Surface exposure dating of young basalts (1–200 ka) in the San Francisco volcanic field (Arizona,USA) using cosmogenic 3He and 21Ne

K–Ar ages of young basalts (<500 ka) are often higher than the actual eruption age, due to low potassium contents and the frequent presence of excess Ar in olivine and pyroxene phenocrysts. Geological studies in the San Francisco and Uinkaret volcanic fields in Arizona have documented the presence of excess ⁴⁰Ar and have concluded that K–Ar ages of young basalts in these fields tend to be inaccurate. This new study in the San Francisco volcanic field presents ³He_c and ²¹Ne_c ages yielded by olivine and pyroxene collected from three Pleistocene basalt flows – the South Sheba (∼190 ka), SP (∼70 ka), and Doney Mountain (∼67 ka) lava flows, – and from one Holocene basalt, the Bonito Lava Flow (∼1.4 ka) at Sunset Crater. These data indicate that, in two of three cases, ⁴⁰Ar/³⁹Ar and K–Ar ages of the young basalts agree well with cosmic-ray surface exposure ages of the same lava flow, thus suggesting that excess ⁴⁰Ar is not always a problem in young basalt flows in the San Francisco volcanic field. The exposure age of the Bonito lava flow agrees within uncertainty with dendrochronological and archeological age determinations. K–Ar and cosmogenic ³He and ²¹Ne ages from the SP flow are in agreement and much older than the OSL age (5.5–6 ka) reported for this lava flow. Furthermore, if the non-cosmogenic ages are assumed to be accurate, the subsequent calculated production rates at South Sheba and SP flow sample sites agree well with values in the literature.     

Late-stage volcano geomorphic evolution of the Pleistocene San Francisco Mountain, Arizona (USA), based on high-resolution DEM analysis and 40Ar/39Ar chronology

The cone-building volcanic activity and subsequent erosion of San Francisco Mountain, AZ, USA, were studied by using high-resolution digital elevation model (DEM) analysis and new ⁴⁰Ar/³⁹Ar dating. By defining remnants or planèzes of the volcano flanks in DEM-derived images, the original edifice can be reconstructed. We propose a two-cone model with adjacent summit vents which were active in different times. The reconstructed cones were 4,460 and 4,350 m high a.s.l., corresponding to ∼2,160 and 2,050 m relative height, respectively. New ⁴⁰Ar/³⁹Ar data allow us to decipher the chronological details of the cone-building activity. We dated the Older and Younger Andesites of the volcano that, according to previous mapping, built the stage 2 and stage 3 stratocones, respectively. The new ⁴⁰Ar/³⁹Ar plateau ages yielded 589–556 ka for the Older and 514–505 ka for the Younger Andesites, supporting their distinct nature with a possible dormant period between. The obtained ages imply an intense final (≤100 ka long) cone-building activity, terminating ∼100 ka earlier than indicated by previous K-Ar ages. Moreover, ⁴⁰Ar/³⁹Ar dating constrains the formation of the Inner Basin, an elliptical depression in the center of the volcano initially created by flank collapse. A 530 ka age (with a ±58.4 ka 2σ error) for a post-depression dacite suggests that the collapse event is geochronologically indistinguishable from the termination of the andesitic cone-building activity. According to our DEM analysis, the original cone of San Francisco Mountain had a volume of about 80 km³. Of this volume, ∼7.5 km³ was removed by the flank collapse and subsequent glacial erosion, creating the present-day enlarged Inner Basin, and ∼2 km³ was removed from the outer valleys by erosion. Based on volumetric analysis and previous and new radiometric ages, the average long-term eruption rate of San Francisco Mountain was ∼0.2 km³/ka, which is a medium rate for long-lived stratovolcanoes. However, according to the new ⁴⁰Ar/³⁹Ar dates for the last ≤100 ka period, the final stratovolcanic activity was characterized by a greater ∼0.3 km³/ka rate.     

40Ar/39Ar dating of the SP and Bar Ten lava flows AZ,USA: Laying the foundation for the SPICE cosmogenic nuclide production-rate calibration project

Precise ⁴⁰Ar/³⁹Ar age determinations made on basalt groundmass collected from the SP and upper and lower Bar Ten lava flows in the San Francisco and Uinkaret volcanic fields of Arizona, USA, yield ages of 72 ± 4, 97 ± 10, and 123 ± 12 ka (2σ; relative to Renne et al., 2010, 2011, full external precision), respectively. Previous ages of the SP lava flow include a K–Ar age of 70 ± 8 ka and OSL ages of 5.5–6 ka. ⁴⁰Ar/³⁹Ar age constraints, relative to the optimization model of Renne et al. (2010, 2011), of 81 ± 50 and 118 ± 64 ka (2σ; full external precision) were previously reported for the upper and lower Bar Ten lava flows, respectively. The new ⁴⁰Ar/³⁹Ar ages are within uncertainty of previous age constraints, and are more robust, accurate, and precise. Preliminary cosmogenic ³He and ²¹Ne production rates from the Bar Ten flows reported by Fenton et al. (2009) are updated here, to account for the improved quality of the ⁴⁰Ar/³⁹Ar data. The new ⁴⁰Ar/³⁹Ar age for the SP flow yields cosmogenic ³He and ²¹Ne production rates for pyroxene (119 ± 8 and 26.8 ± 1.9 at/g/yr; error-weighted mean, 2σ uncertainty; Dunai (2000) scaling method) that are consistent with production rate values reported throughout the literature. The ⁴⁰Ar/³⁹Ar and cosmogenic ³He and ²¹Ne data support field observations indicating the SP flow has undergone negligible erosion. The SP flow contains co-existing phenocrysts of olivine and pyroxene, as well as xenocrysts of quartz in a fine-grained groundmass facilitating cross-calibration of cosmogenic production rates and production-rate (³He, ¹⁰Be, ¹⁴C, ²¹Ne, ²⁶Al, and ³⁶Cl). Thus, we propose the SP flow is an excellent location for a cosmogenic nuclide production-rate calibration site (SPICE: the SP Flow Production-Rate Inter-Calibration Site for Cosmogenic-Nuclide Evaluations).     

Surface exposure dating of Holocene basalt flows and cinder cones in the Kula volcanic field (Western Turkey) using cosmogenic 3He and 10Be

The Kula volcanic field in Western Turkey comprises about 80 cinder cones and associated basaltic lava flows of Quaternary age. Based on geomorphological criteria and K-Ar dating, three eruption phases, β2–β4, were distinguished in previous studies. Human footprints in ash deposits document that the early inhabitants of Anatolia were affected by the volcanic eruptions, but the age of the footprints has been poorly constrained. Here we use ³He and ¹⁰Be exposure dating of olivine phenocrysts and quartz-bearing xenoliths to determine the age of the youngest lava flows and cinder cones. In the western part of the volcanic field, two basalt samples from a 15-km-long block lava flow yielded ³He ages of 1.5 ± 0.3 ka and 2.5 ± 0.4 ka, respectively, with the latter being in good agreement with a ¹⁰Be age of 2.4 ± 0.3 ka for an augen gneiss xenolith from the same flow. A few kilometers farther north, a metasedimentary xenolith from the top of the cinder cone Çakallar Tepe gave a ¹⁰Be age of 11.2 ± 1.1 ka, which dates the last eruption of this cone and also the human footprints in the related ash deposits. In the center of the volcanic field, a basalt sample and a metasedimentary xenolith from another cinder cone gave consistent ³He and ¹⁰Be ages of 2.6 ± 0.4 ka and 2.6 ± 0.3 ka, respectively. Two β4 lava flows in the central and eastern part of the volcanic province yielded ³He ages of 3.3 ± 0.4 ka and 0.9 ± 0.2 ka, respectively. Finally, a relatively well-preserved β3 flow gave a ³He age of ∼13 ka. Taken together, our results demonstrate that the penultimate eruption phase β3 in the Kula volcanic field continued until ∼11 ka, whereas the youngest phase β4 started less than four thousand years ago and may continue in the future.     

Direct dating of Plio-Quaternary pumices by Ar/Ar step-heating and single-grain laser fusion methods: the example of the Monts-Dore massif (Massif Central, France)

K-feldspars separated from Plio-Quaternary pumice flows of the Monts-Dore massif (Massif Central, France) give ⁴⁰Ar/³⁹Ar saddle-shaped age spectra. Laser-probe analysis of hand-picked single grains gives ages in agreement with those of overlying and underlying lava flows previously dated by the conventional K-Ar method. The laser-probe ages are lower than the minimum ages measured on the populations of grains by the step-heating method. As previously suggested by Lo Bello and co-workers for the pumice flow of Neschers belonging to the same volcanic massif, the saddle-shaped age spectra are ascribed to the different Ar-release patterns of two populations of K-feldspars: (1) young sanidine phenocrysts of the pumices; and (2) old K-feldspars plucked from the Hercynian basement during the explosive eruption. Measurements of both the granitic rocks from surrounding areas and the granitic xenoliths included in pumices give ages around 330 Ma and show that most of the xenocrysts included in the pumices did not lose significant amounts of argon during the eruption. With the ⁴⁰Ar/³⁹Ar step-heating method, we were able to detect contamination of Quaternary K-feldspars by Hercynian K-feldspars as low as 0.25. Because pumice flows of the Monts-Dore massif crop out over large areas, these new ages will be useful for establishing a precise stratigraphy of the European Plio-Quaternary.     

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.     

Refining the Quaternary Geomagnetic Instability Time Scale (GITS): Lava flow recordings of the Blake and Post-Blake excursions

The Blake excursion was among the first recognized with directional and intensity behavior known mainly from marine sediment and Chinese loess. Age estimates for the directional shifts in sediments are poorly constrained to about 118−100 ka, i.e., at the marine isotope stage (MIS) 5e/5d boundary. Moreover, sediments at Lac du Bouchet maar, France and along the Portuguese margin reveal what may be a "post-Blake" excursion at about 105−95 ka. The excursional directions are associated with a prominent paleointensity minimum between about 125 and 95 ka in global stacked records. Lava flow recordings of the Blake excursion(s) have, however, been questionable because precise ages required for correlation with these sediment records are lacking. To establish new, independent records of the Blake excursion, and link these into a larger Quaternary GITS, we have undertaken ⁴⁰Ar/³⁹Ar incremental heating and unspiked K-Ar experiments on groundmass from the transitionally magnetized Inzolfato flow on Lipari Island. We also obtained ⁴⁰Ar/³⁹Ar incremental heating results for a lava flow on Amsterdam Island originally thought to record the Mono Lake excursion and from the transitionally magnetized El Calderon basalt flow, New Mexico that was K-Ar dated by Champion et al. (1988) at 128 ± 66 ka.Unspiked K-Ar ages of four samples from the Inzolfato flow are 102.5 ± 4.7, 101.3 ± 3.3, 97.1 ± 2.6, and 96.8 ± 3.1 ka and thus indistinguishable from one another. ⁴⁰Ar/³⁹Ar results are more complex, with three samples yielding discordant age spectra. Based on incremental heating data obtained in both the UW-Madison and Gif-sur-Yvette ⁴⁰Ar/³⁹Ar laboratories, a fourth sample yields six concordant age plateaus and a weighted mean age of 105.2 ± 1.4 ka that we take as the best estimate of time since the flow erupted. Five ⁴⁰Ar/³⁹Ar incremental heating experiments on the Amsterdam Island lava yield a plateau age of 120 ± 12 ka, whereas ages from two sites in the Calderon flow are 112 ± 23 and 101 ± 14 ka, together giving a weighted mean of 104 ± 12 ka. The age of 120 ± 12 ka from Amsterdam Island, though imprecise, correlates with the Blake excursion. In contrast, the 104–105 ka age obtained from both Lipari and New Mexico indicates that these lavas record a younger period of dynamo instability, most probably associated with the post-Blake excursion. These radioisotopic ages are consistent with the astronomical ages of two paleointensity minima in the PISO-1500 global stack. Our findings indicate that the Blake and post-Blake excursions are both global features of past geodynamo behavior and support the hypothesis that Brunhes chron excursions are temporally clustered into two groups of at least a half-dozen each spanning over 220 to 30 ka and 720 to 520 ka.     

<Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating of tuff vents in the Campi Flegrei caldera (southern Italy): toward a new chronostratigraphic reconstruction of the Holocene volcanic activity

The Campi Flegrei hosts numerous monogenetic vents inferred to be younger than the 15 ka Neapolitan Yellow Tuff. Sanidine crystals from the three young Campi Flegrei vents of Fondi di Baia, Bacoli and Nisida were dated using ⁴⁰Ar/³⁹Ar geochronology. These vents, together with several other young edifices, occur roughly along the inner border of the Campi Flegrei caldera, suggesting that the volcanic conduits are controlled by caldera-bounding faults. Plateau ages of ∼9.6 ka (Fondi di Baia), ∼8.6 ka (Bacoli) and ∼3.9 ka (Nisida) indicate eruptive activity during intervals previously interpreted as quiescent. A critical revision, involving calendar age correction of literature ¹⁴C data and available ⁴⁰Ar/³⁹Ar age data, is presented. A new reference chronostratigraphic framework for Holocene Phlegrean activity, which significantly differs from the previously adopted ones, is proposed. This has important implications for understanding the Campi Flegrei eruptive history and, ultimately, for the evaluation of related volcanic risk and hazard, for which the inferred history of its recent activity is generally taken into account.     

40Ar/39Ar geochronology and exhumation of mylonitized metamorphic complex in Changle-Nanao ductile shear zone

New⁴⁰Ar/³⁹Ar plateau ages from rocks of Changle-Nanao ductile shear zone are 107.9 Ma(Mus), 108.2 Ma(Bi), 107.1 Ma(Bi), 109.2 Ma(Hb) and 117.9 Ma(Bi) respectively, which are concordant with their isochron ages and record the formation age of the ductile shear zone. The similarity and apparent overlap of the cooling ages with respective closure temperatures of 5 minerals document initial rapid uplift during 107–118 Ma following the collision between the Min-Tai microcontinent and the Min-Zhe Mesozoic volcanic arc. The⁴⁰Ar/³⁹ Ar plateau ages, K-Ar date of K-feldspar and other geochronologic information suggest that the exhumation rate of the ductile shear zone is about 0.18–1.12 mm/a in the range of 107–70 Ma, which is mainly influenced by tectonic extension.     

First Ar/Ar age of the Ceprano man (central Italy)

The Ceprano calvarium, found in 1994 in Italy and attributed to Homo cepranensis, is one of the most celebrated hominin remains of Europe. It was considered at least 700 ka-old until a recent investigation incorporating magnetostratigraphy and K-Ar ages from the literature assigned to the calvarium an age of ∼450 (+50, −100) ka. Here we pin down the age of the Ceprano calvarium to 353 ± 4 ka (±1σ external) by means of new ⁴⁰Ar/³⁹Ar dating on K-feldspars retrieved from the sediments that hosted the skull. In absence of evidence of reworking, this refined age sinks the conviction that H. cepranensis belonged to human evolution at the Brunhes–Matuyama boundary (c.a. 781 ka). Our refined age indicates that H. cepranensis lived in central Italy probably during the cold period of marine isotope stage (MIS) 10, and that despite his archaic morphology and lack of Neanderthal traits, he was contemporaneous with more advanced species such as H. heidelbergensis.     

Comparative dating of a Bison-bearing late-Pleistocene deposit,Térapa,Sonora, Mexico

A recently discovered Bison-bearing fossil locality at Térapa, Sonora, Mexico, had previously been dated to 440 ± 130 ka using whole rock ⁴⁰Ar/³⁹Ar on a basalt flow that impounds the deposit. This age is considerably older than the accepted age of about 240–160 ka for the migration of Bison into greater North America. The Térapa deposit also contains a mixture of fossils from extralimital or extinct tropical animals and temperate animals. Constraining the age of the deposit is critical to interpret the paleontologic and paleoclimatologic implications of this unique Sonoran fossil locality. Three additional geochronological methods have been applied to this deposit (infrared stimulated luminescence (IRSL), amino acid racemization (AAR), and radiocarbon) and the data from the original ⁴⁰Ar/³⁹Ar age were revisited. The IRSL data suggest that the impounding basalt flow and the sediments that abut it were emplaced 43 ka ago and that the oldest sediments were deposited shortly after. Two radiocarbon ages suggest the fossiliferous sediments were emplaced by 42 ka. Effective diagenetic temperatures inferred from the AAR results, combined with AAR data from a similar-age deposit in southern Arizona, are in accordance with the 40–43 ka age estimates. For the AAR results to corroborate the ⁴⁰Ar/³⁹Ar age, the effective diagenetic temperature for the area would need to be approximately 3 °C, which is unrealistically low for northern Mexico. The new geochronological results suggest the Térapa deposit and fossils are 40–43 ka old. The anomalously old ⁴⁰Ar/³⁹Ar age for the impounding basalt is probably the result of low ⁴⁰Ar* concentrations and inherited ⁴⁰Ar.     

High-precision 40Ar/39Ar dating of pleistocene tuffs and temporal anchoring of the Matuyama-Brunhes boundary

High-precision ⁴⁰Ar/³⁹Ar ages for a series of proximal tuffs from the Toba super-volcano in Indonesia, and the Bishop Tuff and Lava Creek Tuff B in North America have been obtained. Core from Ocean Drilling Project Site 758 in the eastern equatorial Indian Ocean contains discrete tephra layers that we have geochemically correlated to the Young Toba Tuff (73.7 ± 0.3 ka), Middle Toba Tuff (502 ± 0.7 ka) and two eruptions (OTTA and OTTB) related to the Old Toba Tuff (792.4 ± 0.5 and 785.6 ± 0.7 ka, respectively) (⁴⁰Ar/³⁹Ar data reported as full external precision, 1 sigma). Within ODP 758 Termination IX is coincident with OTTB and hence this age tightly constrains the transition from Marine Isotope Stage 19–20 for the Indian Ocean. The core also preserves the location of the Australasian tektites, and the Matuyama-Brunhes boundary with Bayesian age-depth models used to determine the ages of these events, c. 786 and c. 784 ka, respectively. In North America, the Bishop Tuff (766.6 ± 0.4 ka) and Lava Creek Tuff B (627.0 ± 1.5 ka) have quantifiable stratigraphic relationships to the Matuyama-Brunhes boundary. Linear age-depth extrapolation, allowing for uncertainties associated with potential hiatuses in five different terrestrial sections, defines a geomagnetic reversal age of 789 ± 6 ka. Considering our data with respect to the previously published age data for the Matuyama-Brunhes boundary of Sagnotti et al. (2014), we suggest at the level of temporal resolution currently attainable using radioisotopic dating the last reversal of Earths geomagnetic field was isochronous. An overall Matuyama-Brunhes reversal age of 783.4 ± 0.6 ka is calculated, which allowing for inherent uncertainties in the astronomical dating approach, is indistinguishable from the LR04 stack age (780 ± 5 ka) for the geomagnetic boundary. Our high-precision age is 10 ± 2 ka older than the Matuyama-Brunhes boundary age of 773 ± 1 ka, as reported previously by Channell et al. (2010) for Atlantic Ocean records. As ODP 758 features in the LR04 marine stack, the high-precision ⁴⁰Ar/³⁹Ar ages determined here, as well as the Matuyama-Brunhes boundary age, can be used as temporally accurate and precise anchors for the Pleistocene time scale.     

Refinement of the late Quaternary geologic history of Erebus volcano,Antarctica using Ar/Ar and Cl age determinations

Six new ⁴⁰Ar/³⁹Ar and three cosmogenic ³⁶Cl age determinations provide new insight into the late Quaternary eruptive history of Erebus volcano. Anorthoclase from 3 lava flows on the caldera rim have ⁴⁰Ar/³⁹Ar ages of 23 ± 12, 81 ± 3 and 172 ± 10 ka (all uncertainties 2σ). The ages confirm the presence of a second, younger, superimposed caldera near the southwestern margin of the summit plateau and show that eruptive activity has occurred in the summit region for 77 ± 13 ka longer than previously thought. Trachyte from “Ice Station” on the eastern flank is 159 ± 2 ka, similar in age to those at Bomb Peak and Aurora Cliffs. The widespread occurrences of trachyte on the eastern flank of Erebus suggest a major previously unrecognized episode of trachytic volcanism. The trachyte lavas are chemically and isotopically distinct from alkaline lavas erupted contemporaneously in the summit region < 5 km away.     

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.     

Reconciling discrepant chronologies for the geomagnetic excursion in the Mono Basin,California: Insights from new 40Ar/39Ar dating experiments and a revised relative paleointensity correlation

Lacustrine sediments of the Wilson Creek Formation in the Mono Basin, California, record a paleomagnetic field excursion constrained by ¹⁴C and ⁴⁰Ar/³⁹Ar geochronology to have occurred within the last 50 ka. However, ¹⁴C and ⁴⁰Ar/³⁹Ar ages are discordant, making it difficult to distinguish which of two possible excursions during this period, the Mono Lake or Laschamp, is recorded in the Mono Basin. New ⁴⁰Ar/³⁹Ar age determinations from sanidine, as well as the first biotite and obsidian ages, for three of the nineteen rhyolitic ashes intercalated with these sediments are presented and compared to previous ¹⁴C and ⁴⁰Ar/³⁹Ar data sets. Although the sanidine ages of the three ashes are stratigraphically consistent with each other and previously determined ⁴⁰Ar/³⁹Ar ages for other ashes in the Wilson Creek Formation, each is significantly older than ¹⁴C ages obtained from stratigraphically equivalent beds, relative paleointensity field correlations, oxygen isotope records, and glacial histories. These data indicate an absence of juvenile, eruptive crystals and most likely reflect the incorporation of crystals from older volcanic centers or underlying sediment. We examine the strengths and weaknesses of all available geochronologic data for the section exposed at Wilson Creek to arrive at an internally consistent set of age constraints. Using these constraints we propose two new relative paleointensity correlations for the section, both of which indicate that the excursion recorded in the Mono Basin occurred at ～30–34 ka on the Greenland Ice Sheet Project 2 (GISP2) ice core time scale.     

Matuyama-Brunhes reversal and Kamikatsura event on Maui: paleomagnetic directions, Ar/Ar ages and implications

Eighty-nine basaltic lava flows from the northwest wall of Haleakala caldera preserve a concatenated paleomagnetic record of portions of the Matuyama-Brunhes (M-B) reversal and the preceding Kamikatsura event as well as secular variation of the full-polarity reversed and normal geomagnetic field. They provide the most detailed volcanic record to date of the M-B transition. The 24 flows in the transition zone show for the first time transitional virtual geomagnetic poles (VGPs) that move from reverse to normal along the Americas, concluding with an oscillation in the Pacific Ocean to a cluster of VGPs east of New Zealand and back finally to stable polarity in the north polar region. All but one of the 16 Kamikatsura VGPs cluster in central South America. The full-polarity flows, with ⁴⁰Ar/³⁹Ar ages spanning a total of 680 kyr, pass a reversal test and give an average VGP insignificantly different from the rotation axis, with standard deviation consistent with that for other 0-5 Ma lava flows of similar latitude. Precise ⁴⁰Ar/³⁹Ar dating consisting of 31 incremental heating experiments on 12 transitional flows yields weighted mean ages of 775.6±1.9 and 900.3±4.7 ka for the M-B and Kamikatsura transitional flows, respectively. This Matuyama-Brunhes age is ∼16 kyr younger than ages for M-B flows from the Canary Islands, Tahiti and Chile that were dated using exactly the same techniques and standards, suggesting that this polarity transition may have taken considerably longer to complete and been more complex than is generally believed for reversals.     

Ar/Ar and C geochronology of the Albano maar deposits: Implications for defining the age and eruptive style of the most recent explosive activity at Colli Albani Volcanic District,Central Italy

New ⁴⁰Ar/³⁹Ar and ¹⁴C ages have been found for the Albano multiple maar pyroclastic units and underlying paleosols to document the most recent explosive activity in the Colli Albani Volcanic District (CAVD) near Rome, Italy, consisting of seven eruptions (Albano 1 = oldest). Both dating methodologies have been applied on several proximal units and on four mid-distal fall/surge deposits, the latter correlated, according to two current different views, to either the Albano or the Campi di Annibale hydromagmatic center. The ⁴⁰Ar/³⁹Ar ages on leucite phenocrysts from the mid-distal units yielded ages of ca. 72 ka, 73 ka, 41 ka and 36 ka BP, which are indistinguishable from the previously determined ⁴⁰Ar/³⁹Ar ages of the proximal Albano units 1, 2, 5 and 7, thus confirming their stratigraphic correspondence.     

Cosmogenic 21Ne exposure dating of young basaltic lava flows from the Newer Volcanic Province,western Victoria,Australia

Cosmogenic ²¹Ne was utilised to determine exposure ages of young subaerial basaltic lava flows from the Newer Volcanic Province, western Victoria, Australia. The ages (36–53 ka) determined from co-existing cosmogenic ²¹Ne and ³He in olivines separated from basalts are consistent within analytical uncertainties with ages previously determined by cosmogenic ³⁶Cl exposure dating. This paper illustrates the potential of cosmogenic neon exposure ages in studying the eruption, surface morphology, and erosion history of young volcanic rocks, which are difficult to date using other conventional methods, such as K-Ar or ⁴⁰Ar/³⁹Ar dating. The present study demonstrates that combined cosmogenic ³He and ²¹Ne dating, specifically measured cosmogenic ³He/²¹Ne ratios, on the same samples, is powerful for evaluating the validity of calculated cosmogenic ³He and ²¹Ne surface exposure ages.     

Temporal dissection of the Huckleberry Ridge Tuff using the 40Ar/39Ar dating technique

New high-precision single crystal sanidine ⁴⁰Ar/³⁹Ar ages for the Huckleberry Ridge Tuff (HRT), Yellowstone volcanic field, show that the three HRT members (A, B, and C) represent at least two different eruptions. The new ⁴⁰Ar/³⁹Ar ages (all ages calculated relative to the optimisation model of Renne et al., 2011) are: 2.135 ± 0.006 Ma, 2.131 ± 0.008 Ma, and 2.113 ± 0.004 Ma (2σ, full uncertainty propagation), for members A, B and C, respectively. Members A and B are within uncertainty of one another and both are more precise than, but in agreement with, previously published ages. Member C was erupted later than members A and B. HRT members A and B were deposited during the Reunion Normal Polarity Subchron (C2r.1n). Member C was deposited during Subchron C2r.1r. Previously published radiogenic and stable isotope data show that member C was sourced from an isotopically discrete magma with a higher fraction of crustal material than members A and B. The volume of the first HRT eruption is reduced by c. 12% from previous estimates and explosive eruptions from the Yellowstone volcanic field occurred more frequently, producing more homogeneous magma than was previously believed. High-precision ⁴⁰Ar/³⁹Ar dating is key for resolving the eruptive history of Yellowstone, temporal dissection of voluminous ignimbrites, and rigorous investigation of what constitutes a ‘super-eruption’.     

A 13 ± 3 ka age determination of a tholeiite,Pinacate volcanic field,Mexico, and improved methods for Ar/Ar dating of young basaltic rocks

Among the youngest lava flows of the Pinacate volcanic field, Sonora, Mexico, is a large outpouring of tholeiite, the Ives flow. This tube-fed pahoehoe flow contrasts sharply with other Pinacate lavas in its great volume, alkali-poor composition and morphologic features, which include novel small structures named “spatter tubes.” Despite its K-poor character, young age, and the presence of excess ⁴⁰Ar, we determined a ⁴⁰Ar/³⁹Ar age on samples of this flow at 13 ± 3 ka. Such an age determination is made possible via careful monitoring of the mass discrimination of the mass spectrometer and by stacking results from multiple incremental-heating experiments into a single, composite isochron. This age is among the youngest ever to be determined with such precision by the ⁴⁰Ar/³⁹Ar method on a K-poor tholeiite.