The Geochemical Characteristics And Origins Of The Gases In Changbaishan Tianchi Volcanic Region

Changbaishan Tianchi volcano is located in the back-arc enviroment where theEurasian plate and the Pacific plate interact. In other words, it locates in the junction of thenortheastern margin of North China plate and the Meso-Cenozoic volcano tectonic belt,which extends along the coast of the Pacific Ocean in an NE direction. Since2002, morefrequent and tenser earthquakes occurred with the temperature rise of hot springs andrapid changes of local deformation. Hence it is very necessary to strengthen themonitoring of the activities in Tianchi volcanic region.The compositional characteristics and origins of hot spring gases released in Tianchivolcanic region are researched based on measuring the components of Julong and Jinjianghot springs. The geochemical results of the volcanic gases indicate that the temperature(71.1℃), content of He (7×10-6) and CO2(83.02%) and ratio of3He/4He (3.92Ra) ofJulong hot spring are lower than historic values while those of Jinjiang hot spring (58.3℃,133×10-6,94.06%and5.81Ra) increase gradually. The values of δ13CCH4andδ13CC2H6of the volatiles from volcanic rock samples are-48.5‰～-30.6‰and-31.6‰～-21.3‰respectively. Moreover, all the rock samples are in line with the positive carbonic isotopeseries ofδ13CC2H6＜δ13CCH4.Theδ13CCO2value of the volatiles is-27.2‰～-11.4‰, allof which are below-10‰.Totally speaking, the recent observation data (1999-2014) shows that all the above indexes are stable. Moreover, the major, trace and isotopic components of the hot springgases suggest that the gases released by both hot springs are mixtures of crust and mantle:Jinjiang hot spring is not contaminated distinctly by crustal materials whereas the gas fromJulong hot spring contains more components from crust and air compared to mantlematerials. In addition, all the alkane gas and CO2released through step heating from thevolcanic rocks originate organically.