87SR/86SR, δ18O AND NOBLE GASES AS TRACERS OF WATER-ROCK INTERACTION IN THE THEISTAREYKIR GEOTHERMAL FIELD

Theistareykir is the most recently exploited high-temperature (> 300˚C) geothermal field of Iceland. It began production in 2017 with 18 wells drilled at depths from 1,723 m to 2,799 m. Theistareykir is located at the intersection of the North Volcanic Zone (NVZ) – corresponding to a segment of the north-Atlantic mid ocean ridge – and the WNW-ESE-oriented Tjörnes transform zone. This setting created the Theistareykir fissure swarm with associated volcanism between 10 and 2.5 ka. The Theistareykir geothermal reservoir is hosted in tholeiitic basalt lavas and hyaloclastites, with occasional occurrence of silicic volcanic rocks that are moderately to highly altered. The field is recharged by local precipitation, and possibly older glacial water coming from the southern highlands. A noble gas and Sr-isotope survey was carried out between 2017 and 2019 to characterize the fluid circulation. Water was collected from 10 geothermal wells using a field gas/water separator. The ⁸⁷Sr/⁸⁶Sr ratios were measured following a purification step on Sr Resin and using a NuPlasma II MC-ICP-MS. The ⁸⁷Sr/⁸⁶Sr ratios vary significantly from 0.70358 ± 0.00003 to 0.70671 ± 0.00004. The lowest value is close to that of E-MORBs but correlation with noble gases suggests that this endmember has a value closer to 0.7030, pointing instead to a N-MORB source, which is known to dominate the NVZ magmatism. The most radiogenic values are interpreted as resulting from water-rock interactions, as suggested by a linear positive correlation between δ¹⁸O and ⁸⁷Sr/⁸⁶Sr. The ⁸⁷Sr/⁸⁶Sr ratios plotted against ²⁰Ne/³⁶Ar ratios discriminate the sampled wells into two distinct arrays. The covariance among Sr-isotopes, δ¹⁸O and noble gases can be interpreted as mixing between three fluid sources: a fluid with a ⁸⁷Sr/⁸⁶Sr ratio of 0.7030 as measured in local basalts, and ²⁰Ne/³⁶Ar ratio equal to that of solubility equilibrium with the atmosphere (Air Saturated Water); and two fluids with distinct radiogenic ⁸⁷Sr/⁸⁶Sr values and higher ²⁰Ne/³⁶Ar ratios close to that of the atmosphere. The first fluid represents the deep geothermal fluid in the reservoir, while the other two fluids could reflect two generations of glacial meltwater with trapped air and progressively increase of ²⁰Ne/³⁶Ar ratios toward the atmospheric value as interaction with reservoir rocks and Sr exchanges proceed. This ⁸⁷Sr/⁸⁶Sr and ²⁰Ne/³⁶Ar relationship suggests that reservoir changes are due to interactions with fluids belonging to distinct generations.