Isotopic tracing of fluids sources and transfer in the crust

• Main Author: Cardoso, Carolina Dantas
• Other Authors: Centre de Recherches Pétrographiques et Géochimiques (CRPG), Institut national des sciences de l'Univers (INSU - CNRS)-Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS), Lorraine Université d'Excellence (LUE), Université de Lorraine, Raphaël Pik, IMPACT DEEPSURF
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: HAL CCSD 2023
• Subjects: Crustal fluids; Isotopic monitoring; Helium isotopes; Fluides crustaux; Traçage isotopique; Isotopes de l'hélium; [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; Iceland
• Online Access: https://hal.univ-lorraine.fr/tel-04583163; https://hal.univ-lorraine.fr/tel-04583163/document; https://hal.univ-lorraine.fr/tel-04583163/file/DDOC_T_2023_0139_DANTAS_CARDOSO.pdf

Noble gases occur in low concentration on Earth and are relatively inert, making them good tracers of fluid interactions. Contrary to most stable isotopes or major elements commonly used as geochemical tracers, noble gases are less susceptible to water/rock interactions modifications. Due to their widely variable ratios among the three main Earth reservoirs (mantle, continental crust, and atmosphere), the He and Ne isotopic systems are of particular interest, providing information on the source of fluids. The main goal of this thesis was to detect the sources of crustal and geothermal fluids in different geological and geotectonic settings with a specific emphasis on precising transport processes in the crust: : (i) isotopic monitoring and survey in North Iceland, (ii) tracing the source of He in a continental basin in Central France, and (iii) tracing the source of He in the Lake Abhe geothermal field (Djibouti). From the results reported in this thesis, helium isotopes show that mantle input is present in different geotectonic contexts, not necessarily linked to active volcanism or extension, such as the case of off-rift zones in Iceland (Chapter 4) and of the Paris Basin (Chapter 5), the latter inserted in a continental crust setting. In the Lake Abhe geothermal system (Chapter 6), the helium isotopic signature is below the one expected for this segment of the East African Rift System (EARS), where an enriched plume-like endmember was anticipated, as observed a few kilometres away, in SW Afar (∼ 55 km) and Tendaho Graben (∼ 110 km NW). Thus, helium isotopes are a powerful tool to trace the mantle influence and transport complexities at different geological settings. This thesis is organized in seven chapters; the first three of them give background information on the different studies and the the next ones deal with the results and conclusions of such studies. Chapter 1 presents the context of the thesis, the helium isotopes systematics, and target areas. I detail the principle of helium and neon isotopes, ...