GeoScienceWorld
Volume

Advances in 40Ar/39Ar Dating:

From Archaeology to Planetary Sciences

Edited by F. Jourdan, D. F. Mark and C. Verati

Abstract

Decoding the complete history of Earth and our solar system requires the placing of the scattered pages of Earth history in a precise chronological order, and the 40Ar/39Ar dating technique is one of the most trusted dating techniques to do that. The 40Ar/39Ar method has been in use for more than 40 years, and has constantly evolved since then. The steady improvement of the technique is largely due to a better understanding of the K/Ar system, an appreciation of the subtleties of geological material and a continuous refinement of the analytical tools used for isotope extraction and counting. The 40Ar/39Ar method is also one of the most versatile techniques with countless applications in archaeology, tectonics, structural geology, orogenic processes and provenance studies, ore and petroleum genesis, volcanology, weathering processes and climate, and planetary geology. This volume is the first of its kind and covers methodological developments, modelling, data handling, and direct applications of the 40Ar/39Ar technique.

    1. Page 1
    2. Page 9
      Abstract
      Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia and School of Earth Sciences, The University of Queensland, Queensland 4072, Australia (e-mail: Ian.McDougall@anu.edu.au)

      Isotopic dating of geological samples using the K/Ar method and its variant, the 40Ar/39Ar dating technique, provides ages that in favourable circumstances are precise and accurate to within 1%. Limiting factors include the accuracy of the decay constants for 40K and the age of neutron fluence monitor minerals. For rapidly cooled igneous rocks, a K/Ar or 40Ar/39Ar age normally will give a good estimate of the age of eruption, but for slowly cooled igneous rocks or metamorphic rocks an age measured on a sample is likely to be a cooling age. As 40Ar*/39ArK ratios can now be determined often to better than 0.1%, the main limitation on accuracy relates to how well the 40K decay constants are known. Better determination of the β and γ decays of 40K, the basis for the decay constants, is suggested at the present time, rather than adoption of new decay constants linked to another decay scheme. Until improved independently measured decay constants for 40K become available, there may be some circumstances where the newly proposed decay constants need to be used, especially when comparing ages on volcanic igneous rocks with those measured in another system.

    1. Page 21
      Abstract
      Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA and Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA (e-mail: prenne@bgc.org)

      A recent calibration of the 40Ar/39Ar geochronometer is based on an optimization analysis of 40K activity data, isotopic data for the Fish Canyon sanidine (FCs) standard, and pairs of 40Ar/39Ar+238U–206Pb data from selected samples meeting well-documented quality criteria. Inclusion of 238U–206Pb data in the calibration incorporates the precisely known 238U decay constant. Thus, this calibration is inherently consistent with the U–Pb chronometer. Initial presentation of the calibration included an inappropriate datum and should be eschewed in preference to a revision. Compared with previous calibrations, including those focusing mainly on the age of a standard (e.g. astronomical calibrations), the optimization calibration provides superior accuracy in the sense of propagated age uncertainty, particularly for ages much older than the FCs. Recent literature reveals that the optimization-based calibration has been misused and misrepresented in some cases; discussion of these cases clarifies the correct usage of the approach. Apparent conflict between 40Ar/39Ar and 238U–206Pb ages for a Quaternary tuff do not appear to be a result of error in one of the three parameters determined by the optimization approach for the 40Ar/39Ar system. The optimization approach easily accommodates new constraints, but rigorous quality control is needed to maintain accuracy.

    2. Page 33
      Abstract
      Corresponding author (e-mail: f.jourdan@curtin.edu.au)

      The 40Ar/39Ar dating technique requires the transformation of 39K into 39Ar by neutron activation. Neutron activation has undesirable secondary effects such as interfering isotope production, and recoil of 39Ar and 37Ar atoms from their (dominant) targets of K and Ca. In most cases, the grains analysed are large enough (>50 μm) such that the amount of target atoms ejected from the grains is small and has a negligible effect on the ages obtained. However, increasing needs to date fine-grained rocks requires constraining, and in some cases correcting for, the effect of nuclear recoil. Previous quantitative studies of recoil loss focus mostly on 39Ar. However, 37Ar loss can affect the ages of Ca-rich minerals via interference corrections on 36Ar (and, to a lesser extent, 39Ar), yielding lower 40Ar*/39ArK and, thus, an age spuriously too young. New results focused on 37Ar recoil by measuring the apparent age of multi-grain populations of Ca-rich minerals including Fish Canyon plagioclase (FCp) and Hb3gr hornblende, with discrete sizes ranging from 210 to <5 µm. We use previous result on sanidine grains to correct for the 39Ar recoil loss. For the finest fractions, FCp and Hb3gr apparent ages are younger than the 39Ar recoil-corrected ages expected for these minerals, with a maximum deviation of −40% (FCp) and −21% (Hb3gr) reached for grains below 5 μm. We calculate 37Ar-depletion values ranging from approximately 30 to 91% and from approximately 28 to 98% for plagioclase and hornblende, respectively. This results in x0 values (mean thickness of the partial depletion layer) of 3.3±0.4 μm (2σ; FCp) and 3.6±1.4 μm (Hb3gr), significantly higher than suggested by current models. The reason for the substantial 37Ar loss is not well understood, but might be related to the radiation damage caused to the mineral during irradiation. x0 (39Ar) and x0 (37Ar) values obtained in this study, along with crystal dimensions, can be used for correcting 40Ar/39Ar ages from 39Ar and 37Ar recoil loss. We also discuss the relevance of our results to vacuum-encapsulation studies and isotopic redistribution in fine-grained minerals.

      Supplementary material: Annex 1, 2 and 3 are available at http://www.geolsoc.org.uk/SUP18610. Annex 1 and 2: Raw argon data corrected for blank, mass discrimination and radioactive decay for Fish Canyon plagioclase (Annex 1) and Hb3gr hornblende (Annex 2). Annex 3: Step-heating 40Ar/39Ar age spectra for FCp (Fig. A3.1) and Hb3gr (Fig. A3.2).

    3. Page 53
      Abstract
      Department of Earth and Environmental Sciences, University of Michigan, 1100 N. University Ave. Ann Arbor, MI 48109-1005, USA (e-mail: cmhall@umich.edu)

      Advances in the 40Ar/39Ar method using new techniques have led to considerable efforts to improve the accuracy of the calibration of interlaboratory standards. However, the accuracy of the method ultimately relies on the measurement of 40Ar*/39ArK ratios on standards that have been calibrated with the K–Ar method. Usually a 40Ar/39Ar total gas age is assumed to equate to a K–Ar age, but this assumes that there is zero loss of 39Ar due to recoil. Traditional 40Ar/39Ar total gas ages are Ar retention ages and are not strictly comparable to K–Ar ages. Efforts to estimate the importance of this effect on standards have relied on indirect evidence for 39Ar recoil. We report direct measurements of 39Ar recoil for primary and secondary standards using the vacuum-encapsulation technique and show that adjustments to some standard ages may be needed. Revised ages corrected for recoil are given for hornblendes MMhb-1 and Hb3gr, biotites GA1550 and FCT-3 and sanidines FCT-2 and TCR-2. The results show that, in most cases, recoil loss exceeds that which would be expected from grain size and geometry. Internal defects within mineral grains are likely the dominant control on the fraction of recoiled 39Ar lost from these standards.

      Supplementary material: Detailed data tables are available at http://www.geolsoc.org.uk/SUP18587.

    4. Page 63
      Abstract
      Corresponding author (e-mail: leah.morgan@glasgow.ac.uk)

      Sanidine from the Fish Canyon Tuff is regularly used as a neutron flux monitor by 40Ar/39Ar geochronologists. A new sampling of the Fish Canyon Tuff (denoted FCs-EK) has yielded sanidine that relative to FC-2 is confirmed here to have an R value of 0.9997±0.0100, which is indistinguishable from 1. The new sample will allow for continuity as many 40Ar/39Ar laboratories have exhausted their supplies of FC-2. FCs-EK is now available in significant quantities from the Scottish Universities Environmental Research Centre.

      Supplementary material: A table of Ar isotopic data is available at http://www.geolsoc.org.uk/SUP18632.

    5. Page 69
      Abstract
      Corresponding author (e-mail: heri.alexandra@gmail.com)

      Muscovite B4M, distributed in 1961 as an age standard, was ground under ethanol. Five grain size fractions were obtained and characterized by X-ray diffraction. They display a mixing trend between a phengitic (enriched in the fraction <0.2 µm) and a muscovitic component (predominant in the fraction >20 µm). High-pressure phengite is preserved as a relict in retrograde muscovite. Electron microprobe analyses of the distributed mineral separate reveal at least four white mica populations based on Si, Al, Mg, Na, Fe and F. Rb/K ratios vary by one order of magnitude. Rb–Sr analyses link the mineralogical heterogeneity to variable Rb/Sr and 87Sr/86Sr ratios. The grain size fractions define no internal isochron. Relict fine-grained phengite gives older ages than coarse-grained retrograde greenschist facies muscovite. The inverse grain size–age relationship also characterizes 39Ar/40Ar analyses. Cl/K anticorrelates with step ages: Cl-rich coarse muscovite is younger than Cl-poor fine relict phengite. Sr and Ar preserve a similar isotopic inheritance despite peak metamorphism reaching 635±20 °C. A suitable mineral standard requires that its petrological equilibrium first be demonstrated. Relicts and retrograde reaction textures are a guarantee of isotopic disequilibrium and heterogeneous ages within single crystal at the micrometre scale.

      Supplementary material: Electron microprobe results on two grain mounts of the unprocessed B4M separate as distributed and on a whole-rock thin section of Brione gneiss from the teaching collection of the Universität Bern are available at: http://www.geolsoc.org.uk/SUP18590.

    6. Page 79
      Abstract
      Corresponding author (e-mail: d.barfod@suerc.gla.ac.uk)

      Lasers are fundamental tools for sampling in geochemical studies and have found wide application in mass spectrometric sample introduction systems. Here we describe an isotope extraction method for 40Ar/39Ar geochronology using a new scanning CO2 laser system. This method can partially un-mix radiogenic (40Ar*) from trapped argon components and provides an alternative to furnace step-heating methods. A key advantage of the laser scanning method developed at the Scottish Universities Environmental Research Centre (SUERC) is the ability to step-heat samples as large as 100 mg to fusion using low raster speeds, although care must be taken to avoid self-shielding of grains and proper laser targeting. The scanning laser extraction system has the potential for lower overall blanks and the ability to run blanks and calibrations between steps of a heating sequence. This provides better control on system performance and characterization during sample measurement and can result in improved data quality.

      Supplementary material: Ar/Ar data presented in Figures 6a–c and 7 (obsidian) is available at http://www.geolsoc.org.uk/SUP18694.

    7. Page 91
      Abstract
      Corresponding author (e-mail: tmark.harrison@gmail.com)

      It is axiomatic that application of closure theory – the foundation of isotope-based thermochronology – requires an empirical diffusion model. It is therefore surprising that the majority of thermochronological studies have not met this requirement. The advent of the multi-diffusion domain (MDD) model transcended this limitation yielding both diffusion and age information via routine 40Ar/39Ar step-heating of K-feldspar. Observed correlations between age and Arrhenius spectra show that Ar diffusion occurs by the same mechanisms in nature as in the laboratory. Under certain conditions, these data permit the recovery of a unique, cooling history. The community reaction included some unproductive lines of argument but some stimulated refinements of the MDD model that benefited the development of thermochronology. The MDD model was recently applied to muscovite upon recognition that the same diffusion mechanism operates in vacuum step-heating as in nature. The advent of 40K–40Ca closure profile dating opens up a new thermochronological approach. Initial results confirm that muscovite intragrain defects can restrict effective diffusion length scales in white micas from 10–100 s of microns. Our hope for the future of the MDD model is that it be subject to aggressive and sceptical testing by the community in which quantification is valued over assertion.

    8. Page 107
      Abstract
      Università di Milano Bicocca, 20126 Milano, Italy (e-mail: igor@geo.unibe.ch)

      The mechanisms of Ar release from K-feldspar samples in laboratory experiments and during their geological history are assessed here. Modern petrology clearly established that the chemical and isotopic record of minerals is normally dominated by aqueous recrystallization. The laboratory critique is trickier, which explains why so many conflicting approaches have been able to survive long past their expiration date. Current models are evaluated for self-consistency, especially Arrhenian non-linearity which leads to paradoxes. The models’ testable geological predictions suggest that temperature-based downslope extrapolations often overestimate observed geological Ar mobility substantially. An updated interpretation is based on the unrelatedness of geological behaviour to laboratory experiments. The isotopic record of K-feldspar in geological samples is not a unique function of temperature, as recrystallization promoted by aqueous fluids is the predominant mechanism controlling isotope transport. K-feldspar should therefore be viewed as a hygrochronometer. Laboratory degassing proceeds from structural rearrangements and phase transitions such as are observed in situ at high temperature in Na and Pb feldspars. These effects violate the mathematics of an inert Fick's Law matrix and preclude downslope extrapolation. The similar upward-concave non-linear shapes of Arrhenius trajectories of many silicates, hydrous and anhydrous, are likely common manifestations of structural rearrangements in silicate structures.

    9. Page 117
      Abstract
      Corresponding author (e-mail: marnie.forster@anu.edu.au)

      Step-heating experiments in vacuo are routine when conducting 40Ar/39Ar geochronology, including for white mica. White mica can break down, due to dehydroxylation and delamination, so experiments involving mica are often conducted in relative haste, and not with the care and precision necessary when intending to apply multi-diffusion-domain theory to model the results. Here we show, however, that carefully managed step-heating experiments appear to allow release of argon through solid-state diffusion processes alone. We analysed phengite-muscovite intergrowths in high-pressure metamorphic rocks exhumed in and beneath extensional ductile shear zones during continental extension. Such materials often yield Arrhenius plots in which there is a distinct steepening of slope mid-way through the step-heating sequence. This steepening appears to correspond with steps in which release of argon from phengite components dominate. We analysed the data using a computer program (eArgon) and numerically simulated mixing of gas released from multiple diffusion domains. The results suggest that diffusion of 39Ar in phengitic white mica involves radically different diffusion parameters in comparison with muscovite. If these results extrapolate to nature then 40Ar/39Ar geochronology may allow direct dating of white mica mineral growth during metamorphism.

      Supplementary material: Data files A, B and C are available at http://www.geolsoc.org.uk/SUP18619. Data file A C++ computer code used to infer data for an Arrhenius plot, assuming different diffusion geometries. These methods are excerpted from the eArgon computer program used to analyse these data.

      Data file B Analytical methods and procedures used in the laboratory for 40Ar/39Ar geochronology performed on the samples reported.

      Data file C XML formatted data tables for the step-heating experiments reported in this study, in a form that can be read by the eArgon computer program.

    10. Page 137
      Abstract
      Corresponding author (e-mail: jo-anne.wartho@asu.edu)

      We describe the first direct measurements of Ar diffusion and solubility in plagioclases using ultra-violet (UV) laser ablation depth-profiling and noble gas mass spectrometer analyses of experimentally treated (599–1000 °C, 50–200 MPa of Ar) crystal fragments of labradorite and oligoclase. Labradorite 40Ar gain diffusion profiles were measured, yielding an activation energy of 26.72±4.58 kcal mol−1 (118.0±19.16 kJ mol−1) and a frequency factor of 9.77×10−9 (+8.79×10−8, −8.79×10−9) cm2 s−1 (95% confidence). The Ar solubility in labradorite was measured yielding a value of <0.2 ppb bar−1, which is similar to or lower than many rock forming minerals.

      The labradorite diffusion parameters indicate Ar closure temperatures of 211 °C for a spherical diffusion geometry, and 243 °C for a planar diffusion geometry (for 100 µm-diameter grains, with cooling rates of 10 °C Ma−1). The data indicate that labradorite is less Ar retentive than K-feldspar at low temperatures, but more Ar retentive than K-feldspar at high temperatures, corroborating previous work on plagioclase. The relatively slow Ar diffusion rates in labradorite at magmatic temperatures may explain the common observation of older ages in large plagioclase grains in acidic volcanic systems.

      Supplementary material: Details of the UV laser depth profiles obtained from the labradorite and oligoclase samples used in this study are available at http://www.geolsoc.org.uk/SUP18608.

    11. Page 155
      Abstract
      Corresponding author (e-mail: verati@unice.fr)

      The 40Ar/39Ar technique is the most commonly used technique to date basaltic rocks. For basaltic rocks older than about 30 Ma, the dating of plagioclase separates is preferred over groundmass as the latter is susceptible to containing cryptic alteration due to fluid circulations, difficult if not impossible to remove during sample preparation. Alteration under such metamorphic conditions progressively forms K-rich sericite after plagioclase. Owing to its transparency, plagioclase allows a distinction to be made optically between partially–completely altered grains and fresh grains. However, practice shows that grains that contain less than about 1% of sericite are hard to identify under the stereomicroscope. Owing to the high K2O content (c. 10 wt%) of sericite, such compromised grains can have dramatic effects on the age determination of plagioclase.

      Here, we investigate and quantify the effect of sericite on the 40Ar/39Ar age determination of plagioclase using a numerical model with multiple variable parameters. We show that the most influential parameter is the time difference between the crystallization of plagioclase and the sericitization event. We also show that for some continental flood basalts, even 0.1 wt% of sericite can bias the apparent age of a plagioclase separate by several hundred thousand years. The presence of sericite can be identified using a combination of Ca/K ratios, age spectra, and 39Ar and 37Ar degassing curves obtained during a conventional 40Ar/39Ar step-heating procedure. When the age of the fresh plagioclase and its Ca/K ratio are known, the percentage of sericitization and the age of the alteration event can be estimated. Ultimately, above approximately 65% of sericitization, the apparent age measured on the altered plagioclase is within ±1% of the age of the alteration event, with implications for accurately dating low-temperature metamorphism and mineral deposit formations.

      Supplementary material: Further details of calculation are available at http://www.geolsoc.org.uk/SUP18609.

    12. Page 175
      Abstract
      Corresponding author (e-mail: grenville.turner@manchester.ac.uk)

      The earliest publications on 40Ar/39Ar dating recognized the usefulness of 39Ar/37Ar in providing chemical clues to the species releasing argon during stepped heating, while 37Ar measurements are required for interference corrections. Aside from these essential roles, 36Ar, 37Ar and 38Ar have other useful applications. The dominance of Ca as the target nucleus for the production of 38Ar and 36Ar from spallation by cosmic rays has led to the determination of cosmic ray exposure ages from 38Ar/37Ar. 38Ar production from Cl has an important role in the study of ore minerals containing saline fluid inclusions, both in dating and in understanding their genesis. Combining 38Ar/36Ar ratios with micro-thermometric determinations of salinity provides a way to determine absolute concentrations of noble gases in ore fluids. Absorption of cosmic-ray-produced secondary neutrons by 35Cl and 37Cl provides another means to study cosmic ray exposure of meteorites. 36S excesses in meteoritic sodalite provide evidence of now-extinct 36Cl in the early Solar System. Surprisingly, there is little or no evidence of excess 36Ar beyond what can be accounted for by secondary neutron exposure. We have recently devised a method to identify monoisotopic 36Ar from 36Cl decay.

    1. Page 189
      Abstract
      Corresponding author (e-mail: wangfei@mail.iggcas.ac.cn)

      40Ar/39Ar geochronology was carried out on the non-deformed synorogenic granitoid plutons from the Qinling–Dabie Orogen. The new model cooling history (MCH) method is applied to K-feldspar to obtain information about its cooling history. MCH is a quicker and easier method of calculating cooling histories compared with the multi-domain diffusion (MDD) model. Cooling histories indicate that Qinling–Dabie suffered differential uplift and denudation processes since the collision in the Late Triassic. East Qinling uplifted and denuded rapidly from c. 181–187 to c. 150 Ma at a rate of c. 20 °C Ma−1, three times as fast as west Qinling which was exhumed at a rate of c. 6.6 °C Ma−1 from c. 195–190 to c. 100 Ma. Although west and east Qinling started uplift and denudation at the same time, east Qinling reached the 150 °C geothermal line (c. 7.5 km deep) at c. 150 Ma, earlier than west Qinling by c. 50 myr, suggesting that earliest collision and strongest compression occurred in east Qinling. Dabie is characterized with a polyphased process of uplift and denudation, implying that the subduction of south China block in this region was multi-staged. The emplacement of the granitoids and uplift pattern along Qinling–Dabie may be due to delamination of the root of the mountains or the slab break-off. Cooling histories suggest that more mass had been removed from the root of east Qinling than west Qinling and Dabie; the final delamination occurred in Dabie at c. 100 Ma but affected the whole Qinling–Dabie orogen.

    2. Page 207
      Abstract
      Corresponding author (e-mail: christoph.leitner@sbg.ac.at)

      Polyhalite rocks of the evaporitic Haselgebirge Formation are emplaced within a rocksalt–mudrock tectonite. The rheologically weak series served as a major detachment level during nappe stacking of the Northern Calcareous Alps (Eastern Alps). To test the mineral polyhalite [K2Ca2Mg(SO4)4·2H2O] as a useful geochronometer for various diagenetic and deformation fabric types, 40Ar/39Ar age dating was combined with microstructural analysis. Vein infills, polyhalite intergrown with anhydrite and polyhalite within mudrock, crystallized in several stages between c. 235 and 210 Ma. Mylonites of fine-grained polyhalite rock indicate subsequent stages of tectonothermal overprint between c. 155 and 105 Ma, which is roughly consistent with previously measured feldspar and muscovite 40Ar/39Ar data from the region. Illite crystallinity points to temperatures of c. 200 °C. The peak temperature of overprint was at c. 180 °C in the Berchtesgaden mine (vitrinite reflectance, fluid inclusions) and >240 °C in the Altaussee mine (fluid inclusions). These temperatures are below the value of 255 °C, where polyhalite starts to dehydrate. Disturbed age spectra patterns result from multiphase polyhalite growth; however, single phases and completely recrystallized fabrics yield good results. As in the Alpine test case, polyhalite may characteristically serve as a geochronometer for diagenetic and very-low-grade metamorphic processes.

      Supplementary material: A table of used materials and methods and the detailed 40Ar/39Ar step-heating data for polyhalite are available at: http://www.geolsoc.org.uk/SUP18574.

    3. Page 225
      Abstract
      Corresponding author (e-mail: jbenowitz@alaska.edu)

      To address Miocene–present episodic v. persistent exhumation, we utilize a simple graphical procedure that vertically stacks spatially diverse K-feldspar 40Ar/39Ar multi-domain diffusion (MDD) models from the length of the approximately 100 km-long high-peak region of the Eastern Alaska Range. We supply additional constraints with 40Ar/39Ar mica dating because the higher closure-temperature-window places limits on the initiation of rapid Eastern Alaska Range exhumation. We also provide a broad 40Ar/39Ar K-feldspar minimum closure age data set to add more detail on spatial patterns in the regional exhumation history for the Eastern Alaska Range. We find that rapid and persistent exhumation has occurred in the Eastern Alaska Range since about 24 Ma at a long-term rate of approximately 0.9 km/Ma, but that this rapid exhumation is spatially variable through time. Onset of rapid Eastern Alaska Range exhumation is coincident with the initiation of rapid exhumation in SW Alaska, the Western Alaska Range and the Chugach–Saint Elias Range at around 25 Ma, implying a region-wide deformational response to a change in tectonic forcing. The initiation of highly coupled flat-slab subduction of the Yakutat microplate is probably responsible for this prolonged period of rapid exhumation in southern Alaska.

      Supplementary material: Sample locations from the Eastern Alaska Range, and 40Ar/39Ar data tables and age spectrum figures are available at http://www.geolsoc.org.uk/SUP18603.

    4. Page 245
      Abstract
      Corresponding author (e-mail: sidney@ldeo.columbia.edu)

      Iceberg discharges into the North Atlantic are important sources of fresh water, and the sediments they deposit can provide constraints on which sectors of different ice sheets were contributing icebergs. 40Ar/39Ar ages of sand-sized hornblende grains provide useful constraints on IRD (ice-rafted detritus) source areas.

      Heinrich events are intervals of anomalously high percentages of IRD in marine sediment cores of the North Atlantic IRD belt. In contrast to the others, Heinrich event 3 (H3) records a significantly lower flux of IRD. This study compares 40Ar/39Ar hornblende age distributions from the interval around and including H3 in giant gravity core EW9303-GGC31 from Orphan Knoll, in the southern part of the Labrador Sea, with piston core V28-82 in the eastern part of the North Atlantic IRD belt. Collectively, these results confirm that H3 represents a Hudson Strait IRD event, but that it was smaller than during H1, H2, H4 and H5, and therefore comprises only a small fraction of the detritus at the eastern North Atlantic location of V28-82. These results support a previously published interpretation of across-strait ice flow during H3 at Hudson Strait.

      Supplementary material: Appendix 1 is 40Ar/39Ar data from core EW9303-GGC31; Appendix 2 is grain counts across H3 from core V28-82; Appendix 3 is 40Ar/39Ar data from core V28-82; these are available at http://www.geolsoc.org.uk/SUP18631.

    5. Page 265
      Abstract
      Corresponding author (e-mail: sflude@gmail.com)

      New data from a gem-quality feldspar from Itrongay, Madagascar, record naturally occurring 40Ar/39Ar age profiles which can be numerically modelled by invoking a single diffusion mechanism and show that microtexturally simple crystals are capable of recording complex thermal histories. We present the longest directly measured, naturally produced 40Ar*-closure profiles from a single, homogeneous orthoclase feldspar. These data appear to confirm the assumption that laboratory derived diffusion parameters are valid in nature and over geological timescales. Diffusion domains are defined by crystal faces and ancient cracks, thus in gem-quality feldspars the diffusion domain size equates to the physical grain size. The data also illustrate the potential of large, gem-quality feldspars to record detailed thermal histories over tens of millions of years and such samples should be considered for future studies on the slow cooling of continental crust.

      Supplementary material: Ar-isotope data, standards and constants used in calculations and irradiation parameters are available at http://www.geolsoc.org.uk/SUP18720.

    1. Page 277
      Abstract
      Corresponding author (e-mail: suchin@hku.hk)

      Well-preserved Mesozoic terrestrial fossils were discovered in the Haifanggou Formation and the overlying Lanqi Formation (or their correlative strata) in NE China. The recent discoveries of Schmeissneria sinensis and Xingxueanthus sinensis from the middle and upper Jurassic Haifanggou Formation provide evidence that the origin of angiosperms could be predate the Early Cretaceous. In addition to the finding of pre-Cretaceous angiosperms from the Haifanggou Formation, the overlying Lanqi Formation yields a rich and varied terrestrial flora. The high diversity and abundance of the palaeoflora from these formations provide a unique window to understand floral evolution and its diversification in the Mesozoic. Two tuff samples and one andesite sample collected from the Haifanggou and Lanqi formations near Beipiao City, Liaoning, NE China yield robust 40Ar/39Ar age results. Our 40Ar/39Ar age of 166.7 ± 1.0 Ma for plagioclases from one tuff interbedded in the fossiliferous horizons of the middle Haifanggou Formation provides accurate age calibration for the pre-Cretaceous angiosperms for the first time. Moreover, our age results for these fossil-bearing formations will improve our knowledge of the Jurassic environment in general, including the link between plants and atmospheric CO2.

      Supplementary material: Details of analysis procedures, Ar isotopic data corrected for blanks, mass discrimination, radioactive decay and J values are available at: http://www.geolsoc.org.uk/SUP18575.

    2. Page 285
      Abstract
      Corresponding author (e-mail: verati@unice.fr)

      Mineral separates of adularia have been extracted from three samples of highly silicified hydrothermal breccias, newly discovered in the active Bouillante geothermal field (Guadeloupe archipelago), and investigated by 40Ar/39Ar geochronology in order to constrain the timing of geothermal activity in this part of the active Lesser Antilles island arc. The inverse isochron diagram indicates an age of 248±50 ka (2σ) for all adularia from one breccia sample (n=8), with an initial 40Ar/36Ar ratio of atmospheric composition (309±12 (2σ)) attesting that this age is valid. This age is concordant with the weighted mean age of 290±40 ka for the same sample. Adularia from other samples yields concordant ages. The obtained 40Ar/39Ar ages can be related either to the magmatic activity of the Bouillante Volcanic Chain (c. 850–250 ka ago) or to the initiation of the volcanic activity of the active Grande Découverte–Soufrière system (200 ka ago–present day). Our results demonstrate that the Bouillante hydrothermal event is coeval with change in the volcanic pulses previously recognized in the magmatic history of the studied area. The possible duration calculated for this hydrothermal activity requires at least two superposed volcanic pulses to be developed.

    1. Page 297
      Abstract
      Corresponding author (e-mail: Jisun.Park@rutgers.edu)

      The 40Ar/39Ar ages of a group of Martian meteorites called shergottites are systematically older by about 25% or more than ages obtained using Sm–Nd and other radiometric dating methods. The older 40Ar/39Ar ages indicate the presence of 40Ar not derived in situ from the radiogenic decay of 40K. The ‘excess’ argon can be associated with several different components, including the Martian atmosphere and mantle. We discuss the sources of Ar in shergottites, and the methods used to separate and identify them.

    2. Page 317
      Abstract
      Corresponding author (e-mail: ewalton@ualberta.ca)

      Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of NW Africa (NWA) 4797. Shock heating of NWA 4797 completely melted and vesiculated precursor igneous plagioclase, which cooled to an assemblage of plagioclase crystals with interstitial glasses of variable composition (Ca/K ratios). Using a focused ultraviolet laser beam, is has been possible to distinguish between argon isotopic signatures from groundmass minerals (igneous olivine + pyroxene), plagioclase and a shock vein. This study focuses on the potential for this meteorite to shed light on shock ages of shergottites.

      Apparent 40Ar/39Ar ages of groundmass minerals show that there are large amounts of excess argon in this phase, yielding a wide range of calculated ages from 690 ± 30 Ma to several apparent ages older than 4.5 Ga. A traverse of laser-probe extractions across the 1 mm-diameter shock vein in NWA 4797 yielded apparent 40Ar/39Ar ages younger than the groundmass. A signature of the Martian atmosphere, identified by 40Ar/36Ar ratios of 1600–1900, was not found in the NWA 4797 shock vein. This is distinct from other shergottites where the products of shock melting contain a nearly pure sample of Martian atmosphere. We attribute this to a distinct formation mechanism, and hence gas-trapping mechanism, of the NWA 4797 shock vein.

      We undertook 44 analyses of plagioclase areas identified by SEM analysis. Ages ranged from 45 ± 27 to 3771 ± 109 Ma and yield an average age of 375 ± 77 Ma, considerably younger than ages obtained in this study from either the groundmass or the shock vein. A plot of age v. 37Ar/39Ar for plagioclase showed a continuum of ages from the oldest to youngest ages measured. Older ages are correlated with higher Ca/K ratios of plagioclase, indicating contamination from groundmass minerals rich in excess argon. The youngest ages correlate to plagioclase extractions with the lowest Ca/K ratios, interpreted to have crystallized from a nearly pure plagioclase melt with contributions from a K-rich mesostasis. We see no evidence for multiple shock events in NWA 4797. Rather, we favour the interpretation that the cosmic-ray exposure (CRE) age of 3.0±0.5 Ma, obtained on NWA 4797 in this study using cosmogenic 38Ar, approximates the timing of shock melting in this meteorite.

      Supplementary material: Laser probe argon isotopic data for NWA 4797 obtained in this study are available at http://www.geolsoc.org.uk/SUP18602.

    3. Page 333
      Abstract
      Corresponding author (e-mail: tswindle@lpl.arizona.edu)

      Impact ages have now been determined for c. 100 ordinary chondrites, providing information about impacts in the asteroid belt throughout the history of the Solar System. Most notably, there is evidence for (a) impact events in the first 100 Ma of Solar System history, during the accretion of material; (b) an increase in impact ages produced between 3500 and 4100 Ma, presumably related to the suspected ‘lunar cataclysm’; (c) many L chondrites being degassed during the disruption of their parent body slightly less than 500 Ma ago; and (d) at least one younger event in the H chondrites. There are no 40Ar/39Ar ages that clearly match the cosmic ray exposure ages of any meteorites, which means that the collisions that liberated the fragments we now have as meteorites did not produce time–temperature histories capable of degassing radiogenic 40Ar. Since there is no way to know a priori whether an ordinary chondrite impact melt is the result of a recent or ancient event, much of the progress will continue to come from reconnaissance studies of shocked ordinary chondrites.

    4. Page 349
      Abstract
      Corresponding author (e-mail: d.mark@suerc.gla.ac.uk)

      The dating of terrestrial impact craters and impact glasses that exhibit high degrees of mineralogical complexity can be problematic. However, if the maximum potential of the terrestrial impact crater record is to be realized, accurate and precise ages for crater-forming events are critical. Here we report a high-precision 40Ar/39Ar age for the Dellen impact structure, Sweden. Previous radio-isotopic constraints show a wide variation in age as a result of poor sample characterization and analytical approach. A detailed petrographical and mineralogical study provides a solid foundation for interpretation of step-heating 40Ar/39Ar data, culminating in a statistically robust age of 140.82±0.51 Ma (2σ; full external precision) for the Dellen impact event, for which data disfavour an inherited argon component. Primary hydration of the impact melt during cooling–quenching and entrapment of molecular water promoted rapid loss of inherited 40Ar from the impact melt of rhyolitic composition. Duplicate analyses of the water content and ∂D of the glass give similar values for the former (1.9±0.1 μmol mg−1) but unexpectedly low values for the latter (−159±8‰), with scatter beyond the expected analytical reproducibility due to isotopic heterogeneity. This study highlights that the 40Ar/39Ar technique is unrivalled in its ability to precisely and accurately date the products of hypervelocity collisional events.

      Supplementary material: Raw 40Ar/39Ar data are available at http://www.geolsoc.org.uk/SUP18633.

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