A new all-metal induction furnace for noble gas extraction

A new all-metal induction furnace for extraction of all noble gases from pyroxenes, olivines, quartz or barites has been developed at CRPG. It differs in design from other induction furnaces in that the totality of the vacuum vessel is metallic and the induction coil, normally located outside the furnace, has been placed inside the vacuum vessel, with a special radio frequency power feedthrough welded onto a flange. The volume of the crucible is ≈ 15 cm^3 and permits fusion of samples with a mass of up to 1 g. Samples are packed into a metal foil, loaded into a carousel, baked out before analysis, and then sequentially dropped into the Ta-crucible. The low weight of the crucible (≈ 120 g) allows for short and efficient degassing cycles. When the furnace is pumped for the first time after samples loading, short cycles between 500 and 1800 °C at fast heating rates (≈ 400 °C·min^(−1)) are sufficient to achieve very low blanks. The durations of these cycles are range from 30 min for He to up to a few hours for Ne, Kr and Xe. Blanks of He, Kr and Xe (10 min heating durations) and Ne (20 min) in static vacuum are (1.6 ± 1.0) × 10^(−15) mol ^4He (T = 1750 °C), (5.8 ± 2.3) × 10^(−17) mol ^(20)Ne (T = 1500 °C), (2.1 ± 0.3) × 10^(−18) mol ^(84)Kr (T = 1700 °C) and (4.4 ± 0.4) × 10^(−18) mol ^(132)Xe (T = 1700 °C). Argon blanks have not yet been measured

A new all-metal induction furnace for noble gas extraction

A new all-metal induction furnace for extraction of all noble gases from pyroxenes, olivines, quartz or barites has been developed at CRPG. It differs in design from other induction furnaces in that the totality of the vacuum vessel is metallic and the induction coil, normally located outside the furnace, has been placed inside the vacuum vessel, with a special radio frequency power feedthrough welded onto a flange. The volume of the crucible is ≈ 15 cm^3 and permits fusion of samples with a mass of up to 1 g. Samples are packed into a metal foil, loaded into a carousel, baked out before analysis, and then sequentially dropped into the Ta-crucible. The low weight of the crucible (≈ 120 g) allows for short and efficient degassing cycles. When the furnace is pumped for the first time after samples loading, short cycles between 500 and 1800 °C at fast heating rates (≈ 400 °C·min^(−1)) are sufficient to achieve very low blanks. The durations of these cycles are range from 30 min for He to up to a few hours for Ne, Kr and Xe. Blanks of He, Kr and Xe (10 min heating durations) and Ne (20 min) in static vacuum are (1.6 ± 1.0) × 10^(−15) mol ^4He (T = 1750 °C), (5.8 ± 2.3) × 10^(−17) mol ^(20)Ne (T = 1500 °C), (2.1 ± 0.3) × 10^(−18) mol ^(84)Kr (T = 1700 °C) and (4.4 ± 0.4) × 10^(−18) mol ^(132)Xe (T = 1700 °C). Argon blanks have not yet been measured

Toward the feldspar alternative for cosmogenic 10Be applications

The possibility of quantifying surface processes in mafic or volcanic environment using the potentialities offered by the in situ-produced cosmogenic nuclides, and more specifically by the in situ-produced 10Be, is often hampered by the rarity of quartz minerals in the available lithologies. As an alternative to overcome this difficulty, we explore in this work the possibility of relying on feldspar minerals rather that on quartz to perform in situ-produced 10Be measurements in such environments. Our strategy was to cross-calibrate the total production rate of 10Be in feldspar (P10fsp) against the total production rate of 3He in pyroxene (P3px) by measuring 3He and 10Be in cogenetic pyroxene (3Hepx) and feldspar (10Befsp). The samples were collected from eight ignimbritic boulders, exposed from ca 120 to 600 ka at elevations ranging from 800 to 2500 m, along the preserved rock-avalanche deposits of the giant Caquilluco landslide (18°S, 70°W), Southern Peru. Along with data recently published by Blard et al. (2013a) at a close latitude (22°S) but higher elevation (ca. 4000 m), the samples yield a remarkably tight cluster of 3Hepx - 10Befsp total production ratios whose weighted-mean is 35.6 ± 0.5 (1s). The obtained weighted-mean 3Hepx - 10Befsp total production ratio combined with the local 3Hepy total production rate in the high tropical Andes published by Martin et al. (2017) allows to establish a total SLHL 10Be in situ-production rate in feldspar mineral (P10fsp) of 3.57 ± 0.21 at.g-1.yr-1 (scaled for the LSD scaling scheme, the ERA40 atm model and the VDM of Lifton, 2016). Despite the large elevation range covered by the whole dataset (800–4300 m), no significant variation of the 3Hepx - 10Befsp total production ratios in pyroxene and feldspar was evidenced. As an attempt to investigate the effect of the chemical composition of feldspar on the total 10Be production rate, major and trace element concentrations of the studied feldspar samples were analyzed. Unfortunately, giving the low compositional variability of our dataset, this issue is still pending

Toward the feldspar alternative for cosmogenic 10Be applications in mafic environment

In situ-produced 10Be is one of the most commonly used TCN in quantitative geomorphology due to the fact that its production rate is relatively well constrained in the ubiquitous quartz mineral whose integrity minimizes the possibility of contamination by meteoric 10Be. Easily decontaminated from meteoric 10Be, it is in addition reliably measured using the Accelerator Mass Spectrometry technique for which its detection limit is lower than 104 at.g-1. However, volcanic or mafic areas are generally quartz free, which hamper the routine use of 10Be. In the case of a quartz poor lithology, an alternative possibility is to rely on 10Be - feldspars. Two preliminary studies (Kober et al., 2005 and Blard et al., 2013a) already provided promising results, demonstrating that (1) the decontamination protocol classically applied to quartz (Brown et al., 1991) efficiently removes all the meteoric 10Be contamination from the feldspar grains and (2) the total production rate of 10Be in feldspar is 8 to 10 % lower than that in quartz. However, only two samples were analyzed in both studies. In order to better constrain the 10Be in situ-production rate within feldspars, the number of samples analyzed needs to be increased. In this study, we developed a new chemical protocol for the 10Be extraction from feldspar matrices, and to cross-calibrate the total 10Be in situ-production rate in feldspar (P10fsp) against the total 3He production rate in pyroxene (P3px). The cosmogenic 3He and 10Be concentrations were measured, respectively, in pyroxene and feldspar extracted from eight samples of ignimbrite boulders from a giant landslide located between 800 and 2500 m in the high central Andes of Southern Peru. This area is ideally located, since two studies have already determined the local total 3He production rate in pyroxene on the nearby Altiplano (Blard et al., 2013b; Delunel et al., 2016)

Denudation outpaced by crustal thickening in the eastern Tianshan

The modern high topography of the Tianshan resulted from the reactivation of a Paleozoic orogenic belt by the India/Asia collision. Today, the range exhibits tectonically active forelands and intermontane basins. Based on quantitative morphotectonic observations and age constraints derived from cosmogenic 10Be dating, single-grain post-infrared infrared stimulated luminescence (p-IR IRSL) dating and modeling of fault scarp degradation, we quantify the deformation in the Nalati and Bayanbulak intermontane basins in the central Eastern Tianshan. Our results indicate that at least 1.4 mm/yr of horizontal crustal shortening is accommodated within these two basins. This shortening represents over 15% of the 8.5 ± 0.5 mm/yr total shortening rate across the entire range at this longitude. This shortening rate implies that the Eastern Central Tianshan is thickening at a mean rate of ∼1.4 mm/yr, a rate that is significantly higher than the average denudation rate of 0.14 mm/yr derived from our cosmogenic analysis. This discrepancy suggests that the Tianshan range has not yet reached a steady-state topography and remains in a transient state of topographic growth, most likely due to limited denudation rates driven by the arid climate of Central Asia

Les nucléides cosmogéniques in situ (3He, 21Ne, 10Be, 26Al) : des géo/chrono-mètres pour déchiffrer la Terre

Ce manuscrit d’Habilitation à Diriger des Recherches présente une mise en perspective épistémologique et méthodologique de l’utilisation des nucléides cosmogéniques en Sciences de la Terre, le bilan de mon activité scientifique depuis 2002, ainsi que mes projets scientifiques futurs. Il est découpé en 4 chapitres :Chapitre 1 - Introduction : parcours scientifique et CV Chapitre 2 - Les nucléides cosmogéniques : éléments épistémologiques et techniquesCette partie présente 120 ans des progrès scientifiques sur la description des particules cosmiques, les nucléides cosmogéniques et la manière de les utiliser en sciences de la Terre. J’y présente aussi les méthodes actuelles les plus établies pour utiliser les nucléides cosmogéniques en géomorphologie. Chapitre 3 - Résumé de mes travaux de recherche.Mon activité de recherche, polyvalente et pluridisciplinaire, a consisté à développer et utiliser les nucléides cosmogéniques pour résoudre des questions scientifiques en paléoclimatologie et géomorphologie et ainsi améliorer notre connaissance des interactions entre les enveloppes superficielles fluides (atmosphère, cryosphère, hydrosphère) et la Terre solide. Les résultats les plus importants de mon activité de recherche se résument ainsi : •Amélioration significative de la précision des géochronomètres cosmogéniques par le développement des techniques d’analyses de l’3He et du 10Be, la calibration des taux de production en haute altitude, et la mise au point d’un nouveau paléoaltimètre basé sur les nucléides cosmogéniques. •Mise au point des calculateurs en ligne CREp - pour le calcul des âges d’exposition : http://crep.crpg.cnrs-nancy.fr/ - et BASINGA pour le calcul des taux d’érosion. •En couplant une méthode originale associant datation par les nucléides cosmogéniques et modélisation numérique, nous avons caractérisé le retrait des glaciers tropicaux depuis le dernier maximum glaciaire, il y a 20 000 ans. Nos résultats mettent évidence l’amplification altitudinale du refroidissement au Dernier Maximum Glaciaire dans certaines régions (Hawaii), mais aussi permettent de comprendre le rôle des précipitations sur les fluctuations glaciaires, qui se superposent aux variations de températures (Andes Tropicales). •En développant l’analyse des nucléides cosmogéniques dans les séries sédimentaires (Tianshan, Himalaya, Méditerranée), nous avons montré que les fluctuations climatiques n’ont eu qu’un impact limité sur l’érosion, que ce soit au cours du refroidissement Pléistocène, mais aussi lors des fluctuations des cycles glaciaires-interglaciaires du Quaternaire.Chapitre 4 - Projets de recherches pour la période 2021-202

The history of Greenland's ice: Cosmic signature

International audienceGlobal sea levels would rise by several metres if the Greenland Ice Sheet melted completely. Two studies have examined its past behaviour in an effort to evaluate its vulnerability in a warming world — and have come to seemingly conflicting conclusions. Two geochemists and a glaciologist discuss the issues