Lithium isotopes in island arc geothermal systems: Guadeloupe, Martinique (French West Indies) and experimental approach

International audienceWe report lithium (Li) isotopic measurements in seawater-derived waters that were discharged from geothermal wells, thermal springs, and submarine springs located in volcanic island arc areas in Guadeloupe (the Bouillante geothermal field) and Martinique (Lamentin plain and the Diamant areas). While Li isotopic signatures of the geothermal fluids collected from deep reservoirs were found to be homogeneous for a given site, the δ7Li signatures for each of these reservoirs were significantly different. The first low temperature (25-250°C) experiments of Li isotope exchange during seawater/basalt interaction confirmed that Li isotopic exchange is strongly temperature dependent, as previously inferred from natural studies. Li isotopic fractionation ranged from +19.4‰ (Δ solution - solid) at 25°C to +6.7‰ at 250°C. These experiments demonstrated the importance of Li isotopic fractionation during the formation of Li-bearing secondary minerals and allowed us to determine the following empirical relationship between isotopic fractionation and temperature: Δ solution – solid = 7847 / T – 8.093. Application of experimental results and literature data to the Bouillante area suggested that geothermal water was in equilibrium at 250-260°C. It likely has a deep and large reservoir located in the upper sheeted dike complex of the oceanic crust, just below the transition zone between andesite volcanic flows and the basaltic dikes. The upper dike section, from which Li is extracted by hydrothermal fluids, was characterized by light Li isotopic values in the rocks, indicating retention of 6Li by the altered rocks. For the Lamentin and Diamant areas, the geothermal fluids appeared to be in equilibrium with reservoir volcano-sedimentary rocks at 90-120°C and 180°C, respectively. Further evidence for this argument is provided by the fact that only the Na/Li thermometric relationship determined for sedimentary basins yielded temperature values in agreement with those measured or estimated for the reservoir fluids. This suggests the importance of a sedimentary signature in these reservoir rocks. Altogether, this study highlights that the use of Li isotopic systematics is a powerful tool for characterizing the origin of geothermal waters as well as the nature of their reservoir rocks

Behaviour of lithium and its isotopes during weathering in the Mackenzie Basin, Canada

International audienceWe report Li isotopic compositions, for river waters and suspended sediments, of about 40 rivers sampled within the Mackenzie River Basin in northwestern Canada. The aim of this study is to characterize the behaviour of Li and its isotopes during weathering at the scale of a large mixed lithology basin. The Mackenzie River waters display systematically heavier Li isotopic compositions relative to source rocks and suspended sediments. The range in delta Li-7 is larger in dissolved load (from +9.3 parts per thousand to +29.0 parts per thousand) compared to suspended sediments (from 1.7 parts per thousand to +3.2 parts per thousand), which are not significantly different from delta Li-7 values in bedrocks. Our study shows that dissolved Li is essentially derived from the weathering of silicates and that its isotopic composition in the dissolved load is inversely correlated with its relative mobility when compared to Na. The highest enrichment of Li-7 in the dissolved load is reported when Li is not or poorly incorporated in secondary phases after its release into solution by mineral dissolution. This counterintuitive observation is interpreted by the mixing of water types derived from two different weathering regimes producing different Li isotopic compositions within the Mackenzie River Basin. The incipient weathering regime characterizing the Rocky Mountains and the Shield areas produces Li-7 enrichment in the fluid phase that is most simply explained by the precipitation of oxyhydroxide phases fractionating Li isotopes. The second weathering regime is found in the lowland area and produces the lower delta Li-7 waters (but still enriched in Li-7 compared to bedrocks) and the most Li-depleted waters (compared to Na). Fractionation factors suggest that the incorporation of Li in clay minerals is the mechanism that explains the isotopic composition of the lowland rivers. The correlation of boron and lithium concentrations found in the dissolved load of the Mackenzie Rivers suggests that precipitation of clay minerals is favoured by the relatively high residence time of water in groundwater. In the Shield and Rocky Mountains, Li isotopes suggest that clay minerals are not forming and that secondary minerals with stronger affinity for Li-7 appear. Although the weathering mechanisms operating in the Mackenzie Basin need to be characterized more precisely, the Li isotope data reported here clearly show the control of Li isotopes by the weathering intensity. The spatial diversity of weathering regimes, resulting from a complex combination of factors such as topography, geology, climate and hydrology explains, in fine, the spatial distribution of Li isotopic ratios in the large drainage basin of the Mackenzie River. There is no simple relationship between Li isotopic composition and chemical denudation fluxes in the Mackenzie River Basi

The genesis of LCT-type granitic pegmatites, as illustrated by lithium isotopes in micas

International audienceIsotopic compositions in the Monts d’Ambazac Pegmatite Field (French Massif Central) exhibit a narrow range of mica δ7Li values, ranging from -3.6 to + 3.4‰. The value obtained in biotite from the host Saint Sylvestre granite falls within this range (δ7Li = -1.5‰). Lithium concentrations are consistent with the degree of magmatic evolution of each pegmatite type: from 630 ppm in Type II up to 13,500 ppm in the more evolved Type VI pegmatite. Although the rare-element contents e.g., Li, Cs, Ta of the micas are consistent with pegmatite differentiation, δ7Li (‰) are firstly, independent of the degree of magmatic differentiation (independent of pegmatite type) and secondly, independent of the content of Li and other flux-elements such as Be and Cs. Muscovite sampled in pegmatite V from the Chabannes locality is the only pegmatite to exhibit a δ7Li variation from intermediate unit (-1.7‰) to internal pegmatitic unit (+ 3.4‰). The nature of this δ7Li variation suggests that there was extensive fractional crystallisation during the pegmatite’s consolidation. The independence of δ7Li (‰) evolution from the degree of magmatic evolution and the presence of distinct major rare-element bearing phases throughout the pegmatite field tend to confirm that the δ7Li (‰) values recorded in mica are inherited from crustal source rocks common to the granite and pegmatite-forming melts. We propose that the distinct pegmatite subtypes (beryl columbite vs lepidolite-petalite subtypes) observed throughout the Monts d’Ambazac Pegmatite Field reflect the diverse contributions of crustal protoliths. The lack of evidence of surrounding alteration combined with the absence of increasing Li-content within the host granite tend to confirm that the δ7Li values obtained within this pegmatite field are primary, and that no Li-diffusional process and/or mixing-driven Li-isotope fractionation has overprinted these isotopic compositions. In light of these results, the process of partial melting of protoliths enriched in rare-element bearing phases, e.g., mica, garnet, seems to be more responsible for Li-isotope fractionation than Li-diffusion or fractional crystallisation at the temperature of pegmatite consolidation. Finally, we discuss the use of Li isotopic compositions to identify the most highly evolved pegmatitic systems

Catilina, pour combien de temps encore ? Actualités bibliographiques sur un conspirateur trop connu

L. Sergius Catilina et sa conjuration de 63 a.C. ont déjà donné lieu à de nombreux débats. Pourtant, le personnage continue de fasciner (et de diviser) les chercheurs aujourd’hui, en témoigne l’abondance des ouvrages qui s’y intéressent depuis le début de la décennie. Catilina était-il ce criminel insatiable dénoncé par Cicéron et Salluste, ou un réformateur soucieux des intérêts du peuple, devenu révolutionnaire malgré lui ? Sa vie et son action présentent plusieurs zones d’ombre que des hypothèses parfois nouvelles et audacieuses s’attachent à remplir. Au-delà de ces questionnements, la postérité de la conjuration de Catilina offre un intérêt crucial. Souvent utilisée comme exemple de perfidie au cours de l’Histoire, elle se retrouve encore aujourd’hui dans le discours politique car elle pose des problématiques très contemporaines : complots et théories du complot, dénonciation d’une ploutocratie, légitimité des mesures d’état d’urgence.L. Sergius Catiline and his 63’s BC conspiracy have already led to much debate. However, they keep fascinating (and dividing) scholars today. A strong proof of this persistent memory is, among other things, the important number of books dealing with Catiline since Cicero. Was Catiline the insatiable criminal denounced by Cicero and Sallust, or was he a reformer, champion of the people interest, a revolutionary in spite of himself ? Some crucial aspects of his life and action remain mysterious, leading to sometimes bold hypothesis to explore them. Beyond these questions, the memory of Catiline’s conspiracy is of crucial importance. Extensively used as an example of perfidy in History, we can still found the conspiracy today in political discourse, precisely because it raises very contemporary issues : conspiracies and conspiracy theories, denouncing a plutocracy, legitimacy of the state of emergency measures

Chemical weathering of a granitic watershed: coupling Lithium isotopes and reactive transport modeling, preliminary results.

International audienceIn the present study, we report lithium concentrations and Li isotopic compositions for different samples within a granitic watershed (Margeride, France). We investigate unweathered bedrock and samples displaying different stages of weathering in order to characterize Li isotopic fractionation. This was achieved by coupling lithium isotope geochemistry and reactive transport modeling during granite weathering at the scale of this watershed. The following manuscript reports methodology and the preliminary data

The Fontainebleau Sandstone: bleaching, silicification and calcite precipitation under periglacial conditions

It will be show that the typical Fontainebleau Sandstone facies are in fact secondary facies, developed post‐deposition, and related to surface exposure of the formation. Quartzite pans, white sands, and even the mineralogical composition are related to subsurface (groundwater) alterationsconnected to geomorphology and result from exposure during the last glacial periods.The Fontainebleau Sandstone example is extremely spectacular. Yet, such alteration features also exist in many other formations and may extent to 50‐100 m depth. One has to be attentive to recognize them. The goal of the field trip is to bring insight into what is related to primary and to secondary processes in the outcropping formation. This is of prime/particular importance for better understanding of the geology of the Fontainebleau Sandstone … and the geochemical processes that led to make up theses exceptionally pure sandstones mined to be used in all high quality and luxury glass manufactories throughout Europe. and supply guides for prospecting such sand ores.But in a general way, when such alteration happen, it is essential to recognize them before any geochemical study because interpretations, in particular isotopic interpretations may completely diverge as a primary or a secondary hypothesis is envisaged for some facies … like marine deposits vs groundwater precipitation, dry and warm vs glacial climates, sedimentary deposition vs acidic leaching, etc

Boron Isotope Characterization to Design a Frame of Hydrogeological Functioning of a Wetland System (Massif Central, France)

AbstractMulti-isotopic approaches (Li, Sr, O, H), combined with hydrological tools, have been already appliedfor tracing the water and dissolved-element fluxes in a peatland in Central France. Here, we applied B isotopes. The δ11B ratios increase from river draining basalts (∼ 0‰) up to springs bordering the peatland (>+25‰). Peatland groundwaters have intermediate δ11B: 7.8 to 19.4‰. This range is accompanied by an increase in the Ca contents between the river draining basalts and water in the peatland. In aδ11B vs.Ca/B diagram, the role of water rock interaction and present day fertilizer inputs is evidenced, as for Sr isotopes

Lithium isotopes in low and high temperature hydrosystems

International audienceAssessing the origin and behaviour of lithium and the distribution of Li isotopes in hydro-systems is of major importance in order to increase our knowledge of the lithium cycling at the Earth's surface. Lithium is a fluid-mobile element and due to the large relative mass difference between its two stable isotopes, it is subject to significant low and high temperature mass fractionation which provides key information on the nature of water/rock interaction processes. The main objective of the present work is to constrain the behaviour of Li and its isotopes by focusing on three different hydrosystems: rainwaters, river waters and deep geothermal waters