Interactions fluide-roche, conditions physico-chimiques et transferts de matière dans des zones de failles en milieux sédimentaires : exemple de failles chevauchantes pyrénéennes

This work aims to study the physical and chemical parameters that control the mass-transfer and the clays formation and evolution in sedimentary environment thrust faults. Two Pyrenean thrust faults in low metamorphic grade were studied: the Millaris fault (related to Mont Perdu) and the Pic-de-Port-Vieux thrust (related to Gavarnie). In the Millaris fault, the deformation is accompanied mainly by dissolution of the matrix calcite by pressure-solution which induces a volume change of the rock from 20 to 40%. The Pic-de-Port-Vieux thrust records significant changes in the fault core-zone, but in the damaged zone also. In the footwall limestone, a mylonitisation is associated with a partial dissolution of dolomite in the presence of not exceeding 320-340°C fluids. In the hanging-wall pelites, the hematite dissolution by a reducing fluid causes a redox state change of the rock (confirmed by Mössbauer spectroscopy) and chlorite precipitation in synkinematic veins. Oscillatory zoning pattern chlorites located in some shearing veins revealed, by combining chemical mapping microprobe, redox state measurements with μ-XANES and thermometry, cyclic temperature variations of at least 50°C during the crystallization. A seismic valves process could be associated to Pic-de-Port-Vieux thrusting.Ce travail a pour but d’étudier les paramètres physico-chimiques qui contrôlent les transferts de matière ainsi que la formation et l’évolution des argiles dans des failles chevauchantes en environnement sédimentaire. Deux failles chevauchantes pyrénéennes de faible grade métamorphique ont été étudiées : la faille de Millaris (cf. Mont Perdu) et le chevauchement du Pic-de-Port-Vieux (cf. Gavarnie). Dans la faille de Millaris, la déformation s’accompagne principalement d’une dissolution de la calcite matricielle par pression-solution induisant un changement de volume de la roche de 20 à 40%. Le chevauchement du Pic-de-Port-Vieux enregistre des modifications importantes au coeur de la faille mais aussi dans la zone d’endommagement. Dans les calcaires du mur du chevauchement, une mylonitisation est associée à une dissolution partielle des dolomites en présence de fluides ne dépassant pas 320-340°C. Dans les pélites du toit du chevauchement, la dissolution de l’hématite par un fluide réducteur entraine un changement de l’état redox de la roche (confirmé par spectroscopie Mössbauer) et la précipitation de chlorite dans des veines syncinématiques. Des chlorites à zonations chimiques oscillatoires présentes dans certaines veines révèlent, en combinant cartographie chimique à la microsonde, mesures de l’état redox par μXANES et thermométrie, des variations cycliques de température d’au moins 50°C au cours de la cristallisation. Un processus de valves sismiques pourrait donc être associé à la mise en place du chevauchement du Pic de Port Vieux

Quantification of mass transfers and mineralogical transformations in a thrust fault (Monte Perdido thrust unit, southern Pyrenees, Spain)

In fold-and-thrust belts, shortening is mainly accommodated by thrust faults which are preferential zones for recrystallisation and mass transfer. This study focuses on a detachment fault related to the emplacement of the Monte Perdido thrust unit in the southern Pyrenees. The studied fault zone consists of a 10 m thick intensively foliated phyllonite developed within the Millaris marls, of Eocene age. The lithological homogeneity of the hanging wall and footwall allows us to compare the Millaris marls outside the fault zone with the highly deformed marls located in the fault zone and to quantify the chemical, mineralogical and volumetric changes related to deformation processes along the fault. The Millaris marls are composed of detrital quartz, illite, chlorite, minor albite and pyrite, in a micritic calcite matrix. In the fault zone, the cleavage planes are marked by clay minerals and calcite ± chlorite veins attest to fluid–mineral interactions during deformation. The mineral proportions in all samples from both the fault zone and Millaris marls have been quantified by two methods: (1) X-ray diffraction and Rietveld refinement, and (2) bulk chemical analyses as well as microprobe analyses to calculate modal composition. The excellent agreement between the results of these two methods allows us to estimate mineralogical variations using a modification of the Gresens' equation. During fault activation, up to 45 wt% of calcite was lost while the amounts of quartz and chlorite remained unchanged. Illite content remained constant to slightly enriched. The mineralogical variations were coupled with a significant volume loss (up to 45%) mostly due to the dissolution of micritic calcite grains. Deformation was accompanied by pressure solution and phyllosilicates recrystallisation. These processes accommodated slip along the fault. They required fluids as catalyst, but they did not necessitate major chemical transfers

Preconditioning of Specimens - Drying Influence on Alkali-Activated and Geopolymer Mortar

Alkali-activated materials (AAM) are now seriously considered by the cement industry as an economical alternative to Portland cement, especially for its low CO2 footprint. However, their durability still remain to be assessed in more details. The aim of this study is to focus on the sample preconditioning conditions required for testing, especially the drying stage involved in most of the current tests. Four alkali-activated binders were studied: a geopolymer (Na-silicate activated metakaolin), a Na-carbonate activated slag (GGBS), a Na-silicate activated slag and a Na-silicate activated mixture of 50% metakaolin with 50% GGBS. After an endogenous cure of 28 days at 20°C, mortar specimens were dried at different temperatures (from 20°C to 125°C) until mass stabilization. Drying kinetics and released water contents were evaluated, as well as physical, mechanical and mineralogical analyses at the end of drying. Optimal drying temperature for each alkali-activated binder was determined by coupling mechanical strength measurements and mercury intrusion porosimetry. This study revealed that an inappropriate drying temperature could modify the porosity of some classes of AAM, and reduced the compressive strength by up to 30 to 40%. Antagonistic behaviors were observed in the four alkali-activated materials studied, therefore one should be careful about selecting preconditioning protocols for assessing the properties and the durability of these binders

Fluid-rock interactions, physico-chemical conditions and mass transfers in sedimentary environnments fault zones : Pyrenean thrust faults exemple

Ce travail a pour but d’étudier les paramètres physico-chimiques qui contrôlent les transferts de matière ainsi que la formation et l’évolution des argiles dans des failles chevauchantes en environnement sédimentaire. Deux failles chevauchantes pyrénéennes de faible grade métamorphique ont été étudiées : la faille de Millaris (cf. Mont Perdu) et le chevauchement du Pic-de-Port-Vieux (cf. Gavarnie). Dans la faille de Millaris, la déformation s’accompagne principalement d’une dissolution de la calcite matricielle par pression-solution induisant un changement de volume de la roche de 20 à 40%. Le chevauchement du Pic-de-Port-Vieux enregistre des modifications importantes au coeur de la faille mais aussi dans la zone d’endommagement. Dans les calcaires du mur du chevauchement, une mylonitisation est associée à une dissolution partielle des dolomites en présence de fluides ne dépassant pas 320-340°C. Dans les pélites du toit du chevauchement, la dissolution de l’hématite par un fluide réducteur entraine un changement de l’état redox de la roche (confirmé par spectroscopie Mössbauer) et la précipitation de chlorite dans des veines syncinématiques. Des chlorites à zonations chimiques oscillatoires présentes dans certaines veines révèlent, en combinant cartographie chimique à la microsonde, mesures de l’état redox par μXANES et thermométrie, des variations cycliques de température d’au moins 50°C au cours de la cristallisation. Un processus de valves sismiques pourrait donc être associé à la mise en place du chevauchement du Pic de Port Vieux.This work aims to study the physical and chemical parameters that control the mass-transfer and the clays formation and evolution in sedimentary environment thrust faults. Two Pyrenean thrust faults in low metamorphic grade were studied: the Millaris fault (related to Mont Perdu) and the Pic-de-Port-Vieux thrust (related to Gavarnie). In the Millaris fault, the deformation is accompanied mainly by dissolution of the matrix calcite by pressure-solution which induces a volume change of the rock from 20 to 40%. The Pic-de-Port-Vieux thrust records significant changes in the fault core-zone, but in the damaged zone also. In the footwall limestone, a mylonitisation is associated with a partial dissolution of dolomite in the presence of not exceeding 320-340°C fluids. In the hanging-wall pelites, the hematite dissolution by a reducing fluid causes a redox state change of the rock (confirmed by Mössbauer spectroscopy) and chlorite precipitation in synkinematic veins. Oscillatory zoning pattern chlorites located in some shearing veins revealed, by combining chemical mapping microprobe, redox state measurements with μ-XANES and thermometry, cyclic temperature variations of at least 50°C during the crystallization. A seismic valves process could be associated to Pic-de-Port-Vieux thrusting

Al-free di-trioctahedral substitution in chlorite and a ferri-sudoite end-member

International audienceA compilation of Fe 3+-bearing chlorite analyses is used: (1) to investigate the Al-free di-trioctahedral (AFDT) substitution 2Fe 3+ + □ = 3(Mg,Fe 2+) in chlorite; and (2) to estimate the composition of a ferri-sudoite end-member (Si 3 Al)[(Fe 2+ ,Mg) 2 Fe 3+ 2 □Al]O 10 (OH) 8 within the chlorite solid-solution domain. According to our observations, up to two Fe 3+ cations might be allocated in the M2-M3 chlorite sites by the substitution AFDT, which does not involve Al. These unexpected observations were made possible by the development of µXANES techniques allowing in situ measurements of XFe 3+ (Fe 3+ /(Fe 2+ + Fe 3+)) in heterogeneous chlorite. Although further studies are required to confirm the crystallographic position of Fe 3+ and refine its ionic/ magnetic behaviour in chlorite, this development creates opportunities for developing new geothermometers

Nouvelles découvertes minéralogiques à Versilhac, Haute-Loire

National audienc

Du plomb au pays des sucs : les anciennes mines de Versilhac et du Chambonnet, Haute-Loire

National audienc

The elusive ettringite under the high-vacuum SEM - a reflection based on natural samples, the use of Monte Carlo modelling of EDS analyses and an extension to the ettringite group minerals

International audienc

New occurrence of the middle Toarcian Telodactylites eucosmus (Lippi Boncambi, 1947) (Ammonitina) from the NW European realm, and introduction of a new genus.

8 pagesInternational audienceTelodactylites Pinna & Levi-Setti, 1971 was, up to now, considered to be a middle Toarcian ammonite genus restricted to the Mediterranean realm. We here document two specimens from the Variabilis Subzone (middle Toarcian) of the Thouars area (western France), interpreted as extreme variants of “Telodactylites” eucosmus (Lippi Boncambi, 1947). However, Telodactylites Pinna & Levi-Setti, 1971 must be considered a junior synonym of Peronoceras Hyatt, 1867 and its recent usage is based on an illegitimate subsequent change of its type species. A new genus Neotelodactylites is introduced (type species: Peronoceras eucosmum Lippi Boncambi, 1947). The new finds represent firm evidence for the occurrence of Neotelodactylites n. gen. eucosmus in the NW European realm, therefore considerably extending the palaeogeographical distribution of this species and genus

Use of hydraulic binders for reducing sulphate leaching: application to gypsiferous soil sampled in Ile-de-France region (France)

International audiencePolluted soils are a serious environmental risk worldwide and consist of millions of tons of mineral waste to be treated. In order to ensure their sustainable management, various remediation options must be considered. Hydraulic binder treatment is one option that may allow a stabilisation of pollution and thus offer a valorisation as secondary raw materials rather than considering them as waste. In this study, we focused on sulphate-polluted soil and tested the effectiveness of several experimental hydraulic binders. The aim was to transform gypsum into ettringite, a much less soluble sulphate, and therefore to restrict the potential for sulphate pollutant release. The environmental assessment of five formulations using hydraulic binders was compared to the gypsiferous soil before treatment (contaminated in sulphate). The approach was to combine leaching tests with mineralogical quantifications using among others thermogravimetric and XRD methods. In the original soil and in the five formulations, leaching tests indicate sulphate release above environmental standards. However, hydraulic binders promote ettringite formation, as well as a gypsum content reduction as observed by SEM. The stabilisation of sulphates is, however, insufficient, probably as a result of the very high content of gypsum in the unusual soil used. The mineralogical reactions highlighted during the hydration of hydraulic binders are promising; they could pave the way for the development of new industrial mixtures that would have a positive environmental impact by allowing reuse of soils that would otherwise be classified as waste