The isotopic geochemical characteristics and genesis of the CO2 gas pools in the Jiyang depression

Abstract

The CO2 gas pools of the Jiyang Depression are mainly located in the central and southern parts of the Gaoqing-Pingnan deep fault zone, Northwestern margin of the Yangxin subsidiary depression and the Shangdian volcanic dome. These CO2 gas pools contain 69%-97% CO2. The geochemical analyses indicate that the δ13 CCO2 values are varied from -5.67‰ to -3.35‰, the CH4/3 He ratios are varied from 1.01 × 108 to 5.65 × 108, the 3He/4He ratios are varied from 2.80 × 10-6 to 4.49 × 10-6 (i.e. R/Ra = 2-3.21), the 40Ar/36Ar ratios are varied from 317 to 1791, and the CO2/3 He ratio are varied from 0.25 × 109 to 2.61 × 109 respectively. The data above consistently suggest that the CO2 reserved in the Jiyang Depression was mainly sourced from the mantle, with possible influence of partly loosing CO2 in the upward migration process of the mantle sourced CO2, or possible mixing with small part of crust-sourced CO2 gases such as the CO2 produced from metamorphism of carbonate rocks. Besides the qualitative analysis of the CO2 source, many other research works, such as the quantitative analysis of CO2 sources, the forming process of CO2 gas pools and its relationship to the magmatic activities, should be further undertaken.