Geochemical evidence of gas sources of CO2-rich cold springs from Wudalianchi, Northeast China

Abstract

CO2-rich cold springs occur near the active volcanoes at Wudalianchi (五大连池), Northeast China. The springs are rich in CO2, with HCO3− as the predominant anion and have elevated contents of total dissolved solid (TDS) (>1 000 mg/L), Fe2+ (>20 mg/L), Sr (>1 mg/L), and dissolved Si (>20 mg/L). The compositions of escaped and dissolved gases of the springs are similar. The δ13C values of escaped gases and dissolved gases in mineral springs at Wudalianchi vary from −8.77‰ to −4.53‰ and −8.24‰ to −5.26‰, while δ18O values vary from −10.68‰ to −7.65‰ and −10.30‰ to −8.84‰, respectively, indicating the same upper mantle origin of CO2 of escaped gases and dissolved gases in the springs. Carbon and oxygen isotope fractionations and water-CO2 exchange were weak in the process of groundwater flow. The 4He content exceeds 5 000×10−6 cm3·STP/mL in escaped gases of the mineral springs, and the 3He/4He ratios of the escaped and dissolved gases vary from 2.64Ra to 3.87Ra and 1.18Ra to 3.30Ra, respectively. It can be postulated that the CO2 of mineral springs deriving from the magma chamber of the upper mantle moves upward to the surface, to increase the content of 4He in the mineral springs and decrease the ratio of 3He/4He. The helium origin of escaped gases in springs can be calculated with the MORB-crust mixing model, but that in the north spring can be better explained with the MORB-crust-air mixing model due to the effect of mixing with surface water. However, dissolved helium in springs, except the north spring, is better explained with the MORB-crust-ASW mixing model.