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... Albore-Livadie2 56 rated the first principle of the K/Ar dating method. These authors shown that the decay of 40K in fact pro- duces both 40Ar and 40Ca, according to the scheme pre- sented in Figure 2. The decay constants are Ï‚ = 4.962 10-10 yr-1 and ÏÂ = 0.581 10-10 yr-1 for the ways producing 40Ca and 40Ar, respectively (Steiger and Jaeger 1977). ...
Context 2
... age is known with a relative un- certainty of 2% from inter-laboratories comparisons. Though modern mass spectrometers allow to reach an- alytical uncertainties as low as a few permill, the total uncertainty on the age thus should rigourously inte- grate the uncertainty on the age of the standards used as fluence monitors, and hence shoud not be lower than 1-2% at the 1Û level (Coulié et alii 2003). ...
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Citations
... We performed unspiked K-Ar analyses at the GEOPS laboratory (Université Paris-Saclay, France) following the Cassignol-Gillot technique (Cassignol and Gillot, 1982;Gillot and Cornette, 1986;Gillot et al., 2006). Note that the conventional K-Ar technique, which is based on the isotopic dilution approach using an 38 Ar spike with a known number of atoms, is no longer used due to its limitation for dating young (<0.1 Ma) rocks with low (<5%) radiogenic content. ...
... Unfortunately, no age spectra nor isochron plots are shown in Moore et al. (2011) to further investigate the effect of irradiation on these samples. On the other hand, the unspiked Cassignol-Gillot technique (Gillot et al., 2006) necessitates only the measurement of the natural 40 Ar and 36 Ar isotopes. Since it does not require irradiation of the sample prior to analysis, as needed to produce 39 Ar from 39 K for the 40 Ar/ 39 Ar technique, no recoil effects nor correction for reactor-induced production of 36 Ar from 40 Ca (e.g., McDougall and Harrison, 1999) is needed for K-Ar dating. ...
... Thus, the argon measurement process is solely dependent on the sample 40 Ar* content, whose electrical signal is optimized by adjusting the initial sample mass. The measuring process was carried out at the GEOPS laboratory of the Université Paris-Saclay (France), where the detection limit of the mass spectrometer close to 0.1% for 40 Ar* (Quidelleur et al., 2001) allows to date Holocene volcanic rocks with a relatively small uncertainty (Gillot et al., 2006). The potassium (K) concentration is measured independently by ame absorption spectroscopy. ...
The Ecuadorian arc is composed of an unusually high number of volcanoes organized as along-arc alignments and across-arc clusters, over a relatively small area. Although several geochronological studies were carried out in the past three decades, the eruptive history of the central zone of the arc remained poorly documented, preventing the analyses of volcanism initiation of the whole arc. In this study, we present new K-Ar ages obtained from this central area, referred as the Quito segment. These results were then included in an updated comprehensive geochronological database including about 250 ages, allowing us to describe, at the arc scale, the spatial and temporal development of Quaternary volcanic activity in Ecuador. About eighty Quaternary volcanoes are identified in the Ecuadorian Andes, amounting to 45 volcanic complexes with radiometric ages and/or identified as active or potentially active. The volcanic arc developed in three stages marked by increases in the total number of active volcanoes. During the oldest Plio-Early Pleistocene stage, the documented volcanic activity was mostly concentrated in the Eastern Cordillera of the Quito segment, with minor effusive eruptions in the southern Back-Arc. Since ~ 1.4 Ma, the activity spread to the surroundings of the Quito segment and new edifices also appeared in the Western Cordillera and the Inter-Andean Valley. Towards the end of this intermediate stage (i.e., ~ 800 ka), volcanism occurred in isolated areas to the north and south of the Inter-Andean Valley. Finally, the late and current stage was characterized by a remarkable increase in volcanic activity since ~ 600 ka. Approximately 50 volcanoes were active during this stage. The spatial distribution of the Ecuadorian arc volcanism seems to be guided by deep mechanisms and old crustal tectonic structures from the Western Cordillera, whereas the neotectonics seem to influence the development of stratovolcanoes. In addition, we note that the spatial and temporal evolution of volcanism highlights the influence of the Carnegie Ridge and the thermal regime anomaly of the young Nazca crust on the increase of volcanic activity in Ecuador.
... The difference in the 40 Ar/ 39 Ar ratios obtained from the sample and from an air aliquot, measured under identical conditions using a 180 • sector multi-collector mass spectrometer, allows quantification of the 40 Ar* content (%). The detection limit of the mass spectrometer, close to 0.1% for 40 Ar* (Quidelleur et al., 2001), allows dating volcanic products even of Holocene age with a relatively small uncertainty (Gillot et al., 2006). The 40 Ar signal is regularly calibrated with systematic measurements of the HD-B1 standard with an age of 24.18 ± 0.09 Ma (Schwarz and Trieloff, 2007). ...
The unusually high number of volcanoes in the Ecuadorian Arc, located in the deformation zone of the continental North Andean Sliver, coincides with the projection of the major oceanic structures observed in the Nazca Plate, such as the Carnegie Ridge and the Grijalva fracture zone. Although the relationship between this tectonic setting and volcanism has been widely discussed in the literature, their temporal relationship has not been thoroughly investigated due to the lack of geochronological data. We present here 20 new Ksingle bondAr and 2 40Ar/39Ar ages obtained for 7 volcanoes of the central segment of the Ecuadorian arc, which together with previous data show that volcanism in this area started at ∼1.3 Ma. A notable increase in volcanic activity occurred since ∼0.6 Ma, when the formation of a dozen volcanoes occurred in a relatively small area of the central segment. While this arrangement of volcanoes, here referred to as a “volcanic cluster”, appears to be controlled by crustal tectonic structures, the order of onset of these volcanoes and their eruptive activity does not show clear migration patterns over time. However, the presence of older volcanoes in the north of the central segment suggests a possible southward extension of volcanism between ∼1.3 and ∼ 0.6 Ma. Finally, based on the cumulative bulk volumes calculated for the volcanic edifices over time, we infer that the magmatic productivity rate has been roughly constant during the last ∼550 kyr in this area.
... This most recent study revealed the occurrence of Holocene volcanism in Anjouan, but also demonstrated that whole-rock K-Ar dating of Comorian magmatic rocks is challenging due to the presence of xenoliths, which may affect the accuracy of results. In comparison, only 10 ages are available for Mohéli and 2 for Grande Comore (Hajash and Armstrong, 1972;Duncan, 1982, 1983;Nougier et al., 1986), all obtained by K-Ar whole-rock analyses made prior to the development of the K -Ar Cassignol-Gillot technique on groundmass (Gillot and Cornette, 1986;Gillot et al., 2006). Regarding the hypothesis of a geographic migration of volcanism in the Comoros, it is therefore important to ascertain the accuracy of existing ages and to build a robust and more complete geochronological database on these two islands. ...
... Thin sections of the 11 samples (10 subaerial and 1 submarine) selected for K -Ar dating were inspected to determine which fraction size would be the most suitable for analysis based on the size and abundance of phenocrystals ( Fig. S2 in Supplementary material). The K-Ar technique applied to carefully separated groundmass has the main advantage of avoiding the drawbacks of sample irradiation and its recoil effect and interfering production of 36 Ar, which affects the precision of 40 Ar/ 39 Ar dating applied to young (<1 Ma), low-K and high-Ca-rich rocks such as basalts (Gillot et al., 2006). For this reason, the K-Ar technique on groundmass is the most powerful technique for dating volcanic rocks in the Holocene realm in the absence of material suitable for other techniques such as charcoal for 14 C, or zircon for (U -Th)/He. ...
... Indeed, the incorporation of xenocrysts or altered phases may yield calculated K-Ar ages older than the "true" ages. Potassium and Ar measurements were acquired following the unspiked Cassignol-Gillot method (Cassignol and Gillot, 1982;Gillot and Cornette, 1986;Gillot et al., 2006), with duplicated measurements. The K content for each sample was measured via flame-absorption spectrometry with BCR-2 (Raczek et al., 2001; K = 1.481%) and MDO-G (Gillot et al., 1992; K = 3.51%) as reference standards. ...
The Comoros archipelago has attracted renewed attention since 2018 due to the submarine volcano growing east of the island of Mayotte and the associated ongoing seismic crisis. However, the origin of Comorian magmatism remains controversial, as it is either interpreted as related to a hotspot trail, to a fracture zone, or to a plate boundary. Lying in the central part of the archipelago, Anjouan is a key island to better understand the relationship between volcanism and geodynamics. Together with a careful selection of published whole-rock K–Ar ages, our new set of 13 groundmass K–Ar ages on lava flows and one radiocarbon age on a charcoal from a strombolian deposit, allow us to reassess the volcano-tectonic evolution of Anjouan Island. New groundmass K–Ar ages lie within the last 1 Ma, i.e. from 899 ± 14 to 11 ± 1 ka. They suggest that most of the subaerial volcanism in Anjouan is much younger than previously inferred, and occurred as pulses at 900–750 ka, perhaps 530 ka, 230–290 ka, and since 60 ka, with erosional periods in between. Among our new data, one 14C age of 7513–7089 yrs calBCE (9.3 ± 0.2 ka) and five K–Ar ages younger than 60 ka show that recent volcanism occurred in Anjouan. Moreover, the concentration of eruptive vents along a N150° alignment, parallel to the maximum horizontal stress, suggests a strong link between regional tectonics and volcanism. Considering the presence of active volcanoes on both the western and eastern extremities of the Comoros archipelago, our discovery of Holocene activity on Anjouan provides strong arguments against a chronological progression of volcanism along the archipelago, and therefore contradicts the hotspot hypothesis for the origin of volcanism.
Finally, this study provides a robust geochronological timeframe of the different volcanic stages of Anjouan. It demonstrates that Anjouan is an active island and suggests that volcanism and tectonics can both resume at any time.
... is present in the analyzed sample (by xenocrysts, xenoliths, or cumulates). In the first case, the calculated K/Ar age will underestimate the true age of the rock, while in the other two cases, the true age will be overestimated (Gillot et al., 2006). To minimize the chances of erroneous age determination produced by the process described above, careful petrophysical and mineralogical analyses were conducted over all our samples to inspect mineralogy, potential alterations, or contaminations with any inherited magma product. ...
The Southern Central Andes developed though a complex succession of magmatic and deformational episodes, which after more than a century of studies, are still the subject of intense debate. One of the main controversies lies in the ambiguity regarding whether there was a single contractional phase in the Neogene or whether there were multiple contractional phases distributed over the last ~110 My.
We present 33 new K/Ar ages obtained from retroarc subvolcanic intrusives and lava flows and document their crosscutting relations with the host rock strata. These data enabled us to constrain the deformation's timing in several contractional structures along the eastern slope of the southern Central Andes ~36.5°S.
In the hinterland, the timing of compressional deformation has been constrained to the following: between Late Cretaceous and late Miocene age determined by Neogene dikes crosscutting Mesozoic folded strata; pre to syn-late Miocene determined by dikes intruded in the axial surfaces of small anticlines; and post-early Oligocene determined by folded sills.
In the foreland, the timing of compressional deformation has been constrained to between the Late Cretaceous and early Oligocene by dikes crosscutting pre-deformed strata; to the post- Oligocene by folded sills; to the pre- and post-middle Eocene by Eocene to Miocene dikes crosscutting older folded strata; and to the post to syn-middle Miocene by folded lava flows.
We conclude that two pre-Neogene and one Neogene contractional phases, and various retroarc magmatic events have affected this segment of the retroarc. We discuss our observations in relation to previous proposals, separating the tectonic evolution of the area into six tectonic scenarios from the late Early Cretaceous to the present.
... The 40 Ar and 36 Ar have been measured using a multi-collector mass spectrometer, which determines with precision the radiogenic Ar content (% 40 Ar*), comparing the sample with an air aliquot measured in the same analytical conditions (Gillot et al., 2006). The detection limit of the used mass spectrometer is 0.1% for 40 Ar* (Quidelleur et al., 2001), which allows to date rocks as young as 2 ka with only a few centuries of uncertainty (Gillot et al., 2006). ...
... The 40 Ar and 36 Ar have been measured using a multi-collector mass spectrometer, which determines with precision the radiogenic Ar content (% 40 Ar*), comparing the sample with an air aliquot measured in the same analytical conditions (Gillot et al., 2006). The detection limit of the used mass spectrometer is 0.1% for 40 Ar* (Quidelleur et al., 2001), which allows to date rocks as young as 2 ka with only a few centuries of uncertainty (Gillot et al., 2006). The frequent measurements of the HD-B1 standard dated at 24.21 ± 0.32 Ma (Hess and Lippolt, 1994) and refined at 24.18 ± 0.09 Ma (Schwarz and Trieloff, 2007), allow calibration of the 40 Ar signal, combined with frequent measurements of an air pipette compared with the standard. ...
... Another issue for accurate dating using K-Ar or 40 Ar/ 39 Ar is the presence of glass within the matrix. Although glass can be used with success for K-Ar (Gillot et al., 2006) and 40 Ar/ 39 Ar (Morgan et al., 2009) dating, the mobility of K and/or Ar during weathering (Cerling et al., 1985), as well as the effects of hydration and devitrification, might in some cases led to anomalous results (Flude et al., 2010;Hildreth et al., 2017). Samples analyzed here all present a crystallized groundmass with a relatively low glass content, which should limit the problem inherent to K-Ar dating of glass. ...
... Potassium and Ar measurements were acquired following the unspiked Cassignol-Gillot method (Cassignol and Gillot, 1982;Gillot et al., 2006), with duplicated measurements. The K content for each sample was measured via flame-absorption spectrometry with BCR-2 (Raczek et al., 2001; K = 1.481%) and MDO-G (Gillot et al., 1992; K = 3.51%) as reference standards. ...
Dating of submarine volcanic systems is key to understand the history of tectonic and volcanic interactions within the Lesser Antilles volcanic arc. In this study, we investigate the radioisotopic dating of submarine volcanic systems located between Les Saintes (Guadeloupe) and Dominica islands in an intra-arc graben bounded to the west by the active Roseau fault. Submarine lava flows and domes have been sampled with the remotely operated vehicle (ROV) Victor6000 controlled onboard the Ifremer's N/O L'Atalante during the 2017 Subsaintes cruise. We sampled distinct volcanic edifices or sets of edifices in the area, in the previously identified Agoucha and Roseau volcanic complexes, and the Coche and Crawen seamounts, in addition to the basement exposed along the Roseau fault scarp. Pre-degassing of these samples before K-Ar dating has been mandatory to strongly limit their atmospheric argon contamination. Based on twelve new K-Ar ages, we find that the earliest volcanism in the area corresponds to the Agoucha volcanic complex with coeval ages of 4.23 ± 0.06 and 4.17 ± 0.06 Ma. These are also the oldest ages obtained for the recent Lesser Antilles arc to the north of Martinique Island. A lava flow sampled along the Roseau fault scarp yielded an age of 4.12 ± 0.06 Ma, further attesting that early phase of submarine volcanism occurred in the area about 1 Ma earlier than the emergence of Les Saintes Islands, which has been dated onland at ~3 Ma. The Roseau volcanic complex was constructed during a relatively long duration of at least 600 kyr, between 3.13 ± 0.05 and 2.52 ± 0.04 Ma, while a much shorter duration is observed for the smaller Coche and Crawen volcanoes, with ages at about 2.2 and 2.0 Ma, respectively. Together with ages of subaerial lavas from Terre-de-Haut in Les Saintes archipelago, these ages suggest that an intense volcanic activity occurred between 3 and 2 Ma in this part of the recent Lesser Antilles arc. The much younger age of 0.274 ± 0.009 Ma obtained here to the north of Colibri volcano, might be related with the northward magmatism of Dominica Island which displays similar timing and geochemistry. Overall, trace elements data show that Crawen, Coche, Agoucha, and, to a lesser extent, Roseau lavas have strong similarities with Basse-Terre, while pyroclastic units, Colibri and some Roseau lavas, are similar to those from Dominica and Les Saintes islands. Finally, this study shows that dating the submarine volcanic activity, which remains mostly unknown and undated, is a key component to understand the formation of volcanic arcs such as the Lesser Antilles arc.
... Este entorno petrológico puede correlacionarse con rocas de similar aspecto volcánico que aflora en la sierra Diablo (Jones y McKee, 1987;Jones et al., 1995). La mineralogía de los núcleos de estos pozos identifica un vulcanismo continental de composición félsica o intermedia, pero con alteración, lo cual afecta fechar las rocas por algunos métodos isotópicos (Kelley, 2002(Kelley, a, 2002bGillot et al., 2006). ...
El Bloque de Coahuila tiene rocas marinas y continentales del Paleozoico al Reciente que afloran en una vasta región del noreste de México. Este trabajo aporta datos de superficie y subsuelo que constatan la presencia del arco Nazas sobre este bloque. Se describen secciones estratigráficas que indican el proceso de transgresión marina sobre este bloque durante el Aptiano, así como el depósito de un potente espesor de caliza, dolomía y evaporita de edad Albiano y Cenomaniano temprano. La sucesión estratigráfica del Cretácico Superior está compuesta por el paso gradual entre las formaciones Indidura, Parras, Cerro del Pueblo, Cerro Huerta y capas La Soledad que representan depósitos de antepaís (foreland) con ~1000 m de espesor, estas rocas muestran una condensación estratigráfica al confrontar su espesor con secciones estratigráficas similares en las cuencas vecinas de Sabinas, del Centro de México y Chihuahua, las que tienen más de 2000 m de espesor de rocas de la misma edad. En este trabajo hay datos estructurales que se confrontan con ideas previas que consideraban una deformación tenue afectada por fallas de tipo normal. En el Bloque de Coahuila se distinguen tres estilos estructurales: uno está relacionado con pliegues de curvatura amplia, otro está formado por pliegues angostos y alargados, con fallas inversas, este estilo está restringido al borde austral del Bloque de Coahuila en dónde es traslapado tectónicamente por la napa de Parras, está napa sepulta las facies sedimentarias marginales pre-Aptiano de este bloque. Ambos estilos estructurales están estrechamente vinculados con la orientación de estructuras de la fase que deformó al Orógeno mexicano. El tercer estilo es representado por pliegues agudos y geométricamente escalonados, así como conjuntos estructurales con varios ejes burdamente orientados norte-sur, con vergencias opuestas y fallas diversas. Este último estilo estructural no tiene una componente de extensión o acortamiento definida y su deformación posiblemente es posterior a la fase que deformó el Orógeno mexicano. Los datos presentados definen la extensión geográfica del borde suroeste del Bloque de Coahuila.
... The ages of these samples were calculated by measuring their potassium and radiogenic argon ( 40 Ar*) content and using the 40 K/K ratio and the 40 K radioactive decay constants of Steiger and Jäger (1977). We used the unspiked Cassignol-Gillot technique applied to groundmass and plagioclase (Cassignol and Gillot, 1982;Gillot et al., 2006), which is well-suited for Quaternary volcanic products containing low 40 Ar* contents, and that has already been successfully applied in Ecuador (Bablon et al., 2018Santamaria et al., 2022), as well as in other volcanic arcs worldwide (e.g., Germa et al., 2011;Grosse et al., 2018;Lahitte et al., 2019). The 40 Ar* content is derived from comparing the 40 Ar/ 36 Ar ratios of the sample and the atmosphere. ...
The eruptive chronology of arc volcanoes consists of construction stages usually punctuated by large collapse events affecting the edifice. In this paper, we reconstruct the eruptive chronology of Carihuairazo volcano, a Middle Pleistocene edifice from the Ecuadorian segment of the Andean Northern Volcanic Zone. This study is based on extensive fieldwork on both the proximal flanks of the volcano and the medial-to-distal deposits of the nearby Ambato basin, as well as a large dataset of geochronological (⁴⁰K-⁴⁰Ar, ¹⁴C) and geochemical (major and trace element) data. The Basal Carihuairazo edifice is mainly composed of an andesitic lava flow succession dated at 230–200 ka. The resulting edifice suffered a first sector collapse that was responsible for a relatively large (4 ± 1 km³) debris avalanche deposit (DAD-1) that covers the entire Ambato basin. This event occurred between 206 ± 4 and 216 ± 5 ka. Then, Carihuairazo started a succession of construction (Intermediate and Terminal lava flow successions) and destruction stages lasting about 50 ky (i.e., from 200 to 150 ka), which are composed by thick block-and-ash flow deposits (L- and U-BAFD) and debris avalanche deposits (DAD-2). This volcanic succession is recorded in the Ambato basin, interlayered with several tephra fallout deposits (TFD-1 to −4) whose source is the neighbouring Huisla-Mulmul volcanic complex. The current morphology of Carihuairazo results from two additional sector collapses (DAD-3 and -4) that occurred during the past 40–50 ka, i.e., following a long period (at least 100 ka) without volcanic activity. We stress these debris avalanches were not related with magmatic activity. Samples from the Carihuairazo volcano defines a medium-K magmatic trend composed of andesites and dacites with a mineral assemblage of plagioclase, amphibole, ortho- and clino-pyroxene, and Fe–Ti oxides. The evolution of the Carihuairazo edifice, recorded in the medial-distal deposits of the Ambato basin, represents a unique example in the Ecuadorian arc of an edifice that experienced successive destruction and construction stages during a major part of its volcanic history.
... Thus, the independent measurement of K-content, the 40 K relative isotopic abundance in nature and its decay constant (Steiger and Jäger, 1977;Min et al., 2000) allow to calculate the age of the sample. The Cassignol-Gillot technique is adequate to date young calk-alkaline volcanic products that typically exhibit low 40 Ar* contents (Gillot et al., 2006). This technique was successfully applied to the Quaternary Ecuadorian volcanic arc Bablon et al., 2018Bablon et al., , 2019Bablon et al., , 2020, in the Andes (e.g., Germa et al., 2010;Samaniego et al., 2016;Grosse et al., 2018Grosse et al., , 2020Pallares et al., 2019;Mariño et al., 2021), and worldwide (e. g., Germa et al., 2011;Hildenbrand et al., 2018;Dibacto et al., 2020). ...
The youngest volcanism of the Ecuadorian Volcanic Front (Western Cordillera) is mainly dominated by highly explosive events, including the growth and violent destruction of lava domes, and the formation of thick pyroclastic sequences. Deposits associated with such eruptive dynamics have been identified at Iliniza, a compound volcano located in the Western Cordillera with a poorly defined evolutionary history. We present the first K-Ar ages of Iliniza volcano combined with stratigraphic data, numerical reconstructions, and geochemical analyses, providing a new perspective on its evolution. Our results show that Iliniza volcano is much younger than previously proposed. The Iliniza twin-peaked shape is the result of the superposition of two andesitic to dacitic stratovolcanoes. (1) The North Iliniza (NI) edifice was constructed by two lava successions and an intermediate satellite vent, showing a short range of mainly effusive activity between 123 ± 6 and 116 ± 2 ka. (2) The South Iliniza (SI) edifice began its construction through the Lower SI stage when massive lavas dated at ~45 ka formed a basal cone. During the Upper SI stage, the uppermost part of this edifice was destroyed by the highly explosive Jatuncama phase (VEI 5) leaving a 30–40 m-thick ignimbrite sequence. The subsequent extrusion of several dacitic lava domes reconstructed the South Iliniza summit at around 35 ka. The Terminal SI stage corresponds to the emission of several andesitic lavas between 31 ± 4 and 25 ± 3 ka. The Iliniza eruptive activity extended into the Late Pleistocene and Holocene with the extrusion of the Tishigcuchi lava dome, and the emission of the Pongo lava flow dated at 6 ± 4 ka. Based on the proposed eruptive history, we suggest that a revised volcanic hazard assessment of the potentially active Iliniza volcano is required.
