Step-heating Data Research Articles

Three age ranges of Cenozoic basaltic rocks from Lower Silesia (SW Poland) based on 40Ar/39Ar step-heating data

The precise ages of Cenozoic basaltic rocks from 20 localities (24 samples) in south-western Poland were studied by means of the 40Ar/39Ar isotope method. Three phases of volcanic activity were identified in this area. The older, Ruphelian phase took place 31–29 Ma ago and the younger, Aquitanian phase occurred 22–21 Ma ago. Significantly younger ages (4.8–4.6 Ma: Zanclean phase) were obtained for the basalts cropping out in the vicinity of Lądek Zdrój only. Most of the analyzed basalts from the Ruphelian phase are classified as nephelinites. They occur in the area of Lausitz Masif, south of Sudetic Marginal Fault and in the Opole Depression. The volcanic rocks from the Aquitanian phase crop out in the wide area north of the Sudetic Marginal Fault. They are composed of alkali basalts. The youngest phase is represented by the basanites only. The deep discontinuous tectonic structures intersecting the areas of volcanic activity in SW Poland, parallel or oblique to the tectonic stress directions previously reconstructed for the Oligocene and Miocene in the Bohemian Massif, were most probably reactivated by an extension approximately parallel or slightly oblique to them, thus opening pathways for the migration of basaltic magma in the Sudetes and their foreland.

Overview of age constraints for gold mineralization in central and western Newfoundland and new 40Ar/39Ar ages for muscovite from selected auriferous zones

This contribution presents new 40Ar/39Ar laser step-heating data for muscovite associated with four significant orogenic gold-mineralized zones in central and western Newfoundland to build upon existing geochronological constraints for orogenic gold mineralization across much of the Newfoundland Appalachians. Additionally, we provide the first 40Ar/39Ar laser step-heating data for detrital muscovite from gold-mineralized sandstone of the Wigwam Formation (Botwood Group) at the Moosehead gold prospect. Most known gold zones on the isIand occur in proximity to reactivated crustal-scale faults and related structures, and are commonly localized within competent, rigid lithological units (e.g., granitoid rocks); although, some vein systems cut less competent, clastic sedimentary rock sequences. Host rocks range in age from Neoproterozoic to Devonian and may extend into the Carboniferous. Robust Pragian to Emsian (ca. 410–375 Ma) rutile, muscovite and zircon ages constrain the timing of gold mineralization in parts of the Exploits Subzone, whereas less precise age estimates for gold mineralization in the Notre Dame Subzone and Laurentian margin range from Wenlock to Emsian (ca. 433–375 Ma). Collectively, the geochronological data suggest that fluid-pressure cycling associated with gold mineralization in central and western Newfoundland occurred during progressive Siluro-Devonian tectonics associated with the waning stages of the Salinic orogenic cycle and spanning the Acadian and Neoacadian orogenic cycles. Multiple, polyphase, overlapping orogenic events allowed for repeated, gold mineralizing fluid flow events, particularly in proximity to long-lived, reactivated crustal-scale fault zones.

Simulating effects of heterogeneous 4He concentration profiles and radiation damage annealing on whole-grain zircon diffusivity analyses

The retrieval of accurate thermal histories recorded by the (U-Th)/He system relies on empirical estimates of diffusion kinetics from natural minerals, although the difficulty in obtaining samples with homogeneous gas concentrations (required for accurate diffusivities) has limited the collection of such datasets. Whole-grain analyses are relatively time- and cost-effective, but natural 4He concentration heterogeneities caused by α-ejection, diffusional rounding, and/or parent nuclide zonation present an unquantified source of error. We employ a 1D spherical finite difference diffusion model to simulate effects of natural sources of He concentration heterogeneities, with a focus on the zircon (U-Th)/He (ZHe) system. Using He concentration profiles affected by various natural heterogeneities, models consistently predict that diffusion kinetics are strongly underestimated using such methods, especially at low gas release fractions. These underestimations are driven by He depletion near the grain boundary caused by α-ejection, and exacerbated by U and Th depletion. Conversely, models with parent nuclide (and therefore He) enrichment near the grain boundary compensate for ejection-based loss, yielding more accurate diffusion kinetics overall. A comparison of these results to real step-heating data yields few similarities, however, with He releases from real zircons either being linear in Arrhenius space, or having anomalously high diffusivities at low fractional releases. Radiation damage and its annealing are also significant factors affecting He diffusional mechanics. Using diffusivities and annealing as predicted by the current ZHe forward model, cycled step-heating of high-dose zircons (>1 × 1018 α/g) predict that diffusion decreases consistently with increased heating temperatures, similar to observations. The forward model predicts that annealing beyond this threshold dose should increase diffusivity, which is not observed, however. These results suggest that whole-grain step-heating data may provide a reasonable proxy for empirically measuring damage annealing kinetics in zircon. Major discrepancies between model results and real data reveal the need for more cycled-step heating data to understand the full complexities of He diffusion dynamics.

Error Propagation in the Derivation of Noble Gas Diffusion Parameters for Minerals From Step Heating Experiments

AbstractStep heating is an important tool in the derivation of thermochronologic models based on diffusive noble‐gas loss from minerals. This technique investigates noble‐gas diffusion in minerals at length scales of natural crystals. Like any scientific measurement, step heating requires realistic assessment of assumptions and uncertainties; however, uncertainties are often not presented or roughly approximated. Here we propagate errors associated with temperature, step duration, evolved gas quantity, and diffusion domain size. Other potential error sources (e.g., nonideal shapes, fractures, and zonation of aliquots) are not discussed. We provide error propagation formulas for the derivation of diffusivities and diffusion parameters for infinite plane sheet, spherical, and infinite cylinder geometry. We show that a variance in uncertainty (heteroscedasticity) in diffusivity and temperature may lead to erroneous estimates of activation energy (Ea) and frequency factor (D0) and an underestimation of their uncertainties during unweighted regression. The effect becomes significant if the variance in uncertainty exceeds a factor of two to five. The effect of the difference in D0 values is demagnified in the calculation of closure temperature (which calls on lnD0 rather than D0) and its uncertainty. To make this a practical resource, we provide spreadsheets with a step heating data set, diffusivity calculations with associated analytical error propagation formulas, and unweighted and weighted regression models to derive Ea and D0 for all three geometries. We also provide spreadsheets with Monte Carlo and numerical error propagation for plane sheet geometry.

An evidence-based approach to accurate interpretation of 40Ar/39Ar ages from basaltic rocks

Since its inception in the mid-1960s, the 40Ar/39Ar dating technique has been the premier method for determining the eruption ages of basaltic rocks, providing valuable insights into a plethora of terrestrial and planetary processes. Advances in multi-collector mass spectrometry and improved sample preparation procedures are enabling ever-improving analytical precision and clearer evaluation of the isotopic disturbances that affect many basaltic samples and cause discordant 40Ar/39Ar age spectra.Here, we present 40Ar/39Ar step-heating data for multiple samples from two Quaternary basalt flows (0.8038 ± 0.0017 and 2.309 ± 0.009 Ma) of the intraplate Newer Volcanic Province, southeast Australia. A small proportion of these samples give concordant 40Ar/39Ar results, but most are variably discordant. The factors controlling these disturbances and implications for accurate age determination are examined and modelled in both step-heating spectra and inverse isochron space. We demonstrate that the proportion of radiogenic 40Ar (40Ar⁎) present in these samples strongly influences the nature of the discordance reflected in 40Ar/39Ar data. Mass-dependent fractionation appears to have a major influence on low-40Ar⁎ samples, whereas 39Ar recoil loss/redistribution effects are evident in samples with higher 40Ar⁎ proportions. The impact of mass fractionation is quantified via step-heating analyses of unirradiated basalt, whereby a ∼4% difference in 38Ar/36Ar ratios is observed between low- and high-temperature heating steps.On an inverse isochron plot (39Ar/40Ar vs 36Ar/40Ar), isotopic disturbance for groundmass samples primarily manifests as isochron rotation, leading to a negative correlation between initial 40Ar/36Ar ([40Ar/36Ar]i) values and associated 40Ar/39Ar ages. We propose a new framework for the interpretation of 40Ar/39Ar step-heating data for basaltic samples, through judicious evaluation of inverse isochron data, (40Ar/36Ar)i ratios and inverse isochron ages. Results from this study suggest that only samples exhibiting both flat 40Ar/39Ar age spectra and atmospheric (40Ar/36Ar)i ratios yield accurate eruption ages; in the case of more discordant age spectra, intermediate temperature steps with atmospheric (40Ar/36Ar)i ratios may provide the closest approximation of the eruption age.

40Ar/39Ar ages of alkali feldspar xenocrysts constrain the timing of intraplate basaltic volcanism

Abstract Intraplate continental basaltic volcanic provinces (ICBVPs) occur on all continents and represent some of the most enigmatic volcanic systems. Constraints on the origin(s) and evolution of ICBVPs are predicated on detailed knowledge of eruption histories, which are often poorly quantified. Although the 40Ar/39Ar geochronology method has been applied successfully to lava flows, age determinations on scoria cones and maars are more challenging. In this study, we test the potential of entrained anorthoclase megacrysts to yield accurate 40Ar/39Ar ages for scoria cones and maars from the Pliocene-Holocene Newer Volcanic Province (NVP) of south-eastern Australia. The NVP is an ICBVP containing >400 eruption centres with ages spanning 4.6 Ma – 5 ka. K-Ar and 40Ar/39Ar age data exist for a large number of lava flows, but age constraints for scoria cones and maars that lack associated lavas are rare. High-precision 40Ar/39Ar step-heating data were measured on anorthoclase megacrysts from five eruption centres in the NVP. From youngest to oldest, anorthoclase xenocrysts from Mount Noorat (n = 3), Mount Leura (n = 2), Mount Shadwell (n = 3), and The Anakies East Cone (n = 2) yielded reproducible mean inverse isochron ages of 101.75 ± 0.96 ka (2σ), 165.4 ± 1.6 ka (2σ), 353.8 ± 1.9 ka (95% CI), and 2.178 ± 0.005 Ma (95% CI), respectively. In contrast, Lake Keilambete anorthoclase (n = 2) produced discordant age spectra and incongruous isochron/mini-isochron ages of 433.5 ± 7.2 ka (2σ) and 413.4 ± 4.5 ka (2σ). A single anorthoclase megacryst from Mount Franklin gave an age of 126.3 ± 7.2 ka (95% CI). By directly comparing megacryst age results with recently published, high-precision (

Eocene-Miocene igneous activity in Provence (SE France): 40Ar/39Ar data, geochemical-petrological constraints and geodynamic implications

Provence (SE France) was affected by two main phases of sporadic igneous activity during the Cenozoic. New 40Ar/39Ar laser step-heating data constrain the beginning of the oldest phase to late Eocene (40.82±0.73Ma), with activity present until early Miocene (~20Ma). The products are mainly andesites, microdiorites, dacites and basaltic andesites mostly emplaced in the Agay-Estérel area. Major- and trace-element constraints, together with SrNdPb isotopic ratios suggest derivation from a sub-continental lithosphere mantle source variably modified by subduction-related metasomatic processes. The compositions of these rocks overlap those of nearly coeval (emplaced ~38–15Ma) late Eocene-middle Miocene magmatism of Sardinia. The genesis of dacitic rocks cannot be accounted for by simple fractional crystallization alone, and may require interaction of evolved melts with lower crustal lithologies.The youngest phase of igneous activity comprises basaltic volcanic rocks with mildly sodic alkaline affinity emplaced in the Toulon area ~10Myr after the end of the previous subduction-related phase. These rocks show geochemical and isotopic characteristics akin to magmas emplaced in intraplate tectonic settings, indicating a sub-lithospheric HiMu+EM-II mantle source for the magmas, melting approximately in the spinel/garnet-lherzolite transition zone. New 40Ar/39Ar laser step-heating ages place the beginning of the volcanic activity in the late Miocene–Pliocene (5.57±0.09Ma).The emplacement of “anorogenic” igneous rocks a few Myr after rocks of orogenic character is a common feature in the Cenozoic districts of the Central-Western Mediterranean area. The origin of such “anorogenic” rocks can be explained with the activation of different mantle sources not directly modified by subduction-related metasomatic processes, possibly located in the sub-lithospheric mantle, and thus unrelated to the shallower lithospheric mantle source of the “orogenic” magmatism.

Paraná flood basalt volcanism primarily limited to ~ 1 Myr beginning at 135 Ma: New 40Ar/39Ar ages for rocks from Rio Grande do Sul, and critical evaluation of published radiometric data

40Ar/39Ar step heating analyses were carried out on seven rocks (five basalts, an andesite and a rhyolite) from the southern Paraná Province (~28°S–30°S); they yield plateau/isochron ages of 135–134Ma, in good agreement with published step heating data on rocks from the same area. Critical review of laser spot isochron ages for rocks from the Province, ranging from ~140 to 130Ma, are shown to be unreliable estimates of crystallization ages, as the rocks were substantially altered; step heating results on three of these rocks thought to yield good plateau ages, are shown to be incorrect, as a result of a technicality in dating procedures followed. U-Pb ages on zircon and baddeleyite separated from a variety of rock types (~30°S–23°S) fall in the range 135 to 134Ma. All reliable 40Ar/39Ar and U-Pb ages indicate volcanism was sharply focused, initiated at 135Ma, and ~1Myr in duration; no variation of age with either latitude or longitude is noted, Scrutiny of published 40Ar/39Ar ages on the Florianopolis dykes shows they cannot be used as reliable crystallization ages. U-Pb work shows that this dyke swarm was formed coevally with the main part of the Parana province. Most of the published 40Ar/39Ar ages on the Ponta Grossa dyke swarm are unreliable; a few ages appear reliable and suggest the magmatic event in this area, may have postdated the main Paraná pulse by 1–2Myr. A single 40Ar/39Ar age from a high-Nb basalt in the southernmost part (~34°S) of the Paraná at ~135Ma, highlights the need for further radiometric work on other areas of this flood basalt province. The Paraná Province postdates the time of the Jurassic-Cretaceous bound­ary by ~ 10 Myr.

A model for meteoritic and lunar 40Ar/39Ar age spectra: Addressing the conundrum of multi-activation energies

Results of whole-rock 40Ar/39Ar step-heating analyses of extra-terrestrial materials have been used to constrain the timing of impacts in the inner solar system, solidification of the lunar magma ocean, and development of planetary magnetic fields. Despite the importance of understanding these events, the samples we have in hand are non-ideal due to mixed provenance, isotopic disturbances from potentially multiple heating episodes, and laboratory artifacts such as nuclear recoil. Although models to quantitatively assess multi-domain, diffusive 40Ar⁎ loss have long been applied to terrestrial samples, their use on extra-terrestrial materials has been limited. Here we introduce a multi-activation energy, multi-diffusion domain model and apply it to 40Ar/39Ar temperature-cycling, step-heating data for meteoritic and lunar samples. We show that age spectra of extra-terrestrial materials, the Jilin chondrite (K-4) and Apollo 16 lunar breccia (67514,43), yielding seemingly non-ideal behavior commonly interpreted as either laboratory artifacts or localized shock heating of pyroxene, are meaningful and can be understood in context of the presence of multi-diffusion domains containing multiple activation energies. Internally consistent results from both the meteoritic and lunar samples reveal high-temperature/short duration thermal episodes we interpret as due to moderate shock heating.

Argon behaviour in an inverted Barrovian sequence, Sikkim Himalaya: The consequences of temperature and timescale on 40Ar/39Ar mica geochronology

40Ar/39Ar dating of metamorphic rocks sometimes yields complicated datasets which are difficult to interpret in terms of timescales of the metamorphic cycle. Single-grain fusion and step-heating data were obtained for rocks sampled through a major thrust-sense shear zone (the Main Central Thrust) and the associated inverted metamorphic zone in the Sikkim region of the eastern Himalaya. This transect provides a natural laboratory to explore factors influencing apparent 40Ar/39Ar ages in similar lithologies at a variety of metamorphic pressure and temperature (P–T) conditions.The 40Ar/39Ar dataset records progressively younger apparent age populations and a decrease in within-sample dispersion with increasing temperature through the sequence. The white mica populations span ~2–9Ma within each sample in the structurally lower levels (garnet grade) but only ~0–3Ma at structurally higher levels (kyanite-sillimanite grade). Mean white mica single-grain fusion population ages vary from 16.2±3.9Ma (2σ) to 13.2±1.3Ma (2σ) from lowest to highest levels. White mica step-heating data from the same samples yields plateau ages from 14.27±0.13Ma to 12.96±0.05Ma. Biotite yield older apparent age populations with mean single-grain fusion dates varying from 74.7±11.8Ma (2σ) at the lowest structural levels to 18.6±4.7Ma (2σ) at the highest structural levels; the step-heating plateaux are commonly disturbed.Temperatures >600°C at pressures of 0.4–0.8GPa sustained over >5Ma, appear to be required for white mica and biotite ages to be consistent with diffusive, open-system cooling. At lower temperatures, and/or over shorter metamorphic timescales, more 40Ar is retained than results from simple diffusion models suggest. Diffusion modelling of Ar in white mica from the highest structural levels suggests that the high-temperature rocks cooled at a rate of ~50–80°CMa−1, consistent with rapid thrusting, extrusion and exhumation along the Main Central Thrust during the mid-Miocene.

Delamination detection in glass–epoxy composites using step-phase thermography (SPT)

Inspection of Glass/epoxy composite using infrared has always been a challenge. This paper presents the use of step heating thermography to increase the maximum detectable defect depth. Fourier transform is applied to step heating data to obtain phase images. These images use to quantitatively characterize defect depth and dimensions in composites. Desired parameters of step heating test such as time step and heating rate are obtained from finite element modeling of composites. This model provides an estimation of defect depth thermal resistance from early time surface temperature. Therefore, it is possible to predict the best frame in an infrared video. Experimental results show the improvement of defect detectability by step heating thermography.

The timing of sedimentation and Buchan metamorphism in the Grampian Terrane in Scotland from 40 Ar/ 39 Ar apparent age spectra

New 40 Ar/ 39 Ar geochronological data from the Buchan and Barrovian metamorphic sequences on the NE coast of Scotland have been obtained from in vacuo step-heating experiments conducted on white mica extracted from seven samples. The results confirm the synchronous formation of the Barrovian and Buchan metamorphic sequences, with the 40 Ar/ 39 Ar apparent age spectra obtained for samples of andalusite grade and higher recording a range of ages between 470.4 ± 1.8 and 464.6 ± 1.3 Ma. These ages are close to the accepted age for peak metamorphism during the Grampian Orogeny, c. 470 Ma, whereas 40 Ar/ 39 Ar apparent age spectra of white mica from the lower grade cordierite zone near Banff preserve pre-Grampian detrital ages. In these spectra an age of c. 560 Ma is recorded. This indicates that a tectonothermal event occurred at c. 560 Ma and may provide further insight into the timing of sedimentation. These samples also preserve evidence of ages older than 750 Ma. Supplementary material: 40 Ar/ 39 Ar step-heating data tables for all samples, data from analyses of flux monitors, determination of correction factors and mass discrimination used for data reduction and 40 Ar/ 39 Ar step-heating spectra and inverse isochron plots from each sample are available at www.geolsoc.org.uk/SUP18723 .

40Ar/39Ar ages of flood basalt provinces in Russia and China and their possible link to global faunal extinction events: A cautionary tale regarding alteration and loss of 40Ar∗

The hypothesis that the Permo–Triassic boundary (PTB) mass extinctions were caused by flood basalt volcanism in Russia (Siberian Traps) and/or China (the Emeishan Traps) is investigated from the point of view of time of occurrence (40Ar/39Ar ages). Numerous published ages in the literature are rejected as good estimates of the time of crystallization. The filters applied in this respect are (a) statistical reliability of plateau/isochron sections of stepheating data and (b) the alteration state of the material that was dated. Alteration appears to be ubiquitous, unsurprising since most of the material dated was used without acid leaching – a procedure that is effective in yielding fresh(er) samples. Of ∼70 ages in the literature for the main pulse of Siberian Trap volcanism, less than ten prove to be reliable ages. Similar techniques applied to 40Ar/39Ar for the Emeishan Traps, leaves only a single reliable age for the magmatic episode. These ages are compared to both published and new 40Ar/39Ar ages for the PTB as based on analysis of minerals from critical ash beds in China. There is good overlap in the ages (PTB – 250.0±0.1Ma, Siberian Trap lavas – 250.1±0.4Ma), lending credence to a genetic link between the formation of the Siberian Traps and the faunal extinction event at the PTB. A similar link for the formation of the Viluy Traps (Russia) and the Late Devonian extinction event is investigated; only a single reliable 40Ar/39Ar age is available for the Viluy Traps, and falls close to the interpolated age for the Frasnian–Fammenian boundary. The use of the unspiked K–Ar technique to yield accurate ages for such (altered) samples is questioned.A review of U–Pb data pertinent to these problems suggests a close temporal link between the formation of the Siberian Traps and the PTB. Comparison of U–Pb and 40Ar/39Ar ages for the PTB, raises questions about the accuracy of high precision sanidine ages, possibly resulting from very slow leakage of 40Ar∗ from this mineral.

40 Ar/ 39 Ar ages of crystallization and recrystallization of rock-forming polyhalite in Alpine rocksalt deposits

Abstract 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 [K 2 Ca 2 Mg(SO 4 ) 4 ·2H 2 O] as a useful geochronometer for various diagenetic and deformation fabric types, 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar step-heating data for polyhalite are available at: www.geolsoc.org.uk/SUP18574 .

40Ar–39Ar ages for detrital white mica in Meguma terrane, Nova Scotia, Canada: implications for provenance of the Goldenville and Halifax groups

Single-grain 40Ar/39Ar ages are reported for detrital white mica, along with conventional step-heating data for whole rocks, from low-grade metasandstone samples from the Goldenville, Halifax, and Rockville Notch groups in the Meguma terrane of southern Nova Scotia. The majority (166) of single grains from 11 samples yielded ages between ca. 615 and 350 Ma, and the remaining 12 grains yielded ages between ca. 1900 and 870 Ma. The late Neoproterozoic–Paleozoic age distributions are consistent with derivation of sediments from the rapidly uplifted flanks of an active rift, where partial to complete resetting of white mica ages occurred at ca. 520–500 Ma, preceding sediment deposition. The ca. 615 Ma ages may be relics of the original detrital white mica that existed in the source rocks prior to the rifting event. Ages from the Upper Silurian White Rock Formation appear to reflect this same ca. 520–500 Ma event, suggesting that sediments in the White Rock Formation were recycled from the Goldenville and Halifax groups. The older Precambrian ages are inferred to represent white mica in the source region, likely Amazonia. The whole-rock age spectra are discordant, with pronounced age gradients and no well-defined age plateaus. Initial gas released from five of the samples at low laboratory extraction temperatures (ca. 450–500 °C) yielded ages of ca. 260–300 Ma, not seen in the single-grain data, whereas gas released at the highest extraction temperatures yielded ages in the range ca. 510–530 Ma, possibly reflecting the principal result obtained from the single-grain data.

Variation of illite/muscovite 40Ar/39Ar age spectra during progressive low-grade metamorphism: an example from the US Cordillera

40Ar/39Ar step-heating data were collected from micron to submicron grain-sizes of correlative illite- and muscovite-rich Cambrian pelitic rocks from the western United States that range in metamorphic grade from the shallow diagenetic zone (zeolite facies) to the epizone (greenschist facies). With increasing metamorphic grade, maximum ages from 40Ar/39Ar release spectra decrease, as do total gas ages and retention ages. Previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation/recrystallization of neo-formed illite. While recognizing the importance of these processes in evaluating our results, we suggest that the inverse correlation between apparent age and metamorphic grade is controlled, primarily, by thermally activated volume diffusion, analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments. Our results suggest that complete resetting of the illite/muscovite Ar thermochronometer occurs between the high anchizone and epizone, or at roughly 300 °C. This empirical result is in agreement with previous calculations based on muscovite diffusion parameters, which indicate that muscovite grains with radii of 0.05–2 μm should have closure temperatures between 250 and 350 °C. At high anchizone conditions, we observe a reversal in the age/grain-size relationship (the finest grain-size produces the oldest apparent age), which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo-formed illite/muscovite crystallites typically surpass the size of detrital muscovite grains. It is also approximately the stage at which neo-formed illite/muscovite crystallites develop sufficient Ar retentivity to produce geologically meaningful 40Ar/39Ar ages. Results from our sampling transect of Cambrian strata establish a framework for interpreting illite/muscovite 40Ar/39Ar age spectra at different stages of low-grade metamorphism and also illuminate the transformation of illite to muscovite. At Frenchman Mtn., NV, where the Cambrian Bright Angel Formation is at zeolite facies conditions, illite/muscovite 40Ar/39Ar data suggest a detrital muscovite component with an apparent age ≥967 Ma. The correlative Carrara Fm. is at anchizone conditions in the Panamint and Resting Spring Ranges of eastern California, and in these locations, illite/muscovite 40Ar/39Ar data suggest an early Permian episode of subgreenschist facies metamorphism. The same type of data from equivalent strata at epizone conditions (greenschist facies) in the footwall of the Bullfrog/Fluorspar Canyon detachment in southern Nevada reveals a period of slow-to-moderate Late Cretaceous cooling.

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

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

Diamond provenance studies from 40Ar/ 39Ar dating of clinopyroxene inclusions: An example from the west coast of Namibia

The west coast of Namibia is host to substantive detrital diamond deposits located in onshore and offshore beach gravels, desert deflation deposits and lower Orange river terraces. The origin of the Namibian diamonds is controversial, with some studies favouring derivation from distal Cretaceous/Jurassic kimberlites on the Kaapvaal craton, and others arguing that most diamonds originated from proximal Dwyka glacial deposits (~ 300 Ma), which incorporated diamonds from older (≥ 500 Ma), pre-Karoo kimberlites. Previous studies have demonstrated that clinopyroxene inclusions extracted from their host diamonds give 40Ar/ 39Ar ages approaching the time of source kimberlite eruption. This behaviour is attributed to diffusion of argon to lattice defect sites and the diamond/inclusion interface region during mantle residence, with subsequent loss of the latter component on cleaving of the diamond to release the inclusion(s). In this study, we measured 40Ar/ 39Ar ages of extracted clinopyroxene inclusions from Namibian detrital diamonds, in order to determine potential kimberlite sources, craton erosion histories and palaeo-drainage evolution in southern Africa. 40Ar/ 39Ar step-heating data were obtained for eclogitic and peridotitic clinopyroxene inclusions from 50 Namibian diamonds. Low temperature steps produced older apparent ages than high temperature (fusion) steps, consistent with partial retention of pre-eruption argon in defect sites. With one exception, fusion steps yielded younger ages, ranging from 62 ± 30 Ma to 1441 ± 700 Ma. The majority (80%) of inclusions have fusion ages < 300 Ma, indicating that most Namibian detrital diamonds originated from post-Dwyka (< 300 Ma) kimberlites. Six inclusion aliquots (13%) produced ages unique to Cretaceous Group I kimberlites, confirming erosion of diamonds from these sources. The proportion of diamonds sourced from Group II kimberlites is uncertain, although forward modelling suggests roughly equal quantities from Group I and Group II localities. A lesser population of ages close to ~ 240 Ma could indicate minor contributions from syn-Karoo kimberlites (e.g., Jwaneng cluster, Botswana); however derivation from younger kimberlites is considered equally plausible. Based on the ages of known southern African kimberlites, the current results suggest that very few (< 11%), if any, Namibian diamonds were sourced from Dwyka glacial deposits or older sediments. Estimates of erosion levels for southern African kimberlites suggest that Cretaceous Group I and Group II kimberlites in South Africa are the most likely sources of the Namibian diamond deposits. Palaeo-drainage reconstructions are consistent with diamond transport to the Namibian coast by the palaeo-Karoo and modern Orange river systems. If the ~ 240 Ma inclusion ages represent contributions from the Jwaneng kimberlites, this would support models for an earlier Kalahari river with headwaters in southern Botswana.

Cosmogenic helium and neon in mantle xenoliths from the Cameroon Volcanic Line (West Africa): Preliminary observations

Step heating and in-vacuo crushing helium and neon data of alkali basalt-hosted ultramafic xenolith whole rock and mineral separates from the Cameroon Volcanic Line centers of Annobon, Bioko, Oku and Ngaoundere are reported for the first time. Step heating data reveals large 3He/ 4He (up to 118 ± 8 R a, where R a is the atmospheric ratio of 1.4 × 10 −6) and 21Ne/ 22Ne (up to 0.20 ± 0.03) isotopic anomalies resulting from the presence of cosmic ray-derived 3He ( 3He c) and 21Ne ( 21Ne c). Crushing of the same phases releases fluids with morb-like Ne for all the centers considered, but morb-like (Annobon and Ngaoundere) and high-μ-like (Bioko and Oku) 3He/ 4He ratios. 3He c and 21Ne c make up ∼77% and ∼97% respectively of the 3He and excess 21Ne observed in the xenoliths. With 2.44 ± 0.37, the cosmogenic 3He/ 21Ne ratio in the Cameroon Line xenolith olivines is similar to that previously observed. Preliminary 3He c and 21Ne c production rates were calculated for the xenolith localities and used to estimate the preliminary 3He c and 21Ne c xenolith exposure ages. Resulting 3He c and 21Ne c ages are similar and suggest that the Annobon, Oku and Ngaoundere xenoliths erupted between 14 and 195 kyr BP. This is the first cosmonuclide age report of Upper Pleistocene volcanic activity in these centers. The cosmogenic age of the Bioko sample is only ∼25% of its K–Ar age, and suggests (assuming the K–Ar age reflects the true eruption age) erosion of the Bioko lava surface. Rigorous evaluation of the erosion rate and the potentially viable general use of cosmonuclides in rocks of the Cameroon Line for studies of the geomorphic evolution of its volcanic centers need a more cosmogenic nuclide-oriented investigation.

Noble gas and nitrogen isotopic components in Oceanic Island Basalts

We present helium, argon, xenon and nitrogen isotopic data from Oceanic Island Basalts (OIBs), obtained from stepped heating and vacuum crushing experiments. Signatures of 3He/ 4He in the present samples, except for those from the Canary Islands, are significantly higher (up to 20 × the air ratio) than the MORB signature (8.5 × air ratio). The argon ( 40Ar/ 36Ar) and xenon ( 129Xe/ 132Xe and 136Xe/ 132Xe) isotopic data are consistent with a two-component mixture of gases from the mantle (either MORB or OIB source) and air contamination. In the stepwise heating data, δ 15N varies from − 20‰ to + 15‰ in the different heating steps. The vacuum crushing data show a narrower range (− 5‰ to + 8‰) of δ 15N, which, except for one Réunion sample, is somewhat heavier than the totals from the stepped heating experiments. Simultaneous argon and nitrogen isotopic systematics of the present data suggest contributions from mantle (somewhat different from MORB), recycled materials and air contamination. This is also consistent with the 36Ar/ 132Xe elemental ratio. The present stepped heating data are consistent with a nitrogen isotopic component with δ 15N ~ − 25‰ in the OIB mantle. In line with the oxygen isotopic composition of the bulk Earth, this would be consistent with a contribution from enstatite chondrite-like volatiles to the planet's accretion.