Chlorite and corrensite are very common secondary minerals in very low-grade metamorphic or hydrothermally altered mafic rocks. Corrensite, although defined as a 1:1 regularly mixed-layered chlorite/smectite or chlorite/vermiculite, has been considered as a unique mineral phase and might thus be a potential index mineral for evaluating very low metamorphic grade. Many lenticular basaltic rocks, which are intercalated with unmetamorphosed to low-grade metamorphosed Tertiary sedimentary rocks, occur sporadically in the Western foothills and the Central Range in Taiwan. The lenticular basaltic rocks in the Western foothills (diagenetic zone) and the western flank of the Central Range (anchizone to epizone) were subjected to different degrees of hydrothermal alteration and/or metamorphism. However, only few occurrences of corrensite in these basaltic rocks were reported. In the present study, the occurrences and mixed-layering features of corrensite and coexisting chloritic minerals in the basaltic rocks were characterized by utilizing optical microscopy, X-ray diffraction, scanning electron microscopy with energy dispersive spectrometry, and transmission electron microscopy (TEM). The results showed that most of these basaltic rocks contain abundant chloritic minerals occurring as replacements of mafic minerals or interstitial glass, or as vein- or vesicle-filling materials. The chloritic minerals include smectite, corrensite, chlorite, mixed-layered chlorite/smectite, or mixed-layered chlorite/corrensite. The compositions of chlorite, corrensite, and smectite are distinctive in terms of their Si/Al ratios, interlayer cations, and total cation numbers. Corrensite, chlorite, and mixed-layered chlorite/corrensite are common secondary mineral assemblages in the anchizone basaltic rocks, whereas only discrete chlorite occur in the epizone. The basaltic rocks in the diagenetic zone contain different assemblages, such as smectite only, chlorite + smectite + mixed-layered chlorite/smectite + mixed-layered chlorite/corrensite, or chlorite + corrensite + mixed-layered chlorite/corrensite. These different assemblages of chloritic minerals and other secondary mineral assemblages might reflect different stages of hydrothermal alteration. Corrensite was positively identified with TEM lattice-fringe imaging in the diagenetic rocks collected from Nangang, Poneikeng, Shanzijiao, Nanshihjiao, Hsiungkong, Shihsiouping, Fusing, and Northern Shihmen Reservoir areas, and in the studied anchizone rocks from Junghua, Kaopo and Baolai areas. The disappearance of corrensite clearly marks the boundary between the anchizone and epizone basaltic rocks. Corrensite, as a chemically and structurally unique mineral phase, is therefore a potential index mineral in very low-grade metamorphic rocks.

L’objectif de cette thèse est d’étudier le comportement du soufre dans des fluides hydrothermaux soumis à un métamorphisme de bas grade. Elle est séparée en trois études interconnectées couplant étude naturelle et expérimentale : i) une étude du cycle du soufre dans les évaporites carniennes de la "Nappe des Gypses" des Alpes françaises comme cas d’étude naturelle a été réalisée, ii) une étude de la spéciation du soufre dans les inclusions fluides associées à ces évaporites triasiques, et iii) une étude expérimentale du rôle de la spéciation du soufre sur le mécanisme de la Thermo-Réduction des Sulfates (TSR) et l’altération des hydrocarbures associée. La première étude repose sur le couplage d’une approche pétrographique classique et d’une étude détaillée des inclusions fluides, des isotopes stable (δ34S, δ33S, δ36S, Δ33S, Δ36S, δ13C et δ18O) et de la matière organique dans la "Nappe des Gypses". Ces études couplées ont permis d’identifier le processus de TSR comme la réaction contrôlant le comportement du soufre dans cette formation en système fermé. La seconde étude repose sur une nouvelle approche analytique couplant spectroscopie Raman in-situ et platine microthermométrique permettant de réchauffer jusqu’à 300°C les échantillons (naturel et synthétique). Ainsi, à température >100°C la présence de l’ion radicalaire S3- et d’autres espèces polymériques du soufre (Snx) a été mise en évidence dans la “Nappe des Gypses”. Cette étude confirme le rôle indispensable à la TSR de l’ion S3- et des Snx dans les fluides naturels. Cette étude naturelle a été complétée par une étude expérimentale préliminaire qui a permis de mettre en place de nouveaux protocoles expérimentaux en autoclave et en capillaires de silice couplés à des analyses spectroscopiques et isotopiques. Ceci afin de mieux comprendre le rôle de la spéciation du soufre dans les interactions entre une phase aqueuse et un fluide hydrocarboné lors de la TSR. Cette thèse conduit à une meilleure compréhension du rôle de la spéciation du soufre et son lien avec des composés organiques lors de la TSR en condition métamorphique de bas grade. Cela ouvre de nouvelles perspectives de compréhension des formations géologiques basées sur le couplage d’études géologiques classiques et d’études géochimiques plus fines / The aim of this thesis is to study the sulfur behavior in hydrothermal fluids subjected to low grade metamorphism. It is separated into three interconnected studies linking natural and experimental study: i) a study of the sulfur cycle in the Carnian evaporites of the "Nappe des Gypses" from the French Alps as a natural case study is performed, ii) a study of the sulfur speciation in fluid inclusions associated to these Triassic evaporites, and iii) an experimental study of the role of sulfur speciation on the Thermochemical Sulfate Reduction (TSR) mechanisms and the associated hydrocarbons alteration. The first study is based on the coupling of a classical petrographic approach associated to a detailed study of fluid inclusions, stable isotopes (δ34S, δ33S, δ36S, Δ33S, Δ36S, δ13C and δ18O) and organic matter in the “Nappe des Gypses”. The coupling study allows to identify the TSR process as the reaction controlling the behavior of sulfur in this formation in a closed system. The second study is based on a new analytical approach coupling in-situ Raman spectroscopy and microthermometric stage. This technique allows to heat up to 300°C samples (natural and synthetic). Thus, at temperature >100°C, the presence of the radical ion S3- and other polymeric sulfur species (Snx) is demonstrated in the “Nappe des Gypses”. This study confirms the essential role in the TSR process of S3- and Snx species in natural fluids. This study natural was completed by a preliminary experimental study allowed to set up new experimental protocols in autoclave and fused silica capillaries coupled to spectroscopic and isotopic analyses. This to better understood the role of sulfur speciation in the interactions between an aqueous phase and a hydrocarbons fluid during the TSR. This thesis leads to a better understanding of the role of sulfur speciation and its link with organic compounds during TSR under low grade metamorphic conditions. This opens new perspectives to understand geological formations based on coupling of classical geological studies and finer geochemical studies