Discovery of CH4-rich high-pressure fluid inclusions hosted in analcime from Dongying depression, China

It is the first time to report CH4-rich high-pressure fluids in analcime from Dongying depression in eastern China in this paper. The fluid inclusions in volcanic analcime have recorded the characteristics of the fluids which are relative to deuteric hydrotherm of magma and the serpentinization of basalts. The fluid inclusions in analcime can be divided into three types from shape and the ratio of vapor to liquid. The internal pressures of fluid inclusions have been determined by the Raman spectra of methane. The results show that three type fluid inclusions indicate that they were captured under different environmental condition. The maximum internal pressure of these fluid inclusions is 66.01 MPa at room temperature. Previous study shows that the natrolite in alkali basalts changes into analcime at least above 50 MPa and 375 °C. The internal pressures of inclusions at 375 °C have been computed by the equation of state. The maximum internal pressure contrasts to depth reach to 7.529 km. This confirms that the CH4-rich high-pressure fluids in analcime came from a deep environment. We know that the CO2 and H2 in basaltic magma can react to produce CH4 on a metallic iron or iron oxide catalyst. So, the fluid inclusions in analcime may be regarded as a powerful evidence for the formation of methane during serpentinization of basalts and deuteric hydrotherm of magma. Therefore, the discovery of CH4-rich high-pressure fluids in analcime is very significant to study of abiogenic methane.