Extraneous Argon Research Articles

Thermal evolution of Permian post-orogenic extension and Jurassic rifting recorded in the Austroalpine basement (SE Switzerland, N Italy)

The thermal evolution of the continental lithosphere is intimately linked to its tectonic history. In the present contribution, we explore the thermal evolution of the Austroalpine Campo and Grosina units that composed the former necking zone of the Adriatic rifted margin and that are now exposed in the Eastern Alps (SE Switzerland and N Italy). The Campo unit and the overlying Grosina unit are separated by a Jurassic horizontal extensional structure, the Eita shear zone. We report new muscovite and biotite 40Ar/39Ar ages for the Grosina unit (∼ 261 Ma for muscovite and ∼ 246–245 Ma for biotite) and for the Campo unit (∼ 218–178 Ma for muscovite and ∼ 214–171 Ma for biotite). No geologically meaningful amphibole ages were obtained because of contamination by extraneous argon in these minerals. The new results are combined with published ages obtained with different chronometers (UPb, SmNd, RbSr, KAr, 40Ar/39Ar). This dataset reveals that the two units record different thermal evolutions from the Permian post-orogenic extension to the Jurassic rifting. Ages of cooling initiation and associated cooling rates are estimated by Monte Carlo simulations based on DiffArgP. The Grosina unit records a Permo-Triassic slow cooling of 5–10 °C/Myr initiated during Early Permian in relation to the post-Variscan relaxation of the isotherms and cooling of the lithosphere that caused a large-scale thermal subsidence. By contrast, the Campo unit records a fast cooling of 25–100 °C/Myr associated to the tectonic exhumation of basement rocks during the Jurassic rifting and the formation of the Alpine Tethys rifted margin. This fast cooling appears delayed from major basement deformation phases, and likely follows a period of crustal re-heating.

40Ar/39Ar and unspiked 40K-40Ar dating of upper Pleistocene volcanic activity in the Bas-Vivarais (Ardèche, France)

Fifteen basanitic and tephritic flows from Bas-Vivarais, the youngest volcanic field in the French Massif Central together with the Chaîne des Puys, were dated by 40Ar/39Ar and 40K-40Ar on separated groundmass, and studied for paleomagnetism. An almost systematic discrepancy between the two types of ages is observed, the 40K-40Ar method providing ages up to 8.5 times the 40Ar/39Ar ones. Microscopic observations and geochemical analyses lead us to conclude that most of the K-Ar ages measured on Bas-Vivarais samples are in error due to extraneous argon originating from contamination by xenocrysts from disintegrated crustal and mantle xenoliths. However, 40Ar/39Ar experiments do not evidence any excess argon, suggesting two possibilities: 1, the extraneous argon contribution was eliminated during the pre-degassing of the samples at 600°C prior to the step heating experiments, 2 - K-Ar ages may be older because larger quantities of xenocrysts, potential carriers of extraneous argon were involved in the K-Ar experiments than in the 40Ar/39Ar ones. 40Ar/39Ar ages are thus little or not affected by contamination and provide reliable ages for the studied volcanoes.Combined 40Ar/39Ar datings and magnetic directions for each flow point out to three successive stages in the volcanic evolution of Bas-Vivarais. Stage 1, limited to the northern part of the field, has a mean age of 187.3±19.0ka. In its southern part, Stages 2 and 3 emplaced magmas at 31.1±3.9ka and 23.9±8.1ka, respectively. These two last stages are consistent with available 14C dates but not with previous thermoluminescence data.

Extraneous argon in high-pressure metamorphic rocks: Distribution, origin and transport in the Cycladic Blueschist Unit (Greece)

The 40Ar/39Ar geochronological technique has significantly contributed to the study of crustal deformation. However, the frequent occurrence of extraneous argon in high-pressure terrains can make the interpretation of 40Ar/39Ar ages problematic. This contribution attempts to: i) characterize the distribution and origin of extraneous argon in the Cycladic Blueschist Unit (CBU, Greece) by applying the 40Ar/39Ar dating technique on three Cycladic islands, Syros, Tinos and Ios, which span a complete transect of the CBU and ii) discuss regional implications for the tectonometamorphic evolution of the CBU. Our results, combined with ages available in the literature, show that periods of peak-burial conditions (55–49Ma) and syn- (45–38Ma) to post-orogenic exhumation (37–21Ma) are well constrained in the CBU by the 40Ar/39Ar method. Moreover, Variscan apparent ages are preserved in the Cycladic basement of Ios, indicating that the 40Ar/39Ar system was not completely reset during the Eocene metamorphic episode. Additionally, our 40Ar/39Ar data show that the occurrence of extraneous argon is heterogeneously distributed in the CBU. While high-pressure rocks of Syros and Tinos are only locally affected, extraneous argon is clearly evidenced in the CBU of Ios, yielding geologically meaningless ages up to 900Ma. On each island, the most affected ages were obtained in microstructures where fluid circulation is enhanced such as shear bands or strain shadows around garnets. We suggest that the Cycladic Basement, a polymetamorphic continental unit, behaved as an argon reservoir that has been mobilized by fluid circulation at the Eocene. These argon-enriched fluids have then heterogeneously contaminated metamorphic rocks when percolating through the CBU. Finally, we claim that a minimum closure temperature of 550°C must be accepted for the phengites of the CBU and conclude that volume diffusion is not the main process resetting the argon isotopic system in high-pressure rocks compare to recrystallization enhanced by fluid circulation and deformation.

U-series and 40Ar/39Ar ages of Holocene volcanic rocks at Changbaishan volcano, China

Abstract Accurately dating Holocene volcanic rocks poses many challenges but is critical to assessing magmatic evolution and hazard risks at highly active volcanoes. Here we use Ra/Th and 40Ar/39Ar geochronology to date very young eruptions at Changbaishan volcano, northeastern China, a recently active stratovolcano responsible for one of the most voluminous eruptions in the past ∼2000 yr. For Holocene eruptions, 40Ar/39Ar ages are consistently older than those of both independently determined ages and maximum Ra/Th ages. Overall, Ra/Th ages are most consistent with historical accounts and indicate inaccurate 40Ar/39Ar ages that are due to extraneous argon in various forms. Ra/Th geochronology also confirms the highly active nature of Changbaishan and supports the continued presence of trachytic magma residing under the volcano that appeared more than ∼1100 yr ago.

New 40Ar/39Ar dating of the Clearwater Lake impact structures (Québec, Canada) – Not the binary asteroid impact it seems?

The two Clearwater Lake impact structures (Québec, Canada) are generally interpreted as a crater doublet formed by the impact of a binary asteroid. Here, arguments are presented that raise important questions about the proposed double impact scenario. New 40Ar/39Ar dating of two virtually fresh impact melt rock samples from the ⩾36km West Clearwater Lake impact structure yielded two statistically robust Early Permian plateau ages with a weighted mean of 286.2±2.2 (2.6) Ma (2σ; MSWD=0.33; P=0.57). In contrast, 40Ar/39Ar results for two chloritized melt rocks from the ∼26km East Clearwater Lake impact structure produced disturbed age spectra suggestive of a distinct extraneous argon component. Although individually weakly robust, age spectra corrected for the trapped argon component and inverse isochron plots for the East Clearwater melt rocks consistently yielded apparent ages around ∼460–470Ma. No Permian signal was found in either of these melt aliquots. Our new 40Ar/39Ar results reproduce earlier 40Ar/39Ar plateau ages (∼283Ma and ∼465Ma, respectively) for the two impact structures by Bottomley et al. (1990) and are in conflict with a previous, statistically non-robust Rb–Sr age of 287 [293]±26Ma for East Clearwater. The combined cluster of apparent ages of ∼460–470Ma, derived from four different samples across the impact melt sheet, is very unlikely to represent a ‘false age effect’ due to the incorporation of extraneous argon into the melt; instead, it strongly favors a Middle Ordovician age for the East Clearwater impact and impact-induced hydrothermal chloritization. Moreover, the Clearwater impact structures are characterized by different natural remanent magnetizations testifying to separate geologic histories, an effect unexpected in the case of a Permian double impact. Whereas the West Clearwater impact affected Ordovician carbonates incorporated into the impact breccia, drill core reports from the 1960s concluded that clasts of Ordovician sedimentary rocks are seemingly absent in the impact breccia lens of the East Clearwater Lake impact structure, which is overlain by >100m of post-impact sandstones, shales and carbonates. No resolvable impactor contamination has so far been detected in the West Clearwater impact melt rocks, whereas East Clearwater carries a distinct ordinary (possibly L-) chondritic impactor signature in its melt rocks. East Clearwater Lake might thus represent one among a long list of Ordovician impact structures in North America and northern Europe that were presumably formed in response to the L-chondrite asteroid breakup event ∼470Ma ago. Paleogeographic reconstructions show that the Ordovician East Clearwater impact probably occurred in a near-coastal to shallow marine setting, while the Permian West Clearwater impact hit continental Pangaea. Along with the new 40Ar/39Ar data, the paleomagnetic, sedimentologic, and geochemical findings suggest that the close spatial arrangement of the two Clearwater Lakes is most likely pure coincidence. The two impact structures seem to represent a ‘false doublet’ struck by impacts separated by ∼180 million years in time. The new results for the Clearwater Lake impact structures have major implications for the reliable identification of doublet impact craters and the rate of binary asteroid impacts on Earth and on other planetary bodies in the inner Solar System.

High precision multi-collector 40Ar/39Ar dating of young basalts: Mount Rouse volcano (SE Australia) revisited

The acquisition of high precision geochronological data for young basalts, particularly low potassium examples <500 ka old, is a non-trivial task. This is due to low 40Ar radiogenic argon yields, high atmospheric argon levels and, sometimes, extraneous argon contamination. In this study we present new, high precision argon isotopic data for a basalt flow from the Mount Rouse volcano of the Newer Volcanic Province, southeast Australia. These data were generated using a new generation, multi-collector, ARGUSVI mass spectrometer. We show that the multi-collector system achieves an order of magnitude improvement in analytical precision and a concomitant improvement in our ability to detect minor isotopic disturbances, compared to ‘conventional’ single-collector mass spectrometry. We report a revised eruption age of 284.4 ± 1.8 ka (0.6%, 95% CI) for Mount Rouse, based on laser step-heating experiments of several groundmass aliquants (50–100 mg) from a single sample (180–250 μm size fraction). This result is considerably more precise than our previous estimate of 303 ± 13 ka (4.3%, 2σ), obtained using an older generation VG3600 mass spectrometer and larger (∼200 mg) sample aliquants.

Ultra-high precision 40Ar/39Ar ages for Fish Canyon Tuff and Alder Creek Rhyolite sanidine: New dating standards required?

The 40Ar/39Ar dating technique is a high precision (<0.1%) method with wide application to geological samples. However, the method is predicated on the availability of natural mineral standards of known age. Widely used 40Ar/39Ar standards include sanidine from the (ca. 28Ma) Fish Canyon Tuff (FCT) and the (ca. 1.2Ma) Alder Creek Rhyolite (ACR). Despite common usage, the ages of FCT and ACR sanidine have proven contentious, with reported values varying by >2%; well outside the ±0.1% aspiration of EARTHTIME (www.earth-time.org). The current study presents ultra-high precision, multi-collector 40Ar/39Ar results for FCT and ACR sanidine, using a new generation ARGUSVI mass spectrometer. These analyses demonstrate the significantly higher (by a factor of ∼10) level of precision achievable using the ARGUSVI system, compared to most previous studies. Significantly, 40Ar/39Ar step-heating analyses resolve distinct age gradients for both FCT and ACR sanidine. Possible explanations for these gradients include combinations of recoil loss of 39Ar, instrumental bias, isotopic fractionation during step-heating, thermally induced argon loss and/or extraneous argon contamination. Minor recoil loss of 39Ar and partial retention of inherited argon during extended magma residence, are deemed the most plausible causes of the age discordance. Given this assumption, we calculate revised (maximum?) eruption ages of 28.01±0.04Ma (2σ) for FCT and 1.178±0.002Ma (2σ) for ACR, relative to the astronomically calibrated age of A1 Tephra sanidine, Crete [Rivera et al. (2011)Earth Planet Sci. Lett.311, 420–426]. Nonetheless, the ARGUSVI data demonstrate that FCT and ACR sanidine are non-ideal as high precision 40Ar/39Ar standards, with different heating protocols likely to induce inter-laboratory bias. The current findings account for at least some of the scatter in inter-calibration ages reported previously for various 40Ar/39Ar standards. In broader terms, the results mandate a re-evaluation of the astronomically calibrated ages for these and other sanidine samples, while the new generation of multi-collector mass spectrometers provides the means to further evaluate 40Ar/39Ar ages used to define new decay constants and standard ages.

40Ar/39Ar dating of alkali feldspar megacrysts from selected young volcanoes of the Newer Volcanic Province, Victoria

The Newer Volcanic Province (NVP) in Victoria, with extension into south-eastern South Australia, represents the youngest chapter of Cenozoic volcanism in south-eastern Australia. However, most ages have been determined by the potassium–argon (K–Ar) method, and the age data are not comprehensive. In addition, few ages exist for the array of scoria cone volcanoes in the NVP. Seven alkali feldspar samples, mostly anorthoclase megacrysts, from volcanic centres in the NVP were used for 40Ar/39Ar dating in the present study. In geochronological order, with ages quoting 95% confidence limits, locations are Mount Franklin near Daylesford (0.110 ± 0.014 Ma), Red Rock near Alvie (0.116 ± 0.048 Ma), Lake Bullenmerri at Camperdown (0.116 ± 0.019 Ma), Ridge Road Quarry near Daylesford (2.01 ± 0.11 Ma) and Mount Kororoit near Diggers Rest (3.74 ± 0.26 Ma). Two samples from The Anakies, near Bacchus Marsh, produced discordant results suggesting a maximum age of ca. 1.9 Ma. The analyses and reported ages in the present study not only provide new geochronological data for the province, but also elucidate the difficulties in dating very young basalts using the 40Ar/39Ar dating method. These results are consistent with the erosion levels of the scoria volcanoes sampled, and indicate a major episode of explosive volcanic activity at ca. 100 ka. In contrast, the more eroded Mount Kororoit is considered to be ca. 3.7 Ma in age. The age of The Anakies is more equivocal owing to the indicated presence of excess argon and a maximum age of ca. 1.9 Ma is suggested for this locality. Given the latter results and lack of precision obtainable from the younger samples, the possibility remains that other samples contained extraneous argon and that the ages generated are thus maximum eruption ages. Analyses of additional samples from these and other localities will be required to further resolve this issue.

Comment on “New Ar–Ar ages of southern Indian kimberlites and a lamproite and their geochemical evolution” by Osborne et al. [Precambrian Res. 189 (2011) 91–103

Osborne et al. (2011; Precambrian Res. 189, 91–103) report new 40Ar/39Ar age results for two kimberlites (Muligiripalle pipe 5 and Tummatapalle pipe 13) and one lamproite (Pochampalle), from the Archaean Dharwar craton in southern India. Previous studies indicate that kimberlites and related rocks are highly susceptible to extraneous argon contamination, leading to anomalously old ages; although reliable 40Ar/39Ar ages can be obtained from step-heating analyses of groundmass phlogopite grains. Osborne et al. (2011) carried out single-step 40Ar/39Ar fusion analyses on phlogopite phenocrysts and xenocrysts. This approach is not favoured, because it does not allow identification of argon loss/gain effects, thus complicating data interpretation and reducing confidence in the reliability of the reported ages. In particular, it is suggested that the Pochampalle lamproite was emplaced <1400Ma ago, rather than ca.1500Ma, as suggested by Osborne et al. (2011). It is recommended that 40Ar/39Ar geochronology of kimberlite and lamproites include step-heating analyses of matrix phlogopite grains, with anomalous or contentious results verified using other dating methods.

New 40Ar/ 39Ar ages for selected young (

The Pliocene-Holocene Newer Volcanic Province (NVP) of southeastern Australia is an extensive, relatively well-preserved, intra-plate basaltic lava field containing more than 400 eruptive centres. This study reports new, high-precision 40Ar/ 39Ar ages for six young (300–600 ka) basalt flows from the NVP and is part of a broader initiative to constrain the extent, duration, episodicity and causation of NVP volcanism. Six fresh, holocrystalline alkali basalt flows were selected from the Warrnambool-Port Fairy area in the Western Plains sub-province for 40Ar/ 39Ar dating. These flows were chosen on the basis of pre-existing K-Ar age constraints, which, although variable, indicated eruption during a period of apparent relative volcanic quiescence (0.8–0.06 Ma). 40Ar/ 39Ar ages were measured on multiple aliquots of whole rock basalt samples. Three separate flows from the Mount Rouse volcanic field yielded concordant 40Ar/ 39Ar age results, with a mean eruption age of 303 ± 13 ka (95% CI). An older weighted mean age of 382 ± 24 ka (2σ) was obtained for one sample from the central Rouse-Port Fairy Flow, suggesting extraneous argon contamination. Two basalt flows from the Mount Warrnambool volcano also yielded analogous results, with an average 40Ar/ 39Ar age of 542 ± 17 ka (95% CI). The results confirm volcanic activity during the interval of relative quiescence. Most previous K-Ar ages for these flows are generally older than the weighted mean 40Ar/ 39Ar ages, suggesting the presence of extraneous 40Ar. This study demonstrates the suitability of the 40Ar/ 39Ar incremental-heating method to obtain precise eruption ages for young, holocrystalline alkali basalt samples in the NVP.

Early Yellowstone hotspot magmatism and gold metallogeny

High-grade epithermal gold deposits in the Northern Great Basin have long been associated with regional Miocene basaltic to rhyolitic volcanism. Previous models for the low-sulfidation epithermal gold ores in this region have generally portrayed the bimodal magmas as a source of heat to drive large-scale convection of meteoritic water that leached gold from crustal sources and deposited it in hydrothermal vein systems, or required that the gold evolve from fractionated silicic magmas. New data of the present study indicate a more direct genetic link to the plume-related basaltic magmas of the region. Laser 40Ar/ 39Ar incremental heating plateau ages for single crystals of adularia from several of these low-sulfidation epithermal gold deposits range from 16.6 Ma to 15.5 Ma. Adularia from the Jumbo deposit yields three concordant plateau ages with a combined statistical result of 16.54 ± 0.04 Ma (95% confidence level, MSWD = 0.23). Plateau ages for adularia from other deposits in the region, and from gold-bearing veins in the Owyhee Mountains of southwestern Idaho, yield similar ages up to ~16.5 Ma, however some veins are as young as ca. 15.5 Ma and the grain-to-grain ages for a given sample can vary by up to ca. 0.5 Ma. Observed variations in age among the adularia crystals of a given rock sample indicate varying amounts of extraneous argon, and also loss of radiogenic 40Ar, among the population of grains for a particular sample. The single-crystal results are interpreted to indicate a 16.5–15.5 Ma interval for formation of gold-bearing adularia veins in the region. The initiation and duration of this gold-forming event appears contemporaneous (within uncertainties) with the basaltic volcanism at the Steens Mountain section and an ensuing one-million-year episode of basaltic volcanism from multiple centers in the region ( Brueseke et al., 2007). Trace amounts of lead are alloyed with gold in the deposits studied. The isotopic compositions of this lead are not compatible with regional crustal units that host the gold ores, or the silicic igneous lithologies of the region, but have the same lead isotopic composition as basalts of the earliest Yellowstone plume (represented by the earliest lavas of the Columbia River basalt province, the Steens basalts, and Stonyford Volcanic Complex; Hanan et al., 2008). We propose that the gold studied and its traces of alloyed lead were derived together from the mantle, released from basaltic magma chambers of the province, and carried by low-density fluids into shallow geothermal systems during the earliest stages of Yellowstone hotspot magmatism.

Exhumation of the Saualpe eclogite unit, Eastern Alps: constraints from 40Ar/39Ar ages and structural investigations

The Cretaceous Eclogite-Gneiss unit and its tectonic overburden (Micaschist, Phyllite and Lower Magdalensberg units) and the underlying Preims subunit of the Saualpe, Eastern Alps, have been investigated in order to constrain the mode of exhumation of the type locality of eclogites. 40Ar/39Ar ages of white mica from the eclogite-bearing unit suggest rapid, uniform cooling and exhumation between 86 and 78 Ma (Santonian-Campanian). Overlying units show upwards increasingly older ages with an age of 261.7 ± 1.4 Ma in the uppermost, low-grade metamorphic unit (Lower Magdalensberg unit). We consider this Permian age as geologically significant and to record a Permian tectonic event. Rocks of phyllite and micaschist units along western margins of the Saualpe block yield amphibole and white mica ages ranging from 123 to 130 Ma. These are considered to closely date the age of nappe stacking, whereas a single biotite age of 66–68 Ma from a shear zone is interpreted to date retrogression during normal faulting. Biotite and amphibole of Micaschist and Eclogite-Gneiss units show variable contents of extraneous argon. Consequently, their ages are in part geologically meaningless whereas other samples yield meaningful ages. The white mica ages from the Eclogite-Gneiss unit range from 78 to 85 Ma and argue for cooling through ca. 400 °C during the time as the westerly adjacent Upper Cretaceous Krappfeld collapse basin formed. The Preims subunit with paragneiss and marbles is considered to represent a large synmetamorphic shear zone at the base of the overthrusting Eclogite-Gneiss unit. The unit comprises a flat-lying foliation and a SE-trending lineation. This zone is interpreted to represent a zone of top-NW thrusting. A major ductile low-angle normal fault with top to ESE shear has been detected between the Eclogite-Gneiss and overlying units, and between the Micaschist and Phyllite units. The ductile thrust at the base and the low-angle normal fault at the top are considered to confine a NW-ward extruding high-pressure wedge. The new observations argue for rapid exhumation of a subducted high-pressure wedge within a subduction channel. Rapid surface erosion of the exhuming wedge might have facilitated exhumation. Eroded sedimentary rocks are preserved within adjacent Gosau basins, although only pebbles of low-grade metamorphic rocks of the uppermost tectonic unit can be found in these basins.

Limiting Effects of UHP Metamorphism on Length Scales of Oxygen, Hydrogen, and Argon Isotope Exchange: An Example from the Qinglongshan UHP Eclogites, Sulu Terrain, China

New and previously published stable isotope (oxygen and hydrogen) and 40Ar/39Ar data are compared from selected outcrops of the Sulu UHP terrain near Qinglongshan (Jiangsu Province, China). These rocks exhibit unusually low, heterogeneous δ18O and δD values acquired in a Neoproterozoic geothermal system despite undergoing Triassic (220-240 Ma) UHP metamorphism. Incremental heating 40Ar/39Ar analyses of muscovite, biotite, and K-feldspar from metagranite, quartzite, and gneiss (all metamorphosed to the coesite-eclogite facies) yield cooling ages between 190 and 204 Ma. In contrast, phengite 40Ar/39Ar data from eclogite, quartzite, and gneiss contain variable amounts of extraneous argon, consistent with inheritance from a Neoproterozoic protolith. Plots comparing phengite δ18O, δD, and 40Ar/39Ar total gas ages from different lithologies within individual Qinglongshan outcrops only meters apart highlight significant inter-outcrop isotopic heterogeneities (ΔDphengite = 26‰; Δ18Ophengite = 8.8‰; Δ40Ar/39Arphengite = 664 Ma); however maximum intra-outcrop isotopic variations between lithologies are limited (ΔDphengite = 5‰; Δ18Ophengite = 1.8‰). The oxygen and hydrogen isotopic variations are interpreted to reflect primary isotopic heterogeneities acquired during Neoproterozoic hydrothermal fluid circulation with cold-climate meteoric waters. The limited intra-outcrop isotopic variations suggest that extensive isotopic exchange occurred during UHP metamorphism within discrete outcrops, irrespective of lithology. Likewise, extraneous argon within the phengites reflects inheritance from a Neoproterozoic protolith, and the age variations between outcrops is probably due to differential argon loss during thermal and baric equilibration accompanying differential exhumation following UHP metamorphism. The retention of extraneous argon in phengite is partially controlled by the host rock lithology, inasmuch as as the armoring effects of basaltic eclogite are greater than quartzite or gneiss. Collectively, these data indicate that the length-scale of isotopic exchange is defined by contiguous blocks of coherent rock. Isotopic exchange during UHP metamorphism occurred between different lithologies restricted to discrete blocks within an accretionary prism, but did not communicate with other blocks. This study underscores the closed system (outcrop scale) behavior of isotopic exchange that can occur during continental subduction, collision, and uplift under UHP conditions.

40Ar– 39Ar dating of pseudotachylytes: the effect of clast-hosted extraneous argon in Cenozoic fault-generated friction melts from the West Antarctic Rift System

Fault-generated pseudotachylytes are the product of frictional melting during high-velocity slip associated with coseismic faulting. 40Ar– 39Ar dating of pseudotachylytes represents a powerful tool to directly determine the age of brittle faulting which is otherwise dated only indirectly. However, the pronounced spatial heterogeneity of most endogenically generated pseudotachylytes, due to the intimate coexistence of unmelted mineral clasts from the source rock, frictional glass and neogenic igneous minerals, often precludes a straightforward interpretation of argon data. This requires careful evaluation of the role of clasts in plaguing the age record of the pseudotachylyte matrix. This study exploits the full potential of the 40Ar– 39Ar method, using laser step-heating and laser in situ techniques in conjunction with textural and chemical characterisation at the microscale to disentangle the complexity of the first record of pseudotachylytes generated in the right-lateral fault systems which dissect the western shoulder of the West Antarctic Rift System. Pseudotachylytes occur as fault and injection veins. They exhibit glassy matrices with a potassium feldspar-like composition, and invariably contain, at a millimeter to microscopic scale, unmelted quartz and subordinate feldspars from the source rock. Injection veins characteristically contain much fewer mineral clasts. In situ ultraviolet (UV) laser analyses indicate that quartz from the host rock and from clasts within the fault veins contains significant amounts (∼10–50 ppb) of parentless 40Ar, most probably hosted in microscopic to submicroscopic fluid inclusions, associated with high Cl/K ratios. Infrared laser step-heating experiments on fault vein pseudotachylyte matrices yield strongly discordant age spectra with an overall saddle shape characterised by a minimum at ∼34 Ma, unrealistically old ages at high temperatures associated with high Cl/K ratios and total gas ages of 41.5–44.3 Ma. Petrographic data and the pronounced compositional similarity between in situ UV laser data on quartz and the high-temperature end member of step-heating analyses suggest that quartz is the main contaminant of the pseudotachylyte age record. In situ laserprobe analyses on pseudotachylyte matrices give concordant ages at ∼34 Ma for an injection vein and systematically older ages, clustering at ∼40–48 Ma, for fault veins. Although the age cluster fits into the Cenozoic tectonic framework of the region, it is an artefact due to the low spatial resolution of the argon laserprobe compared to the size and spatial distribution of quartz clasts within the pseudotachylyte. The isochron age of 34.11±0.96 Ma, derived from in situ data from the injection vein, overlaps with the emplacement age of syn-tectonic dykes from nearby areas and is interpreted to date a single episode of coseismic faulting. Regionally, the age of the studied pseudotachylytes represents the first direct onshore evidence of right-lateral strike–slip fault system activity in Victoria Land during the Cenozoic.

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

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

Distribution and significance of extraneous argon in UHP eclogite (Sulu terrain, China): insight from in situ 40Ar/ 39Ar UV-laser ablation analysis

In situ 40Ar/ 39Ar UV-laser ablation analyses of garnet, omphacite, epidote, phengite, barroisite and symplectite in a sample of UHP eclogite from Qinglongshan (Sulu terrain, China) reveal extremely heterogeneous argon isotopic ratios. Results from the 220–240 Ma eclogite indicate a significant and variable extraneous argon signature that is restricted to the early-formed phengite and epidote and later-formed barroisite. Individual, compositionally zoned, phengite grains record a large variation in 40Ar/ 39Ar dates (300–1100 Ma) that are positively correlated with celadonite content. These data are consistent with retrograde chemical exchange and argon isotopic repartitioning in response to decreasing partial pressures of argon during decompression. Consistent with the observation of atmospheric argon ratios in garnet, omphacite and symplectite these data support an interpretation that the extraneous Ar was incorporated early in the crystallization history of the eclogite, and not from a late infiltrating fluid. The ultimate source of the extraneous argon remains speculative, however on the basis of supporting stable oxygen and hydrogen isotope data from this locality, which support a closed isotopic system, it is inferred to have been inherited from the protolith of the UHP eclogite.

A re-examination of the influence of composition on argon retentivity in metamorphic calcic amphiboles

The effect of composition on argon retention in metamorphic hornblende has been re-evaluated by the 40 Ar 39 Ar step heating method utilizing the same samples from a K/Ar study in which a relationship between hornblende composition and argon retention was inferred. Incremental heating of the hornblendes yields 40 Ar 39 Ar age plateaus or near plateaus between 1081 ± 3 and 1112 ± 3 Ma. These data support a simple cooling history through argon closure following high-grade metamorphism, consistent with recent geochronological studies of the Bamble Sector of Norway. No clear evidence of extraneous argon is preserved in the 40 Ar 39 Ar data nor is there any sign of a thermal disturbance following argon closure. Contrary to the K/Ar results, the 40 Ar 39 Ar data show no relationship between hornblende cooling age and composition. The results of this investigation, together with those from other studies, suggest that the composition of metamorphic hornblende is not an important factor in controlling its argon closure temperature. Published studies inferring a compositional dependence on the argon closure temperature in metamorphic hornblende are commonly based on analytical data from impure, retrogressed, altered, recrystallized, or microstructurally complex hornblendes, which are not representative of the diffusive properties of a pure hornblende. Such mineralogical and structural complexities may form both above and below the closure temperature of hornblende, which may give rise to variable hornblende ages from rocks that have experienced the same geologic history. Such complexities forming at high temperature may contribute to a modified or variable characteristic grain size(s) of diffusion, resulting in a modified “complex hornblende” closure temperature. These complexities may contribute to low apparent “hornblende” ages if formed below its (true) closure temperature. In addition, variable hornblende ages may be obtained if fine-grained mineral intergrowths are present and undergo recoil-related phenomena during irradiation. Geologically meaningful hornblende ages are possible only when proper care is taken in sample selection and characterization.

40Ar/39Ar Geochronology of Post‐Valles Caldera Rhyolites, Jemez Volcanic Field, New Mexico

The40Ar/39Ar dating of postcollapse rhyolites in the Valles Caldera (1140 kyr) defines five intervals during which rhyolite domes, flows, and pyroclastic rocks were emplaced from ring fracture vents. Previously, ages were based on limited K‐Ar dating and stratigraphic relations. The 40Ar/39Ar dates are concordant with the previous dates for about half of the samples; others have ages significantly older (7–25%) despite evidence for small amounts of xenocrystic material. Eruptive activity occurred during relatively brief periods at 1133, 973‐915, 800‐787, 557‐521, and ∼300‐170 kyr separated by quiescent intervals of ∼115–300 kyr. Isochrons defined by single‐crystal sanidine fusions indicate trapped 40Ar/39Ar ranging from ∼282 to 326. Sanidine analyses yield reliable and consistent results, whereas biotite and glass data may be ambiguous. Biotite may have low radiogenic yields, contain variable amounts of extraneous argon, or define apparent isochrons discordant with cogenetic sanidine. Unaltered, unhydrated glass in one case yields an isochron concordant with coexisting sanidine, whereas in another is undateable due to large amounts of non‐radiogenic argon. The revised chronology has implications for the evolution of the Jemez magmatic system as well as for the geomagnetic polarity timescale. The discrete periods of volcanism, relations among rhyolites erupted within these intervals, and the duration of the intervening quiescent intervals are consistent with the generation, evolution, and eruption of several distinct magmas following caldera collapse. In addition, the 40Ar/39Ar data suggest revision of the Brunhes ‐ Matuyama polarity reversal boundary from the currently accepted 730 kyr to 780 kyr.

40Ar/39Ar thermochronology and Alleghanian development of the southernmost Appalachian Piedmont, Alabama and southwest Georgia

40Ar/39Ar age spectra of hornblende, muscovite, and microcline, and total fusion ages of biotite from metamorphic rocks of the Inner Piedmont, Pine Mountain, and Uchee belts are reported. Mineral cooling ages from the eastern part of the Inner Piedmont are as follows: hornblende, 320 Ma; muscovite, 296 Ma; biotite, 293 Ma; and microcline (diffusional release patterns) Tmax = 267 Ma, Tmin = 234 Ma. A 347 Ma hornblende spectrum from the highest Inner Piedmont structural level sampled is the oldest date determined and implies earlier passage of this level through the 500 °C isotherm. Most release spectra from Pine Mountain belt units are discordant with little or no apparent geologic meaning. Modified saddle-shaped release patterns for hornblende indicate extraneous argon with a maximum age of ∼358 Ma. Muscovite from the Pine Mountain belt cover sequence is 286 Ma (plateau age), and one from the underlying Grenville basement is 277 Ma (correlation age), indicating cooling below the 350 °C isotherm. Plateau ages on Uchee belt rocks are as follows: hornblende, from 297 to 288 Ma; muscovite, 285 Ma; biotite, 276 Ma; and microcline Tmax = 261 Ma, Tmin 230 Ma. Muscovite fish from a Bartletts Ferry fault zone phyllonite have a plateau age of 283 Ma. The 40Ar/39Ar results combined with other geologic data indicate that (1) a large part of the southern and Inner Piedmonts of Alabama and southwest Georgia experienced a late Paleozoic amphibolite-facies thermal and deformational event contemporaneous with the Alleghanian orogeny observed in the foreland; (2) the tectonic development of this event, characterized by initial crustal thickening followed by right-slip and normal-slip movements, is grossly similar to that described for the amphibolite-facies Alleghanian belt in the eastern Piedmont of South Carolina and Georgia; and (3) extensional movements along the flanks of the Pine Mountain window occurred between ca. 277 Ma and the Late Triassic-Early Jurassic and thus may reflect latest Alleghanian extensional collapse or Mesozoic rifting.

Cadomian vs. Variscan evolution of the Ossa-Morena zone (SW Iberia): field and 40Ar/ 39Ar mineral age constraints

Six hornblende and ten muscovite concentrates have been dated from three contrasting tectonic units exposed within the Ossa-Morena zone of the Iberian Massif. These include: (1) the Obejo-Valsequillo domain (north of the Badajoz-Córdoba shear zone); (2) the Sierra Albarrana structural unit (immediately south of the Badajoz-Córdoba shear zone); and (3) the thermal some exposed within the Olivenza-Monesterio antiform (Zafra-Monesterio domain). Hornblende from foliated amphibolite in the Obejo-Valsequillo domain (Siera Negra unit) displays internally discordant 40Ar/ 39Ar age spectra as a result of both intracrystalline contamination with extraneous argon and slight rejuvenation. 36Ar/ 40Ar vs. 39Ar/ 40Ar isotope-correlations are generally well-defined and yield ages of c. 550–560 Ma. These are interpreted to date cooling following late Precambrian-early Paleozoic (Cadomian) tectonothermal activity. This was followed by slight Variscan rejuvenation at c. 375–400 Ma. Muscovite from this unit displays internally discordant age spectra which reflect more extensive Variscan rejuvenation of intracrystalline argon systems which had initially cooled below appropriate blocking temperatures at c. 560 Ma. Hornblende and muscovite from the Sierra Albarrana domain (Sierra Albarrana Group) record 40Ar/ 39Ar plateau and isotope-correlation ages which range between c. 392 Ma and 351 Ma. These results indicate that complete Variscan rejuvenation was followed by relatively rapid post-metamorphic cooling. Hornblende separated from various lithologie elements exposed in the Olivenza-Monesterio Antiform yield variable results. Hornblende within a mafic xenolith in the Monesterio Granodiorite records an isotope-correlation age of c. 553 Ma which is interpreted to date the last cooling through appropriate argon retention temperatures. Hornblende within amphibolite of the Montemolin Series (lower Serie Negra) records internally discordant age spectra which reflect extensive Variscan (c. 390–400 Ma) rejuvenation of intracrystalline argon systems which had initially recorded ages > c. 500 Ma. Muscovite was concentrated from several lithologie elements in this area, including: (1) mylonitic Monesterio Granodiorite; (2) metasedimentary rocks of the Montemolin succession (displaying high-grade, sillimanite-potassium feldspar-bearing assemblages); and (3) migmatitic schist of the Siere Negra Group. These muscovite concentrates display similarly discordant 40Ar/ 39Ar age spectra which suggest significant but incomplete Variscan rejuvenation occurred at c. 400 Ma. This appears to have affected intracrystalline argon systems which had initially cooled through appropriate closure temperatures sometime prior to c. 450 Ma following an initial high-grade (Cadomian?) metamorphism. The 40Ar/ 39Ar results clearly reflect a variable, complex, and polymetamorphic evolution for the Ossa-Morena zone. Internal consistency of data within each regional tectonic unit compared with marked contrast between the units provides evidence of the extremely heterogeneous Variscan tectonothermal overprint. The results demonstrate that the present tectonic architecture of the Ossa-Morena Zone was not solely the result of late Variscan wrench tectonics, but, instead, a consequence of processes active since at least the Lower-Middle Devonian. Juxtaposition of regional structural units with contrasting tectonothermal histories, and recording variable rates and extents of uplift are consistent with maintainence of an overall transpressional Variscan tectonic regime.