High-precision 40Ar/39Ar age of the Jänisjärvi impact structure (Russia)

The ～ 14 km diameter Jänisjärvi impact structure is located in Svecofennian Proterozoic terrain in the southeastern part of the Baltic shield, Karelia, Russia. Previous radioisotopic dating attempts gave K/Ar and ⁴⁰Ar/³⁹Ar ages of 700 ± 5 Ma and 698 ± 22 Ma, respectively, with both results being difficult to interpret. Recent paleomagnetic results have challenged these ages and proposed instead ages of either 500 Ma or 850–900 Ma. In order to better constrain the age of the Jänisjärvi impact structure, we present new ⁴⁰Ar/³⁹Ar data for the Jänisjärvi impact melt rock. We obtained five concordant isochron ages that yield a combined isochron age of 682 ± 4 Ma (2σ) with a MSWD of 1.2, P = 0.14, and ⁴⁰Ar/³⁶Ar intercept of 475 ± 3. We suggest that this date indicates the age of the impact and therefore can be used in conjunction with existing paleomagnetic results to define the position of the Baltica paleocontinent at that time. Argon isotopic results imply that melt homogenization was achieved at the hundred-micrometer scale certainly, because of the low-silica content of the molten target rock that allows fast ⁴⁰Ar^? diffusion in the melt. However, the large range of F(⁴⁰Ar^?_inherited) (4.1% to 11.0%) observed for seven grains shows that complete isotopic homogenization was not reached at the centimeter and perhaps the millimeter scale. The F(⁴⁰Ar^?_inherited) results are also in good agreement with previous Rb and Sr isotopic data.     

The isotopic composition of atmospheric argon and 40Ar/39Ar geochronology: Time for a change?

A redetermination of the isotopic composition of atmospheric argon by Lee, J.-Y., Marti, K., Severinghaus, J.P., Kawamura, K., Yoo, H.-S., Lee, J.B., Kim, J.S. [2006. A redetermination of the isotopic abundances of atmospheric Ar. Geochimica et Cosmochimica Acta 70, 4507–4512] represents the first refinement since the work of Nier [1950. A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon, and potassium. Physical Reviews 77, 789–793]. The new ⁴⁰Ar:³⁸Ar:³⁶Ar proportions imply <1% adjustments to ⁴⁰Ar/³⁹Ar ages in all but exceptional cases of very young and/or K-poor and/or Ca-rich samples, or cases in which samples are grossly under- or over-irradiated. Analytical protocols employing atmospheric argon to determine mass discrimination corrections are insensitive to the effects of revision on the air correction, but are subject to non-negligible adjustments arising from expanded heavy to light isotope ratios attending the increased mass discrimination correction. The competing effects of increased ⁴⁰Ar/³⁹Ar and ⁴⁰Ar/³⁷Ar ratios render the adjustments a function of sample chemistry and neutron irradiation parameters. The improved precision of atmospheric ⁴⁰Ar/³⁶Ar and ³⁸Ar/³⁶Ar permits increasingly sensitive detection of departures from atmospheric values. Non-atmospheric initial ⁴⁰Ar/³⁶Ar values are increasingly well-documented in volcanic materials, including subatmospheric values correlated with ³⁸Ar/³⁶Ar in a trend consistent with kinetic mass fractionation whereby incomplete equilibration between magma and atmosphere favors light isotope enrichment in the magma. The detailed mechanism(s) of such fractionation are unclear and must be clarified by further study. A detectable increase in atmospheric ⁴⁰Ar/³⁶Ar in the past 800 ka [Bender, M.L., Barnett, B., Dreyfus, G., Jouzel, J., Porcelli, D., 2008. The contemporary degassing rate of ⁴⁰Ar from the Earth. Proceedings of the National Academy of Sciences 105, 8232–8237] suggests that ages of late Quaternary (e.g., <100 ka) materials incorporating large amounts of atmospheric argon such as biotite may be underestimated by as much as 100% if a modern atmospheric ⁴⁰Ar/³⁶Ar value is erroneously assumed, unless air argon is used to determine mass discrimination. Further evaluation of the evolution of paleoatmospheric ⁴⁰Ar/³⁶Ar, and the fidelity with which argon trapped in igneous materials reflects this, would be very productive. The use of isochrons rather than model (e.g., plateau) ages mitigates the vagaries associated with uncertain trapped argon isotope ratios, and the importance of strategies to derive statistically valid isochrons is underscored.     

Quantification of Ar recoil ejection from GA1550 biotite during neutron irradiation as a function of grain dimensions

This study presents the first measurement of ³⁹Ar recoil ejection loss from individual, dimensionally characterized mineral grains due to neutron irradiation, and reveals the extent to which this recoil loss is problematic for ⁴⁰Ar/³⁹Ar dating. Using the well-characterized biotite standard GA1550, known to have between grain reproducibility of ⁴⁰Ar*/³⁹Ar_K of order 0.1%, we measured the thicknesses (3-210 μm) and surface areas (0.07-0.90 mm²) of 159 grains selected to span the dimensional range represented in the aliquot. Thinner grains with high surface area/volume (SA/V) reveal elevated ⁴⁰Ar/³⁹Ar, as much as 26% higher than thicker grains expected to suffer proportionately negligible depletion. Although the thinner grains yield intrinsically less precise measurements due to small ³⁹Ar ion beams, a regular decrease in net recoil loss with increasing biotite grain thickness is clear for grains thinner than ca. 50 μm. Grains thicker than 50 μm reveal essentially no ³⁹Ar loss within analytical uncertainties. The measured ³⁹Ar loss spectrum is significantly higher than predicted by previous modeling approaches. These results suggest a practical threshold of ca. 50 μm grain thickness for biotites, and probably other phyllosilicates, irradiated with ²³⁵U fission spectrum neutrons in order to avoid recoil artifacts. Poor agreement between our data and simulation results indicates that recoil displacement models should be revisited in order to resolve the discrepancy. Further empirical work to determine the recoil loss of ³⁹Ar in other minerals is important not only for routine age measurements, but also to shed more light on the role of recoil in multi-diffusion domain theory and other thermochronologic applications exploiting variable diffusion radii and/or grain size effects.     

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.     

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.