Formation and evolution of the Changbaishan volcanic geothermal system in a convergent plate boundary back-arc region constrained by boron isotope and gas data

Abstract

The goal of this study is to analyze fluid geochemistry, boron isotope and gas data to reassess the formation and evolution processes of the Changbaishan volcanic geothermal system (Jilin, China). Due to the presence of hot fumaroles (with temperatures of higher than 100 °C) and sinters, we believe that phase separation is occurring underground, which is also indicated by the characteristics of the B and gas data. Based on their hydrochemical data and B/Cl ratios, the thermal water samples in this area can be divided into three groups: 1) group 1 is steam-heated/condensate water (group 1-1: JJ, JLQ; group 1-2: SBDG, XRQ); 2) group 2 is karst water (CR3 and LSD); and 3) group 3 is residual water (CR1 and CR2). The preliminary discrimination of samples using B and gas data implies that group 1-1 around the crater is controlled by a second phase separation process, which is induced by the input of hot magmatic material; group 1-2 in the basin area is controlled by the transformation of clay minerals, and the XRQ water sample shows contamination by sewage; group 2 represents karst water that has leached metamorphic rock and been directly heated by the input of hot magmatic volatiles; and group 3 is the result of one seawater phase separation process. The mixing and fluid-rock interaction models based on the mass balance of boron concentration and isotope imply that the phase separation and input of magmatic volatiles are more important than fluid-rock interactions in the formation of the Changbaishan geothermal system, and the first phase separation process in the deep carbonate reservoir occurred in a late stage, with approximately 75%–87.5% of water becoming vapor. Additionally, the Rayleigh distillation of B isotope implies that new magma is being generated below this area, with the low B concentrations and high boron isotopic values of its gases implying that the mixed component may be derived from the subducted Pacific plate (e.g., marine sediment). The addition of “excess heat” due to the continued subduction of the Pacific plate has further encouraged the phase separation process, and the subduction process has also accelerated the transition of the hydrothermal system into a non-hydrothermal system.