentrations of Cl in minerals. The accuracy is limited by lack of an adequate standard. LMP analyses of standard Mmhb- 1 indicate its Cl concentration is quite heterogeneous and is not a useful standard for LMP Cl analyses. In natural phlogopite and hydrothermally treated (HT) biotite Cl concentration gradients have been measured with the LMP. The zonation probably reflects partial re-equilibration by diffusive anion exchange with Cl transport occurring more rapidly parallel to the silicate sheets than perpendicular to the sheets. The LMP also detected 40Ar gradients in the phlogopite and HT biotite, suggesting that the rate of Cl transport is comparable to that for Ar diffusion. LMP analyses also indicate that 40Ar migration is more rapid parallel to the silicate sheets than perpendicular to the sheets and that the radius of diffusion is on the order of the grain size. The latter observation apparently contradicts the relatively much smaller estimates of the radius of diffusion for Ar in biotite (∼ 150 μm) inferred from 40Ar analyses of bulk separates of hydrothermally treated biotite. The discrepancy may be related to the numerous pores occurring at layer terminations and pinch outs and at the edges of Guinier-Preston zones detected by high-resolution transmission electron microscopy (HRTEM) and which appear to be more abundant in the HT biotite than in the original untreated sample biotite. If these pores are sufficiently large to permit a conduit for rapid Cl and Ar transport via pipe diffusion, then they may be the cause of the apparently enhanced diffusivity of 40Ar observed in large-grain-size fractions of hydrothermally treated biotite.

Argon solubility and diffusion in silica glass: Implications for the solution behavior of molecular gases

Argon solubility and diffusivity in SiO 2 glass have been determined from experiments at pressures of 200 to 3725 bars and temperatures of 400 to 900°C. Samples contained in unsealed capsules were exposed to Ar gas used as the pressure-transmitting medium in cold-seal reaction vessels. Electron microprobe analysis of diffusion-controlled concentration gradients of Ar in the quenched samples allows determination of Ar diffusivity and solubility over the range of investigated conditions. The temperature dependence of Ar diffusivity ( D) between 400 and 900°C at ∼ 1200 bars is well described by the Arrhenius relationship D = D o · exp(− E/ RT) with Log 10 D o = −5.06 ± 0.31 (cm 2/sec) and an activation energy for diffusion E, of 24.1 ± 1.3 kcal/mol. Argon diffusivity appears to be concentration independent up to at least ∼ 1.3 wt% Ar, and any pressure effect on Ar diffusivity is within the error of the measurements. Argon solubility at 700°C increases linearly with increasing pressure up to ∼ 1500 bars (0.35 wt%) and can be described by Henry's law ( f Ar = K H · X Ar ) with K H = 8.06 (±0.2) · 10 5 bar, f Ar = Ar fugacity in bars, and X Ar = mole fraction dissolved Ar in Si 0.5O. Above 1500 bars the solubility is lower than would be predicted from a linear (Henrian) extrapolation of the low P data. At constant pressure Ar solubility decreases with increasing temperature. Thermodynamic analysis of the Ar solubility data using the assumption that Ar activity is equivalent to X Ar yields an estimated partial molar volume for Ar in silica glass of 16.4 ± 1.4 cm 3/mol and an enthalpy of solution of −4.76 ± 0.26 kcal/mol at 1 bar. The data may be equally well described by a solution model in which the gas atoms are considered to occupy a certain population of sites or holes that are available to them in the glass structure. Fitting of the solubility data at 700°C to such a model with the assumption of ΔV = 0 indicates 5.74 · 10 20 sites/cm 3 of SiO 2 glass, corresponding to ∼5% of the total number of holes in a randomized cristobalite-like structure. However, without an independent estimate of the number of available sites in the glass this solution is non-unique, and the data may be fit by a family of models with both positive and negative values for ΔV. Higher pressure solubility studies offer the best means to further investigate the applicability of such solution models for molecular or atomic gases dissolved in melts, glasses, and crystals.

Forgotten major elements: Hydrogen and oxygen variation in biotite from metapelites

Variations in the H{sup +} and O{sup 2{minus}} content of minerals have been virtually ignored since the advent of the electron microprobe in the mid-1960s. Such studies do not serve as a basis for consideration of the variation of other major constituents such as Fe{sup 3+}, H{sup +}, and O{sup 2{minus}}. The authors report results of a comprehensive study of all major element compositional variation in biotite using samples from a suite of metapelites ranging from lower garnet to sillimanite + K-feldspar grades. The data show that Fe{sup 3+} remains constant within analytical error while percentages by weight of H{sub 2}O and O decrease and increase, respectively, with increasing metamorphic grade. H{sup +} and O{sup 2{minus}} contents of biotite appear to be highly sensitive to temperature and pressure. Samples from low metamorphic grades contain bulk H{sup +} contents > 4 H{sup +}/formula unit. Excess H{sup +} probably occupies vacancies in the interlayer sites as either H{sub 2}O or H{sub 3}O. These results will have an impact on geoscience disciplines that make assumptions about stoichiometry of hydrous phases. For example, the temperature range of reactions involving the breakdown of hydrous phases with more or less H{sup +} than predicted will change,more » in some cases dramatically, and any calculations involving water activities must be modified to consider H{sup +} variation. Partitioning of H{sup +} among coexisting hydrous phases ({delta}D studies) may be affected by both amount of H{sup +} and its location and bonding (interlayer H{sub 2}O or H{sub 3}O vs. OH{sup {minus}}). Ar diffusivities calculated for {sup 40}Ar/{sup 39}Ar work will be influenced by the H{sup +} occupancy in the interlayer sites. Cation substitution models for H{sup +}-bearing phases need to consider exchanges involving both H{sup +} and O{sup 2{minus}}.« less

Discordant 40Ar 39Ar ages from the Musgrave Ranges, central Australia: Implications for the significance of hornblende 40Ar 39Ar spectra

Hornblende and biotite from granulites in the Musgrave Ranges, central Australia, yield variable 40Ar 39Ar bulk ages even at the scale of a single outcrop. There is no obvious relationship between age, chemical composition, sample location, or textural characteristics of the minerals. We interpret the age variation as due to the incorporation of variable quantities of excess Ar. However, the age spectra of the minerals do not show the typical gain profiles commonly associated with excess Ar and isotope correlation diagrams do not indicate trapped Ar of non-atmospheric composition. Instead, some of the hornblendes and all the biotites show relatively simple age spectra similar to those commonly interpreted in terms of volume diffusion. These observations lead us to argue that, as is commonly the case with biotites, not all hornblende age spectra with simple patterns, similar to those predicted by diffusion theory, can be interpreted unambiguously in terms of geologically significant Ar concentration profiles. More specifically, we conclude that a relatively simple hornblende age spectrum does not necessarily indicate the absence of excess Ar. Despite their complexity, our results show that post-metamorphic cooling of the granulites was very slow. Although the incorporated excess Ar hinders the reconstruction of a more precise thermal history, the data show that following granulite-facies metamorphism ≈ 1200 Ma ago, the terrane did not cool below temperatures at which hornblende and biotite close to Ar diffusion until after ≈ 930 and ≈ 690 Ma, respectively.

Diffusion of 40Ar in metamorphic hornblende

Isothermal, hydrothermal experiments were performed on two compositionally contrasting hornblendes from amphibolites in order to examine Ar diffusion behavior in metamorphic hornblendes. Ten experiments on sample RF were performed at temperatures of 750°C, 800°C, and 850°C and pressures of 1 kbar using measured grain radii of 158, 101, and 34 μm. Eight experiments on sample 118576 were performed under the same conditions using measured grain radii of 145, 77, and 25 μm. Minor (<5%) alteration was observed in high temperature runs. Diffusion coefficients were calculated from measured radiogenic 40Ar loss following treatment assuming a spherical geometry for the mineral aggregate. Diffusivities calculated for different grain sizes vary by up to an order of magnitude for a given temperature indicating that the effective diffusion radius was less than the measured grain radius. Diffusivities for RF and 118576 calculated for grain radii of 101 and 145 μm, respectively, form a linear array on an Arrhenius diagram with slopes indicating activation energies of ∼ 60 kcal/mol. No correlation between Mg number (100 Mg/(Mg+Fe)) and activation energy was observed. Diffusivities calculated for these experiments are higher than previously reported results from similar experiments performed on hornblendes. A comparison of results for 34 μm splits from these two studies indicates higher apparent diffusivities (by a factor of 5), which probably result from observed phyllosilicate inter-growths (chlorite) and/or exsolution lamellae that partition the metamorphic hornblendes into smaller subdomains. Diffusivities calculated for experiments performed on 65 μm and 34 μm splits of 40Ar/39Ar standard MMhb-1 at 800°C and 1 kbar are consistent with a previously reported activation energy of 65 kcal/mol. Arrhenius parameters which emerge from the empirical model of Fortier and Giletti (1989) agree with experimental results to within analytical uncertainty. Although results of these experiments support previously reported estimates of the activation energy of 40Ar in hornblende (∼60 kcal/mol), phyllosilicate intergrowths and/or microstructures such as exsolution lamellae within the two metamorphic hornblendes result in extremely small diffusion domains, which may lead to lower Ar retentivities and lower closure temperatures. The effective diffusion dimension for 40Ar in hornblende is not likely to be defined by dislocations but rather by some larger structure within the crystal. TEM and SEM studies may provide some insight into the effective diffusion dimension for 40Ar in amphiboles, thereby enabling better estimates of closure temperatures and more precise temperature-time reconstructions.

Diffusion of 40Ar and 39Ar in irradiated orthoclase

The important concerns of whether neutron irradiation affects Ar diffusional behavior in minerals and whether the diffusivities of natural radiogenic 40Ar and induced 39Ar are identical are considered. Both issues are addressed with isothermal and incremental-heating experiments on natural, homogeneous orthoclase from Benson Mines which was subjected to irradiation similar to typical 40Ar 39Ar measurements. Previous study of this feldspar shows that laboratory 40Ar loss occurs by volume diffusion with physical grain sizes as the effective transport dimensions following a single Arrhenius relation. Isothermal-heating experiments (600 to 800°C, 4 to 21 days) on irradiated feldspar show the same loss and apparent 40Ar diffusion coefficients, within uncertainty, as unirradiated sample. For these heating times and temperatures, the experiments indicate that the defects accompanying irradiation have only very minor, if any, effects on Ar behavior with regard to both diffusion kinetics and effective transport domains and that 39Ar and 40Ar diffusivities do not differ radically. The results suggest intrinsic control of Ar diffusion by a single mechanism for all isotopes, regardless of origin, in this specimen. Step-heating experiments yield essentially flat spectra but with minor yet significant apparent discordance which is the same for different grain sizes. The spectra show high ages for small, initial low-temperature fractions and a slight increase in age with progressive Ar release. While these variations are potentially explicable by sample heterogeneities, the preferred explanation is that they result from 39Ar recoil loss and a slightly higher diffusivity of 39Ar relative to 40Ar. The relative behavior is consistent with a kinetic mass effect in which the diffusion rates of 39Ar and 40Ar are inversely proportional to the square root of mass according to rate theory. The results have important implications for diffusion processes in feldspar and the application of the 40Ar 39Ar method.