Temperature of paleo- to modern self-sealing within a continental rift basin: The fluid inclusion data (Soultz-sous-Forêts, Rhine graben, France)
Dubois, Michel; Ayt Ougougdal, Mοhamed; Meere, Patrick; Royer, Jean-Jacques; Boiron, Marie-Christine; Cathelineau, Michel
European Journal of Mineralogy Volume 8 Number 5 (1996), p. 1065 - 1080
published: Oct 30, 1996
manuscript accepted: May 22, 1996
manuscript received: Feb 16, 1995
ArtNo. ESP147050805014, Price: 29.00 €
Abstract Self-sealing of open microstructures and mesostructures characterizes evolving fluid flow in the sedimentary cover as well as in the granitic basement of the Rhine continental rift basin in the region of Soultz. This area currently experiences active fluid migration. A detailed petrographic and microthermometric study of paleofluids trapped as fluid inclusions in the structures (veins filled by quartz or barite, healed microfissures) and authigenic minerals (quartz overgrowths in sandstones, and euhedral quartz in altered granite plagioclases) reveals a succession of contrasting P-T-X conditions for self-sealing of the rocks. The data have been obtained from rock cores sampled at different depths along the Hot Dry Rock Soultz-sous-Forêts drillhole EPS-1, extending from the Triassic sandstone cover down to the granitic basement. Several stages of paleofluid trapping are distinguished as a function of the type of host mineral or microstructure. The earliest fluid stage (probably late Hercynian) is mostly recorded in healed fissures (fluid inclusion planes) affecting the granite quartz grains, but also in some intragranitic early quartz-carbonate veinlets: i) rare CO2-(N2)- H2O fluids, ii) a succession of fluids of moderate salinity (2-7 wt% eq. NaCl) trapped under a large range of temperatures (minimum temperature of 180 to 340°Q. The second main stage of mineral crystallization corresponds to a more recent crystallization of quartz cement in sandstones, and of quartz-barite veins hosted by granite or sandstone. This stage is probably related to post- Oligocene fluid flows. Fluids trapped in these authigenic minerals display rather similar features to the modern brines (temperature in the range of 130 to 160°C, large range of salinity) encountered in drillholes of the area. The temperature-depth profile obtained from recent vein infillings is close to the profile measured after drilling, indicating a recent to active self-sealing, or a rather monotonous thermal regime during the latest fluid migration period. The large variations of salinities observed in some samples suggest a heterogeneous mixing process between low salinity fluids (less than 5 wt% eq. NaCl) and brines. Fluid inclusion data indicate a deep penetration of fluids equilibrated with sediments, and large scale fluid movements between the sedimentary cover and the basement.