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1.
The effects of deformation on radiogenic argon (40Ar∗) retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite. Cylindrical cores, ∼15 mm in length and 6.25 mm in diameter, were drilled from granite collected from the South Armorican Massif in northwestern France, loaded into gold capsules, and weld-sealed in the presence of excess water. The samples were deformed at a pressure of 10 kb and a temperature of 600 °C over a period 29 of hours within a solid medium assembly in a Griggs-type triaxial hydraulic deformation apparatus. Overall shortening in the experiments was approximately 10%. Transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking, cracking, and grain segmentation of the micas.Infrared (IR) laser (CO2) heating of individual 1.5-2.5 mm diameter grains of muscovite and biotite separated from the undeformed granite yield well-defined 40Ar/39Ar plateau ages of 311 ± 2 Ma (2σ). Identical experiments on single grains separated from the experimentally deformed granite yield results indicating 40Ar∗ loss of 0-35% in muscovite and 2-3% 40Ar∗ loss in biotite. Intragrain in situ ultraviolet (UV) laser ablation 40Ar/39Ar ages (±4-10%, 1σ) of deformed muscovites range from 309 ± 13 to 264 ± 7 Ma, consistent with 0-16% 40Ar∗ loss relative to the undeformed muscovite. The in situ UV laser ablation 40Ar/39Ar ages of deformed biotite vary from 301 to 217 Ma, consistent with up to 32% 40Ar∗ loss. No spatial correlation is observed between in situ40Ar/39Ar age and position within individual grains. Using available argon diffusion data for muscovite the observed 40Ar∗ loss in the experimentally treated muscovite can be utilized to predict average 40Ar∗ diffusion dimensions. Maximum 40Ar/39Ar ages obtained by UV laser ablation overlap those of the undeformed muscovite, indicating argon loss of <1% and an average effective grain radius for 40Ar∗ diffusion ?700 μm. The UV laser ablation and IR laser incremental 40Ar/39Ar ages indicating 40Ar∗ loss of 16% and 35%, respectively, are consistent with an average diffusion radius ?100 μm. These results support a hypothesis of grain-scale 40Ar∗ diffusion distances in undeformed mica and a heterogeneous mechanical reduction in the intragrain effective diffusion length scale for 40Ar∗ in deformed mica. Reduction in the effective diffusion length scale in naturally deformed samples occurs most probably through production of mesoscopic and submicroscopic defects such as, e.g., stacking faults. A network of interconnected defects, continuously forming and annealing during dynamic deformation likely plays an important role in controlling both 40Ar∗ retention and intragrain distribution in deformed mica. Intragrain 40Ar/39Ar ages, when combined with estimates of diffusion kinetics and distances, may provide a means of establishing thermochronological histories from individual micas.  相似文献   

2.
In a recent study, Qiu and Wijbrans (2006) [Qiu, H.-N. and Wijbrans, J. R. (2006). Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: new insights from 40Ar/39Ar dating by stepwise crushing. Geochimica et Cosmochimica Acta70, 2354-2370.] analyzed Ar-isotopes extracted by crushing garnets from the Dabie Shan Bixiling eclogite and claimed to have constrained a pre-Triassic (∼450 Ma) episode of UHP metamorphism from primary fluid inclusions.However, in the absence of careful sample characterization and stepped heating analyses, the reported ages are more easily explained as experimental artifacts related to Ar extracted from either mineral inclusions or the interface sites between mineral inclusions and the garnet matrix: Dabie Shan garnets commonly contain mineral impurities such as K-rich omphacite and/or K-feldspar.If Dabie Shan UHP metamorphism is of the generally accepted Triassic age (210-240 Ma), the apparent age of a phengite sample in equilibrium with the garnet can be explained by the presence of an extraneous 40Ar∗ component with mean 40Ar/36Ar value of ∼5000. This value is similar to the composition of extraneous 40Ar∗ in other eclogite facies terrane.  相似文献   

3.
The thermal histories of Martian meteorite are important for the interpretation of petrologic, geochemical, geochronological, and paleomagnetic constraints that they provide on the evolution of Mars. In this paper, we quantify 40Ar/39Ar ages and Ar diffusion kinetics of Martian meteorites Allan Hills (ALH) 84001, Nakhla, and Miller Range (MIL) 03346. We constrain the thermal history of each meteorite and discuss the resulting implications for their petrology, paleomagnetism, and geochronology. Maskelynite in ALH 84001 yields a 40Ar/39Ar isochron age of 4163 ± 35 Ma, which is indistinguishable from recent Pb-Pb (Bouvier et al., 2009a) and Lu-Hf ages (Lapen et al., 2010). The high precision of this result arises from clear resolution of a reproducible trapped 40Ar/36Ar component in maskelynite in ALH 84001 (40Ar/36Ar = 632 ± 90). The maskelynite 40Ar/39Ar age predates the Late Heavy Bombardment and likely represents the time at which the original natural remanent magnetization (NRM) component observed in ALH 84001 was acquired. Nakhla and MIL 03346 yield 40Ar/39Ar isochron ages of 1332 ± 24 and 1339 ± 8 Ma, respectively, which we interpret to date crystallization. Multi-phase, multi-domain diffusion models constrained by the observed Ar diffusion kinetics and 40Ar/39Ar age spectra suggest that localized regions within both ALH 84001 and Nakhla were intensely heated for brief durations during shock events at 1158 ± 110 and 913 ± 9 Ma, respectively. These ages may date the marginal melting of pyroxene in each rock, mobilization of carbonates and maskelynite in ALH 84001, and NRM overprints observed in ALH 84001. The inferred peak temperatures of the shock heating events (>1400 °C) are sufficient to mobilize Ar, Sr, and Pb in constituent minerals, which may explain some of the dispersion observed in 40Ar/39Ar, Rb-Sr, and U-Th-Pb data toward ages younger than ∼4.1 Ga. The data also place conservative upper bounds on the long-duration residence temperatures of the ALH 84001 and Nakhla protolith to be  °C and  °C over the last ∼4.16 Ga and ∼1.35 Ga, respectively. MIL 03346 has apparently not experienced significant shock-heating since it crystallized, consistent with the fact that various chronometers yield concordant ages.  相似文献   

4.
Plagioclase is not only the most abundant mineral in the Earth’s crust, but is present in almost all terrestrial tectonic settings and is widespread in most extraterrestrial material. Applying the K-Ar system to this common mineral would provide a powerful tool for quantifying thermal histories in a wide variety of settings. Nonetheless, plagioclase has rarely been used for thermochronometry, largely due to difficulties in simultaneously acquiring precise geochronologic data and quantifying argon diffusion kinetics from a mineral with low-K concentration. Here we describe an analytical technique that generates high-precision 40Ar/39Ar data and quantifies Ar diffusion kinetics of low-K minerals. We present results of five diffusion experiments conducted on single crystals of plagioclase from the Bushveld Complex, South Africa. The observed diffusion kinetics yield internally consistent thermochronological constraints, indicating that plagioclase is a reliable thermochronometer. Individual grains have activation energies of 155-178 kJ/mol and ln(D0/a2) varies between 3.5 and 6.5. These diffusion parameters correspond to closure temperatures of 225-300 °C, for a 10 °C/Ma cooling rate. Age spectra generally conform to single-domain diffusive loss profiles, suggesting that grain-scale diffusion dominates argon transport in this fairly simple plagioclase. Conjointly examining several single-grain analyses enables us to distinguish episodic reheating from slow cooling and indicates that the Bushveld Complex cooled rapidly and monotonically from magmatic temperature to <300 °C over 3 Ma, followed by protracted cooling to ambient crustal temperatures of 150-200 °C over ∼600 Ma.  相似文献   

5.
Studies of meteorites are based mostly on samples that fell to Earth in the recent past (i.e., a few million years at most). The Morokweng LL-chondrite meteorite is a particularly interesting specimen as its fall is much older (ca. 145 Ma) than most other meteorites and because it is the only macro-meteorite clast (width intersected in drill core: 25 cm) found in a melt sheet of a large impact structure. When applied to the Morokweng meteorite, 40Ar/39Ar thermochronology provides an opportunity to study (1) effects associated with pre-impact and post-impact processes and (2) collision events within a potentially distinct and as yet unsampled asteroid population.A single multi-grain aliquot yielded an inverse isochron age of 625 ± 163 Ma. This suggests a major in-space collisional event at this time. We have modeled the diffusion of 40Ar∗ within the meteorite and plagioclase during and after the ∼145 Ma impact on Earth to tentatively explain why pre-terrestrial impact 40Ar∗ has been preserved within the plagioclase grains. The ∼145 Ma terrestrial impact age is recorded in the low-retentivity sites of the meteorite plagioclase grains that yielded a composite inverse isochron age at 141 ± 15 Ma and thus, confirms that age information about major (terrestrial or extraterrestrial) impacts can be recorded in the K-rich mineral phases of a meteorite and measured by the 40Ar/39Ar technique. More studies on fossil meteorites need to be carried out to understand if the rough 0.6 Ga age proposed here corresponds to major LL-chondrite asteroid population destructions or, rather, to an isolated collision event.  相似文献   

6.
22 hornblende K-Ar ages and 10 39Ar-40Ar spectra were obtained for hornblende garbenschists from the Western Tauern Window. The post-kinematic amphiboles were produced during the late Alpine prograde metamorphism (6–10 kb and 500–570° C). Two nearly potassiumfree cummingtonites rimming hornblende yield K-Ar ages of 120 Ma, while the 20 tschermakitic hornblendes scatter between 17 and 37 Ma. The reason for this scatter is excess Ar, possibly incorporated into amphiboles during healing of fractures, now traceable by trails of fluid inclusions. Excess Ar is semiquantitatively corrected for by combining cogenetic hornblende and cummingtonite with K-Ar isochrons. It can be quantified in 4 out of 10 hornblendes by 39Ar-40Ar stepwise heating experiments. Ages of 18–20 Ma result for corrected hornblendes. The retentivity of 40Ar, after correction for excess, shows no correlation with chemistry within the narrow compositional range observed; rather, it shows intriguing correlations with irregularities in Ca/K spectra, pointing to a microstructurally controlled mechanism for Ar loss. This observation leads to a critical evaluation of the closure temperature constant, which apparently depends on an incompletely known number of mineralogical and environmental parameters. In particular those 39Ar-40Ar release spectra which yield low temperature steps with younger ages than the plateaus are not interpretable in terms of a synchronous closure. This gives evidence that loss of radiogenic isotopes proceeds by a more complex mechanism than simple volume diffusion through isotropic media.  相似文献   

7.
40Ar/39Ar and K-Ar geochronology have long suffered from large systematic errors arising from imprecise K and Ar isotopic data for standards and imprecisely determined decay constants for the branched decay of 40K by electron capture and β− emission. This study presents a statistical optimization approach allowing constraints from 40K activity data, K-Ar isotopic data, and pairs of 238U-206Pb and 40Ar/39Ar data for rigorously selected rocks to be used as inputs for estimating the partial decay constants (λε and λβ) of 40K and the 40Ar∗/40K ratio (κFCs) of the widely used Fish Canyon sanidine (FCs) standard. This yields values of κFCs = (1.6418 ± 0.0045) × 10−3, λε = (0.5755 ± 0.0016) × 10−10 a−1 and λβ = (4.9737 ± 0.0093) × 10−10 a−1. These results improve uncertainties in the decay constants by a factor of >4 relative to values derived from activity data alone. Uncertainties in these variables determined by our approach are moderately to highly correlated (cov(κFCs, λε) = 7.1889 × 10−19, cov(κFCs, λβ) = −7.1390 × 10−19, cov(λε, λβ) = −3.4497 × 10−26) and one must take account of the covariances in error propagation by either linear or Monte Carlo methods. 40Ar/39Ar age errors estimated from these results are significantly reduced relative to previous calibrations. Also, age errors are smaller for a comparable level of isotopic measurement precision than those produced by the 238U/206Pb system, because the 40Ar/39Ar system is now jointly calibrated by both the 40K and 238U decay constants, and because λε(40K) < λ(238U). Based on this new calibration, the age of the widely used Fish Canyon sanidine standard is 28.305 ± 0.036 Ma. The increased accuracy of 40Ar/39Ar ages is now adequate to provide meaningful validation of high-precision U/Pb or astronomical tuning ages in cases where closed system behavior of K and Ar can be established.  相似文献   

8.
Important He and Ar isotope studies on rocks and minerals, relevant to the geochemical and degassing history of the Earth, are often hampered by insufficient knowledge of the retentivity of different types of sites in minerals (inclusions, matrix) for these species, and of the relative importance of radiogenic and trapped components and possible differences in their behavior.To identify sites of noble gas isotopes, shed some light on their origin and estimate their residence times in olivine, which is a mineral considered as a good natural sampler, we investigated 2.5 Ga old ultramafic rocks from the Monche Pluton (Kola Peninsula, north-east part of the Baltic shield) using several extraction methods: crushing, fusion, slow step-wise and rapid incremental heating. Previous studies indicated that these rocks contain mainly trapped noble gases; however, to constrain the possible contribution of in-situ generated radiogenic helium, U and Th concentrations were also measured in the samples.The helium release pattern obtained by relatively fast (∼1.5 h long) incremental heating of olivine includes three distinct release peaks for helium: a low-temperature (600 °C) l-peak, a middle (800-1100 °C) m-peak and a high-temperature (∼1400 °C) h-peak. However, helium extraction from a powdered aliquot of the same olivine yields mainly the middle m-peak indicating that gases released in the l- and h-peaks occupy gas-liquid inclusions opened in the course of crushing and grinding. Moreover, slow step-wise heating (14 h) also results in a broad He release peak but in two well-separated l- and h-peaks of non-atmospheric 40Ar∗. This feature implies helium migration from l- and h-vesicles into the matrix m during long step-wise heating experiments, whereas less movable Ar remains in inclusions at even relatively high almost-magmatic temperatures.Using a simple phenomenological model envisaging the three different residence sites for noble gases, both fast- and slow-heating release patterns for 40Ar∗ and He, including those for the crushed sample, could be reproduced. The diffusion parameters inferred from the modeling of olivine (D0 = 2.4 × 10−2 cm2 s−1 and Ea = 133 kJ mol−1) are similar to those published by Shuster et al. (2003) and Blard et al. (2008). The high matrix/fluid solubility coefficient for helium, HHe ∼ 0.01, exceeds estimates reported by Trull and Kurz (1993); however, the product DHe(T) × HHe, the “permeability” (that governs He migration in vesicles + matrix composed materials), is very similar to their value. Extrapolation to the ambient temperature (0 °C) gives long and similar helium residence times in l- and h-vesicles, exceeding 1010 yrs, and even longer time scales ∼1016 yrs are obtained for the helium residence in the matrix. Therefore, at low temperatures our samples may be considered as excellent samplers of trapped volatile species, including helium.  相似文献   

9.
40Ar/39Ar step-heating data were collected from micron to submicron grain-sizes of correlative illite- and muscovite-rich Cambrian pelitic rocks from the western United States that range in metamorphic grade from the shallow diagenetic zone (zeolite facies) to the epizone (greenschist facies). With increasing metamorphic grade, maximum ages from 40Ar/39Ar release spectra decrease, as do total gas ages and retention ages. Previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation/recrystallization of neo-formed illite. While recognizing the importance of these processes in evaluating our results, we suggest that the inverse correlation between apparent age and metamorphic grade is controlled, primarily, by thermally activated volume diffusion, analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments. Our results suggest that complete resetting of the illite/muscovite Ar thermochronometer occurs between the high anchizone and epizone, or at roughly 300 °C. This empirical result is in agreement with previous calculations based on muscovite diffusion parameters, which indicate that muscovite grains with radii of 0.05–2 μm should have closure temperatures between 250 and 350 °C. At high anchizone conditions, we observe a reversal in the age/grain-size relationship (the finest grain-size produces the oldest apparent age), which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo-formed illite/muscovite crystallites typically surpass the size of detrital muscovite grains. It is also approximately the stage at which neo-formed illite/muscovite crystallites develop sufficient Ar retentivity to produce geologically meaningful 40Ar/39Ar ages. Results from our sampling transect of Cambrian strata establish a framework for interpreting illite/muscovite 40Ar/39Ar age spectra at different stages of low-grade metamorphism and also illuminate the transformation of illite to muscovite. At Frenchman Mtn., NV, where the Cambrian Bright Angel Formation is at zeolite facies conditions, illite/muscovite 40Ar/39Ar data suggest a detrital muscovite component with an apparent age ≥967 Ma. The correlative Carrara Fm. is at anchizone conditions in the Panamint and Resting Spring Ranges of eastern California, and in these locations, illite/muscovite 40Ar/39Ar data suggest an early Permian episode of subgreenschist facies metamorphism. The same type of data from equivalent strata at epizone conditions (greenschist facies) in the footwall of the Bullfrog/Fluorspar Canyon detachment in southern Nevada reveals a period of slow-to-moderate Late Cretaceous cooling.  相似文献   

10.
Rapid Pb-Pb dating of natural rutile crystals by laser ablation multiple-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) is investigated as a tool for constraining geological temperature-time histories. LA-MC-ICPMS was used to analyse Pb isotopes in rutile from granulite-facies rocks from the Reynolds Range, Northern Territory, Australia. The resultant ages were compared with previous U-Pb zircon and monazite age determinations and new mica (muscovite, phlogopite, and biotite) Rb-Sr ages from the same metamorphic terrane. Rutile crystals ranging in size from 3.5 to 0.05 mm with ?20 ppm Pb were ablated with a 300-25 μm diameter laser beam. Crystals larger than 0.5 mm yielded sufficiently precise 206Pb/204Pb and 207Pb/204Pb ratios to correct for the presence of common Pb, and individual rutile crystals often exhibited sufficient Pb isotopic heterogeneity to allow isochron calculations to be performed on replicate analyses of a single crystal. The mean of 12 isochron ages is 1544 ± 8 Ma (2 SD), with isochron ages for single crystals having uncertainties as low as ±1.3 Myr (2 SD). The 207Pb-206Pb ages calculated without correction for common Pb are typically <0.5% higher than the common-Pb-corrected isochron ages reflecting the very minor amounts of common Pb present in the rutile. The LA-MC-ICPMS method described samples only the outer 0.1-0.2 mm of the rutile crystals, resulting in a grain size-independent apparent closure temperature (Tc) for Pb diffusion in rutile that is less than the Tc of monazite ?0.1 mm in diameter, but significantly higher than the Rb-Sr system in muscovite (550 °C), phlogopite (435 °C) and biotite (400 °C). Even small rutile crystals are extremely resistant to isotopic resetting. For the established slow cooling rate of ca. 3 °C/Myr, the Tc for Pb diffusion in the analysed rutile is ca. 630 °C. This is in excellent agreement with recent experimental results that indicate that rutile has a higher Tc than previously thought (ca. 600-640 °C for rutile 0.1-0.2 mm diameter cooled at 3 °C/Myr; near 600 °C [Cherniak D.J., 2000. Pb diffusion in rutile. Contrib. Mineral. Petrol. 139, 198-207], versus 400 °C [Mezger, K., Hanson G.N., Bohlen S.R., 1989a. High precision U-Pb ages of metamorphic rutile: applications to the cooling history of high-grade terranes. Earth Planet. Sci. Lett. 96, 106-118.] for 1 °C/Myr), and with current Tc estimates for monazite and other high temperature geochronometers, which have been revised upwards in recent years. The new rutile ages, together with the other geochronological data from the region, support the interpretation that the Reynolds Range underwent prolonged slow cooling on a conductive geotherm, under nearly steady-state conditions. Slow cooling at ca. 3 °C/Myr persisted for at least 40 Myr followed the peak of high-T/low-P metamorphism to granulite-facies conditions, and probably continued at ca. 2-3 °C/Myr for ca. 200 Myr overall.  相似文献   

11.
In the last decade the zircon (U-Th)/He (ZHe) thermochronometer has been applied to a variety of geologic problems. Although bulk diffusion coefficients for He in zircon are available from laboratory step-heating experiments, little is known about the diffusion mechanism(s) and their dependence on the crystallographic structure of zircon. Here, we investigate the diffusion of He in perfectly crystalline zircon using atomistic simulation methods that provide insights into the structural pathways of He migration in zircon. Empirical force fields and quantum-mechanical calculations reveal that the energy barriers for He diffusion are strongly dependent on structure. The most favorable pathway for He diffusion is the [0 0 1] direction through the open channels parallel to the c-axis (, activation energy for tracer diffusion of a He atom along [0 0 1]). In contrast, energy barriers are higher in other directions where narrower channels for He diffusion are identified, such as [1 0 0], [1 0 1], and [1 1 0] (ΔE∗ of 44.8, 101.7, and 421.3 kJ mol−1, respectively). Molecular dynamics simulations are in agreement with these results and provide additional insight in the diffusion mechanisms along different crystallographic directions, as well as the temperature dependence. Below the closure temperature of He in zircon [Tc ∼ 180 °C, Reiners P. W., Spell T. L., Nicolescu S., and Zanetti K. A. (2004) Zircon (U-Th)/He thermochronometry: He diffusion and comparisons with Ar-40/Ar-39 dating. Geochim. Cosmochim. Acta68, 1857-1887], diffusion is anisotropic as He moves preferentially along the [0 0 1] direction, and calculated tracer diffusivities along the two most favorable directions differ by approximately five orders of magnitude (D[001]/D[100] ∼ 105, at T = 25 °C). Above this temperature, He atoms start to hop between adjacent [0 0 1] channels, along [1 0 0] and [0 1 0] directions (perpendicular to the c-axis). The diffusion along [1 0 0] and [0 1 0] is thermally activated, such that at higher temperatures, He diffusion in zircon becomes nearly isotropic (D[001]/D[100] ∼ 10, at T = 580 °C). These results suggest that the anisotropic nature of He diffusion at temperatures near the closure temperature should be considered in future diffusivity experiments. Furthermore, care should be taken when making geologic interpretations (e.g., exhumation rates, timing of cooling, etc.) from this thermochronometer until the effects of anisotropic diffusion on bulk ages and closure temperature estimates are better quantified.  相似文献   

12.
The 40Ar/39Ar dating technique requires the activation of 39Ar via neutron irradiation. The energy produced by the reaction is transferred to the daughter atom as kinetic energy and triggers its displacement, known as the recoil effect. Significant amounts of 39Ar and 37Ar can be lost from minerals leading to spurious ages and biased age spectra. Through two experiments, we present direct measurement of the recoil-induced 39Ar and 37Ar losses on Fish Canyon sanidine and plagioclase. We use multi-grain populations with discrete sizes ranging from 210 to <5 μm. One population consists of a mixture between sanidine and plagioclase, and the other includes pure sanidine.We show that 39Ar loss (depletion factor) for sanidine is ∼3% for the smallest fraction. Age spectra of fractions smaller than ∼50 μm show slight departure from flat plateau-age spectrum usually observed for large sanidine. This departure is roughly proportional to the size of the grain but does not show typical 39Ar loss age spectra. The calculated thickness of the total depletion layer d0(sanidine) is 0.035 ± 0.012 (2σ). This is equivalent to a mean depth of the partial depletion layer (x0) of 0.070 ± 0.024 μm. The latter value is indistinguishable from previous values of ∼0.07-0.09 μm obtained by argon implantation experiments and simulation results.We show that it is possible to adequately correct ages from 39Ar ejection loss provided that the d0-value and the size range of the minerals are sufficiently constrained. As exemplified by similar calculations performed on results obtained in a similar study of GA1550 biotite [Paine J. H., Nomade S., and Renne P. R. (2006) Quantification of 39Ar recoil ejection from GA1550 biotite during neutron irradiation as a function of grain dimensions. Geochim. Cosmochim. Acta70, 1507-1517.], the d0(biotite) is 0.46 ± 0.06 μm. The significant difference between empirical results on biotite and sanidine, along with different simulation results, suggests that for biotite, crystal structures and lattice defects of the stopping medium and possibly subsequent thermal degassing (due to ∼150-200 °C temperature in the reactor or extraction line bake out) must play an important role in 39Ar loss.The second experiment suggests that 37Ar recoil can substantially affect the age via the interference corrections with results that suggest up to ∼98% of 37Ar can be ejected from the ∼5 μm grain dimension.Further investigation of silicates of various compositions and structures are required to better understand (and correct) the recoil and recoil-induced effects on both 39Ar and 37Ar and their influences on 40Ar/39Ar dating.  相似文献   

13.
Thirty-five illite and muscovite concentrates were extracted from Triassic and Permian claystones, shales, slates and phyllites along a cross-section from the diagenetic Alpine foreland (Tabular Jura and borehole samples beneath the Molasse Basin) to the anchi- and epimetamorphic Helvetic Zone of the Central Alps. Concentrates and thin sections were investigated by microscopic, X-ray, infrared, Mössbauer, thermal (DTA and TG), wet chemical, electron microprobe, K-Ar, Rb-Sr, 40Ar/39Ar and stable isotope methods.With increasing metamorphic grade based on illite crystallinity data (XRD and IR) the following continuous changes are observed: (i) the 1Md2M1 polymorph transformation is completed in the higher grade anchizone; (ii) K2O increases from 6–8 wt. % (diagenetic zone) to 8.5–10% (anchizone) to 10–11.5% (epizone), reflecting an increase in the total negative layer charge from 1.2 to 2.0; (iii) a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone/ epizone boundary; iv) a shift of an endothermic peak in differential thermal curves from 500 to 750° C; (v) K-Ar and Rb-Sr apparent ages of the fraction <2 m decrease from the diagenetic zone to the epizone, K-Ar ages being generally lower than Rb-Sr ages. The critical temperature for total Ar resetting is estimated to be 260±30° C. K-Ar and Rb-Sr ages become concordant when the anchizone/ epizone boundary is approached. The stable isotope data, on the other hand, show no change with metamorphic grade but are dependent on stratigraphic age.These results suggest that the prograde evolution from 1 Md illite to 2M1 muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals, however this is not a recrystallization process. This restructuration is completed approximately at the anchizone/epizone boundary. The isotopic data indicate significant diffusive loss of 40Ar and 87Sr prior to any observable lattice reorganization. The restructuration progressively introduces a consistent repartition of Ar and K in the mineral lattices and is outlined by the 40Ar/39Ar age spectra.Concordant K-Ar and Rb-Sr ages of around 35-30 Ma. with concomitant concordant 40Ar/39Ar release spectra are representative for the main phase of Alpine metamorphism (Calanda phase) in the Glarus Alps. A second age group between 25 and 20 Ma. can probably be attributed to movements along the Glarus thrust (Ruchi phase), while values down to 9 Ma., in regions with higher metamorphic conditions, suggest thermal conditions persisting at least until the middle Tortonian.  相似文献   

14.
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15.
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 m. Eight experiments on sample 118576 were performed under the same conditions using measured grain radii of 145, 77, and 25 m. 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 m, 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 m 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 m and 34 m 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.  相似文献   

16.
To document the plume-ridge interaction at the Galapagos Spreading Center (GSC), we present neon 3-isotope analyses performed on vesicles-trapped volatiles of MORB glasses dredged along the ridge axis between 86°W and 98°W. 4He-40Ar∗-CO2 relative abundances were also measured in order to study gas loss in this context and discern between source and degassing effects. Neon isotopic compositions are in the MORB range with 21Ne/22Ne ratios extrapolated to the 20Ne/22Ne mantle ratio of 12.5 varying between 0.053 and 0.072. Unradiogenic plume-like compositions were not measured. The 4He-40Ar∗-CO2 relative abundances are highly variable along the ridge, for example 4He/40Ar∗ ratio varies between 3 and 433, but these variations can be fully explained by a simple model of Rayleigh distillation, with a single volatile source composition for the entire GSC. Magma fractional crystallisation, which increases in the plume influenced zone, seems to be the main motor for degassing. As other geochemical and geophysical studies indicate a significant plume influence on the GSC, these results suggest the plume component feeding the ridge is either degassed or else different from the plume core.  相似文献   

17.
The release of irradiation-produced noble gas isotopes (38ArCl, 80KrBr, 128XeI and 39ArK) during in vacuo crushing scapolite has been investigated and is compared to quartz. Three thousand crushing strokes released 98% of fluid inclusion-hosted noble gas from quartz. In comparison, 3000 crushing strokes released only 4% of the lattice-hosted 38ArCl from a scapolite gem. In vacuo crushing released lattice Ar preferentially relative to lattice Kr or Xe and prolonged crushing released 88% of the lattice-hosted noble gas in 96,000 crushing strokes. We suggest fast diffusion pathways generated by crushing are an important noble gas release mechanism and we demonstrate two applications of prolonged in vacuo crushing on irradiated scapolite.Firstly, scapolite molar Br/Cl and I/Cl values are shown to vary over a similar range as crustal fluids. The Cl-rich scapolite gem from Hunza, Pakistan has Br/Cl of 0.5–0.6 × 10−3 and I/Cl values of 0.3–2 × 10−6, that are similar to fluids that have dissolved evaporites. In contrast, three out of four skarn-related scapolites from the Canadian Grenville Province have molar Br/Cl values of 1.5–2.4 × 10−3, and I/Cl values of 11–24 × 10−6, that are broadly consistent with skarn formation by magmatic fluids. The fourth Grenvillian scapolite, with only 0.02 wt% Cl, has an exceptionally elevated molar Br/Cl value of up to 54 × 10−3 and I/Cl of 284 × 10−6. It is unclear if these values reflect the composition of fluids formed during metamorphism or preferential incorporation of Br and I in Cl-poor meionitic scapolite.Secondly, the Grenvillian scapolites give plateau ages of between 830 Ma and 400 Ma. The oldest ages post-date regional skarn formation by 200 Myr, but are similar to feldspar cooling ages in the Province. The age variation in these samples is attributed to a combination of factors including variable thermal history and the presence of mineral sub-grains in some of the samples. These sub-grains control the release of 39ArK, 38ArCl and 40Ar* during in vacuo crushing as well as the samples 40Ar* retentivity in nature. Scapolite is suggested as a possible analogue for K-feldspar in thermochronologic studies.  相似文献   

18.
The volatization of Rhenium (Re) from melts of natural basalt, dacite and a synthetic composition in the CaO-MgO-Al2O3-SiO2 system has been investigated at 0.1 MPa and 1250-1350 °C over a range of fO2 conditions from log fO2 = −10 to −0.68. Experiments were conducted using open top Pt crucibles doped with Re and Yb. Analysis of quenched glasses by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) normal to the melt/gas interface showed concentration profiles for Re, to which a semi-infinite one-dimensional diffusion model could be applied to extract diffusion coefficients (D). The results show Re diffusivity in basalt at 1300 °C in air is log DRe = −7.2 ± 0.3 cm2/s and increases to log DRe = −6.6 ± 0.3 cm2/s when trace amounts of Cl were added to the starting material. At fO2 conditions below the nickel-nickel oxide (NNO) buffer Re diffusivity decreases to and to in dacitic melt. In the CMAS composition, . The diffusivity of Re is comparable to Ar and CO2 in basalt at 500 MPa favoring its release as a volatile. Our results support the contention that subaerial degassing is the cause of lower Re concentrations in arc-type and ocean island basalts compared to mid-ocean ridge basalts.  相似文献   

19.
Highly turbid alkali feldspars from the Loch Ainort granite (59 Ma), Isle of Skye, have been analysed using the 40Ar-39Ar method to obtain chronological and chemical (K, Cl, Br, I) information concerning their origin and hydrothermal alteration. Three methods of gas extraction have been applied to neutron-irradiated samples: laser probe spot fusion of feldspars, in vacuo crushing of a feldspar/quartz separate, and laser stepped heating of the crushed residue. Apparent ages obtained by laser probe spot fusion are mostly similar to the 59 Ma intrusion age. Analyses of relatively pristine regions give some high apparent ages (>59 Ma) indicating the presence of small amounts of 40ArE (excess 40Ar). Crushing releases significant amounts of 40ArE, but is dominated by an 40ArA (atmospheric 40Ar) component. 84Kr/36Ar values obtained by crushing are higher than air and are consistent with air equilibration with fresh water at low temperature 20°C). Therefore, 40ArA was most probably introduced as palaeoatmospheric argon dissolved in the circulating hydrothermal fluids that interacted with the granite, thus supporting a meteoric origin for the fluids. Stepped heating gives a flat age spectrum and an age of 56±4 Ma. Crushing and stepped heating both released significant amounts of halogens with high Br/Cl and I/Cl ratios; excess Xe is also present as indicated by the high 132Xe/36Ar values. It seems likely that the halogen (and possibly Xe) enrichments resulted from interaction of the meteoric fluids with Jurassic sedimentary country rocks.  相似文献   

20.
The 40Ar/39Ar dating technique is based on the knowledge of the age of neutron fluence monitors (standards). Recent investigations have improved the accuracy and precision of the ages of most of the Phanerozoic-aged standards (e.g. Fish Canyon Tuff sanidine (FCs), Alder Creek sanidine, GA1550 biotite and LP-6 biotite); however, no specific study has been undertaken on the older standards (i.e. Hb3gr hornblende and NL-25 hornblende) generally used to date Precambrian, high Ca/K, and/or meteoritic rocks.In this study, we show that Hb3gr hornblende is relatively homogenous in age, composition (Ca/K) and atmospheric contamination at the single grain level. The mean standard deviation of the 40Ar?/39ArK (F-value) derived from this study is 0.49%, comparable to the most homogeneous standards. The intercalibration factor (which allows direct comparison between standards) between Hb3gr and FCs is RFCsHb3gr = 51.945 ± 0.167. Using an age of 28.02 Ma for FCs, the age of Hb3gr derived from the R-value is 1073.6 ± 5.3 Ma (1σ; internal error only) and ± 8.8 Ma (including all sources of error). This age is indistinguishable within uncertainty from the K/Ar age previously reported at 1072 ± 11 Ma [Turner G., Huneke, J.C., Podosek, F.A., Wasserburg, G.J., 1971. 40Ar-39Ar ages and cosmic ray exposure ages of Apollo 14 samples. Earth Planet. Sci. Lett. 12, 19-35].The R-value determined in this study can also be used to intercalibrate FCs if we consider the K/Ar date of 1072 Ma as a reference age for Hb3gr. We derive an age of 27.95 ± 0.19 Ma (1σ; internal error only) for FCs which is in agreement with the previous determinations. Altogether, this shows that Hb3gr is a suitable standard for 40Ar/39Ar geochronology.  相似文献   

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