Blake excursion at Vulcano (Aeolian Islands, Italy): Revised K-Ar and 40 Ar/ 39 Ar ages

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40 Ar/ 39 Ar age of cryptochron C2r.2r-1 as recorded in a lava sequence within the Ko'olau volcano (Hawaii, USA)

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De la justesse des âges K-Ar : exemple de la datation de deux dômes trachytiques du Gölcük (Turquie)

International audienceLe Gölcük, situé au sud de la ville d'Isparta (Turquie), est un volcan composite, construit en trois phases durant le Pléistocène (Alici et al., 1998 ; Platevoet et al., 2008 ; Elitok et al., 2010). Les études réalisées par Platevoet et al. (2008) et Schmitt et al. (2014) se sont intéressées en partie à la datation de deux dômes mis en place au sein de la caldeira du Gölcük. Les âges K-Ar (Platevoet et al., 2008) de ces dômes trachytiques (Küçükçesme Tepe : 52 ± 2 ka et Pilav Tepe : 24 ± 2 ka) sont significativement plus vieux que les âges (U-Th)/He sur zircons obtenus par Schmitt et al. (2014) étant respectivement de 14,1 ± 0,5 et de 12,9 ± 0,4 ka. Nous avons testé l'hypothèse proposée par Schmitt et al. (2014) qui est celle d'un excès d' 40 Ar * faussant les âges K-Ar en datant ces mêmes trachytes par la méthode 40 Ar/ 39 Ar. Nos âges 40 Ar/ 39 Ar sur mésostase confirment les âges (U-TH)/He sur zircons et datent l'activité magmatique la plus récente du Gölcük à 13,4 ± 0,3 ka. Nous proposons que l'erreur sur les âges K-Ar soit liée à un excès d'argon probablement porté par des clinopyroxènes associés à la mésostase. ABSTRACT QUESTIONING THE ACCURACY OF K-AR AGES: EXAMPLE OF THE DATING OF TWO TRACHYTIC DOMES FROM THE GÖLCÜK (TURKEY) The Gölcük volcano, located south to the city of Isparta (Turkey), built-up in three stages during the Pleistocene (Alici et al., 1998; Platevoet et al., 2008; Elitok et al., 2010). A reliable and accurate dating of the most recent volcanic stage is crucial to assess the potential hazards for the infrastructures and the populations. In this context, Platevoet et al. (2008) and Schmitt et al. (2014) applied respectively the K-Ar and the (U-Th)/He methods to date the youngest volcanic stage of the Gölcük and obtained contrasted results. Based on the K-Ar dating of two trachytic domes (Kücükcesme Tepe et Pilav Tepe), the post-caldera stage is dated between 52 ± 2 ka and 24 ± 2 ka by Platevoet et al. (2008). When the (U-Th)/He method is applied on zircons from the same trachytic domes, Schmitt et al. (2014) date the post-caldera stage around 14.1 ± 0.5-12.9 ± 0.4 ka. Schmitt et al. (2014) proposed that the K-Ar ages were erroneously too old, probably because of excess 40 Ar*. We have tested this hypothesis by dating groundmass splits of these trachytes using the 40 Ar/ 39 Ar method. Our 40 Ar/ 39 Ar ages reproduce the (U-Th)/He ages on zircons and date the youngest volcanic stage of the Gölcük at 13.4 ± 0.3 ka. Confirming the hypothesis of Schmitt et al. (2014), we propose that the error on the K-Ar ages is related to excess argon probably trapped in small fragments of clinopyroxene attached to the groundmass

New core-top Mg/Ca calibration of multiple benthic foraminiferal species: Thermometry of the thermocline water in tropical western Atlantic

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New 40 Ar/ 39 Ar constraints for the “Grande Nappe”: The largest rhyolitic eruption from the Mont-Dore Massif (French Massif Central)

International audienceSince the 1960s, an early explosive activity in the Mont-Dore Massif is associated with a major pyroclastic rhyolitic eruption (5–7 km3) known as the “Grande Nappe” (GN). This event, linked to the formation of a 6-km-diameter cryptic caldera named “Haute Dordogne”, was before our investigation dated by 40Ar/39Ar at 3.07 ± 0.04 Ma. Our new single-crystal laser fusion 40Ar/39Ar dates obtained on two outcrops of the GN (Rochefort-Montagne and Ludières) questioned several hypotheses made concerning this “landmark” event of the Mont-Dore Massif history. We demonstrate that: (1) the GN rhyolitic eruption has occurred much later than previously estimated (i.e. 2.77 ± 0.02–0.07 Ma full external uncertainties); (2) the correlation made between the Vendeix rhyolitic complexes (intra-caldera position) dated back to 2.74 ± 0.04 Ma and the GN is proposed; (3) xenocryst contamination could be very high (i.e. 70% for the Rochefort-Montagne GN outcrop) and explains the noticeable older age obtained previously; (4) a link between the GN eruption and the formation of a caldera is questionable; the hypothesis of a northward-oriented blast channeled eastward toward the paleo-Allier River is thus proposed

Tephrochronology of the Mont-Dore volcanic Massif (Massif Central, France): new 40Ar/39Ar constraints on the Late Pliocene and Early Pleistocene activity

International audienceThe Mont-Dore Massif (500 km2), the youngest stratovolcano of the French Massif Central, consists of two volcanic edifices: the Guéry and the Sancy. To improve our knowledge of the oldest explosive stages of the Mont-Dore Massif, we studied 40Ar/39Ar-dated (through single-grain laser and step-heating experiments) 11 pyroclastic units from the Guéry stratovolcano. We demonstrate that the explosive history of the Guéry can be divided into four cycles of explosive eruption activity between 3.09 and 1.46 Ma (G.I to G.IV). We have also ascertained that deposits associated with the 3.1-3.0-Ma rhyolitic activity, which includes the 5-km3 "Grande Nappe" ignimbrite, are not recorded in the central part of the Mont-Dore Massif. All the pyroclastites found in the left bank of the Dordogne River belong to a later explosive phase (2.86-2.58 Ma, G.II) and were channelled down into valleys or topographic lows where they are currently nested. This later activity also gave rise to most of the volcanic products in the Perrier Plateau (30 km east of the Mont-Dore Massif); three quarters of the volcano-sedimentary sequence (up to 100 m thick) was emplaced within less than 20 ky, associated with several flank collapses in the northeastern part of the Guéry. The age of the "Fournet flora" (2.69 ± 0.01 Ma) found within an ash bed belonging to G.II suggests that temperate forests already existed in the French Massif Central before the Pliocene/Pleistocene boundary. The Guéry's third explosive eruption activity cycle (G.III) lasted between 2.36 and 1.91 Ma. It encompassed the Guéry Lake and Morangie pumice and ash deposits, as well as seven other important events recorded as centimetric ash beds some 60 to 100 km southeast of the Massif in the Velay region. We propose a general tephrochronology for the Mont-Dore stratovolcano covering the last 3.1 My. This chronology is based on 44 40Ar/39Ar-dated events belonging to eight explosive eruption cycles each lasting between 100 and 200 ky. The occurrence of only one pumice deposit in the 800-ky period between 1.9 and 1.1 Ma suggests that volcanic explosive activity was strongly reduced or quiescent

40 Ar/ 39 Ar and unspiked 40 K- 40 Ar dating of upper Pleistocene volcanic activity in the Bas-Vivarais (Ardèche, France)

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