Abstract Geological mapping in the Ogaden region of SE Ethiopia, integrated with aeromagnetic data, has revealed a large dyke swarm extending SSE more than 600 km from the southern Afar margin across the Somali Plate to the Ethiopia–Somalia border. 40 Ar/ 39 Ar age dating shows that emplacement occurred at 24–27 Ma, contemporaneous with early rifting and dyking in the Red Sea. Slab-pull forces generated at the Zagros subduction zone strained the lithosphere in the Afro-Arabian plate, and dyking began to extend south from the Red Sea at c. 27 Ma, extending across Afar, fed by a plume-related magma source, rather than the rift-related source prevailing along the Red Sea. Immediately south of the Afar margin, the dyke system was emplaced along the Precambrian Marda Fault Zone, and the continuation across the Ogaden may have been controlled by lithospheric weakness associated with a splay of the Marda Fault. We suggest that the Ogaden Dyke Swarm is a zone of crustal dilation continuing the Red Sea trend across the Horn of Africa and constituting the original third ‘arm’ of the Afar triple junction. Geochemical and geochronological analyses indicate that basaltic outpourings from the Ogaden Dyke Swarm flowed at least as far east as the Ethiopia–Somalia border and emanated from the same magma source as the Ethiopian flood basalts, which had erupted earlier at c. 30 Ma. Dykes are emergent only occasionally and are marked at the surface by linear sand-filled troughs varying from 2 to 20 m deep caused by tensional collapse above the dyke tip. Magnetic anomalies associated with the dykes vary in width up to 1 km and likely identify dyke zones. Supplementary material: Geological map of the Marda Fault Zone and surroundings; argon dating analytical protocol and isotopic data corrected from baseline; petrographic description of representative samples of the Ogaden Dyke Swarm and analytical protocol used for geochemistry are available at http://www.geolsoc.org.uk/SUP18829 .

Abstract Isotopic dating of geological samples using the K/Ar method and its variant, the 40 Ar/ 39 Ar 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 40 K and the age of neutron fluence monitor minerals. For rapidly cooled igneous rocks, a K/Ar or 40 Ar/ 39 Ar 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 40 Ar*/ 39 Ar K ratios can now be determined often to better than 0.1%, the main limitation on accuracy relates to how well the 40 K decay constants are known. Better determination of the β − and γ decays of 40 K, 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 40 K 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.

Abstract A recent calibration of the 40 Ar/ 39 Ar geochronometer is based on an optimization analysis of 40 K activity data, isotopic data for the Fish Canyon sanidine (FCs) standard, and pairs of 40 Ar/ 39 Ar+ 238 U– 206 Pb data from selected samples meeting well-documented quality criteria. Inclusion of 238 U– 206 Pb data in the calibration incorporates the precisely known 238 U 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 40 Ar/ 39 Ar and 238 U– 206 Pb 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 40 Ar/ 39 Ar system. The optimization approach easily accommodates new constraints, but rigorous quality control is needed to maintain accuracy.

Abstract The 40 Ar/ 39 Ar dating technique requires the transformation of 39 K into 39 Ar by neutron activation. Neutron activation has undesirable secondary effects such as interfering isotope production, and recoil of 39 Ar and 37 Ar 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 39 Ar. However, 37 Ar loss can affect the ages of Ca-rich minerals via interference corrections on 36 Ar (and, to a lesser extent, 39 Ar), yielding lower 40 Ar*/ 39 Ar K and, thus, an age spuriously too young. New results focused on 37 Ar 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 39 Ar recoil loss. For the finest fractions, FCp and Hb3gr apparent ages are younger than the 39 Ar 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 37 Ar-depletion values ranging from approximately 30 to 91% and from approximately 28 to 98% for plagioclase and hornblende, respectively. This results in x 0 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 37 Ar loss is not well understood, but might be related to the radiation damage caused to the mineral during irradiation. x 0 ( 39 Ar) and x 0 ( 37 Ar) values obtained in this study, along with crystal dimensions, can be used for correcting 40 Ar/ 39 Ar ages from 39 Ar and 37 Ar 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 40 Ar/ 39 Ar age spectra for FCp (Fig. A3.1) and Hb3gr (Fig. A3.2).

Abstract Advances in the 40 Ar/ 39 Ar 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 40 Ar*/ 39 Ar K ratios on standards that have been calibrated with the K–Ar method. Usually a 40 Ar/ 39 Ar total gas age is assumed to equate to a K–Ar age, but this assumes that there is zero loss of 39 Ar due to recoil. Traditional 40 Ar/ 39 Ar 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 39 Ar recoil. We report direct measurements of 39 Ar 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 39 Ar lost from these standards. Supplementary material: Detailed data tables are available at http://www.geolsoc.org.uk/SUP18587 .

Abstract Sanidine from the Fish Canyon Tuff is regularly used as a neutron flux monitor by 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 .

Abstract 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 87 Sr/ 86 Sr 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 39 Ar/ 40 Ar 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 .

Abstract 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 40 Ar/ 39 Ar geochronology using a new scanning CO 2 laser system. This method can partially un-mix radiogenic ( 40 Ar*) 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 .

Abstract 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 40 Ar/ 39 Ar 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 40 K– 40 Ca 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.

Abstract 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.

Abstract Step-heating experiments in vacuo are routine when conducting 40 Ar/ 39 Ar 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 39 Ar in phengitic white mica involves radically different diffusion parameters in comparison with muscovite. If these results extrapolate to nature then 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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.

Abstract 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 40 Ar 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 ) cm 2 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 .