Competition of equilibrium and kinetic silicon isotope fractionation during silica precipitation from acidic to alkaline pH solutions in geothermal systems

Abstract

As an important volcanic geothermal region in China, Tengchong belongs to the Mediterranean-Himalayan geothermal belt and is characterized by a wide distribution of geothermal systems. The study area- Rehai- is the sole geothermal area in Tengchong in mainland China that discharges both acidic and alkaline hot springs with pH values ranging from 1.6 to 9.7 at temperatures between 50 and 97 °C. Natural geothermal environments with large variations in pH and temperature are ideal study sites to investigate silicon (Si) isotope fractionation mechanisms which are still insufficiently understood. In this study, we investigated 3 different geothermal systems, which have distinct fluid characteristics: (1) type I fluid from Diretiyan Spring (pH: 1.6–2.5, HSO4 water, saturated with respect to quartz), (2) type II fluid from Zhenzhuquan Spring (pH: 6.4, NaClSO4 water, saturated with respect to quartz), and (3) type III fluid from Dagunguo Spring (pH: 7.8–9.7, NaCl water, oversaturated with respect to amorphous silica: SiO2am). The mechanisms controlling Si isotope fractionation during Si precipitation differ significantly between the three fluid types. In type I fluid, equilibrium fractionation controls Si isotope variability during silica precipitation, causing the highest natural Si isotope equilibrium fractionation (Δ30Si) detected to date, ranging in Δ30Siquartz-fluid from +1.09‰ to +2.78‰. The equilibrium isotope fractionation is essentially caused by balanced precipitation and dissolution rates and high ionic strength of the geothermal fluids. In type II fluid, amorphous silica formed during evaporation and/or cooling of the spring water during capillary rise and a kinetically controlled, uni-directional Si isotope fractionation is observed. In type III fluid, the Δ30SiSiO2am-fluid ranges from −1.57‰ to +0.55‰ at the temperature range of 80–100 °C. The Δ30SiSiO2am-fluid show a shift from kinetic to equilibrium controlled fractionation and demonstrate that isotopic re-equilibration can reset the original kinetic Si isotope values in dependence of the hydrochemical conditions like pH, temperature and ionic strength as well as reaction time. Here we show that isotopic re-equilibration after solid formation can shift the isotopic signatures originally inherited during precipitation, challenging the interpretation of Si isotope values in the ancient rock record.