39Ar recoil losses and presolar ages in Allende inclusions

Ar analyses are reported for five coarse-grained, Ca-Al-rich inclusions from the Allende meteorite. The samples were neutron-irradiated in individual evacuated ampoules, and the Ar gas in the ampoules as well as the samples was analyzed. A large fraction (up to 60%) of the³⁹Ar from³⁹K (n, p) reactions was lost out of the inclusions into the ampoules. The³⁹Ar losses resulted in substantial increases in the apparent⁴⁰Ar-³⁹Ar ages of the samples.³⁹Ar recoil loss during neutron-irradiation is a major effect and must be accounted for in⁴⁰Ar-³⁹Ar dating. All of the Allende inclusions studied contained substantial trapped³⁶Ar. The origin of the trapped³⁶Ar is unknown, and the possible presence of trapped⁴⁰Ar cannot be excluded. Ar measurements on Allende inclusions which have yielded anomalously old ages must be re-examined in the context of³⁹Ar recoil loss and possible contributions of trapped⁴⁰Ar. Allende inclusions appear on both accounts to be poor candidates to search for relicts of presolar materials with well-defined K/Ar ages.     

A novel purification technique for noble gas isotope analyses of authigenic minerals

Noble and active gases are released from geological samples during gas extraction for noble gas isotope analyses. The active gases should be removed before inletting to mass spectrometers for the analyses. The normal noble gas preparation systems can clean up most geological samples. However, authigenic minerals from sedimentary rocks in oil/gas fields contain organic matter, which cannot be cleaned up by the normal preparation systems and thus influence the noble gas analyses. We introduce a novel gas purification system(PRC patent No. ZL201320117751.2), which includes several reversible purification pumps with different absorbing and degassing temperatures. It can well clean up water steam, carbon dioxide and organic gases. Mica minerals are often used for ~40Ar/~39 Ar dating. A muscovite sample(2082MS) which could not be cleaned up by the normal preparation system with two SAES NP10~#174; getters, becomes the test sample for a comparative experiment in this study. The experiment is assigned into 4 sections with the organic gas removal system(OGRS) "Closed/Opened" in turn. When the OGRS is closed only with two NP10 getters for purification, the ~40Ar intensities increase in curves with inlet time because of impurities, the ~40Ar/~39 Ar dating results yield age errors about ±2%–±1%(2σ). When the OGRS is opened for purification, in contrast, the ~40 Ar intensities decrease linearly with inlet time. This indicates that the gases have been cleaned up effectively, and the 4~0Ar/~39 Ar results yield ages with errors in ±0.4%. The OGRS is very helpful to obtain high-quality analysis data.     

Dating young MORB of the Central Indian Ridge (19°S): Unspiked K-Ar technique limitations versus 40Ar/39Ar incremental heating method

We have applied the unspiked K-Ar and the ⁴⁰Ar/³⁹Ar methods to samples precisely collected and localised, on both Central Indian Ridge flanks, to test their effectiveness and reliability when applied to the dating of recent (i.e. less than 1 Ma) MORBs. Twenty six samples) from the sixty five samples collected every ∼500 m up to the Brunhes-Matuyama boundary on both ridge flanks, were selected based on their distance from the ridge axis. Therefore, we can evaluate whether the isotopic ages are a good indicator of the crystallisation age by considering their geographic position with respect to the ridge axis (zero age) and the B/M magnetic boundary. Direct comparison of the isotopic and model ages shows that only 9 out of 26 samples were successfully dated. The GIMNAUT – MORB's test case amply demonstrates that the unspiked K-Ar technique, when applied to submerged volcanic samples, is subject to potentially defective assumptions of trapped atmospheric argon, excess/fractionated argon and extremely sensitive to alteration. Although the unspiked K-Ar technique is theoretically capable to produce high precision ages, the comparison with the ⁴⁰Ar/³⁹Ar techniques reveals that only 15% (i.e. 4 samples out of 26) of the ages obtained here are geologically meaningful. Five of the seven ⁴⁰Ar/³⁹Ar incremental heating experiments provide meaningful ages. Because potential sources of systematic errors such as excess ⁴⁰Ar*, recoil of ³⁹Ar_K and ³⁷Ar_Ca can be identified and because effects of alteration are significantly reduced by the pre-heating of the samples up to 500–600°c, the ⁴⁰Ar/³⁹Ar incremental heating method appears to be the method of choice to date MORBs.     

Age of the Hawaiian-Emperor bend

⁴⁰Ar/³⁹Ar age data on alkalic and tholeiitic basalts from Diakakuji and Kinmei Seamounts in the vicinity of the Hawaiian-Emperor bend indicate that these volcanoes are about 41 and 39 m.y. old, respectively. Combined with previously published age data on Yuryaku and Ko¯ko Seamounts, the new data indicate that the best age for the bend is 42.0 ± 1.4 m.y.Petrochemical data indicate that the volcanic rocks recovered from bend seamounts are indistinguishable from Hawaiian volcanic rocks, strengthening the hypothesis that the Hawaiian-Emperor bend is part of the Hawaiian volcanic chain.⁴⁰Ar/³⁹Ar total fusion ages on altered whole-rock basalt samples are consistent with feldspar ages and with⁴⁰Ar/³⁹Ar incremental heating data and appear to reflect the crystallization ages of the samples even though conventional K-Ar ages are significantly younger. The cause of this effect is not known but it may be due to low-temperature loss of³⁹Ar from nonretentive montmorillonite clays that have also lost⁴⁰Ar.     

Archaeological age constraints from extrusion ages of obsidian: Examples from the Middle Awash,Ethiopia

Extrusion ages of archaeological obsidian, especially as determined by the ⁴⁰Ar/³⁹Ar method, can provide reliable maximum ages for tool manufacture. In at least one case in the Middle Awash of Ethiopia, freshly extruded obsidian was used for tool making, resulting in useful maximum ages for site occupation. Hydration resulting in mobility of K and/or Ar in glass, and recoil artifacts produced by neutron irradiation, fatally affect most glass shards from volcanic ashes. The much lower surface area to volume ratio of most archaeological obsidian, however, indicates that the affected areas can be manually removed prior to analysis and the recoil and hydration problems can be easily overcome. A more important issue in dating obsidian is that of possible mass-dependent kinetic isotope fractionation during or subsequent to quenching of volcanic glasses. This is evidenced in some cases by sub-atmospheric initial ⁴⁰Ar/³⁶Ar ratios, and more generally in sub-atmospheric ³⁸Ar/³⁶Ar. Resulting bias can be avoided through the use of isochron ages, which do not entail the assumption of an initial value of ⁴⁰Ar/³⁶Ar as is required for plateau ages. Since step heating of glasses often yields limited variability in ⁴⁰Ar:³⁹Ar:³⁶Ar (and therefore little spread on isochrons), another approach is to use an average value for initial ⁴⁰Ar/³⁶Ar, with concomitantly larger uncertainty than is associated with atmospheric ⁴⁰Ar/³⁶Ar, when calculating a plateau age. The ³⁸Ar/³⁶Ar of an un-irradiated subset of our samples validates the inference of kinetic fractionation, and potentially provides a basis for determining initial ⁴⁰Ar/³⁶Ar in samples that fail to yield isochrons, but only in samples lacking magmatic excess ⁴⁰Ar. These approaches allow us to reliably apply the ⁴⁰Ar/³⁹Ar method to volcanic glasses, which has resulted in maximum ages for archaeological sites that are not amenable to traditional geochronological methods. ⁴⁰Ar/³⁹Ar geochronology can also provide information on the geological provenance of the raw material used for tool making, especially when combined with geochemical data.     

Studies on40Ar/39Ar thermochronology of strike-slip time of the Tan-Lu fault zone and their tectonic implications

Samples of mylonite, ultramylonite and phyllonite were collected from 5 localities in the Anhui part of the Tan-Lu fault zone for⁴⁰Ar/³⁹Ar chronological studies. Among them 4 samples from 3 localities on the eastern margin of the Dabie orogenic belt yielded⁴⁰Ar/³⁹Ar plateau ages of 128 —132 Ma; and 2 samples from the western margin of the Zhangbalin uplift and eastern margin of the Bengbu uplift gave the same⁴⁰Ar/³⁹Ar plateau ages of 120 Ma. Isochron analyses and other lines of evidence suggest that the data are reliable. The data are interpreted as cooling ages of sinistral strike-slip deformation of the Tan-Lu fault zone. The younger ages from the north might be related to slower strike-slip rising. These results indicate that the large-scale left-lateral displacement in the Tan-Lu fault zone took place in the Early Cretaceous, rather than in Late Triassic (Indosinian) as proposed by some geologists. Therefore, this fault zone is an intracontinental wrench fault rather than a transform fault or suture line developed during formation of the Dabie orogenic belt.     

The evolution of excess argon in alpine biotites — A40Ar-39Ar analysis

Alpine biotites containing excess⁴⁰Ar have been analysed by step-heating argon analysis of both neutron irradiated and unirradiated samples. In addition to age spectra the data are discussed in terms of the thermal release of⁴⁰Ar,³⁹Ar,³⁷Ar and³⁶Ar and also displayed on a correlation plot of³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar which is used to interpret the data and present a model of isotopic evolution during metamorphic cooling. This diagram overcomes misleading complications of isochron plots. The samples exhibit the following argon systematics: (1) flat age spectra for 80–90%³⁹Ar release with anomalously old ages but early gas fractions that approximate the accepted cooling ages; (2) each sample shows decreasing³⁶Ar/⁴⁰Ar with increasing temperature of heating step with three samples having a negative correlation of³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar and one a positive correlation; (3) there appear to be two³⁶Ar components, one released at high temperatures and correlated with radiogenic⁴⁰Ar and one released at low temperatures which is not correlated with radiogenic⁴⁰Ar; and (4) there is no significant effect of neutron irradiation on the release of⁴⁰Ar and³⁶Ar.Interpretation suggests that these biotites contain a record of the evolution and isotopic composition of ambient argon retained within the metamorphic host rocks during cooling. After incorporation of argon of high⁴⁰Ar/³⁶Ar another argon component, of atmospheric composition, was retained at lower temperature and argon partial pressures.     

Diffusion artifacts in dating by stepwise thermal release of rare gases

Age plateaux and isochrons in the ⁴⁰Ar-³⁹Ar and similar dating techniques can be severely altered by processes changing the geometric distribution of one isotope relative to the other. Age plateaux and isochrons can even be generated entirely as experimental artifacts. Alterations of ⁴⁰Ar-³⁹Ar plateau ages by recoil redistribution of ³⁹Ar, incorporation of trapped ⁴⁰Ar and prior ⁴⁰Ar loss provide significant examples.10% shifts in isotopic ratios are very easily obtained and would result in errors in ⁴⁰Ar-³⁹Ar plateau ages of 4 AE old samples of ～100 m.y., which is comparable to the age differences which must be resolved to develop early lunar and solar system chronology. The possible occurrence of diffusion artifacts must be evaluated in every case to establish that ages and age differences obtained by stepwise thermal release analyses are real.All studies involving the stepwise thermal extraction of multiple isotopic components may show similar diffusion artifacts. Constant isotopic compositions may be obtained during thermal release which do not represent the actual compositions of sample reservoirs.     

New 40Ar/39Ar ages for selected young (<1 Ma) basalt flows of the Newer Volcanic Province,southeastern Australia

The Pliocene-Holocene Newer Volcanic Province (NVP) of southeastern Australia is an extensive, relatively well-preserved, intra-plate basaltic lava field containing more than 400 eruptive centres. This study reports new, high-precision ⁴⁰Ar/³⁹Ar ages for six young (300–600 ka) basalt flows from the NVP and is part of a broader initiative to constrain the extent, duration, episodicity and causation of NVP volcanism. Six fresh, holocrystalline alkali basalt flows were selected from the Warrnambool-Port Fairy area in the Western Plains sub-province for ⁴⁰Ar/³⁹Ar dating. These flows were chosen on the basis of pre-existing K-Ar age constraints, which, although variable, indicated eruption during a period of apparent relative volcanic quiescence (0.8–0.06 Ma).⁴⁰Ar/³⁹Ar ages were measured on multiple aliquots of whole rock basalt samples. Three separate flows from the Mount Rouse volcanic field yielded concordant ⁴⁰Ar/³⁹Ar age results, with a mean eruption age of 303 ± 13 ka (95% CI). An older weighted mean age of 382 ± 24 ka (2σ) was obtained for one sample from the central Rouse-Port Fairy Flow, suggesting extraneous argon contamination. Two basalt flows from the Mount Warrnambool volcano also yielded analogous results, with an average ⁴⁰Ar/³⁹Ar age of 542 ± 17 ka (95% CI). The results confirm volcanic activity during the interval of relative quiescence. Most previous K-Ar ages for these flows are generally older than the weighted mean ⁴⁰Ar/³⁹Ar ages, suggesting the presence of extraneous ⁴⁰Ar. This study demonstrates the suitability of the ⁴⁰Ar/³⁹Ar incremental-heating method to obtain precise eruption ages for young, holocrystalline alkali basalt samples in the NVP.     

40Ar/39Ar and U-Th-Pb dating of separated clasts from the Abee E4 chondrite

Determinations of⁴⁰Ar/³⁹Ar and U-Th-Pb are reported for three clasts from the Abee (E4) enstatite chondrite, which has been the object of extensive consortium investigations. The clasts give⁴⁰Ar/³⁹Ar plateau ages and/or maximum ages of 4.5 Gy, whereas two of the clasts give average ages of 4.4 Gy. Within the range of 4.4–4.5 Gy these data do not resolve any possible age differences among the three clasts.²⁰⁶Pb measured in these clasts is only ～1.5–2.5% radiogenic, which leads to relatively large uncertainties in the Pb isochron age and in the²⁰⁷Pb/²⁰⁶Pb model ages. The Pb data indicate that the initial²⁰⁷Pb/²⁰⁶Pb was no more than 0.08±0.07% higher than this ratio in Can?on Diablo troilite. The U-Th-Pb data are consistent with the interpretation that initial formation of these clasts occurred 4.58 Gy ago and that the clasts have since remained closed systems, but are contaminated with terrestrial Pb. The⁴⁰Ar/³⁹Ar ages could be gas retention ages after clast formation or impact degassing ages. The thermal history of Abee deduced from Ar data appears consistent with that deduced from magnetic data, and suggests that various Abee components experienced separate histories until brecciation no later than 4.4 Gy ago, and experienced no appreciable subsequent heating.     

几种全岩样品40Ar-39Ar年龄谱的探讨

探讨了玄武岩、辉绿岩和断层泥中粘土矿物3种全岩样品在40Ar-39Ar分析中的某些问题。利用快中子照射过程中不同矿物发生不均匀Ar丟失的模式解释了玄武岩40Ar-39Ar全熔年龄与K-Ar年龄间的差异。对辉绿岩样品的40Ar-39Ar分析确定了辉绿岩脉的侵入时代,并说明该地区其它辉绿岩样品K-Ar年龄的离散是受后期变质作用的影响。对于断层泥中粘土矿物,快中子照射中39Ar的反冲丢失导致<1μm粒级部分的40Ar-39Ar全熔年龄偏高。<1μm和2～10μm粒级样品的40Ar-39Ar年龄谱中,低温区较低的阶段年龄可能是断层后期较强烈活动的结果。     

40Ar/39Ar age and thermal history of the Kirin chondrite

The Kirin meteorite, a large (2800kg) H5 chondrite, fell in Kirin Province, China in 1976. A sample from each of the two largest fragments (K-1, K-2) yield⁴⁰Ar/³⁹Ar total fusion ages of 3.63 ± 0.02b.y. and 2.78 ± 0.02b.y. respectively.⁴⁰Ar/³⁹Ar age spectra show typical diffusional argon loss profiles. Maximum apparent ages of 4.36 b.y. (K-1) and ～4.0 b.y. (K-2) are interpreted as possible minimum estimates for the age of crystallization of the parent body.The⁴⁰Ar/³⁹Ar ages found for gas released at low temperature are about 2.2 b.y. for K-1 and about 0.5 b.y. for K-2, suggesting that this meteorite may have suffered two discrete collisional events that caused degassing of radiogenic argon. Modelling of possible thermal events in the parent body indicates that samples K-1 and K-2 were at a depth of less than 3 m from the base of an impact melt of a thickness less than 7 m and separated by no more than ～2 m from one another at the time of the heating event about 0.5 b.y. ago. Further, the duration of heating was probably less than a few years.Calculations from³⁸Ar data yield exposure ages for samples K-1 and K-2 of about 5 m.y., similar to that found for many other H chondrites.     

Potassium argon dating of gamma-irradiated minerals

A method for measuring potassium-argon ages making use of the reaction³⁹K(λ, n)³⁸K^β+→³⁸Ar to indirectly determine potassium is discussed. In principle, it is closely analogous to the⁴⁰Ar/³⁹Ar dating method and should possess all of the attributes of that technique. It is demonstrated that precise dating of mica samples with ages between 15 my and 1000 my can be carried out, and a discussion of potentially interfering reactions suggests that it may be possible to extend the method to the problem of induced argon isotopic dating of calcium-rich minerals.     

Combined unspiked K–Ar and 40Ar/39Ar dating applied to the dating of 80–260 ka old Icelandic sub-glacial rhyolites

We have used two techniques (i.e. K–Ar and ⁴⁰Ar/³⁹Ar) on Icelandic obsidian samples to produce and more specially to estimate the quality and accuracy of the ages that can be obtained. Following a meticulous protocol, we were able to date six rhyolitic eruptions with an accuracy 7 to 40 times better than those obtained previously. Among these six rhyolites are the first published K–Ar and ⁴⁰Ar/³⁹Ar ages of Krafla.The combined K–Ar and ⁴⁰Ar/³⁹Ar approach produces not only highly precise but also accurate ages. Such high precision makes it possible to produce accurate reconstructions of ice thickness at a given location and time, to test whether there was a possible link between deglaciation and rhyolitic volcanism onset in Iceland, and to explore other possible applications of the ⁴⁰Ar/³⁹Ar dating method to paleo-environmental and paleo-climatic reconstruction at Iceland's latitude.Then, we investigate, by combining geochemistry (i.e. determination of major and trace element composition) and geochronology (i.e. dating of rhyolitic eruptions via K–Ar and ⁴⁰Ar/³⁹Ar dating) for a number of Icelandic rhyolitic volcanoes whose activity could be recorded in North Atlantic sedimentary cores as well as in Arctic ice. The aim of this approach is to provide new independent anchors and correlations between climate records. Of the six dated eruptions, we propose that one is record in North Atlantic sediments, the Loðmundur eruption that constitutes one of the Kerlingarfjöll tuyas, which we date at 189.9 ± 1.1 ka and assume to be the source of the tephra recognized in core MD04-2822 at a depth of 3630–3631 cm.     

Argon in Apollo 15 green glass spherules (15426):40Ar39Ar age and trapped argon

We report the results of thermal-release argon analyses of neutron-irradiated green glass spherules separated from lunar sample 15426. The gas-retention age, as determined by the⁴⁰Ar³⁹Ar method, is (3.38 ± 0.06) X 10⁹yr. This age is similar to those of local mare basalts and distinct from the ages of Appenine Front samples recovered from the same region as 15426. Trapped argon is present in near-surface regions of the spherules, and can be resolved into at least two components requiring separate origins, a shallow (0.1 μ) component with⁴⁰Ar/³⁹Ar > 30, and a deeper (2 μ) component with ⁴⁰Ar/³⁶Ar= 2.9. The ratio of trapped⁴⁰Ar to³⁶Ar is higher than found in any lunar soil and suggests that the trapped gas was implanted early in the spherules' history. The cosmic-ray exposure age is 300 my.     

39Ar-40Ar ages of samples from the Apollo 17 station 7 boulder and implications for its formation

³⁹Ar-⁴⁰Ar ages and³⁷Ar-³⁸Ar exposure ages of samples representing four different lithologies of the Apollo 17 station 7 boulder were measured. The age of the dark veinlet material77015of3.98 ± 0.04AE is interpreted as representing the time of intrusion of this veinlet into the 77215 clast. The data obtained so far indicate that the vesicular basalt 77135 formed 100–200 m.y. later. However, this has to be confirmed by³⁹Ar-⁴⁰Ar investigations on separated mineral and/or grain-size fractions. A small clast enclosed in the 77135 basalt gives a well-defined high temperature age of3.99 ± 0.02AE. A sample of the noritic clast 77215 gave4.04 ± 0.03AE, the highest age found so far in this boulder. The³⁹Ar-⁴⁰Ar ages obtained are in agreement with the age relationships deduced from the stratigraphic evidence.Taking into account the shielding by the boulder itself, an average³⁷Ar-³⁸Ar exposure age of(27.5 ± 2.5)m.y. is obtained for the samples collected from the boulder.     

Testing the applicability of vacuum-encapsulated 40Ar/39Ar geochronology to pedogenic palygorskite and sepiolite

The difficulty of isolating intact, mineralogically pure pedogenic crystals from cemented soil is one of the most significant obstacles to quantifying rates of soil formation, geomorphic processes, and climate change in arid regions. We evaluate the applicability of vacuum encapsulated ⁴⁰Ar/³⁹Ar geochronology to pedogenic palygorskite and sepiolite extracted from the 4 to 5 Ma, extant Mormon Mesa petrocalcic soil-geomorphic surface of southern Nevada, and from the 780 ka to 2 Ma Jornada Experimental Range La Mesa soil-geomorphic surface near Las Cruces, New Mexico. Selective dissolution of cements using NaOAc and Tiron, accompanied by particle size fractionation, was used to isolate the pedogenic Mg-phyllosilicates. Scanning electron microscopy, inductively-coupled plasma spectrometry, X-ray diffraction, gas chromatograph mass spectrometry, and Ar isotope analysis were used to determine whether extraction impacted palygorskite/sepiolite suitability for ⁴⁰Ar/³⁹Ar geochronology. We found no adverse morphological or mineralogical effects, but meaningful ages could not be obtained due to small amounts of old, detrital phyllosilicates in the samples. While the potential of pedogenic palygorskite and/or sepiolite for geochronology now seems limited, results from this study may prove relevant for samples from other, non-pedogenic surface environments. It is hoped that this work will encourage further research towards successful ⁴⁰Ar/³⁹Ar geochronology of pedogenic phyllosilicates, as well as inform future geochemical or isotopic studies of individual pedogenic mineral species.     

Data reporting norms for 40Ar/39Ar geochronology

Data reported in ⁴⁰Ar/³⁹Ar geochronology studies are commonly insufficient to allow computation of ages. This deficiency renders it difficult to compare ages based on different standards or constants, and often hinders critical evaluation of the results. Herein are presented an enumeration of the data that should be reported in all ⁴⁰Ar/³⁹Ar studies, including a discussion in support of these requirements. The minimum required data are identified and distinguished from parameters that are useful but may be derived from them by calculation. Finally, recommendations are made for metadata needed to document age calculations (e.g., from age spectrum or isochron analyses).     

Intercalibration of the Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile and WiscAr 40Ar/39Ar laboratories for Quaternary dating

Accurate and precise dating of Quaternary lavas and pyroclastic flow or fall deposits is essential for understanding the evolution of active volcanoes and providing context for future eruptions and hazard assessment. The ⁴⁰Ar/³⁹Ar method is commonly employed to date these volcanic materials, however, dating young (<150 ka) K₂O-poor materials can be challenging owing to low radiogenic ⁴⁰Ar* contents that can be difficult to distinguish from trapped atmospheric argon. To address this challenge, a collaborative intercalibration exercise involving the University of Wisconsin-Madison WiscAr Laboratory and the ⁴⁰Ar/³⁹Ar Laboratory of the Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile was conducted on a common set of samples with the aim of refining our methods and optimizing precision and accuracy of age determinations. Groundmass and plagioclase samples were analyzed on a 5-collector Noblesse ion counting mass spectrometer in the WiscAr lab, whereas measurements in the SERNAGEOMIN lab were performed using an ARGUS VI spectrometer equipped with faraday detectors and one compact discrete dynode electron multiplier. Samples for the intercalibration were collected jointly from three Andean Southern Volcanic Zone volcanoes to evaluate the capability of each laboratory to date different materials. Samples from lava flows with 1.0–3.2 wt % K₂O from Planchon-Peteroa volcanic complex and with <1.0 wt % K₂O from Calbuco Volcano that are the focus of ongoing geological studies were measured in both laboratories. Single crystals of plagioclase (0.6–1.0 wt% K₂O) were measured from the voluminous Diamante (Pudahuel) ignimbrite sourced from the Diamante Caldera. Multiple rounds of experiments were conducted including co-irradiation of samples at Oregon State University, as well as irradiations using the CCHEN reactor in Chile to investigate differences in neutron fluence parameters. As a result, SERNAGEOMIN has modified long-used protocols for the CCHEN reactor so that Quaternary samples may be irradiated for periods of time most appropriate for their age. Although less precise than plateau ages, the isochron ages generated in the two laboratories agree at 2σ for each sample. Six of six co-irradiated samples from Planchon-Peteroa yield plateau ages that also show inter-lab agreement at 2σ. The low K₂O lavas from Calbuco proved more challenging with only three out of five plateau ages in agreement between labs. SERNAGEOMIN blanks were higher and more variable in Calbuco experiments, thus, differences in the variability of the measured ³⁶Ar blanks between the two laboratories may explain the discrepancy in plateau ages. Analysis of single plagioclase crystals from the Diamante Ignimbrite show excellent agreement between labs for both weighted mean apparent ages and isochron ages. We favor an isochron age for the ignimbrite of 132.4 ± 2.2 ka, however, discrepancies in results between samples from three different outcrops present an interesting geochronologic problem that warrants further study. Overall, the consistency of the results between labs is promising. These new precise age determinations significantly improve our understanding of the temporal evolution of these active volcanoes.     

Ar/Ar geochronology of Neogene phreatomagmatic volcanism in the western Pannonian Basin,Hungary

Neogene alkaline basaltic volcanic fields in the western Pannonian Basin, Hungary, including the Bakony–Balaton Highland and the Little Hungarian Plain volcanic fields are the erosional remnants of clusters of small-volume, possibly monogenetic volcanoes. Moderately to strongly eroded maars, tuff rings, scoria cones, and associated lava flows span an age range of ca. 6 Myr as previously determined by the K/Ar method. High resolution ⁴⁰Ar/³⁹Ar plateau ages on 18 samples have been obtained to determine the age range for the western Pannonian Basin Neogene intracontinental volcanic province. The new ⁴⁰Ar/³⁹Ar age determinations confirm the previously obtained K/Ar ages in the sense that no systematic biases were found between the two data sets. However, our study also serves to illustrate the inherent advantages of the ⁴⁰Ar/³⁹Ar technique: greater analytical precision, and internal tests for reliability of the obtained results provide more stringent constraints on reconstructions of the magmatic evolution of the volcanic field. Periods of increased activity with multiple eruptions occurred at ca. 7.95 Ma, 4.10 Ma, 3.80 Ma and 3.00 Ma.