干旱区生态环境知识资源中心

Both deep seismic sounding (DSS) and magnetotelluric sounding (MT) data suggest that there exist still magma chambers within crust in Tengchong volcanic field (TVF) present-day, but there are differences on their number and spatial distribution and their complete picture still lack good understanding. MT data suggests that TVF is an asthenosphere-upwelling and lithosphere-thinning area, but the spatial extent of the thinning area still lack adequate constraints. By sampling, istopic analyzing of free gases from thermal springs in TVF and its periphery area, and utilizing some previous investigation data, we obtain He-3/He-4 ratio data of 75 thermal springs. By atmosphere contamination correcting, mantle-derived proportion calculating and spatial interpolation, we obtain spatial distribution map arid time diagram of release intensity of mantle-derived volatiles in TVF. After investigating the characterization of spatial arid temporal distribution of mantle-derived helium release according helium isotope tracing principle, we found that there are 1 band and 3 blocks of high He-3/He-4 ratio in spatial distribution pattern in TVF. With He-3/He-4 >= 1 Ra (Ra denotes the atmospheric He-3/He-4 ratio value, 1. 39 x 10(-6)), mantle-derived helium proportion >= 15% as boundary conditions, the abnormal release area of mantle-derived volatiles in TVF shows an entire zonal area, which is a north-south strike strip, north to south 100km long, east to west 50km wide. In the whole piece of abnormal area inside, the intensity of mantle-derived volatile release can be divided into 3 different areas. (1) The Tengchong County-Rehai and periphery in central TVF, where the He-3/He-4 ratio reaches more than 5.5Ra, the mantle-derived helium proportion above 70%, are the most strong release area. (2) The Qushi and periphery in north TVF, where the He-3/He-4 more than 4. 5Ra, the mantle-derived helium proportion more than 50%, are second strong release area; (3) The Withe-Puchuari-Xinhua in southern TVF, where the He-3/He-4 more than 2Ra, the mantle-derived helium proportion more than 25%, are the weakest release area. In temporal distribution pattern, mantle-derived volatiles release intensity in TVF has been incresing, and the rate of He-3/He-4 ratio (Ra) increasing in the 3rd releases area is significantly larger than the other two. Our results suggest that, the spatial distribution pattern of intensity of mantle-derived volatiles release in TVF is the most direct reflection of the spatial scale and intensity of asthenosphere upwelling (lithospheric thinning) in the region, the size of thinning zone is roughly north to south 100km long, east to west 50km wide. There are 3 magma chambers within crust in TVF. The 1st one locates beneath Tengchong County-Rehai area, the 2nd one locates beneath Mazhan-Qushi area, and the 3rd one locates beneath Wuhe-Longjiang-TuantianPuchuan-Xinhua area. These 3 magma chambers constantly being fed with mantle melt materials, among which, it is the most obvious for the 1st one to be fed continuously with mantle melt materials. The 1st magma chamber, which concentrates mantle-derived volatiles release, relative geothermal gradient, deformation and seismicity and so on abnormal in one, being accepting feeding from mamtlederived magma, being of strongest activity, most likely to erupt, needs the key surveillance.

The 2nd magma chamber, which release intensity of mantle-derived volatiles is also noticeable, also being possibly accepting feeding from mamtle-derived magma, should be strengthened in monitoring. The 3rd magma chamber, which has the biggest size, and is shallowest, where the current feeding of mantle-derived magma increased quickly, is must be paid some attention.