Wijbrans et al 2011

Pierre-Yves Gillot,· Anthony Hildenbrand,· Jean-Claude Lefèvre, Claude Albore Livadie

The K/Ar geochronometer is appropriate to date geological materials ranging in age from several thousands to several billions of years. Two complementary techniques of the K/Ar method are classically used: 1. the K/Ar conventional technique, in which potassium and argon are measured separately from a homogeneous preparation of the sample, and 2. the 40Ar/39Ar technique, which consists in the combined measurement of radiogenic argon (40Ar*), and argon produced from potassium (39Ar K ) during irradiation of the sample in a fast neutron nuclear reactor. We here review the principle, the analytical procedures, the advantages and the limitations of the two techniques. More specifically, we focuss on the unspiked K/Ar Casignol-Gillot technique, which allows to detect tiny amounts of radiogenic argon (as low as 0.1%) and is thus especially suitable to date very recent volcanic products, even basic in composition. Applications of this technique to young volcanic eruptions in southern Italy are presented, including the Avelino sub-historical explosive event from Monte Somma-Vesuvius. Sanidines separated from a grey pumice layer sampled independently in the sites of Nola and Cava Novasche yield a mean age of 3840 ± 180 yr, in perfect agreemeent with calibrated ams radiocarbon ages achieved recently on charcoal fragments (Albore-Livadie et alii 1998). This demonstrates the effectiveness of the K/Ar method to date accurately volcanic episodes up to historical periods, with absolute uncertainties presently reaching a few centuries.

Quaternary Geochronology, 2011

Absolute chronologies of active volcanoes and consequently timescales for eruptive behaviour and magma production form a quantitative basis for understanding the risk of volcanoes.

Bulletin of Volcanology, 2012

2009

New 40 Ar/ 39 Ar and 14 C ages have been found for the Albano multiple maar pyroclastic units and underlying 26 paleosols to document the most recent explosive activity in the Colli Albani Volcanic District (CAVD) near 27 Rome, Italy, consisting of seven eruptions (Albano 1^=^oldest). Both dating methodologies have been applied 28 on several proximal units and on four mid-distal fall/surge deposits, the latter correlated, according to two 29 current different views, to either the Albano or the Campi di Annibale hydromagmatic center. The 40 Ar/ 39 Ar 30 ages on leucite phenocrysts from the mid-distal units yielded ages of ca. 72 ka, 73 ka, 41 ka and 36 ka BP, 31 which are indistinguishable from the previously determined 40 Ar/ 39 Ar ages of the proximal Albano units 1, 2, 32 5 and 7, thus confirming their stratigraphic correspondence. 33 Twenty-one 14 C ages of the paleosols beneath Albano units 3, 5, 6 and 7 were found for samples collected 34 from 13 proximal and distal sections, some of which were the same sections sampled for 40 Ar/ 39 Ar 35 measurements. The 14 C ages were found to be stratigraphically inconsistent and highly scattered, and were 36 systematically younger than the 40 Ar/ 39 Ar ages, ranging^from 35 ka^to 3 ka. Considering the significant 37 consistence of the 40 Ar/ 39 Ar chronological framework, we interpret the scattered and contradictory 14 C ages 38 to be the result of a variable contamination of the paleosols by younger organic carbon deriving from the 39 superficial soil horizons. 40 These results suggest that multiple isotopic systems anchored to a robust stratigraphic framework may need 41 to be employed to determine accurately the geochronology of the CAVD as well as other volcanic districts. 42

Terra Nova, 2005

Recent geological studies performed at Etna allow reassessing the stratigraphic frame of the volcano where distinct evolutionary phases are defined. This stratigraphic reconstruction was chronologically constrained on the basis of a limited number of U–Th and K–Ar age determinations whose uncertainty margins are sometimes too wide. For this reason, we successfully adopted at Etna the 40Ar/39Ar technique that allowed obtaining more precise age determinations. The incremental heating technique also gives information on sample homogeneity, and potential problems of trapped argon. Five samples were collected from stratigraphically well-controlled volcanic units in order to chronologically define the transition between the fissure-type volcanism of the Timpe phase to the central volcanism of the Valle del Bove Centers. Isotopic ages with an uncertainty margin of 2–4% have been obtained emphasizing that this transition occurred (130–126 ka) without significant temporal hiatus.

Geology

Many volcanoes worldwide still have poorly resolved eruption histories, with the date of the last eruption often undetermined. One such example is Ascension Island, where the timing of the last eruption, and consequently, the activity status of the volcano, is unclear. Here, we use the 40 Ar/ 39 Ar dating technique to resolve ages of the three youngest lava flows on the island, which are hawaiites and mugearite with 1.5-1.9 wt% K2O. In dating these lavas, we provide the first evidence of Holocene volcanic activity on Ascension (0.51 ± 0.18ka; 0.55 ± 0.12 ka; 1.64 ± 0.37 ka), determining that it should be classed as an active volcanic system. In addition, we demonstrate that the 40 Ar/ 39 Ar method can reproducibly date mafic lava flows younger than 1 ka, decreasing the gap between recorded history and geological dating. These results offer new prospects for determining patterns of late-Holocene volcanic activity; critical for accurate volcanic hazard assessment.

Geological Magazine, 2012

The city of Naples can be considered part of the Campi Flegrei volcanic field, and deposits within the urban area record many autochthonous pre-to post-caldera eruptions. Age measurements were carried out using 40 Ar-39 Ar dating techniques on samples from small monogenetic vents and more widely distributed tephra layers. The 40 Ar-39 Ar ages on feldspar phenocrysts yielded ages of c. 16 ka and 22 ka for events older than the Neapolitan Yellow Tuff caldera-forming eruption (15 ka), and ages of c. 40 ka, 53 ka and 78 ka for events older than the Campanian Ignimbrite caldera-forming eruption (39 ka). The oldest age obtained is 18 ka older than previous dates for pyroclastic deposits cropping out along the northern rim of Campi Flegrei. The results of this study allow us to divide the Campi Flegrei volcanic history into four main, geochronologically distinct eruptive cycles. A new period, the Paleoflegrei, occurred before 74-78 ka and has been proposed to better discriminate the ancient volcanism in the volcanic field. The eruptive history of Campi Flegrei extends possibly further back than this, but the products of previous eruptions are difficult to date owing to the lack of fresh juvenile clasts. These new geochronological data, together with recently published ages related to young volcanic edifices located in the city of Naples (Nisida volcano, 3.9 ka) testify to persistent activity over a period of at least 80 ka, with an average eruption recurrence interval of ∼ 555 years within and adjacent to this densely populated city.

Journal of Volcanology and Geothermal Research, 1996

The chronology of the most significant eruptions at the island of Ischia in the last 5000 years has been studied by means of Accelerator Mass Spectrometry to obtain 14C data. The results are in good agreement with stratigraphical and archeological constraints. This allows interpretation of the measured dates as ages of the eruptions which produced the deposits overlying or incorporating the sampled paleosols and charcoal fragments, respectively. These ages define the timing of volcanism in the last 5000 years, suggesting that the recent volcanic activity at Ischia is characterized by periods of very intense volcanism alternating to periods of quiescence. This timing of the volcanism is correlated with the behaviour of the magmatic system and dynamics of resurgence of the Mt. Epomeo block.

Earth and Planetary Science Letters, 2005

This study investigates the reliability of a paleoaltimeter based on the elevation dependency of the cosmogenic nuclides production rate. The proposed method relies on bfossilQ cosmogenic record. Indeed, measuring the cosmogenic nuclide concentration in an exposed and then buried lava flow combined with an independent estimate (by K-Ar or 40 Ar / 39 Ar dating) of the past exposure duration theoretically allows recording bfossilQ cosmogenic production rates, which can thus be converted in paleoelevations. This approach was tested by measuring cosmogenic 3 He ( 3 He c ) in olivines and clinopyroxenes of Quaternary K-Ar dated basaltic flows of Mount Etna volcano (Sicily, 388N). The agreement within uncertainties (1r V 500 m) between current and cosmogenic-derived elevations demonstrates the paleoaltimeter reliability. However, dating imprecision, Earth's paleomagnetic fluctuations and erosion contemporary to the flow exposure are parameters that may affect the accuracy and the precision of the method on older geological scales. Their respective influences were thus carefully evaluated and discussed to conclude that the paleoaltimeter can potentially reach resolutions better than~1000 m, even for Cenozoic records. D

Palaeogeography Palaeoclimatology Palaeoecology, 2000

New geochronological data are reported for two key ash beds interbedded in Upper Miocene (Messinian)–Early Pleistocene rocks at Maccarone, Bellante, and Mosciano S. Angelo on the Adriatic side of the Italian peninsula. Major element chemistry of glass shards was determined with electron microprobe analysis on the younger key bed. The older, Messinian-age ash bed, sampled at Maccarone, yields a corrected glass fission-track age of 5.38±0.42 Ma. The younger ash bed, sampled at Mosciano S. Angelo and at two localities at Bellante, yields corrected glass ages of ∼2.1 Ma, similar to ages previously determined on Pliocene–Pleistocene ash collected along the Ionic coast and on the Tyrrhenian side of Italy. Apatite fission-track dating of one of the Bellante samples confirms the glass age, whereas apatite from the Mosciano S. Angelo sample yields an excessively old age due to the presence in the sample of a significant component of detrital grains. An unusual bimodal spontaneous track-diameter distribution is found in the Maccarone glass, similar to the distributions previously obtained on other Messinian-age tephra samples from the same region. This anomalous track-diameter distribution is interpreted as a thermal disturbance coeval with the volcanic activity that produced the Pliocene–Pleistocene key volcanic ash bed sampled at Bellante and Mosciano S. Angelo. The contemporaneity of this ash with the event detected by the Maccarone glass suggests that it was related to the tectonic phase which marked, with numerous erosive levels distributed along the whole Apennine region, the end of the Pliocene sedimentary cycle and the onset of the Pleistocene cycle.