39Ar Release Research Articles

The in vacuo release of Ar from minerals: 2. The role of structural modifications of K-feldspar during heating revealed by Raman microprobe analyses

The release of Ar during stepwise heating 39Ar40Ar dating experiments may be controlled by Fick's Law diffusion in an inert matrix, and/or by structural modifications of the host mineral. A ca. 1 mm3 irradiated cleavage fragment of a low sanidine crystal from Itrongay, Madagascar was degassed isothermally at 888 ± 2 °C for 264 h, acquiring 67 stepwise 39Ar release data. The 39Ar release was observed to follow a smooth sigmoid curve and not a line as would be predicted by Fick's Law. If the 39Ar release is controlled by crystallographic changes, the implication is that these changes undergo time-dependent variations. The long-term bulk degassing, being the sum of various structural modifications, approximates a Fickian behavior that is not verified in detail in short-term experiments, as it averages over different 39Ar release regimes. This would make the downslope extrapolation of laboratory data to geological conditions highly underconstrained.In order to constrain the behavior of the crystal structure of K-feldspar during laboratory heating, we measured the Raman spectra of a different, ca. 1 mm3 cleavage fragment of the same irradiated sanidine crystal. The sample was heated in air in a Linkam heating stage, and Raman spectra were acquired at temperatures increasing from 300 to 1000 °C, including a 6-h isothermal run at 900 ± 1 °C.Raman modes between 50 and 1200 cm−1 were observed to record two kinds of change, defining robust trends. The positions of most peaks were shifted towards lower wavenumbers (lower energies) and broadened; in addition, the relative heights of different peaks showed robust variation trends. The larger changes coincide with discrete temperature increases, but all changes also progressed over time at constant temperature. These peaks mirror the excitation of phonon modes, which are associated with interatomic bond stretching and deformation, with Si,Al ordering and with deformation and rotation of SiO2 tetrahedra. Most, but by far not all, of the change is reversible (such as e.g. the differential activation of phonon modes), but irreversible structure modifications (such as e.g. Al,Si disordering) are also recorded. We conclude that the K-feldspar structure violates mathematical requirements of matrix inertness during laboratory heating.

Pathways for 39Ar loss during step-heating of alkali feldspar megacrysts from the Shap granite (UK): Combined evidence from diffusion experiments and characterisation of heating-induced texture modifications

This study aims to identify the dominant fast pathways for 39Ar loss during step-heating 40Ar/39Ar analysis of alkali feldspar from the Shap granite (UK). In our analysis we combined step-heating 40Ar/39Ar data acquired from variably sized feldspar fragments (0.4 to 2.2 mm in the shortest dimension) and optical and electron microscopy observations of microtextural modifications caused by heating in a muffle furnace in air. Our optical and electron microscopy results suggest that laboratory heating of the Shap feldspar causes it to fracture. The resulting cracks form an interconnected network, which may link with some of previously documented nanoscale defects such as nanotunnels, slots, pull aparts and bubbles. Our step-heating 40Ar/39Ar analyses yielded non-linear Arrhenius trajectories of 39Ar release. The non-linearity is best explained by heating-induced reduction of the effective diffusion length due to fracturing, likely with some contribution from nanotunnels, pull-aparts, slots and bubbles. Incoherent grain boundaries apparently played a minor role in providing fast pathways for 39Ar loss, if any. Importantly, unambiguous evidence for a reduction of the diffusion length during step-heating was only observed in experiments with large fragments (≥1 mm in the shortest dimension). The Arrhenius trajectory obtained from the smaller fragments (≤0.5 mm in the shortest dimension) appears to be consistent with the presence of variably-sized non-interacting intra-grain diffusion domains that existed prior to mineral separation. Heating-induced fracturing must be taken into account when interpreting 40Ar/39Ar dates of alkali feldspar, with particular focus on the following two points. First, this process is clearly inconsistent with the underlying assumptions of commonly utilised multi-diffusion domain (MDD) theory, although it can result in 39Ar release patterns that appear consistent with them. Therefore, it is important to identify on a case by case basis the dominant fast pathways for 39Ar loss from alkali feldspar during step-heating before applying MDD theory. Second, if heating-induced fracturing is ubiquitous in alkali feldspar and affects even gem-quality samples, then presently available diffusion parameters of Ar in alkali feldspar structure may be inaccurate to some degree, since all of them were obtained without considering the presence of cracks.

40Ar-39Ar step heating ages of North American tektites and of impact melt rock samples from the Chesapeake Bay impact structure

Abstract This study presents 40Ar-39Ar step heating ages of four North American tektites (three bediasites and one georgiaite) and of two groundmass samples extracted at different depths from clast-rich impact melt rocks (CB-W61 and CB-W84) recovered by the USGS-ICDP Eyreville B drill-core about 9 km from the centre of the Chesapeake Bay impact structure. Radiometric age determination on both North American tektites and impact melt rocks from within Chesapeake Bay crater offers the first possibility to confirm the origin of these tektites. For this aim, argon isotopic data from 13 samples/aliquots of tektite rims, cores and bulk, and 4 samples/aliquots from two impact melt rocks were obtained over 15 to 26 step heating extractions. Age spectra of all tektite samples show plateaux comprising 62–98% of the 39Ar release over consecutive intermediate and high temperature heating steps. Few low temperature extractions indicate excess 40Ar. Inverse isochron 40Ar/36Ar intercepts of tektite samples are indistinguishable from air (295.5). However, impact melt rock spectra presented complex Ar-release affecting primarily the low temperature heating-steps. Inverse isochrones indicate excess argon from which the 40Ar/36Ar intercept was used to correct the age calculation. CB-W61 and CB-W61-2 40Ar/36Ar intercepts are 354.5 ± 2.5 and 327.2 ± 6.3, respectively, and those for CB-W84 and CB-W84-2 are 332.0 ± 7.3 and 329.6 ± 5.6, respectively. The inverse isochron weighted mean age (according to currently suggested K-decay constants revisions by Schwarz et al. (2011) and Renne et al. (2011)) for all four tektites is 34.86 ± 0.25 Ma (MSWD = 0.96, P = 0.41; n = 4) and for the two impact melt rocks is 37.16 ± 3.65 Ma (MSWD = 0.83, P = 0.36). The combined tektite and impact melt rocks isochron mean age of 34.86 ± 0.23 (0.32) Ma (MSWD = 0.87, P = 0.43) is slightly – though not significantly – higher than the plateau mean age of 34.55 ± 0.27 (0.36) Ma (MSWD = 0.66, P = 0.62). Placing this age in the Global Stratotype Section and Point (GSSP) marine section exposed at Massignano, Italy, it falls below the Eocene/Oligocene (E/O) boundary overlapping with the 10.28 m Ir-anomaly. These results agree within errors with previously reported ages of 35.20 ± 0.54 Ma, especially those derived from K-Ar and Ar-Ar total fusion analysis. An age of 34.86 ± 0.32 Ma sets the Chesapeake Bay impact event close to the youngest of the three Ir anomalies at ∼35.0 Ma in the case the impactor was Ir-rich (e.g, a chondrite, primitive achondrite, stony-iron or iron meteorite). The concordance with the E/O boundary at ∼33.9 Ma seems only marginally possible, and only if the Ir contribution from the ejecta were, potentially, due either to its small amount becoming diluted in the geologic record or the impactor being Ir poor, e.g., of differentiated achondritic composition. This study also brings to front the need to re-establish the stratigraphic and palaeo-magnetic correlations across the globe for the Ir-anomalies and the magneto-stratigraphy during the mid- to late-Eocene and early-Oligocene, and the need to re-evaluate the markers for the Eocene-Oligocene boundary.

Fast cooling following a Late Triassic metamorphic and magmatic pulse: implications for the tectonic evolution of the Korean collision belt

We discuss the evolution of Korea in the context of a relatively short-lived, tectonically induced, magmatic and metamorphic pulse that affected large portions of the crust of the peninsula's southern part during the Late Triassic. Recent 40Ar/39Ar single grain laser step-heating dates imply a prolonged metamorphic recrystallization between 243 and 220Ma, which occurred in distinct phases that were not coeval throughout the peninsula. We obtained identical plateau ages between 231.4±0.8 and 228.9±0.8Ma (1σ; 85–95% 39Ar release) on single grains of detrital muscovite from Jurassic sandstones (Gimpo Group). A literature review shows that the ages of detrital muscovites are identical to: (1) concordant 40Ar/39Ar ages of biotite (228Ma) and amphibole (230Ma) in amphibolites of the Deokjeongri Gneiss Formation and the Weolhyeonri Complex, pointing to very rapid cooling of 100–150°C/Ma, and (2) 231–229Ma muscovite from the low-grade metamorphic mid-Paleozoic turbidites of the Taean Formation. The efficiency of cooling is further underlined by the near-coincidence of these 40Ar/39Ar ages with 243–229Ma (average: 234.6Ma) zircon U–Pb ages in the Gyeonggi Massif and the Hongseong belt, in the literature. It is argued that the Late Triassic magmatic and metamorphic pulse is superimposed on an earlier tectono-metamorphic event, possibly related to collision, indicated by: (1) ~243–237Ma muscovite ages, or age components in age spectra, and (2) two generations of folds and associated tectonic foliations truncated by ~229.5-Ma-old syenites and earlier mafic dykes. The Late Triassic thermal pulse could have been the result of post-collisional delamination of the lower crust and uppermost mantle, and/or oceanic slab break-off, which is also suggested by almost coeval, widespread mantle-sourced Mg-rich potassic magmatism. Continuing ductile deformation is shown by mylonitization of Late Triassic magmatic rocks; an ~220Ma muscovite age may be related to this.

First 40Ar/39Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India

Lateritic surface processes have shaped large platform and cratons of the tropical belt. Constraining the timing of such processes is crucial to decipher their role in cratonic morphogenesis and their response to long-term climatic change and lithospheric deformation. Weathering histories have been documented for South America, Africa and Australia, but precise time constraints of the lateritic weathering processes in South India are still lacking. We present 40Ar/39Ar ages of supergene cryptomelane (K–Mn oxide) formed in the Sandur Mn ore deposits exposed on the highest lateritic paleolandsurface that once covered the Mysore plateau and the adjacent Deccan Traps. Significant 40Ar/39Ar ages are estimated between ∼36 and ∼26 Ma from well-defined plateaus in step heating 39Ar release spectra and from best-fitted inverse isochrones. These ages constitute firm time constraints that document intense late Eocene–Oligocene lateritic weathering over Peninsular India under the influence of warm and wet climate comparable to that prevailing in tropical humid forests. These results imply that Southern India was weathered between ∼36 and 26 Ma and may have been dissected mostly in the Neogene.

K-feldspar hygrochronology

The significance of the multi-isotopic record preserved in K-feldspars is assessed on samples from the Aar metagranite, Central Alps, Switzerland having very tight independent geological constraints. Stepwise leaching reveals that two diachronically grown K-feldspar generations coexist: Kfs-1 (⩾35Ma old, Ca-poor, Rb–Cl-rich, with low 87Sr/86Sr and high 206Pb/204Pb) and Kfs-2 (⩽10Ma old, antithetic isotopic signatures deriving from external fluids). Microtextures imaged by cathodoluminescence, backscattered electrons, and electron probe microanalysis are patchy and chemically heterogeneous, with pronounced enrichments in Ba in the retrogressed regions. This confirms the simultaneous presence of fluid-dominated retrogression and recrystallization and isotopic inheritance. The staircase-shaped 40Ar/39Ar age spectrum correlates with the Ca/K and Cl/K signatures. This reflects a mixture of heterochemical K-feldspar generations, and not an intracrystalline Ar gradient caused by diffusion. The shape of the age spectrum and the in vacuo release kinetics proceed from entirely different physical and geological phenomena.What K-feldspars can be effectively used for is to constrain the timing of the fluids that interacted with them by multi-isotopic analyses, rather than to model a “cooling history” from 39Ar release alone. The identification of multiple mineral generations by imaging combined with multi-isotopic analysis enables the accurate dating of the events of a multistage evolution after the initial crystallization of the rock in which the minerals occur.

Recovery of muscovite age gradients by 40Ar/39Ar vacuum furnace step-heating analysis

40Ar/39Ar furnace step heating analysis of 3 size fractions of Crazy Basin muscovite was performed using ‘conventional’ (12 steps) and ‘detailed’ (29 steps) heating schedules. The intracrystalline age gradients previously documented by laser spot ablation analysis were reproduced in the step-heating age spectra. Total gas and plateau ages from ‘conventional’ results correlate positively with grain size indicating that muscovite preserves natural dimensions during sample processing and the crystal size influences argon retention during geologic cooling. Age spectra from ‘detailed’ analyses exhibit incrementally increasing ages over the low-temperature portion of the gas release (first ~10%) followed by more gently increasing ages over the remainder of the gas release. The ages in the ‘detailed’ spectra span the range previously documented by laser spot ablation analysis including the core ages reported by Hames and Bowring (1994) indicating the age zonation in the mica crystals was not homogenized during furnace heating. Arrhenius diagrams constructed from the incremental 39Ar release during the ‘detailed’ experiments yield consistent results with those recently reported by Harrison et al. (2009). Interpreting the initial Arrhenius array as subdomains yields domain distribution plots (log r/ro vs. % 39Ar released) exhibiting strong correlation with the shape of the associated age spectra. This correlation supports the assertion that the same process controlling diffusion during geologic cooling is active during vacuum furnace degassing in the laboratory. Therefore, muscovite step-heating data, may afford recovery of thermal histories.

Laser argon dating of melt breccias from the Siljan impact structure, Sweden: Implications for a possible relationship to Late Devonian extinction events

Abstract— In earlier studies, the 65-75 km diameter Siljan impact structure in Sweden has been linked to the Late Devonian mass extinction event. The Siljan impact event has previously been dated by K-Ar and Ar-Ar chronology at 342-368 Ma, with the commonly quoted age being 362.7 ± 2.2 Ma (2 s̀, recalculated using currently accepted decay constants). Until recently, the accepted age for the Frasnian/Famennian boundary and associated extinction event was 364 Ma, which is within error limits of this earlier Siljan age. Here we report new Ar-Ar ages extracted by laser spot and laser step heating techniques for several melt breccia samples from Siljan (interpreted to be impact melt breccia). The analytical results show some scatter, which is greater in samples with more extensive alteration; these samples generally yield younger ages. The two samples with the least alteration yield the most reproducible weighted mean ages: one yielded a laser spot age of 377.2 ± 2.5 Ma (95% confidence limits) and the other yielded both a laser spot age of 376.1 ± 2.8 Ma (95% confidence limits) and a laser stepped heating plateau age over 70.6% 39Ar release of 377.5 ± 2.4 Ma (2 s̀). Our conservative estimate for the age of Siljan is 377 ± 2 Ma (95% confidence limits), which is significantly different from both the previously accepted age for the Frasnian/Famennian (F/F) boundary and the previously quoted age of Siljan. However, the age of the F/F boundary has recently been revised to 374.5 ± 2.6 Ma by the International Commission for Stratigraphy, which is, within error, the same as our new age. However, the currently available age data are not proof that there was a connection between the Siljan impact event and the F/F boundary extinction. This new result highlights the dual problems of dating meteorite impacts where fine-grained melt rocks are often all that can be isotopically dated, and constraining the absolute age of biostratigraphic boundaries, which can only be constrained by age extrapolation. Further work is required to develop and improve the terrestrial impact age record and test whether or not the terrestrial impact flux increased significantly at certain times, perhaps resulting in major extinction events in Earth's biostratigraphic record.

An 40 Ar/ 39 Ar, Rb/Sr, and stable isotope study of micas in low-grade fold-and-thrust belt: an example from the Swiss Helvetic Alps

White micas in carbonate-rich tectonites and a few other rock types of large thrusts in the Swiss Helvetic fold-and-thrust belt have been analyzed by 40Ar/39Ar and Rb/Sr techniques to better constrain the timing of Alpine deformation for this region. Incremental 40Ar/39Ar heating experiments of 25 weakly metamorphosed (anchizone to low greenschist) samples yield plateau and staircase spectra. We interpret most of the staircase release spectra result from variable mixtures of syntectonic (neoformed) and detrital micas. The range in dates obtained within individual spectra depends primarily on the duration of mica nucleation and growth, and relative proportions of neoformed and detrital mica. Rb/Sr analyses of 12 samples yield dates of ca. 10–39 Ma (excluding one anomalously young sample). These dates are slightly younger than the 40Ar/39Ar total gas dates obtained for the same samples. The Rb/Sr dates were calculated using initial 87Sr/86Sr ratios obtained from the carbonate-dominated host rocks, which are higher than normal Mesozoic carbonate values due to exchange with fluids of higher 87Sr/86Sr ratios (and lower 18O/16O ratios). Model dates calculated using 87Sr/86Sr values typical of Mesozoic marine carbonates more closely approximate the 40Ar/39Ar total gas dates for most of the samples. The similarities of Rb/Sr and 40Ar/39Ar total gas dates are consistent with limited amounts of detrital mica in the samples. The δ18O values range from 24–15‰ (VSMOW) for 2–6 μm micas and 27–16‰ for the carbonate host rocks. The carbonate values are significantly lower than their protolith values due to localized fluid-rock interaction and fluid flow along most thrust surfaces. Although most calcite-mica pairs are not in oxygen isotope equilibrium at temperatures of ca. 200–400 °C, their isotopic fractionations are indicative of either 1) partial exchange between the minerals and a common external fluid, or 2) growth or isotopic exchange of the mica with the carbonate after the carbonate had isotopically exchanged with an external fluid. The geological significance of these results is not easily or uniquely determined, and exemplifies the difficulties inherent in dating very fine-grained micas of highly deformed tectonites in low-grade metamorphic terranes. Two generalizations can be made regarding the dates obtained from the Helvetic thrusts: 1) samples from the two highest thrusts (Mt. Gond and Sublage) have all of their 40Ar/39Ar steps above 20 Ma, and 2) most samples from the deepest Helvetic thrusts have steps (often accounting for more than 80% of 39Ar release) between 15 and 25 Ma. These dates are consistent with the order of thrusting in the foreland-imbricating system and increase proportions of neoformed to detrital mica in the more metamorphosed hinterland and deeply buried portions of the nappe pile. Individual thrusts accommodated the majority of their displacement during their initial incorporation into the foreland-imbricating system, and some thrusts remained active or were reactivated down to 15 Ma.

Systematic analysis of K-feldspar 40Ar/39Ar step heating results II: relevance of laboratory argon diffusion properties to nature

We examine a database containing the results of 40Ar/39Ar step-heating experiments performed on 194 basement K-feldspars to recover thermal history information. Qualitative examination of 40Ar/39Ar systematics reveals that about half of the K-feldspars examined are sufficiently well behaved to be suitable for thermal history analysis. Correlation algorithms are developed to quantitatively assess the degree to which age and 39Ar release spectra are compatible with the same volume diffusion process. Upon applying these methods, we find that 65% of all samples yield correlation coefficients in excess of 0.8, whereas roughly 40% give values above 0.9. We further compare the observed correlation behavior with that predicted from the multidiffusion domain model and find good agreement for samples with correlation coefficients above 0.9. In contrast, hydrous phases unstable under in vacuo heating and K-feldspars with highly disturbed age spectra yield poorly correlated age and diffusion properties. The high degree of correlation exhibited by the majority of K-feldspars we have analyzed validates extrapolation of experimentally determined diffusion properties to conditions attending natural Ar loss within the crust. Despite this, a significant number of basement K-feldspars analyzed by the step-heating method yield 40Ar/39Ar systematics that are clearly problematic for thermal history analysis. We numerically explore the effects of low-temperature alteration of K-feldspar on thermochronological analysis and identify a range of conditions under which information is progressively lost. Finally, we demonstrate the insensitivity of thermal history calculations to detailed knowledge of the diffusion mechanism by introducing the heterogeneous diffusion model. We find that the multidiffusion domain approach can successfully recover imposed thermal histories from heterogeneous diffusion-type crystals and conclude that most details of the interpretive model employed are of secondary importance. The only requirement for recovering thermal histories from K-feldspar 40Ar/39Ar step-heating results is that argon loss proceeds by volume diffusion and that laboratory argon release adequately mimics the natural diffusion boundaries and mechanisms—a requirement implicitly met by those samples exhibiting high degrees of correlation.

Excess argon incorporation in phengite of the Mulhacén Complex: submicroscopic illitization and fluid ingress during late Miocene extension in the Betic Zone, south-eastern Spain

40Ar/ 39Ar induction furnace and laser step-heating of well-crystallised post-tectonic phengitic mica single grains from gneisses of the Mulhacén Complex with an early Alpine tectonic fabric has resulted in: (1) highly scattered integrated ages, (2) an abnormally high atmospheric contamination and (3) often anomalously old apparent ages during early 39Ar release that is associated with a high 36Ar AIR and 37Ar Ca contamination. This low-temperature excess argon ( 40Ar XS) component is probably released from carbonate formed during slight alteration of the mica. More than 50% of the samples yielded plateau ages ranging from 15.8±0.4 to 90.1±1.0 Ma. Samples taken only a few metres apart may differ in age by as much as 50 Ma; a grain that was split over the basal plane yielded plateau ages for each half that differ by 12%. The age variation on these different scales is explained by heterogeneous 40Ar XS incorporation during a period with a high transient partial argon pressure in the metamorphic fluid, resulting from a late stage reheating event. The very swift cooling of 50–100°C/Ma during exhumation of the Mulhacén Complex concomitant with late Miocene extension may have prevented the equilibration of different 40Ar XS levels in the mica. HRTEM images of the oldest and youngest phengite specimens show that at least 20% of the lattice is affected by submicroscopic illitisation, which is concentrated in several micrometer wide zones and veins that cross-cut the basal cleavage. These are made up of aggregates of 0.07–0.30 μm thick crystallites of three illitic micas types, which are chemically and structurally progressively closer to pure illite and occur in different textures. The oldest specimen is affected most severely as the veins contain newly formed (pseudo)illite that does not inherit its crystallographic orientation and chemistry from the host mica, in contrast to the youngest sample. HRTEM–AEM analyses revealed that phengite and the different illitic micas may be depleted in K. The oldest sample is derived from a coarse-grained augen gneiss with extensively developed hydraulic cracks, which are lacking in the youngest sample, a fine-grained mylonitic gneiss. Fluid–rock interaction and consequently sub-microscopic illitization were therefore more intense in the coarser-grained rocks. Growth of the illitic micas in equilibrium with a high partial 40Ar pressure could account for 40Ar XS incorporation in K-vacancies and other lattice imperfections. Variation in illitisation and associated textural dissimilarities between the oldest and youngest mica permit the different levels of 40Ar XS incorporation that account for the observed age discordance. The finding of 40Ar XS plateau ages, despite the degassing of intimately intergrown micaceous minerals, is interpreted by gas release invoked by in-vacuo chemical and structural changes that led to a joint collapse of the lattices of phengite and the illitic micas between 800°C and 1000°C.

39Ar- 40Ar ages and thermal history of mesosiderites

Determinations of 39Ar- 40Ar ages for plagioclase separates of three clasts from the Vaca Muerta and Mount Padbury mesosiderites give similar results. Ages increase monotonically by ∼0.2–0.3 Ga when plotted against increasing 39Ar release from the samples and give average values of ∼3.95 Ga. When combined with revised ages for mesosiderite analyses previously reported, nineteen analyses of thirteen mesosiderites give an average Ar-Ar age of 3.94 ± 0.10 Ga. The average age at 90% 39Ar release is 4.13 ±0.14 Ga, and the ages at 10% 39Ar release range over 3.32–3.97 Ga. It seems unlikely that the 39Ar- 40Ar ages were produced by direct impact heating, but they probably are independent evidence for slow cooling deep within the mesosiderite parent body. We conclude that the sloped age plateaus and the significant range in Ar-Ar closure temperatures calculated for these mesosiderites (∼75–340°C) are due to significant differences in Ar diffusion properties among samples. From the Ar diffusion data, we estimate an approximate mesosiderite cooling rate of ∼0.2°C/Ma, which is similar to the preferred cooling rate of 0.1–0.5°C/Ma derived by others from Ni diffusion and metal textures. The 39Ar/ 40Ar ages seem incompatible with very slow cooling rates of <0.2°C/Ma, unless the closure temperatures for Ar diffusion and formation of metal cooling textures on the parent body substantially overlap. Any possible role of fragmentation of the mesosiderite parent body in producing the younger Ar-Ar ages and the very slow cooling rates is unclear.

Disturbed 40Ar 39Ar systematics in hydrothermal biotite and hornblende at the Scotia gold mine, Western Australia: Evidence for argon loss associated with post-mineralisation fluid movement

Homblende and biotite that formed during gold mineralisation at the Scotia mine, Western Australia, have erratic 40Ar 39Ar release spectra and total gas ages that are ∼200–900 million year younger than the ca. 2600–2620 Ma minimum age of gold mineralisation, as given by 40Ar 39Ar plateau (muscovite) ages of crosscutting pegmatite dykes. Analysed hornblendes are dominated by magnesio hornblende but also contain small domains of ferro-actinolitic hornblende, actinolitic hornblende, and actinolite. Biotite also appears to be substantially altered to chlorite along cleavage planes. Relatively young apparent ages and high K/Ca ratios of argon released from hornblendes at temperatures less than ∼1000°C are interpreted to be the result of degassing of contaminant biotite. However, this cannot totally explain the young ages of hornblendes. Gas fractions released at furnace temperatures above 1000 C, where the effect of biotite degassing is demonstrably negligible, still have apparent ages that are ∼200–900 million years younger than the age of muscovite from post-gold pegmatite dykes. The close proximity of disturbed hydrothermal hornblende samples to apparently undisturbed pegmatite muscovite samples (less than a few metres in some cases) is difficult to reconcile with argon loss in hydrothermal hornblende being the product of thermally-driven volume diffusion. Given a suitable thermal history, argon loss could occur preferentially in hornblendes if (1) the closure (for slow cooling) and blocking (for reheating) temperatures of hydrothermal hornblendes were lower than published estimates, as has been observed in structurally complex metamorphic hornblendes and/or (2) the closure and blocking temperature of pegmatite muscovite were higher than commonly estimated. However, neither of these interpretations can easily explain the large variation in hornblende ages. It is instead suggested that argon loss occurred during mineral-fluid interaction during movement of a retrograde fluid along the mineralised lode structures and that this occurred at ambient temperatures below the blocking temperature of pegmatite muscovite. There is abundant geological evidence for the passage of such a fluid at the Scotia mine, including the presence of numerous late brittle fractures containing retrogressive low-temperature mineral assemblages. Late fluid movement is probably related to Proterozoic orogenic activity along the nearby southeastern margin of the Yilgarn Craton. The difference in argon systematics between hydrothermal minerals and pegmatite muscovite is largely ascribed to the relatively low permeability of the more massive pegmatite dykes (with respect to ore zones) preventing fluid egress to muscovite samples. The variations in ages of hydrothermal minerals are probably related to the extent of fluid/mineral interaction as this is a function of parameters, such as fluid/rock ratio, fluid P-T-X conditions, permeability, and mineral microstructures that may vary on short time and length scales. Recognition of possible argon loss in hornblende via fluid interaction is important for the interpretation of 40Ar- 39Ar systematics in environments, such as many hydrothermal ore deposits, where minerals may be exposed to fluids after crystallisation.

Thermal evolution and slip history of the Renbu Zedong Thrust, southeastern Tibet

Cretaceous granitoids of the Gangdese batholith, southeastern Tibet, were overthrust by upper greenschist to epidote‐amphibolite facies Tethyan rocks derived from the Indian shelf along the north directed (∼30° dip) Renbu Zedong Thrust (RZT). Thermochronological results obtained from a NE‐SW transect near Lian Xian show evidence of thermal effects related to thrusting. Granitoids immediately beneath the RZT exhibit considerable recrystallization to greenschist facies assemblages. Biotite and K‐feldspar 40Ar/39Ar ages measured along the traverse into the footwall increase systematically away from the RZT. The timing of initial upward displacement of the RZT hanging wall is constrained to have occurred at ∼18 Ma from the hornblende ages and the two K‐feldspar samples closest to the fault. K‐feldspars up to 15 km from the thrust yield 9–12 Ma ages for the initial ∼20% of 39Ar release. Distal samples yield 40Ar/39Ar ages nearly as old as the 70–110 Ma ion microprobe 206Pb*/238U ages determined for coexisting zircons. We have integrated our thermal history results with numerical heat flow models and found that while reheating to 320–280°C at shallow (∼7 km) levels due to rapid (&gt;15 mm/yr) slip along the RZT at ∼10 Ma is capable of explaining the initial portion of the K‐feldspar age spectra, a prior common thermal history experienced by all samples cannot satisfactorily account for all the 40Ar/39Ar results. Instead, we find our thermal history constraints to be more completely explained by a numerical model in which (1) rocks currently at the surface originated from different depths,(2) footwall samples in close proximity to the RZT experienced fault drag from 19 to 15 Ma and (3) postthrusting denudation of the region involving localized tilting occurred at ∼10 Ma. The minimum average slip rate and displacement along the ramp during this period are 2 mm/yr and 12 km, respectively, but are likely to have been greater. The cooling episode recorded in all the K‐feldspar age spectra beginning at ∼10 Ma may either reflect denudation following regional uplift due to displacement along the ramp of the Main Himalayan Thrust or topographic collapse following cessation of RZT thrusting.

The Inverse Problem of Argon Diffusion from Minerals: Determination of Kinetic Parameters from Stepwise-Heating Experiments

Results of two routine 40Ar/39Ar stepwise heating experiments on a biotite and a basanite are interpreted in terms of Fick's and Arrhenius' laws. Both patterns represent a saddle-shaped 39Ar release. Argon isotope spectra are suggested to be controlled by the activation energy of diffusion E and the frequency factor Do . The activation energy of 39Ar is lower than the one of 40Ar. This results in a preferable release of 40Ar relatively to 39Ar at high-temperature steps and an increasing high-temperature wing in the saddle-shaped age spectrum. At low temperatures, considerable losses and irregularities in release of mainly 39Ar are observed, which cause the decreasing low-temperature wing in the “saddle”. The suggestion of argon losses (mainly of 39Ar) from a loose, “unstable” zone of the mineral structures becomes justified. The n-irradiation of the samples and the shift of E of 39Ar towards lower values seems to explain the saddle-shaped age-spectra often encountered in 40Ar/39Ar-geochronometry.

The collisional history of the HED parent body inferred from 40Ar 39Ar ages of eucrites

In the framework of a consortium studying small solar system bodies we analysed matrix samples and individual clasts from five eucrites—Stannern, Pasamonte, ALHA-76005, ALHA-78132 and ALHA-80102—by high resolution stepwise heating 40Ar 39Ar technique. Our aim was to determine the duration of secondary processes like brecciation and recrystallization caused by collisional events, which were accompanied by rather weak reheating and thus by rather small effects on the KAr system. For this reason all age spectra are complex. The apparent ages increase with increasing degassing temperatures and, in some cases, are additionally disturbed by 39Ar recoil. Thus, the low temperature ages provide estimates of the time of the last thermal disturbing event while the high temperature ages give lower limits for the last total degassing of the rocks. Only a granulitic breccia clast from polymict eucrite Pasamonte yields a plateau age of 4.245 ± 0.021 Ga, which probably dates the formation of its equigranular texture. The K Ca spectra for all samples are constant for the first 70% of the 39Ar release and then level off by about one order of magnitude. Finally, we are not able to prove or dismiss a cataclysmic bombardment of the HED parent body comparable to that of the Earth's moon. Nevertheless, a rather short duration of impact metamorphism seems unlikely. Completed by the data from other studies, our results support a period of intense bombardment of the HED parent body totally resetting the KAr system of the rocks ∼ 4.4-3.9 Ga ago. It was followed by a time of less intense impact induced metamorphism causing only partial loss of radiogenic 40Ar ∼ 3.9-2.8 Ga ago.

Formula omitted] analysis of K-feldspars from Cretaceous granitic rocks in Japan: Significance of perthitization in Ar loss

40 Ar 39 Ar spectrum ages have been determined for five K-feldspars from Cretaceous granitic rocks in Japan. Three samples yield flat age spectra, giving plateau ages of 60.1 ± 1.1 Ma (94.3% of integrated 39Ar) for the Naegi granite of the Sanyo Belt, and 101.9 ± 2.5 Ma (95.5%) and 103.3 ± 1.7 Ma (96.1%) for granitic rocks from northeast Japan. The results suggest that these granitic rocks experienced a simple and rapid cooling history. Two feldspars from granitic rocks of the Ryoke Metamorphic Belt in the vicinity of the Median Tectonic Line give two plateau ages each: 61.1 ± 1.0 Ma (46.0%) and 66.0 ± 1.1 Ma (32.4%) for low-temperature steps; and 64.6 ± 1.0 Ma (51.6%) and 71.2 ± 1.1 Ma (65.4%) for high-temperature steps, though plateau ages for high-temperature steps are less well defined. This age pattern may suggest a later thermal event related to the Median Tectonic Line. The cooling rates calculated from KAr ages of coexisting mineral pairs range from 17° to 32°C Ma −1 for granitic rocks from the Sanyo and Ryoke Belts, whereas mineral ages for granitic rocks from northeast Japan are nearly equal, suggesting a very rapid cooling. Diffusion parameters of K-feldspars calculated from the 39Ar release yield activation energy, E, ranging from 22.3 to 53.4 kcal mol −1, and D 0 a 2 ranging from 0.0297 to 1.73·10 5 s −1. The closure temperatures are calculated to be from 115° to 331°C. The activation energy and closure temperature are inversely correlated to the degree of perthitization, which is the decisive factor for Ar loss from K-feldspar.

KAr and 40Ar/ 39Ar evidence for a Transamazonian age (2030-1970 Ma) for the granites and emerald-bearing K-metasomatites from Campo Formoso and Carnaíba (Bahia, Brazil)

The Campo Formoso and Carnaíba granites belong to a suite of middle Proterozoic magmatic rocks located in the northern part of the São Francisco craton. They intrude the Archaen basement and Lower Proterozoic Jacobina volcanosedimentary series. Emerald-bearing K-metasomatites in the mining districts of Campo Formoso and Carnaíba are developed within serpentinites at the contact with granite-related pegmatitic veins. KAr and 40Ar/ 39Ar measurements were performed on biotites and deuteric muscovites from these two granites, and phlogopites from the K-metasomatites. For the Campo Formoso granite, the biotites yield ages between 1875 ± 45 Ma and 1908 ± 47 Ma (2 σ) and the muscovites yield ages of 1897 ± 34 Ma and 2040 ± 24 Ma (2 σ). For the Carnaíba granite, the biotites and muscovites fit isochrons with ages of 1888 ± 32 and 1979 ± 28 Ma (2 σ), respectively. In contrast, phlogopites from emerald-bearing metasomatites display KAr ages that spread between 1900 and 2000 Ma with an isochron of 1973 ± 20 Ma (2 σ) for Carnaíba. Generally, the youngest biotite and phlogopite ages occur for specimens where these minerals are chloritized. 40Ar/ 39Ar step heating release spectra are complex but give integrated ages in good agreement with the KAr ages. The least disturbed spectrum permits assignment of an age of 2032 ± 10 Ma (2 σ)for the first granitic pulse of the emplacement of the Campo Formoso composite pluton. Since in Carnaíba, deuteric muscovites and chlorite-free phlogopites give similar KAr ages, 1979 ± 28 and 1973 ± 20 Ma (2 σ) respectively, we conclude that emerald mineralization is contemporaneous with the pervasive muscovitization of the granite. The 1979 ± 28 Ma (2 σ) age obtained by KAr on muscovite represents the best estimate of the Carnaíba granite cooling age. A model invoking the pervasive alteration of the upper part of the granitic cupola along the pegmatite veins and serpentinites by the muscovitizing fluids is proposed for the formation of emerald-bearing K-metasomatites. The disturbances of 40Ar/ 39Ar release spectra testify to the existence of a hydrothermal heating that overprinted the KAr clock of biotite and to a less extent phlogopite. This event is clearly subsequent to the Transamazonian granitization and emerald mineralization and consequently younger than 1973 ± 20 Ma. Due to the lack of structural evidences for a Braziliano event (700-500 Ma) in this region, we tentatively propose a Transamazonian age (1900 Ma) for the thermal overprint.

40Ar/ 39Ar polyorogenic mineral age record in the northern Mauritanide orogen, West Africa

Northern segments of the Mauritanide orogen in the Akjoujt region are dominated by suprastructural nappe complexes. These structurally overlie other tectonic elements, including: (1) autochthonous/parautochthonous foreland units composed of late Proterozoic-late Paleozoic (Frasnian) sedimentary cover sequences, and unconformably underlying crystalline basement (late Archean-middle Proterozoic) of the West African Shield, (2) parautochthonous successions largely represented by low-grade metasedimentary rocks, and (3) allochthonous infrastructural complexes with characteristics similar to those of late Proterozoic-early Paleozoic (“Pan-African”) tectonic elements exposed in central and southern parts of the Mauritanide orogen. Amphibole from various structural levels of the Ijibiten and Akjoujt suprastructural nappe complexes generally display internally discordant, “saddle-shaped” 40Ar/ 39Ar release spectra suggestive of extraneous (excess) argon contamination. One sample records a well-defined, intermediate-temperature plateau of ~2420 Ma. Muscovite within lower and intermediate structural levels of the complexes displays variably discordant 40Ar/ 39Ar age spectra which suggest that variable thermal rejuvenation of older (up to ~2600 Ma) mineral systems occurred during a ca. 310-300 Ma metamorphism. Uppermost structural levels of the complexes are represented, in part, by penetratively mylonitic metagranite. Muscovite within these rocks displays internally concordant 40Ar/ 39Ar age spectra which define plateau ages of ~300–310 Ma. There is no indication in either amphibole or muscovite results of any late Proterozoic-early Paleozoic (Pan-African) thermal record. Together with field relationships, the 40Ar/ 39Ar results suggest that structural units of the suprastructural nappe complexes are, in part, composed of variably overprinted basement rocks. These structurally overlie allochthonous Pan-African sequences and therefore must have been derived west of and now largely conceal, the Pan-African orogen. The complexes were transported eastward to higher structural levels during late Paleozoic orogenesis. The extent of late Paleozoic thrusting within the Akjoujt region contrasts with more limited transport distances documented southward in the orogen. This contrast probably reflects a kinematic influence of the Reguibat promentory of the West African Shield, which during the late Paleozoic collision of Gondwana and Laurentia effected extensive compression in proximal sequences.

The 40Ar/39Ar thermochronometry for slowly cooled samples having a distribution of diffusion domain sizes

Many 40Ar/39Ar age spectra for alkali feldspars are significantly different from the model age spectra calculated for slowly cooled samples composed of diffusion domains of a single size, and the Arrhenius plots for these samples show departures from linearity that are inconsistent with diffusion from domains of equal size. The most plausible explanation for these discrepancies is the existence of a distribution of diffusion domain sizes. We have extended the single‐diffusion‐domain closure model of Dodson so that it applies to minerals with a distribution of domain sizes and have used it to explain many commonly observed features of 40Ar/39Ar age spectra and Arrhenius plots for 39Ar loss during step heating. For samples with a distribution of diffusion domain sizes, the form of the 39Ar Arrhenius curve is a function of the heating schedule (i.e., the temperature and duration of the steps used), and thus different heating schedules will result in different curves for the same sample. This effect can be used to confirm the existence of a distribution of diffusion domain sizes and to optimize the information contained in the Arrhenius plot. The multiple diffusion domain size model is used to reinterpret the age spectra, Arrhenius plots, and cooling history of three feldspars from the Chain of Ponds pluton, northwestern Maine, earlier interpreted assuming a single domain size. Interpreting the 40Ar/39Ar and 39Ar released during step heating in terms of a single domain size gives rise to a large discrepancy between the cooling rate determined from the age and closure temperature of the three samples compared to the cooling rate required to explain the shape of the individual age spectra. The single domain size model fails also to account for the observed departures from linearity of the Arrhenius plots. We show that a particular domain size distribution in each sample can explain in detail both the shape of the age spectra and the Arrhenius plots, and results in the three samples defining a common cooling history. There is thus good evidence for the three alkali feldspar samples studied here that the thermally activated diffusion measured by 39Ar release during step heating in the laboratory is also the mechanism responsible for argon loss or retention in the natural setting.