Ar Diffusion Research Articles

Paleoproterozoic Age Relationships in the Three Bluffs Archean Iron Formation-Hosted Gold Deposit, Committee Bay Greenstone Belt, Nunavut, Canada

The Three Bluffs gold deposit is located in the Committee Bay greenstone belt, which forms part of the Rae domain of the western Churchill province, Nunavut, Canada. Gold mineralization is hosted by iron formation of the Neoarchean volcanosedimentary Prince Albert Group, and is associated with silicification (quartz veining) and sulfidation of magnetite and other Fe-rich minerals. Conventional U-Pb zircon dating of a conformable dacite unit within the volcanosedimentary host-rock sequence and a crosscutting diorite intrusion confirm a ~2.7 Ga age for deposition of the supracrustal package. U-Pb monazite dates and Pb isotopic analyses of sulfides were obtained by laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS). A subset of U-Pb monazite (1813.8 ± 8.7 Ma) and Re-Os arsenopyrite (1822 ± 21 Ma) dates, combined with a Pb-Pb secondary errorchron age for pyrite and arsenopyrite (1829 ± 77 Ma), suggest that gold mineralization associated with sulfidation of the iron formation occurred at ~1815 Ma, prior to high-grade (upper amphibolite facies) tectonometamorphism in the Three Bluffs area (D 2TB /(M 2TB ). An ~1815 Ma age for deposit formation is broadly consistent with evidence from elsewhere in the western Churchill province and to the southwest in Manitoba and Saskatchewan for a late Trans-Hudson (1.9–1.8 Ga) gold mineralizing event. The majority of U-Pb monazite ages form a second population at 1780.6 ± 4.2 Ma, similar to the age of the majority of Re-Os arsenopyrite analyses (1763 ± 11 Ma). These dates are thought to reflect the timing of peak M 2TB metamorphism. 40 Ar/ 39 Ar dating of amphibole, biotite, and muscovite yielded plateau ages ranging from 1723.8 ± 9.0 Ma to 1710 ± 17 Ma, which are interpreted to record the timing of postpeak metamorphic cooling to below the respective closure temperatures for Ar diffusion in these minerals.

Timing of brittle deformation within the Nojima fault zone, Japan

K–Ar ages from clay fault gouges of outcrop and core samples (UG500 well) from the Nojima fault zone, Awaji Island, Japan are consistent internally with established field constraints, and with apatite and zircon fission track ages, demonstrating the suitability of this method for dating of brittle deformation. Six unspiked and thirty conventional K–Ar data of clay minerals separated from gouge zones were analysed. Characterized by protoliths of high-grade metamorphic and magmatic rocks, the samples from the Nojima fault zone offer the opportunity to distinct identify newly grown illite in the fault planes. All dated sample fractions were characterized by XRD, SEM and TEM. K–Ar ages of 29 separates have an age range from 63.4±1.3Ma (Early Palaeocene–Danian) to 42.2±1.0 (Palaeogene–Middle Eocene–Lutetian). Some <0.1 and <0.4μm fractions of samples in close proximity to a pseudotachylyte zone are thermally influenced and indicate loss of radiogenic Ar. The K–Ar data from illite support a model that the Nojima fault zone was initiated ∼55Ma ago based on ZFTA data. The data confirm elevated temperatures and a heterogeneous thermal history within the study area. The <2μm ages of the outcropping samples document the age of gouge formation, whereas the <0.1 and <0.4μm illite fractions suggest the influence of a secondary thermal heating event probably caused by circulation of hot fluids within the fault zone about 31–38Ma ago and even a potential influence of Quaternary faulting. Ar diffusion models support elevated temperatures for outcrop samples collected near the Hirabayashi site. The thermal anomalies documented in the outcrop samples could not be observed in the UG500 core samples supporting further a heterogeneous thermal history model within the Nojima fault zone.

Ar, CO 2 and H 2O diffusion in silica glasses at 2 kbar pressure

The diffusion of volatiles into silica glasses was studied by sorption experiments in argon gas pressure vessels at 2 kbar and temperatures from 521 to 1097 °C. In the experiments either Ar and H 2O (AH series, samples in open containers), or CO 2 and H 2O (CH series, samples in closed noble metal capsules with silver oxalate added as source for CO 2) could exchange between the fluid and the glasses. Both sample assemblies were run simultaneously to enable direct comparison of diffusion data. Profiles were recorded on sections perpendicular to the initial surfaces by electron microprobe (Ar) and FTIR spectroscopy (CO 2 and H 2O). Ar diffusion is systematically slower by 0.17 log units in average in water-rich silica glass (∼ 0.05 wt.% H 2O, Suprasil ®) than in water-poor silica glasses (< 0.01 wt.% H 2O, Infrasil ® and Herasil ®) while the activation energy for argon diffusion agrees within error. This effect is attributed to partial closure of doorways for argon in the silicate network by the formation of hydroxyl groups. CO 2 is dissolved only as molecular species in silica glasses and its diffusivity is about the same as that of argon, similar as found for rhyolitic glasses. Dissolved H 2O was detected in the run products exclusively in form of silanol groups. The H 2O profiles show systematic deviations from error function shape. The shape of the profiles could originate from either a positive correlation of water diffusivity to water content or a continuous decrease in water fugacity in the fluid. H 2O was only a minor component in the fluids and the condition of an infinite reservoir was not necessarily always ensured during the whole run. Assuming negligible decrease of water fugacity during experiment, diffusion coefficients of H 2O ( D H2O) were determined by a Boltzmann–Matano analysis. The data are consistent with D H2O being proportional to the concentration of dissolved water.

Argon diffusion in plagioclase and implications for thermochronometry: A case study from the Bushveld Complex, South Africa

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( D 0/ a 2) 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.

Using argon as a temporal tracer of large-scale geologic processes

Argon (Ar) has proven to be one of the most useful elements in understanding the timing and duration of geological processes. Because of its chemical and physical properties, Ar is a particularly sensitive indicator of temperature variations associated with large-scale geological events. Ar mobility in minerals is commonly described using solid-state diffusion theory (volume diffusion), which has been used extensively to model diffusion profiles and closure temperatures in minerals. Because 40Ar is the radiogenic daughter product of radioactive 40K and therefore occurs in many common rock-forming minerals, Ar has been used effectively as a temporal tracer to understand a wide variety of geological phenomena. Ar isotopes have been applied with great success in three broad fields of earth science—the origin and evolution of the earth's atmosphere, the tectonothermal history of geologic terranes, and the timing and duration of large-scale tectonic events. From Ar isotopic data, the earth's atmosphere is now known to be mainly secondary in nature, although evolutionary models have undergone a recent paradigm shift due to geophysical evidence, moving away from the earlier notion of a chemically layered upper and lower mantle to a whole-earth mantle structure that may be chemically inhomogeneous. Although recent models are growing in sophistication by integrating additional information and measurements reflecting the behaviour of other noble gases through time, the temporal evolution of the earth's atmosphere still remains ambiguous because measured compositions of ancient atmosphere are sparse and remain difficult to obtain. 40Ar/ 39Ar thermochronology is one of the most commonly used tools in tectonothermal investigations because of the large range of Ar closure temperatures recorded by various common K-bearing minerals; although more recent thermochronological models, such as multidiffusion domain (MDD) modelling, remain controversial, the combination of 40Ar/ 39Ar thermochronology with both higher and lower temperature thermochronological methods utilizing other isotopic systems makes this an extremely versatile method of understanding the temperature–time evolution of geologic regions. Recent developments involving the numerical modelling of Ar diffusion coupled with the geodynamic modelling of orogens have led to new ways of estimating the timing and duration of a complete tectonic cycle of subduction and exhumation; by applying these new methods to rocks from the Bergen Arcs in Norway, studies have shown that this cycle can be so rapid that the subducted and exhumed crust can remain relatively cool, never achieving thermal equilibrium with its ambient environment throughout the entire process. In summary, the versatility of the 40Ar/ 39Ar dating method combined with the effective application of various techniques (e.g. laser spot-dating and step-heating) and a detailed understanding of the diffusional behaviour of Ar has proven to be a powerful research tool in elucidating the timing and duration of many large-scale earth-system processes.

Ar, CCl4 and C6H6 adsorption outside and inside of the bundles of multi-walled carbon nanotubes—simulation study

This is the first paper reporting the results of systematic study of the adsorption of Ar, C(6)H(6) and CCl(4) on the bundles of closed and opened multi-walled carbon nanotubes. Using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, we also study the effect of the introducing defects in the external and internal walls of osculating and separated nanotubes on Ar diffusion and on adsorption of all three adsorbates. The Ar diffusion coefficients obtained are very sensitive to the presence of defects. Simulated isotherms are discussed to show the relation between the shapes of the high resolution alpha(s)-plots and the mechanisms of adsorption. From obtained data, as well as from geometric considerations, from the VEGA ZZ package, and from simulations (ASA), the values of surface areas of all nanotubes are calculated and compared with those obtained using the most popular adsorption methods (BET, alpha(s) and the A,B,C-points). We show that the adsorption value for the C-point of the isotherm should be taken for the calculation of the specific surface area of carbon nanotubes to obtain a value which approaches the absolute geometric surface area. A fully packed monolayer is not created at the A-, B- or C-points of the isotherm; however, the number of molecules adsorbed at the latter point is closest to the number of molecules in the monolayer as calculated via the ASA method, the VEGA ZZ package or from geometric considerations.

Diffusion of 40Ar in muscovite

Hydrothermal treatment of closely sized muscovite aggregates in a piston-cylinder apparatus induced 40Ar ∗ loss that is revealed in 40Ar/ 39Ar step heating spectra. Age spectra and Arrhenius data, however, differ from that expected from a single diffusion length scale. A numerical model of episodic loss assuming the presence of multiple diffusion domains yields excellent fits between synthetic and actual degassing spectra. We used this model to isolate 40Ar ∗ loss from the grains that remained intact during hydrothermal treatment at 10 kbar permitting calculation of diffusion coefficients in the temperature range 730-600 °C. Diffusion data generated in this manner yield an activation energy ( E) of 63 ± 7 kcal/mol and frequency factor ( D o) of 2.3 + 70 - 2.2 cm 2/s. Experiments at 20 kbar yield diffusivities lower by about an order of magnitude and correspond to an activation volume of ∼14 cm 3/mol. Together, these parameters predict substantially greater retentivity of Ar in muscovite than previously assumed and correspond to a closure temperature ( T c) of 425 °C for a 100 μm radius grain cooling at 10 °C/Ma at 10 kbar ( T c = 405 °C at 5 kbar. Age and log ( r/ r o) spectra for the run products show strong correlations indicating that muscovites can retain Ar diffusion boundaries and mechanisms that define their natural retentivity during vacuum step heating. This may permit the application of high resolution, continuous 40Ar/ 39Ar thermochronology to low grade, regionally metamorphosed terranes.

Dhofar 378 Martian shergottite: Evidence of early shock melting

Shock heating of the Dhofar 378 (Dho 378) Martian shergottite produced melting, vesiculation, and flow of the plagioclase, which upon cooling recrystallized into complex textures. Heating experiments on the similar Zagami shergottite indicate that Dho 378 was shock heated to 1000–1100°C and was cooled at ∼2.5°C/h. An 39Ar‐40Ar analysis of Dho 378 plagioclase indicates different Ar diffusion domains and K/Ca ratios. The lower‐temperature phase defines an Ar‐Ar isochron age of 141 ± 32 Ma. The higher‐temperature phase released more 40Ar but does not define an age. The meteorite's thermal history was examined by constructing a generic model to compare cooling rates for objects of different sizes against fractional diffusion loss of Ar for different cooling times. Using gas diffusion parameter values measured for Dho 378, this model indicates that it is improbable that the major shock heating event occurred at the time that Dho 378 was ejected from Mars ∼3 Ma ago. Rather, we suggest that the time of shock heating is probably given by its Ar‐Ar age. For Dho 378 to cool sufficiently fast not to lose most of its 40Ar ∼3 Ma ago would require it to have been ejected into space as an impossibly small object. Larger and more reasonable Mars ejection sizes indicate that Dho 378 should have lost most of its 40Ar. On the basis of plagioclase texture and Ar data, we suggest that a major impact event ∼141 Ma ago melted Dho 378 plagioclase, degassed most of its 40Ar, and deposited it in crater ejecta to cool. A smaller and later impact ejected it into space ∼3 Ma ago.

39Ar– 40Ar age and thermal history of martian dunite NWA 2737

We report an 39Ar– 40Ar age determination of a whole rock sample of the olivine-rich, martian meteorite Northwest Africa (NWA) 2737. Those extractions releasing 0–48% of the 39Ar define an 39Ar– 40Ar isochron age of 160–190 Ma, when evaluated in various ways. Higher temperature extractions show increasing ages that eventually exceed the reported Sm–Nd age of 1.42 Ga. At least part of this excess 40Ar may have been shock implanted from the martian atmosphere. We considered two possible interpretations of the Ar–Ar isochron age, utilizing the measured Ar diffusion characteristics of NWA 2737 and a thermal model, which relates Ar diffusion to the size of a cooling object after shock heating. One interpretation, that 40Ar was only partially degassed by an impact event ~ 11 Ma ago (the CRE age), appears possible only if NWA 2737 was shock-heated to temperatures > 600 °C and was ejected from Mars as an object a few 10 s of cm in diameter. The second interpretation, which we prefer, is that NWA experienced an earlier, more intense shock event, which left it residing in a warm ejecta layer, and a less intense event ~ 11 Ma ago, which ejected it into space. Our evaluation would require NWA 2737 to have been heated by this first event to a temperature of ~ 300–500 °C and buried in ejecta to a depth of ~ 1–20 m. These conclusions are compared to model constraints on meteorite ejection from Mars reported in the literature. The second, Mars-ejection impact ~ 11 Ma ago probably heated NWA 2737 to no more than ~ 400 °C. NWA 2737 demonstrates that some martian meteorites probably experienced shock heating in events that did not eject them into space.

39Ar‐40Ar dating of the Zagami Martian shergottite and implications for magma origin of excess 40Ar

Abstract— The Zagami shergottite experienced a complex, petrogenetic formation history (McCoy et al. 1992, 1999). Like several shergottites, Zagami contains excess 40Ar relative to its formation age. To understand the origin of this excess 40Ar, we made 39Ar‐40Ar analyses on plagioclase and pyroxene minerals from two phases representing different stages in the magma evolution. Surprisingly, all these separates show similar concentrations of excess 40Ar, ˜1 × 10−6 cm3/g. We present arguments against this excess 40Ar having been introduced from the Martian atmosphere as impact glass. We also present evidence against excess 40Ar being a partially degassed residue from a basalt that actually formed ˜4 Gyr ago. We utilize our experimental data on Ar diffusion in Zagami and evidence that it was shock‐heated to only ˜70 °C, and we assume this heating occurred during an ejection from Mars ˜3 Myr ago. With these constraints, thermal considerations necessitates either that its ejected mass was impossibly large, or that its shock‐heating temperature was an order of magnitude higher than that measured. We suggest that this excess 40Ar was inherited from the Zagami magma, and that it was introduced into the magma either by degassing of a larger volume of material or by early assimilation of old, K‐rich crustal material. Similar concentrations of excess 40Ar in the analyzed separates imply that this magma maintained a relatively constant 40Ar concentration throughout its crystallization. This likely occurred through volatile degassing as the magma rose toward the surface and lithostatic pressure was released. These concepts have implications for excess 40Ar in other shergottites.

Measurement of the Ar diffusion coefficient in graphite at high temperature by the ISOL method

This work has been carried out at GANIL within the ambit of the TARGISOL European collaboration which aims to study the relevant variables governing the release of radioactive elements from targets in an ISOL system. This work shows how it has been possible to extract diffusion coefficients for 35Ar atoms diffusing out of graphite targets from release time measurements by using an analytic description of the release times. The diffusion coefficients and efficiencies are presented and compared with results obtained using a “continuous” method.

Ar–Ar dating of authigenic K-feldspar: Quantitative modelling of radiogenic argon-loss through subgrain boundary networks

We have analysed two distinct generations of authigenic K-feldspar in Fucoid Bed sandstones from An-t-Sron and Skiag Bridge, NW Highlands, Scotland, which have experienced post-growth heating to levels in excess of the predicted Ar-closure temperature. Authigenic K-feldspars show microtextural similarities to patch perthites; that is subgrains separated by dislocation-rich boundary networks that potentially act as fast diffusion pathways for radiogenic argon. The two generations of authigenic K-feldspar in the Fucoid Bed sandstones can be distinguished by different microtextural zones, bulk mineral compositions, fluid-inclusion populations, and inferred temperatures and chemistries of parent fluids. Ar–Ar age data obtained using high-resolution ultraviolet laser ablation, show that the first cementing generation is Ordovician and the second cementing generation is Silurian. Modelling of Ar diffusion using subgrain size as the effective diffusion dimension and a simplified tectono–thermal thrust model assuming transient heating of the Fucoid Beds is inconsistent with observed data. Removal of heat from the thrust zone through rapid flushing of heated fluids rather than transient heating can be invoked to explain the observed Ar–Ar ages for both generations of cement. Alternatively, Ar-diffusion modelling using overgrowth thickness as the effective diffusion dimension instead of subgrain size also yields models that are consistent with both the Fucoid Bed palaeothermal maxima and determined Ar–Ar age ages for the two generations of K-feldspar cement. Based on this alternate explanation, we propose a theoretical microtextural model that highlights fundamental differences between the microtextures of deuterically formed patch perthites and authigenic K-feldspars, explaining the apparent robustness of authigenic K-feldspar with respect to Ar-retention.

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

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

40Ar– 39Ar dating of plagioclase grain size separates from silicate inclusions in IAB iron meteorites and implications for the thermochronological evolution of the IAB parent body

In order to better constrain the thermochronological evolution of the IAB parent body we performed a 40Ar/ 39Ar age study on individual silicate inclusions of the IAB irons Caddo County, Campo del Cielo, Landes, and Ocotillo. In contrast to earlier studies, several plagioclase separates of different grain sizes and quality grades were extracted from each inclusion to reduce the complexity of the age spectra and study the influence of these parameters on the Ar–Ar ages. In nearly all inclusions we found significantly different Ar–Ar ages among the separates (Caddo County: 4.472 ± 0.02–4.562 ± 0.02 Ga; Campo del Cielo 2: 4.362 ± 0.04–4.442 ± 0.03 Ga; Landes 2: 4.412 ± 0.05–4.522 ± 0.04 Ga; Ocotillo: 4.382 ± 0.04–4.462 ± 0.03 Ga). These ages were calculated using the new 40K decay constant published by [Mundil R., Renne P. R., Min K. and Ludwig K. R. (2006) Resolvable miscalibration of the 40Ar/ 39Ar geochronometer. Eos Trans. AGU 87, Fall Meet. Suppl., Abstract V21A-0543]. The ages did not systematically correlate with the respective grain size of the separate as expected, i.e., smaller grains did not necessarily show younger ages due to later closure to Ar diffusion or easier re-opening of the system in the course of a reheating event compared to larger grains. Based on the large range of Ar–Ar ages we suggest that the individual inclusions are composed of silicate grains from different locations within the IAB parent body. While some grains remained in a hot (deep) environment that allowed Ar diffusion over an extended time period—in some cases combined with grain coarsening—, others cooled significantly earlier (near surface) through the K/Ar blocking temperature. These different grains where brought together during an impact followed by mixing and reassembly of the debris as proposed by Benedix et al. [Benedix G. K., McCoy T. J., Keil K. and Love S. G. (2000) A petrologic study of the IAB iron meteorites: constraints on the formation of the IAB-Winonaite parent body. Meteorit. Planet. Sci. 35, 1127–1141]. Due to rapid cooling after the impact some of the age differences among the grains could be preserved. Based mainly on our Caddo County Ar–Ar age information, the IAB parent body was destroyed by impact and reassembled between ∼4.5 and 4.47 Ga. However, IAB silicate Ar–Ar ages should depend much more on the pre- and post-impact cooling rate and burial depth than on the time of the actual impact. This is supported by a compilation of our and literature IAB and winonaite Ar–Ar ages ranging smoothly from the time of accretion of the chondritic IAB parent body down to the time of its final cooling through the K–Ar blocking temperature after impact and reassembly, instead of showing a peak in Ar–Ar ages at the time of the destructive impact.

High-temperature Mars-to-Earth transfer of meteorite ALH84001

Martian meteorites provide crucial insights into Martian evolution and interplanetary mass transfer, including the potential for exogenesis. ALH84001 is the oldest Martian meteorite discovered so far, and has been used to derive important conclusions about Martian surface temperatures and very low-temperature Mars-to-Earth transfer. To better constrain the thermal evolution and shock metamorphic history of ALH84001, we applied (U–Th)/He thermochronometry to single grains of phosphate (merrillite) from ALH84001. The (U–Th)/He ages of individual phosphate grains in ALH84001 range from 60 Ma to 1.8 Ga, with a weighted mean of ~830 Ma. This broad age distribution reflects multiple diffusion domains, and requires a relatively high-temperature resetting event younger than ∼ 60 Ma. These new data are combined with the published whole-rock (maskelynite as a main Ar reservoir) 40Ar/ 39Ar age spectra which show 5–8% fractional loss of radiogenic 40Ar since 4.0 Ga. He diffusion in both terrestrial and extraterrestrial apatite has a significantly higher activation energy (138 ∼ 184 kJ/mol) than Ar diffusion in maskelynite (75 kJ/mol), leading to an important “kinetic crossover” in fractional loss contours for these systems. Taken together, the phosphate (U–Th)/He and whole-rock 40Ar/ 39Ar ages require both very low surface temperatures on Mars, and one or more short-lived, high-temperature, shock events after 4.0 Ga. We suggest that the last shock event occurred with ejection of ALH84001 from Mars, and reached a peak temperature of approximately 400 °C. These results undermine the proposed low-temperature ejection hypothesis for ALH84001, but support long-lived extremely cold Martian surface temperatures.

Whole-rock Ar-Ar dating for low-grade metavolcanics within the Dabie orogen and its geological significance

The genetic relationship between lowgrade and ultrahigh-pressure (UHP) metamorphic units in the interior of the Dabie orogen has been controversial with respect to preservation of volcanic texture during continental subduction to mantle depths. In order to resolve this issue, whole-rock Ar-Ar dating was carried out for greenschist-facies metatuff that is in contact with UHP eclogite in Yuexi County, Anhui Province. One sample gave a plateau age of 784.4±2.0 Ma and an isochron age of 785.0±4.7 Ma, and the other sample a plateau age of 770.9±2.0 Ma and an isochron age of 769.5±3.1 Ma. It appears that the Ar isotopic system was not disturbed since the volcanic eruption at the middle Neoproterozoic. Because of the low closure temperature of Ar diffusion in volcanic rocks, retention of the Neoproterozoic ages indicates that the low-grade metavolcanics did not experience high-grade metamorphism up to eclogite-facies conditions during the Triassic subduction of continental crust. Furthermore, the Neoproterozoic Ar-Ar ages are also in agreement with protolith ages of UHP metaigneous rocks in the Dabie orogen. Therefore, the metatuff is interpreted as a tectonic relict that was scraped off during the Triassic subduction and thus was not subducted to mantle depths like the adjacent eclogite. This lends support to the model for crustal detachment between cover and basement during continental collision. A tectonic melange model is proposed to explain the occurrence of contrasting grades of metamorphic rock within the UHP metamorphic zone.

The Ulvö Gabbro Complex of the 1.27-1.25 Ga Central Scandinavian Dolerite Group (CSDG): Intrusive age, magmatic setting and metamorphic history

The Ulvö Gabbro Complex (UGC) belongs to the 1.27-1.25 Ga Central Scandinavian Dolerite Group (CSDG), which manifests a major event of intra-cratonic mafic magmatism possibly related to break-up between Laurentia and Baltica. Lopolithic, layered intrusions of the UGC outcrop in east central Sweden and west central Finland. Fresh baddeleyite grains in a pegmatoidal gabbro were dated by the TIMS U-Pb method. The age of 1256.2±1.1 Ma is interpreted to date crystallisation of the UGC, and is consistent with the age of other mafic intrusions of the CSDG in Västerbotten and south-western Finland. Previously reported K-Ar biotite data confirm that the UGC was not heated above the blocking temperature for Ar diffusion in biotite (∼250[ddot]C) after ∼1.2 Ga. This suggests that UGC was virtually unaffected by later geological events and that magmatic textures and mineralogy have not been modified after solidification.

Geochronology and isotope transport systematics in a subsurface granite from the Larderello–Travale geothermal system (Italy)

A monzogranite, here referred to as the Montieri pluton, was encountered along two depth profiles from ca. 2 to 4 km depth in wells of the Larderello–Travale geothermal system, Italy. We obtained high-resolution petrographic and microchemical data on seven fresh samples of the Montieri pluton. These data are required to correctly interpret the combined isotope data for the Rb–Sr and K–Ar systems. Biotite and K-feldspar from all samples were analyzed by Rb–Sr and 39Ar– 40Ar. Rb–Sr analyses gave extremely variable two-point Rb–Sr apparent ages and Sr initials, indicating that biotite did not equilibrate with feldspar. Bulk dissolution of K-feldspars does not define a Rb–Sr alignment that could have proved a common origin of all K-feldspar samples from a single magma batch. The leachable fractions of K-feldspar separates have much more homogeneous 87Sr/ 86Sr ratios around 0.712, which may reflect the isotopic composition of a late-stage circulating fluid. Because of the anatectic origin of the granites from the Larderello–Travale geothermal system, it is likely that every K-feldspar separate contains at least three isotopically distinct feldspar generations: relics of the Hercynian gneiss basement, Pliocene magmatic minerals, and hydrothermal retrogression products. Such heterogeneity can be confirmed and quantified by petrographical observations and electron microprobe analyses. The 39Ar– 40Ar age spectra of the biotite samples show some internal discordance. Because of deviations from stoichiometry, biotite discordance can be attributed both to chlorite intergrowths, as predicted from the active fluid circulation, and to the presence of multiple biotite generations, such as have already been documented from other micas from the Larderello–Travale geothermal system and as indeed confirmed by high-resolution petrography. Total K–Ar biotite ages on cluster around 3 Ma and broadly agree with a Rb–Sr age obtained by regressing only the biotite analyses. This suggests that the Montieri pluton, whose known volume is at least 25 km 3, was emplaced at that time. The K-feldspar spectra are strongly discordant; no step age approaches the zero-age which would be predicted by Ar diffusivity modelling. The extraneous Ar in the feldspars is not excess Ar, but inherited Ar instead. Ar inheritance in the feldspar separates correlates with inheritance of Sr. Arrhenius trajectories of the apparent Ar diffusivities for all seven feldspars are astonishingly identical, despite the heterochemism amongst samples. However, the apparent diffusivity calculated from the step-heating experiments led to irreproducible models for the thermal history along the depth profile, as well as time-scales shorter than the historical record. This indicates that Ar diffusivities calculated from laboratory experiments must not be extrapolated to geological conditions.

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

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

Estimates of Ar diffusion and solubility in leucite and nepheline: Electron microprobe imaging of Ar distribution in a mineral

Leucite is unique among minerals that have been exploited for K-Ar and Ar-Ar dating, in that it exhibits a major phase change at 645-665 degrees C, across which Ar diffusion and solubility can be studied. In addition, well-developed twinning occurs only in the low temperature form, offering further opportunity to examine the effects of crystallographic change upon Ar diffusion. Nepheline was studied to compare the effect of its I-dimensional c-axis oriented lattice channels with the 3-dimensional channels of leucite. The amounts of Ar introduced into leucite were far higher than those observed in feldspars, allowing its analysis by electron microprobe spot traverses and X-ray maps. This has provided the first high spatial resolution 2-dimensional study of Ar distribution in a K-bearing mineral. Extreme changes in Ar solubility were observed across the leucite phase transition, with Ar solubilities jumping from similar to 70 ppm (at 1 kbar) in the tetragonal (low-temperature) form to similar to 750 ppm (at I kbar) in the cubic (high-temperature) form. Argon penetration profiles were complex and estimates of the diffusion rates show that they are more rapid than in K-feldspar. In addition, spikes of high Ar concentration were observed in both forms, suggesting extended defects or micro-inclusions formed argon traps in the structure. No concentration changes could be correlated to leucite twin planes, suggesting that twin planes did not act as fast pathways for Ar movement. Nepheline yielded a much lower Ar solubility of 0.15-0.31 ppm at 1 kbar.