The isotopic evidence of gas source of the cold mineral springs with high pCO2 in Wudalianchi

Abstract

Active volcanoes are often associated with thermal springs, but some cold mineral springs exist near the active volcanoes in Wudalianchi, Heilongjiang Province, China, and these cold spas are rich in CO2. Water analysis shows that these HCO3 type mineral springs have higher mineralization degree ( > 1000 mg/L) and are rich in CO2, Fe2+(>20 mg/L), Sr(> 1 mg/L), dissolved Si (>20 rag/L) with lower temperature(< 10 ~ = and pH < 6.5. Dissolved and evolved gases of all these spas have the same composition, and are composed mainly of CO2 with minor N2 and 02. The ~13C and 81SO of dissolved and evolved CO2 in these spas are almost identical, and all 613C values are between -2-9, so consistent with the ~13C of the inclusions in basalt of upper mantle (-4-8). We presume that the CO2 of mineral springs derives from magma chamber of the upper mantle and comes through magma channels and faults in the Wudalianchi active volcanic area. While 4He content exceeds 5000x10 "6 cm 3 STP/L in evolved gas of these spas, and 3He/aHe ratios in the dissolved and evolved gases vary between 2.6-3.8 Ra (where Ra is the atmosphere ratio of 3He/4He=l.4x 10 -6 ), respectively. Higher contents of 4He in these spas enable us to make it sure that the gas of these mineral springs mainly derives from magma chamber of the upper mantle, and the ratio of 3He/aHe lends further evidence. Because the ratio of 3He/4He of upper mantle is 8.4 Ra, one third of He in these spas attributes to the upper mantle based on simple mixing at least.