CO2 gas pools in Jiyang sag,China

The CO₂ gas pools of Jiyang sag are located along the Gaoqing–Pingnan fault within a region of alkaline basalts. The concentration of CO₂ in the gas pools is in the range of 68.85–96.99%. All of the geochemical tracers for the CO₂ gas pools support the suggestion that CO₂ was mainly derived from mantle degassing. The δ¹³C values of CO₂ in the gas pools are in the range of −5.67–−3.41‰, which are higher than those of organogenic CO₂, and near to those of abiogenic CO₂. Their ³He/⁴He ratios are 2.80–4.47×10⁶, i.e. the R/Ra ratios are 2.00–3.19, showing that the Jiyang sag had undergone strong mantle degassing. CO₂/³He ratios are 0.59–0.89×10⁹, which are identical to those for N-MORB, indicating that CO₂ in these CO₂ gas pools was mainly derived from the mantle. Accompanying the intrusion of mantle-derived magma, the mantle-derived CO₂ migrated upwards along deep faults and was trapped in advantageous structures forming gas pools.