No abstract

The current lithospheric structure of the Betic Cordillera results from active geodynamic system related to slab retreat slowdown in western Mediterranean. A sharp change in lithospheric thickness has been imaged beneath the Betic Cordillera, potentially resulting from a near‐vertical subduction‐transform‐edge‐propagator fault toward the surface with possible mantle influx. In this study, we use helium isotopic composition of bubbling and dissolved gases in groundwater samples of the central part of the Betic Cordillera to evaluate the origin of gases and to set constraints on its lithospheric structure. We found that helium isotopic composition have a dominant radiogenic component with a mantle‐derived He contribution reaching mainly 1% for the investigated area. Estimation of He diffusion within the ductile crust indicates that this process is potentially to slow to explain the low mantle‐derived He contribution measured at the surface. A new analysis of the available data of the crustal metamorphic complexes allows us to suggest that the crust could be dissociated from the mantle with no evidence of asthenospheric influx. The weak mantle He signature could reflect a mantle material earlier incorporated in the crustal metamorphic complexes of the Betic Cordillera during their exhumation. In light of mass balance calculations, we propose that the slight 3He excess observed in the present‐day fluids might result from a fossil mantle signature diluted by local radiogenic production over time.

3There is growing interest in geothermal energy, which is considered as an efficient energy 4 solution to mitigate rising atmospheric CO2. Besides known high enthalpy geothermal systems, 5 increasing attention is paid to low temperature geothermal systems, as they are suitable for local 6 use. Although geothermal production seems to be an environmentally advantageous renewable 7 energy, it might result in significant CO2 emissions. In this study, we investigate the relationship 8 between temperature, fugacity of CO2 (fCO2), and mineral buffers in the reservoir conditions, 9 taking the low-to medium-enthalpy thermal waters in the Central Betic Cordillera as case 10 study. Using geochemical modeling, three main groups of waters have been identified 11 depending on temperature, buffering mineral assemblage, and fCO2 in their reservoir. A group 12 of waters with a reservoir temperature ranging from 70 to 90°C and located in the intramountain 13 sedimentary basins shows a fCO2 in depth ranging from ~ 6 x10 -2 and 6 x10 -1 . The reservoir 14 chemistry of this water group seems to be mainly controlled by carbonates and evaporites 15 displaying a fCO2 variation between depth and surface (ΔfCO2) of 10 -1 . Another intermediate 16 group of waters, located in an active extension zone, displays lower temperature (50-60°C) and 17 fCO2 in the reservoir (from 10 -3 to 10 -2 ). Finally, the third group of waters, located on the 18 metamorphic complexes contacts, show the highest estimated temperatures (130 -140°C) and 19 fCO2 in the reservoir (1 to 10 2 ). The two latter groups suggest increasing buffering effect of 20 alumino-silicates, in addition to carbonates and quartz. Therefore, we evidenced a strong 21 relationship between temperature and fCO2 in the reservoir as well as the potential mineral 22 buffers. We discussed the potential of geothermal systems as clean energy source based on the 23 estimation of the CO2 emissions generated by the investigated thermal systems for a practical 24 case of household heating. 25 chemical water-rock interactions, is an important parameter to consider as it "buffers" the fluid 51 chemical composition and the CO2 degassing. Therefore, understanding the relationship 52 between the mineral buffer, the fugacity of CO2 (fCO2) (or its partial pressure of CO2, pCO2), 53 and the temperature in the reservoir, is crucial to estimate the CO2 release in geothermal 54 systems. These links between fCO2 and reservoir temperature have been subject to Chiodini 55 and collaborators' investigations since the early 1990's, although they focused mainly on 56 volcanic geothermal systems (e.g., Chiodini et al.