Intercalibration of the Hb3gr Ar/Ar dating standard

The ⁴⁰Ar/³⁹Ar dating technique is based on the knowledge of the age of neutron fluence monitors (standards). Recent investigations have improved the accuracy and precision of the ages of most of the Phanerozoic-aged standards (e.g. Fish Canyon Tuff sanidine (FCs), Alder Creek sanidine, GA1550 biotite and LP-6 biotite); however, no specific study has been undertaken on the older standards (i.e. Hb3gr hornblende and NL-25 hornblende) generally used to date Precambrian, high Ca/K, and/or meteoritic rocks.In this study, we show that Hb3gr hornblende is relatively homogenous in age, composition (Ca/K) and atmospheric contamination at the single grain level. The mean standard deviation of the ⁴⁰Ar^?/³⁹Ar_K (F-value) derived from this study is 0.49%, comparable to the most homogeneous standards. The intercalibration factor (which allows direct comparison between standards) between Hb3gr and FCs is R_FCs^Hb3gr = 51.945 ± 0.167. Using an age of 28.02 Ma for FCs, the age of Hb3gr derived from the R-value is 1073.6 ± 5.3 Ma (1σ; internal error only) and ± 8.8 Ma (including all sources of error). This age is indistinguishable within uncertainty from the K/Ar age previously reported at 1072 ± 11 Ma [Turner G., Huneke, J.C., Podosek, F.A., Wasserburg, G.J., 1971. ⁴⁰Ar-³⁹Ar ages and cosmic ray exposure ages of Apollo 14 samples. Earth Planet. Sci. Lett. 12, 19-35].The R-value determined in this study can also be used to intercalibrate FCs if we consider the K/Ar date of 1072 Ma as a reference age for Hb3gr. We derive an age of 27.95 ± 0.19 Ma (1σ; internal error only) for FCs which is in agreement with the previous determinations. Altogether, this shows that Hb3gr is a suitable standard for ⁴⁰Ar/³⁹Ar geochronology.     

Paleozoic ages and excess Ar in garnets from the Bixiling eclogite in Dabieshan, China: New insights from Ar/Ar dating by stepwise crushing

The ⁴⁰Ar/³⁹Ar stepwise crushing technique is applied for the first time to date garnet from ultra-high-pressure metamorphic (UHPM) eclogites. Three garnet samples from the Bixiling eclogites analyzed by ⁴⁰Ar/³⁹Ar stepwise crushing yield regular, predictable age spectra, and a clear separation between excess ⁴⁰Ar and concordant plateau and isochron ages. All three age spectra begin with high apparent ages followed by step by step decreasing ages, and finally age plateaux with apparent ages in the range from 427 ± 20 to 444 ± 10 Ma. The data points constituting the age plateaux yield excellent isochrons with radiogenic intercept ages ranging from 448 ± 34 to 459 ± 58 Ma, corresponding to initial ⁴⁰Ar/³⁶Ar ratios from 292.1 ± 4.5 to 294.5 ± 6.7, statistically indistinguishable from the modern air. The high initial ages are interpreted to derive from secondary fluid inclusions containing excess ⁴⁰Ar, whereas the plateau ages are attributed to gas from small primary fluid inclusions without significant excess ⁴⁰Ar. The plateau ages are interpreted to approximate the time of garnet growth during initial UHPM metamorphism. Phengite analyzed by laser stepwise heating yielded a complicated two-saddle age spectrum with a scattered isochron corresponding to age of 463 ± 116 Ma and initial ⁴⁰Ar/³⁶Ar ratio of 1843 ± 1740 indicative of the presence of extraneous ⁴⁰Ar within phengite. These concordant isochron ages measured on minerals diagnostic of eclogite grade metamorphism strongly suggest that Dabie UHPM eclogites were first formed in the early Paleozoic, during the same event that caused the Qinling-Northern Qaidam Basin-Altyn Tagh eclogites.     

Ar/Ar thermochronology of the fossil LL6-chondrite from the Morokweng crater, South Africa

Studies of meteorites are based mostly on samples that fell to Earth in the recent past (i.e., a few million years at most). The Morokweng LL-chondrite meteorite is a particularly interesting specimen as its fall is much older (ca. 145 Ma) than most other meteorites and because it is the only macro-meteorite clast (width intersected in drill core: 25 cm) found in a melt sheet of a large impact structure. When applied to the Morokweng meteorite, ⁴⁰Ar/³⁹Ar thermochronology provides an opportunity to study (1) effects associated with pre-impact and post-impact processes and (2) collision events within a potentially distinct and as yet unsampled asteroid population.A single multi-grain aliquot yielded an inverse isochron age of 625 ± 163 Ma. This suggests a major in-space collisional event at this time. We have modeled the diffusion of ⁴⁰Ar^∗ within the meteorite and plagioclase during and after the ∼145 Ma impact on Earth to tentatively explain why pre-terrestrial impact ⁴⁰Ar^∗ has been preserved within the plagioclase grains. The ∼145 Ma terrestrial impact age is recorded in the low-retentivity sites of the meteorite plagioclase grains that yielded a composite inverse isochron age at 141 ± 15 Ma and thus, confirms that age information about major (terrestrial or extraterrestrial) impacts can be recorded in the K-rich mineral phases of a meteorite and measured by the ⁴⁰Ar/³⁹Ar technique. More studies on fossil meteorites need to be carried out to understand if the rough 0.6 Ga age proposed here corresponds to major LL-chondrite asteroid population destructions or, rather, to an isolated collision event.     

Ar-Ar ages of martian nakhlites

We report ³⁹Ar-⁴⁰Ar ages of whole rock (WR) and plagioclase and pyroxene mineral separates of nakhlites MIL 03346 and Y-000593, and of WR samples of nakhlites NWA 998 and Nakhla. All age spectra are complex and indicate variable degrees of ³⁹Ar recoil and variable amounts of trapped ⁴⁰Ar in the samples. Thus, we examine possible Ar-Ar ages in several ways. From consideration of both limited plateau ages and isochron ages, we prefer Ar-Ar ages of NWA 998 = 1334 ± 11 Ma, MIL 03346 = 1368 ± 83 Ma (mesostasis) and 1334 ± 54 Ma (pyroxene), Y-000593 = 1367 ± 7 Ma, and Nakhla = 1357 ± 11 Ma, (2σ errors). For NWA 998 and MIL 03346 the Ar-Ar ages are within uncertainties of preliminary Rb-Sr isochron ages reported in the literature. These Ar-Ar ages for Y-000593 and Nakhla are several Ma older than Sm-Nd ages reported in the literature. We conclude that the major factor in producing Ar-Ar ages slightly too old is the presence of small amounts of trapped martian or terrestrial ⁴⁰Ar on weathered grain surfaces that was degassed along with the first several percent of ³⁹Ar. A total K-⁴⁰Ar isochron for WR and mineral data from five nakhlites analyzed by us, plus Lafayette data in the literature, gives an isochron age of 1325 ± 18 Ma (2σ). We emphasize the precision of this isochron over the value of the isochron age. Our Ar-Ar data are consistent with a common formation age for nakhlites. The cosmic-ray exposure (CRE) age for NWA 998 of ∼12 Ma is also similar to CRE ages for other nakhlites.     

Ar loss in experimentally deformed muscovite and biotite with implications for Ar/Ar geochronology of naturally deformed rocks

The effects of deformation on radiogenic argon (⁴⁰Ar^∗) retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite. Cylindrical cores, ∼15 mm in length and 6.25 mm in diameter, were drilled from granite collected from the South Armorican Massif in northwestern France, loaded into gold capsules, and weld-sealed in the presence of excess water. The samples were deformed at a pressure of 10 kb and a temperature of 600 °C over a period 29 of hours within a solid medium assembly in a Griggs-type triaxial hydraulic deformation apparatus. Overall shortening in the experiments was approximately 10%. Transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking, cracking, and grain segmentation of the micas.Infrared (IR) laser (CO₂) heating of individual 1.5-2.5 mm diameter grains of muscovite and biotite separated from the undeformed granite yield well-defined ⁴⁰Ar/³⁹Ar plateau ages of 311 ± 2 Ma (2σ). Identical experiments on single grains separated from the experimentally deformed granite yield results indicating ⁴⁰Ar^∗ loss of 0-35% in muscovite and 2-3% ⁴⁰Ar^∗ loss in biotite. Intragrain in situ ultraviolet (UV) laser ablation ⁴⁰Ar/³⁹Ar ages (±4-10%, 1σ) of deformed muscovites range from 309 ± 13 to 264 ± 7 Ma, consistent with 0-16% ⁴⁰Ar^∗ loss relative to the undeformed muscovite. The in situ UV laser ablation ⁴⁰Ar/³⁹Ar ages of deformed biotite vary from 301 to 217 Ma, consistent with up to 32% ⁴⁰Ar^∗ loss. No spatial correlation is observed between in situ⁴⁰Ar/³⁹Ar age and position within individual grains. Using available argon diffusion data for muscovite the observed ⁴⁰Ar^∗ loss in the experimentally treated muscovite can be utilized to predict average ⁴⁰Ar^∗ diffusion dimensions. Maximum ⁴⁰Ar/³⁹Ar ages obtained by UV laser ablation overlap those of the undeformed muscovite, indicating argon loss of <1% and an average effective grain radius for ⁴⁰Ar^∗ diffusion ?700 μm. The UV laser ablation and IR laser incremental ⁴⁰Ar/³⁹Ar ages indicating ⁴⁰Ar^∗ loss of 16% and 35%, respectively, are consistent with an average diffusion radius ?100 μm. These results support a hypothesis of grain-scale ⁴⁰Ar^∗ diffusion distances in undeformed mica and a heterogeneous mechanical reduction in the intragrain effective diffusion length scale for ⁴⁰Ar^∗ in deformed mica. Reduction in the effective diffusion length scale in naturally deformed samples occurs most probably through production of mesoscopic and submicroscopic defects such as, e.g., stacking faults. A network of interconnected defects, continuously forming and annealing during dynamic deformation likely plays an important role in controlling both ⁴⁰Ar^∗ retention and intragrain distribution in deformed mica. Intragrain ⁴⁰Ar/³⁹Ar ages, when combined with estimates of diffusion kinetics and distances, may provide a means of establishing thermochronological histories from individual micas.     

Thermal history of the Ecstall pluton from Ar/Ar geochronology and thermal modeling

New ⁴⁰Ar/³⁹Ar thermochronology results and thermal modeling support the hypothesis of Hollister et al. (2004), that reheating of the mid-Cretaceous Ecstall pluton by intrusion of the Coast Mountains Batholith (CMB) was responsible for spatially variable remagnetization of the Ecstall pluton. ⁴⁰Ar/³⁹Ar ages from hornblende and biotite from 12 locations along the Skeena River across the northern part of the Ecstall pluton decrease with proximity to the Quottoon plutonic complex, the nearest member of the CMB to the Ecstall pluton. The oldest ⁴⁰Ar/³⁹Ar ages are found farthest from the Quottoon plutonic complex, and are 90 ± 3 Ma for hornblende, and 77.9 ± 1.2 Ma for biotite. The youngest ⁴⁰Ar/³⁹Ar ages are found closest to the Quottoon plutonic complex, and are 51.6 ± 1.2 Ma for hornblende, and 45.3 ± 1.7 Ma for biotite. No obvious relationship between grain size and age is seen in the Ecstall pluton biotites. Spatial trends in ⁴⁰Ar/³⁹Ar ages are consistent with model results for reheating by a thermal wall at the location of the Quottoon plutonic complex. Although no unique solution is suggested, our results indicate that the most appropriate thermal history for the Ecstall pluton includes both reheating and northeast side up tilting of the Ecstall pluton associated with intrusion of the Quottoon plutonic complex. Estimates of northward translation from shallow paleomagnetic inclinations in the western part of the Ecstall pluton are reduced to ∼3000 km, consistent with the Baja-BC hypothesis, when northeast side up tilting is accounted for.     

Shock implantation of Martian atmospheric argon in four basaltic shergottites: A laser probe Ar/Ar investigation

Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of two Martian basalts (Los Angeles and Zagami) and two Martian olivine-phyric basalts (Dar al Gani (DaG) 476 and North West Africa (NWA) 1068). With a ∼50 μm diameter focused infrared laser beam, it has been possible to distinguish between argon isotopic signatures from host rock (matrix) minerals and localized shock melt products (pockets and veins). The concentrations of argon in analyzed phases from all four meteorites have been quantified using the measured J values, ⁴⁰Ar/³⁹Ar ratios and K₂O wt% in each phase. Melt pockets contain, on average, 10 times more gas (7-24 ppb ⁴⁰Ar) than shock veins and matrix minerals (0.3-3 ppb ⁴⁰Ar). The ⁴⁰Ar/³⁶Ar ratio of the Martian atmosphere, estimated from melt pocket argon extractions corrected for cosmogenic ³⁶Ar, is: Los Angeles (∼1852), Zagami (∼1744) and NWA 1068 (∼1403). In addition, Los Angeles shows evidence for variable mixing of two distinct trapped noble gas reservoirs: (1) Martian atmosphere in melt pockets, and (2) a trapped component, possibly Martian interior (⁴⁰Ar/³⁶Ar: 480-490) in matrix minerals. Average apparent ⁴⁰Ar/³⁹Ar ages determined for matrix minerals in the four analyzed meteorites are 1290 Ma (Los Angeles), 692 Ma (Zagami), 515 Ma (NWA 1068) and 1427 Ma (DaG 476). These ⁴⁰Ar/³⁹Ar apparent ages are substantially older than the ∼170-474 Ma radiometric ages given by other isotope dating techniques and reveal the presence of trapped ⁴⁰Ar. Cosmic ray exposure (CRE) ages were measured using spallogenic ³⁶Ar and ³⁸Ar production. Los Angeles (3.1 ± 0.2 Ma), Zagami (2.9 ± 0.4 Ma) and NWA 1068 (2.0 ± 0.5 Ma) yielded ages within the range of previous determinations. DaG 476, however, yielded a young CRE age (0.7 ± 0.25 Ma), attributed to terrestrial alteration. The high spatial variation of argon indicates that the incorporation of Martian atmospheric argon into near-surface rocks is controlled by localized glass-bearing melts produced by shock processes. In particular, the larger (mm-size) melt pockets contain near end-member Martian atmospheric argon. Based on petrography, composition and argon isotopic data we conclude that the investigated melt pockets formed by localized in situ shock melting associated with ejection. Three processes may have led to atmosphere incorporation: (1) argon implantation due to atmospheric shock front collision with the Martian surface, (2) transformation of an atmosphere-filled cavity into a localized melt zone, and (3) shock implantation of atmosphere trapped in cracks, pores and fissures.     

Evidence for shock heating and constraints on Martian surface temperatures revealed by Ar/Ar thermochronometry of Martian meteorites

The thermal histories of Martian meteorite are important for the interpretation of petrologic, geochemical, geochronological, and paleomagnetic constraints that they provide on the evolution of Mars. In this paper, we quantify ⁴⁰Ar/³⁹Ar ages and Ar diffusion kinetics of Martian meteorites Allan Hills (ALH) 84001, Nakhla, and Miller Range (MIL) 03346. We constrain the thermal history of each meteorite and discuss the resulting implications for their petrology, paleomagnetism, and geochronology. Maskelynite in ALH 84001 yields a ⁴⁰Ar/³⁹Ar isochron age of 4163 ± 35 Ma, which is indistinguishable from recent Pb-Pb (Bouvier et al., 2009a) and Lu-Hf ages (Lapen et al., 2010). The high precision of this result arises from clear resolution of a reproducible trapped ⁴⁰Ar/³⁶Ar component in maskelynite in ALH 84001 (⁴⁰Ar/³⁶Ar = 632 ± 90). The maskelynite ⁴⁰Ar/³⁹Ar age predates the Late Heavy Bombardment and likely represents the time at which the original natural remanent magnetization (NRM) component observed in ALH 84001 was acquired. Nakhla and MIL 03346 yield ⁴⁰Ar/³⁹Ar isochron ages of 1332 ± 24 and 1339 ± 8 Ma, respectively, which we interpret to date crystallization. Multi-phase, multi-domain diffusion models constrained by the observed Ar diffusion kinetics and ⁴⁰Ar/³⁹Ar age spectra suggest that localized regions within both ALH 84001 and Nakhla were intensely heated for brief durations during shock events at 1158 ± 110 and 913 ± 9 Ma, respectively. These ages may date the marginal melting of pyroxene in each rock, mobilization of carbonates and maskelynite in ALH 84001, and NRM overprints observed in ALH 84001. The inferred peak temperatures of the shock heating events (>1400 °C) are sufficient to mobilize Ar, Sr, and Pb in constituent minerals, which may explain some of the dispersion observed in ⁴⁰Ar/³⁹Ar, Rb-Sr, and U-Th-Pb data toward ages younger than ∼4.1 Ga. The data also place conservative upper bounds on the long-duration residence temperatures of the ALH 84001 and Nakhla protolith to be  °C and  °C over the last ∼4.16 Ga and ∼1.35 Ga, respectively. MIL 03346 has apparently not experienced significant shock-heating since it crystallized, consistent with the fact that various chronometers yield concordant ages.     

Combined U-Th/He and Ar/Ar geochronology of post-shield lavas from the Mauna Kea and Kohala volcanoes, Hawaii

Late Quaternary, post-shield lavas from the Mauna Kea and Kohala volcanoes on the Big Island of Hawaii have been dated using the ⁴⁰Ar/³⁹Ar and U-Th/He methods. The objective of the study is to compare the recently demonstrated U-Th/He age method, which uses basaltic olivine phenocrysts, with ⁴⁰Ar/³⁹Ar ages measured on groundmass from the same samples. As a corollary, the age data also increase the precision of the chronology of volcanism on the Big Island. For the U-Th/He ages, U, Th and He concentrations and isotopes were measured to account for U-series disequilibrium and initial He. Single analyses U-Th/He ages for Hamakua lavas from Mauna Kea are 87 ± 40 to 119 ± 23 ka (2σ uncertainties), which are in general equal to or younger than ⁴⁰Ar/³⁹Ar ages. Basalt from the Polulu sequence on Kohala gives a U-Th/He age of 354 ± 54 ka and a ⁴⁰Ar/³⁹Ar age of 450 ± 40 ka. All of the U-Th/He ages, and all but one spurious ⁴⁰Ar/³⁹Ar ages conform to the previously proposed stratigraphy and published ¹⁴C and K-Ar ages. The ages also compare favorably to U-Th whole rock-olivine ages calculated from ²³⁸U-²³⁰Th disequilibria. The U-Th/He and ⁴⁰Ar/³⁹Ar results agree best where there is a relatively large amount of radiogenic ⁴⁰Ar (>10%), and where the ⁴⁰Ar/³⁶Ar intercept calculated from the Ar isochron diagram is close to the atmospheric value. In two cases, it is not clear why U-Th/He and ⁴⁰Ar/³⁹Ar ages do not agree within uncertainty. U-Th/He and ⁴⁰Ar/³⁹Ar results diverge the most on a low-K transitional tholeiitic basalt with abundant olivine. For the most alkalic basalts with negligible olivine phenocrysts, U-Th/He ages were unattainable while ⁴⁰Ar/³⁹Ar results provide good precision even on ages as low as 19 ± 4 ka. Hence, the strengths and weaknesses of the U-Th/He and ⁴⁰Ar/³⁹Ar methods are complimentary for basalts with ages of order 100-500 ka.     

Successive deformation episodes along the Lungmu Co zone, west-central Tibet

Field study, thermochronology and geochemistry of the east Lungmu Co (LMC) range highlight some of the geological events that shaped western Tibet. The LMC fault zone has long been interpreted as the boundary between the Tianshuihai terrane of Laurasian affinity and the Qiangtang block of Gondwanian affinity. In the LMC range, the Paleozoic series is intruded by the Mangtsa leucogranite whose zircon have a U/Pb age of 116.9 ± 1 Ma and by mafic rocks with U/Pb zircon ages ranging from 116.9 ± 1 to 95.1 ± 1.7 Ma. Geochemistry of the mafic rocks indicates that they have been emplaced in a supra-subduction zone setting, probably the north dipping Nujiang suture zone. ⁴⁰Ar/³⁹Ar micas ages of the granite indicate that cooling below ~ 350 °C occurred between 105 and 85 Ma. ⁴⁰Ar/³⁹Ar K-feldspar data suggest a fast cooling event at 60-55 Ma, which we relate to the reactivation of the LMC suture zone as a thrust at the onset of the India-Eurasia collision. The last, and still active, deformation event corresponds to left-lateral strike-slip faulting along the ENE-WSW LMC fault.     

Comment on ‘Paleozoic ages and excess Ar in garnets from the Bixiling eclogite in Dabieshan, China: New insights from Ar/Ar dating by stepwise crushing by Hua-Ning Qiu and J.R. Wijbrans’

In a recent study, Qiu and Wijbrans (2006) [Qiu, H.-N. and Wijbrans, J. R. (2006). Paleozoic ages and excess ⁴⁰Ar in garnets from the Bixiling eclogite in Dabieshan, China: new insights from ⁴⁰Ar/³⁹Ar dating by stepwise crushing. Geochimica et Cosmochimica Acta70, 2354-2370.] analyzed Ar-isotopes extracted by crushing garnets from the Dabie Shan Bixiling eclogite and claimed to have constrained a pre-Triassic (∼450 Ma) episode of UHP metamorphism from primary fluid inclusions.However, in the absence of careful sample characterization and stepped heating analyses, the reported ages are more easily explained as experimental artifacts related to Ar extracted from either mineral inclusions or the interface sites between mineral inclusions and the garnet matrix: Dabie Shan garnets commonly contain mineral impurities such as K-rich omphacite and/or K-feldspar.If Dabie Shan UHP metamorphism is of the generally accepted Triassic age (210-240 Ma), the apparent age of a phengite sample in equilibrium with the garnet can be explained by the presence of an extraneous ⁴⁰Ar^∗ component with mean ⁴⁰Ar/³⁶Ar value of ∼5000. This value is similar to the composition of extraneous ⁴⁰Ar^∗ in other eclogite facies terrane.     

甘肃花牛山东钾长花岗岩40Ar/39Ar同位素年龄及其地质意义

对甘肃花牛山东复式花岗岩体中钾长花岗岩的钾长石进行了详细的⁴⁰Ar/³⁹Ar同位素年龄测定,11个加热阶段所获数据构成-条相关关系非常好的直线,其对应的等时线年龄为194.25±1.96 Ma(2σ).⁴⁰Ar/³⁶Ar初始值为288.87±2.17(2σ),接近于尼尔值(295.5).鉴于该岩体形成之后未受到明显的构造-岩浆活动或其它热事件的影响,因此,194 Ma代表了钾长花岗岩钾长石的形成时代.花牛山东钾长花岗岩是燕山早期构造-岩浆活动的产物.由此推断,花牛山地区构造-岩浆活动时间不是印支期,更不是海西期,而是中生代燕山早期.     

Comparison between grain size and multi-mineral Ar/Ar thermochronology

In the case of volume diffusion, the closure temperature of a mineral is function of, among other factors, the characteristic diffusion dimension, which can be approximated by the grain size of the mineral analysed for grains smaller than or similar in size to the diffusion domains. The theoretical possibility of single mineral grain size thermochronology had been demonstrated empirically in earlier studies, mostly using biotite. In order to examine the potential of this method, it was tested alongside the widely used multi-mineral ⁴⁰Ar/³⁹Ar thermochronology. The sample comes from the granitic McLean pluton, in the south section of the Grenville orogeny. Seven grain size separates of biotite (ranging between 90 and 1000 μm), eight size fractions of amphibole (between 63 and 1000 μm), and three size fractions of K-feldspar (250-600 μm) were extracted and dated by the laser step-heating ⁴⁰Ar/³⁹Ar method. The total gas ages obtained behave as theoretically predicted, with increasing ages for increasing grain sizes, including for K-feldspar, but with the exception of the smallest and the largest grains for biotite and amphibole. The calculated cooling rates are ca. 0.7 °C/Ma for K-feldspar, ca. 2.5 °C/Ma for biotite, and ca. 11 °C/Ma for amphibole, corresponding very well to a monotonic cooling of the McLean pluton. A quick initial thermal re-equilibration with the cooler host-rocks is followed by a much slower cooling on a thermal path parallel to that of the Frontenac Terrain situated immediately to the southeast. The validity of the single mineral grain size thermochronology is demonstrated by comparison with the thermal evolution of the adjacent units and with the cooling history derived from a multi-mineral thermochronology, suggesting that it can be routinely used. The application of this method can be hampered by insufficiently low analytical uncertainties.     

Age calibration of the Fish Canyon sanidine Ar/Ar dating standard using primary K-Ar standards

The ⁴⁰Ar/³⁹Ar dating technique requires the use of neutron fluence monitors (standards). Precise calibrations of these standards are crucial to decrease the uncertainties associated with ⁴⁰Ar/³⁹Ar dates. Optimal calibration of ⁴⁰Ar/³⁹Ar standards should be based on K/Ar standards having independent isotope dilution measurements of ⁴⁰K and ⁴⁰Ar*, based on independent isotope tracers (spikes) because this offers the possibility to eliminate random interlaboratory errors. In this study, we calibrate the widely used Fish Canyon sanidine (FCs) standard based on four primary K/Ar standards (GA-1550, Hb3gr, NL-25, and GHC-305) on which K and Ar* concentrations have been determined in different labs with independently calibrated tracers. We obtained a mean age of 28.03 ± 0.08 Ma (1σ; neglecting uncertainties of the ⁴⁰K decay constants) for FCs, based on the decay constant recommended by Steiger and Jäger [Steiger R.H., Jäger. E. 1977. Subcommission on geochronology: convention of the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett.36, 359-362.]. This age corresponds to a mean ⁴⁰Ar*/⁴⁰K value of (1.6407 ± 0.0047) × 10⁻³. We also discuss several criteria that prevent the use of previous calibrations of FCs based on other primary standards (LP-6, SB-3 and MMhb-1). The age of FCs obtained in this study is based on the ⁴⁰K decay constants of Steiger and Jäger (1977) but we anticipate the imminent need for revision of the value and precision of the ⁴⁰K decay constants (representing the main source of uncertainties in ⁴⁰Ar/³⁹Ar dating). The ⁴⁰Ar*/⁴⁰K result of FCs obtained in this study allows therefore a rapid calibration of the age of FCs with uncertainties at the 0.29% level but perhaps more importantly this value is independent of any particular value of the ⁴⁰K decay constants and may be used in the future in conjunction with revised decay constants.     

论河北丰宁牛圈银(金)矿床的成矿时限问题

对河北牛圈银(金)矿区细粒钾长花岗岩株和含矿隐爆角砾岩墙钾长石样品进行了详细的40Ar/39Ar同位素年龄测定。细粒钾长花岗岩株钾长石样品主要加热阶段所获得数据构成一条相关性很好的直线,其对应的同位素等时线年龄值为(216.6±3.2)Ma(2σ),40Ar/39Ar初始比值为297±28。相比之下,含矿隐爆角砾岩墙钾长石样品特定加热阶段(1200～1300℃和1360～1450℃)所获数据同样也构成两条直线,其对应的同位素等到时线年龄值分别为(227.9±4.8)Ma(2σ)和(216.7±6.1)Ma(2σ),40Ar/39Ar初始比值分别为307±41和269±39。考虑到所测样品均采自未受到明显后期构造-岩浆活动或其他热事件影响的岩(矿)体,因此,细粒钾长花岗岩株和含矿隐爆角砾岩墙的形成时代为216Ma,与印支期富钾岩浆作用具有密切成因联系。根据含矿隐爆角砾岩墙的野外地质证据,结合矿物共生组合关系和钾长石同位素年龄数值,可以推测牛圈银(金)矿区早期成矿作用发生在(227.9±4.8)Ma,而主要成矿期为(216.7±6.1)Ma,是印支期多阶段富碱质成矿流体活动的产物。牛圈银(金)矿区成矿作用时间的厘定对于提高该区成矿理论研究水平和寻找隐伏金属矿床均具重要意义。     

Re-Os and Mo isotope systematics of black shales from the Middle Proterozoic Velkerri and Wollogorang Formations, McArthur Basin, northern Australia

Joint application of the Mo isotope paleoredox proxy and Re-Os deposition-age geochronometer to euxinic black shales has potential for tracing the evolution of ocean redox chemistry over geological time. Here, we report new Re-Os and Mo isotope data for the Mesoproterozoic Velkerri Formation (Roper Group) and Paleoproterozoic Wollogorang Formation (Tawallah Group), McArthur Basin, northern Australia. New Re-Os ages of 1361 ± 21 Ma (2σ, n = 14, mean square of weighted deviates [MSWD] = 1.3, Model 1) and 1417 ± 29 Ma (2σ, n = 12, MSWD = 1.3, Model 1) constrain the depositional age of the Velkerri Formation and its contained biomarkers, as well as acritarchs and microfossils from the Roper Group. Black shales from the upper Velkerri Formation have high Mo abundances (105-119 ppm) and degree of pyritization [DOP] values (0.90-0.92) implying quantitative conversion of molybdate (MoO₄²⁻) to thiomolybdate (MoS₄²⁻) in overlying bottom waters. The average δ^97/95Mo (0.72 ± 0.10‰, 2σ, n = 6) of these shales is consistent with previous data, but represents a significantly more precise determination for global seawater δ^97/95Mo at 1.4 Ga. This value is lighter than present-day seawater by ∼0.85‰ and reflects expanded strongly euxinic deep ocean conditions ([H₂S]_aq > 11 μM) relative to oxic, suboxic, and weakly/intermittently euxinic ([H₂S]_aq < 11 μM) marine deposition in the 1.4 Ga oceans. Mass-balance modelling suggests Mo removal into strongly euxinic and oxic sediments may have comprised 30-70% and less than 15%, respectively, of the oceanic Mo sink at 1.4 Ga as opposed to 5% and 35% today, respectively.The Re-Os radioisotope system in organic-rich shales serves as a test for post-depositional alteration that could affect the usefulness of paleoredox tracers such as Mo stable isotopes. Re-Os isotope data for the Wollogorang Formation black shales are scattered and yield a highly imprecise date of 1359 ± 150 Ma (2σ, n = 21, MSWD = 85, Model 3). This age is younger than U-Pb zircon ages from interbedded tuffs that constrain the age of deposition at ca. 1730 Ma. In conjunction with previous petrological, geochemical, and paleomagnetic data, the Re-Os isotope data suggest hydrothermal fluid flow through the Wollogorang Formation, possibly associated with formation of the ca. 1640 Ma McArthur River Pb-Zn-Ag sedimentary exhalative deposit, resulted in post-depositional mobilization of Re and Os. Based on the degree of deviation of the Re-Os data from a 1730 Ma reference line, open-system behavior of Re and Os was greatest near the base of the black shale unit. Wollogorang Formation black shales are enriched in Mo (41-58 ppm), but are characterized by variable δ^97/95Mo (0.3-0.8‰) and DOP (0.57-0.92). The lightest δ^97/95Mo values occur near the base of the black shale unit. Based on the Re-Os systematics, hydrothermal fluids have probably overprinted the authigenic δ^97/95Mo signature in those shales. However, the heaviest δ^97/95Mo values in the Wollogorang Formation come from stratigraphically higher shales, and are similar to those observed for the Velkerri Formation, and thus may reflect seawater δ^97/95Mo at 1.73 Ga.     

Comment on “Paleozoic ages and excess Ar in garnets from the Bixiling eclogite in Dabieshan, China: New insights from Ar/Ar dating by stepwise crushing” by Qiu and Wijbrans (2006)

Qiu and Wijbrans [Qiu H.-N. and Wijbrans J. R. (2006) Paleozoic ages and excess ⁴⁰Ar in garnets from the Bixiling eclogite in Dabieshan, China: new insights from ⁴⁰Ar/³⁹Ar dating by stepwise crushing. Geochim. Cosmochim. Acta70, 2354-2370] present three Ar-Ar age spectra for fluid inclusions in garnet from eclogite at Bixiling in the Dabie orogen, east-central China. These Paleozoic ages of 427 ± 20 to 444 ± 10 Ma are interpreted to represent the first formation of Dabie ultrahigh-pressure (UHP) eclogite and thus require subduction of Yangtze crust to have started much earlier than previously accepted. However, no petrographic evidence, such as mineral inclusions in the garnet relating to the particular metamorphic conditions, is presented to substantiate the proposed UHP metamorphic event. Because garnet growth is not uniquely responsible for UHP eclogite-facies metamorphism, a distinction between UHP and high-pressure (HP) metamorphic events must be made in the interpretation of geochronological results. Available data from mineral Sm-Nd and zircon U-Pb dating of eclogites from the same area have firmly established that the UHP eclogite-facies metamorphism took place at Triassic. Neither the age of UHP metamorphism nor the timing of continental collision is reliably constrained by their presented data; the fluid inclusions in garnet must contain inherited ⁴⁰Ar from UHP eclogite precursor, without considerable resetting of the Ar-Ar isotopic system during Triassic UHP metamorphism. Therefore, their data are either meaningless, or at best viewed as the age of garnet growth by low-T/HP blueschist/eclogite-facies metamorphism of the UHP eclogite precursor during arc-continent collision in the early Paleozoic. Furthermore, it is critical for metamorphic geochronology to substantiate the timing of UHP metamorphic event by means of zircon U-Pb in situ dating on coesite-bearing domains of metamorphically grown zircon.     

Ar and Ar recoil loss during neutron irradiation of sanidine and plagioclase

The ⁴⁰Ar/³⁹Ar dating technique requires the activation of ³⁹Ar via neutron irradiation. The energy produced by the reaction is transferred to the daughter atom as kinetic energy and triggers its displacement, known as the recoil effect. Significant amounts of ³⁹Ar and ³⁷Ar can be lost from minerals leading to spurious ages and biased age spectra. Through two experiments, we present direct measurement of the recoil-induced ³⁹Ar and ³⁷Ar losses on Fish Canyon sanidine and plagioclase. We use multi-grain populations with discrete sizes ranging from 210 to <5 μm. One population consists of a mixture between sanidine and plagioclase, and the other includes pure sanidine.We show that ³⁹Ar loss (depletion factor) for sanidine is ∼3% for the smallest fraction. Age spectra of fractions smaller than ∼50 μm show slight departure from flat plateau-age spectrum usually observed for large sanidine. This departure is roughly proportional to the size of the grain but does not show typical ³⁹Ar loss age spectra. The calculated thickness of the total depletion layer d₀(sanidine) is 0.035 ± 0.012 (2σ). This is equivalent to a mean depth of the partial depletion layer (x₀) of 0.070 ± 0.024 μm. The latter value is indistinguishable from previous values of ∼0.07-0.09 μm obtained by argon implantation experiments and simulation results.We show that it is possible to adequately correct ages from ³⁹Ar ejection loss provided that the d₀-value and the size range of the minerals are sufficiently constrained. As exemplified by similar calculations performed on results obtained in a similar study of GA1550 biotite [Paine J. H., Nomade S., and Renne P. R. (2006) Quantification of ³⁹Ar recoil ejection from GA1550 biotite during neutron irradiation as a function of grain dimensions. Geochim. Cosmochim. Acta70, 1507-1517.], the d₀(biotite) is 0.46 ± 0.06 μm. The significant difference between empirical results on biotite and sanidine, along with different simulation results, suggests that for biotite, crystal structures and lattice defects of the stopping medium and possibly subsequent thermal degassing (due to ∼150-200 °C temperature in the reactor or extraction line bake out) must play an important role in ³⁹Ar loss.The second experiment suggests that ³⁷Ar recoil can substantially affect the age via the interference corrections with results that suggest up to ∼98% of ³⁷Ar can be ejected from the ∼5 μm grain dimension.Further investigation of silicates of various compositions and structures are required to better understand (and correct) the recoil and recoil-induced effects on both ³⁹Ar and ³⁷Ar and their influences on ⁴⁰Ar/³⁹Ar dating.     

Ar-Ar laser dating of tektites from the Cheb Basin (Czech Republic): Evidence for coevality with moldavites and influence of the dating standard on the age of the Ries impact

Moldavites (Central European tektites) are genetically related to the impact event that produced the ∼24-km diameter Ries crater in Germany, representing one of the youngest large impact structures on Earth. Although several geochronological studies have been completed, there is still no agreement among ⁴⁰Ar-³⁹Ar ages on both moldavites and glasses from Ries suevites. Even recently published data yielded within-sample mean ages with a nominal spread of more than 0.6 Ma (14.24-14.88 Ma). This age spread, which significantly exceeds current internal errors, must be in part ascribed to geological and/or analytical causes.This study reports the results of a detailed geochronological investigation of moldavites from the Cheb area (Czech Republic), which have never been dated before, and, for comparison, of two samples from type localities, one in southern Bohemia and the other in western Moravia. We used ⁴⁰Ar-³⁹Ar laser step-heating and total fusion techniques in conjunction with microscale petrographic and chemical characterization. In addition, with the purpose of ascertaining the influence of the dating standards on the age of the Ries impact and making data from this study and literature consistent with the now widely used Fish Canyon sanidine (FCs) standard, we performed a direct calibration of multi-grain splits of the Fish Canyon biotite (FCT-3) with FCs. The intercalibration factors (), determined for eight stack positions in one of the three performed irradiations, were indistinguishable within errors and gave an arithmetic mean and a standard deviation of 1.0086 ± 0.0031 (±2σ), in agreement with previous works suggesting that biotite from the Fish Canyon Tuff is somewhat older (∼0.8%) than the coexisting sanidine.Laser total fusion analysis of milligram to sub-milligram splits of five tektite samples from the Cheb area yielded mostly concordant intrasample ⁴⁰Ar-³⁹Ar ages, and within-sample weighted mean ages of 14.66 ± 0.08-14.75 ± 0.12 Ma (±2σ internal errors, ages relative to FCs) that overlap within errors. These ages match those obtained for samples from western Moravia (14.66 ± 0.08 Ma) and southern Bohemia (14.68 ± 0.11 Ma), supporting the genetic link between Cheb Basin tektites and moldavites, and, consequently, between Cheb Basin tektites and the Ries impact. In contrast to samples from the Cheb area and Moravia, ⁴⁰Ar-³⁹Ar ages from total fusion experiments on the Bohemian specimen ranged widely from ∼14.6 to ∼17.0 Ma. Older apparent ages, however, were systematically obtained from fragments characterized by visible surface alteration. Laser step-heating experiments, although displaying slightly disturbed age profiles, were in line with total fusion analyses and yielded well-defined plateau ages of 14.64 ± 0.11-14.71 ± 0.11 Ma (±2σ internal errors, ages relative to FCs).A thorough comparison of our and previous ⁴⁰Ar-³⁹Ar ages on both moldavites and Ries suevite glasses, recalculated relative to the ⁴⁰Ar^∗/⁴⁰K ratio recently determined for FCs using intercalibration factors available in or derivable from the literature, reveals some inconsistencies which may be ascribed to either geological or analytical causes. Based on our data, decay constants in current use in geochronology, and ages calculated relative to FCs, we infer that the age of moldavites is 14.68 ± 0.11 Ma (±2σ, neglecting uncertainties in the ⁴⁰K decay constants).     

The problem of inherited Ar* in dating impact glass by the Ar/Ar method: Evidence from the Tswaing impact crater (South Africa)

The Tswaing meteorite impact crater is a 1.13 km diameter structure located in the 2.05 Ga Nebo granite of the Bushveld Complex. The impact age had previously been determined by fission track dating to 220 ± 104 ka. ⁴⁰Ar/³⁹Ar step-heating and total fusion experiments performed on single- and multi-grain impact glass aliquots gave apparent ages ranging from 1.0 ± 0.3 Ma to 204 ± 6 Ma. These “ages” indicate that the radiogenic Ar derived from the target rocks has not been completely degassed as a result of the impact process, despite fusion of the target material. Results of step-heating experiments imply that the trapped within the glass is located in two distinct reservoirs thought to be the glass matrix and fluid/vapor inclusions (or un-melted residual clasts). Calculations assuming an age of 0.2 ± 0.1 Ma for Tswaing (fission track data) reveal that the amount of inherited ⁴⁰Ar*() relative to the pre-impact concentration varies from 0.015% to 4.15%. The spread defined by likely reflects the various quench rates experienced by the glass, most certainly due to the pre-impact position of the sample relative to the center of the crater. We compare the influence of on the apparent ⁴⁰Ar/³⁹Ar age determination of five impact structures. Our calculations show that the main characteristic controlling the age offset (for a given proportion of ) is the age difference between the impact and the target rocks (i.e., the ⁴⁰Ar* concentration in the target rock). The buffer effect for a given crater structure can be predicted knowing the age of the basement and having a rough estimation of the age of the crater structure itself. The occurrence of is likely influenced by (1) the degree of polymerization (i.e., silicate structure complexity) of the target rock and presumably related to the diffusivity of Ar in the melt and glass, (2) the Ar partial pressure at the grain boundary, (3) the quantity of energy involved in the impact, and (4) the porosity of the target rocks. For glass that inevitably suffers inherited and/or excess ⁴⁰Ar*, the use of the inverse isochron technique can be appropriate but should be applied with careful statistical treatment.