Penetration depth of meteoric water in orogenic geothermal systems

Abstract

Warm springs emanating from deep-reaching faults in orogenic belts with high topography and orographic precipitation attest to circulation of meteoric water through crystalline bedrock. The depth to which this circulation occurs is unclear, yet it is important for the cooling history of exhuming orogens, for the exploitation potential of orogenic geothermal systems, and for the seismicity of regional faults. The orogenic geothermal system at Grimsel Pass, Swiss Alps, is manifested by warm springs with a clear isotopic fingerprint of high-altitude meteoric recharge. Their water composition and their occurrence within a 3 Ma fossil upflow zone render them particularly favorable for estimating the temperature along the deep flow path via geochemical modeling. Because the background geotherm has remained stable at 25 °C/km and other heat sources are unavailable, the penetration depth can be derived from the deep-water temperature. We thus estimated the base of the Grimsel system to be at 230-250 °C and 9-10 km depth. We propose that deep temperatures in such systems, particularly those with normal background geotherms (<30 °C/km), have been systematically underestimated. Consequently, far more enthalpy may be accessible for geothermal energy exploitation around the upflow zones than previously thought. Further, the prevalence of recent earthquake foci at ≤10 km beneath Grimsel suggests that meteoric water is involved in the seismicity of the host faults. Our results therefore call for reappraisal of the heat budget and the role of meteoric water in seismogenesis in uplifting orogens.

Quantification of 3‐D Thermal Anomalies From Surface Observations of an Orogenic Geothermal System (Grimsel Pass, Swiss Alps)

“…The compositions of the thermal waters discharging at that site could be matched by performing forward reactive‐transport simulations that treat thermodynamic and kinetic processes of mineral dissolution and precipitation along the upflow path. By combining the calculated 250 °C minimum temperature of water–rock equilibration along the deep flow path with the current geothermal gradient of 25 °C/km, this approach confirmed a penetration depth of at least 10 km (Diamond et al, ), thereby bridging the results obtained by classic solute geothermometry and petrologic studies.…”

“…All the thermal springs contain 3 H (bomb tritium); thus, they represent mixtures between an old, deeply circulated geothermal end‐member that is 3 H‐free, and a young, cold‐water end‐member that is 3 H‐bearing. In the warm spring with the highest solute concentration, the measured 3 H activity defines a mixing ratio of about 1:1 (Diamond et al, ). Combining this ratio with the 14 C concentration shows that the deep geothermal end‐member is 14 C‐free and therefore more than 30 ka old (Waber et al, ).…”

“…Thus, a topographically induced hydraulic head gradient is interpreted to be the main hydraulic force driving ascent of the geothermal water from depth. Reactive‐transport modeling, constrained by the mineralogy of the Pliocene hydrothermally altered breccia, shows that the infiltrating meteoric water reaches a temperature of at least 211 °C and more likely on the order of 230–250 °C along the deepest reaches of its flow path (Diamond et al, ).…”

“…Considering geothermal gradients typical of waning orogens (25–30 °C/km), the reservoir temperatures in meteoric‐dominated systems suggest penetration of meteoric water to less than 6‐km depth. Recently, Diamond et al () took an alternative geochemical modeling approach to determine the penetration depth of meteoric water at the Grimsel Pass geothermal site in the Swiss Alps. The compositions of the thermal waters discharging at that site could be matched by performing forward reactive‐transport simulations that treat thermodynamic and kinetic processes of mineral dissolution and precipitation along the upflow path.…”

Geochronological and thermometric evidence of unusually hot fluids in an Alpine fissure of Lauzière granite (Belledonne, Western Alps)

“…The closure temperature is considered as the temperature at which the fission-track system closes to the loss of fission tracks by annealing and is applicable in the case of monotonic cooling (Dodson, 1973). The idea is that no fission tracks are preserved in the zircon crystals at elevated ambient temperatures, but start to be retained as soon as the crystal cools below the effective closure temperature.…”