Chemical and isotopic constraints on the origin of saline waters from a hot spring in the eastern coastal area of China

Abstract

The Jimo hot spring in Shandong Peninsula, China, is a typical coastal geothermal system. The geothermal water has a very high salinity (10.8 g/L) and the origin of the salt is key to utilization of this geothermal resource. A systematic investigation of the hydrochemistry and isotopes (δ11B, 87Sr/86Sr, δ18O, δD, 13CDIC and 14CDIC) of water from 14 geothermal wells was conducted. The results show that the geothermal waters from the high-temperature center and eastern part of the geothermal field are Cl-Na·Ca-type waters with Br/Cl (8 × 10−4–1.0 × 10−3), Na/Cl (0.63–0.70) and δ11B values (15.9–17.2‰) that are lower than those of seawater. The western geothermal waters are dominated by Cl-Na-type waters with Br/Cl and Na/Cl values similar to those of seawater. The depleted δD and δ18O compositions and the corrected 14CDIC age suggest that Jimo geothermal waters are mixtures of late Pleistocene to early Holocene and younger meteoric waters. An improved Br/Cl-Na/Cl diagram, ion mass balance calculations and δ11B values indicate that halite and K-salt dissolution and subsequent cation exchange formed the dominant Cl-Na·Ca-type geothermal water, and this was then modified into a small amount of Cl-Na-type water in the western area by mixing with minor seawater entrapped in the unconsolidated sediments. The 87Sr/86Sr ratios (0.710613–0.710726) of the geothermal waters reflect water–rock reactions in the sandstone. The improved Br/Cl-Na/Cl diagram, Piper plot and boron isotopic dataset containing saline waters from coastal geothermal systems worldwide further confirm that the salinity in the Jimo geothermal water originated from dissolution of marine evaporites.