Changbaishan Tianchi volcano geothermal system：Magma chamber and hidden high-temperature geothermal resources

The Changbaishan Tianchi volcano is a Late Cenozoic central eruptive composite layered volcano. It has experienced multiple large-scale eruptions since the Holocene, and there is still modern magmatic thermal disturbance in the depth. Therefore, it is a potential high-temperature geothermal resource area in China. The Changbaishan Tianchi volcano is an intraplate volcano which has undergone three forming stages of shield volcano, cinder cone and ignimbrite. The shield-forming stage: Late Pliocene-Early Pleistocene, basaltic magma erupted from Changbaishan Tianchi volcano and adjacent craters overflowed radially, forming a shield-shaped volcanic lava platform. The cone-forming stage: In the Mid-Late Pleistocene, the coarse rock lava formed by multiple eruptions overflowed from the centre of the Tianchi crater and accumulated on the volcanic shield basalt, forming the Tianchi volcanic cone. The ignimbrite-forming stage: In the Holocene, the activity of Tianchi volcanic reached a new peak, and an explosive eruption occurred at its centre. The pyroclastic flow accumulates and consolidates to form ignimbrite, which is covered on the volcanic cone in a sheet shape.

Active magma chambers still exist beneath Changbaishan Tianchi volcano. The shallow crustal magma chambers provide sufficient heat for underground hydrothermal activity. The groundwater surrounding Tianchi volcano is primarily composed of pore and fissure water in the Late Pleistocene basalt, which is characterised by thick aquifers, good connectivity and an abundance of water. At the periphery of Tianchi volcano, the thickness of basalt thins towards the periphery, and groundwater runoff from the Tianchi volcanic cone to the periphery is impeded by the surrounding bedrock, forming a ringed groundwater overflow zone. The groundwater is heated by high-temperature magma gas （CO₂） at depth and then rises along fractures under stratigraphic pressure to form hot springs at the surface. There are three thermal spring groups associated with modern volcanic activity around the Tianchi crater lake in Changbaishan area. The genesis of all thermal spring groups is related to high-temperature magmatic gases from magma sacs. However, the maximum temperature at watering places of the hot spring groups is 82℃, and there is no high-temperature hydrothermal activity.

The hydrogeological conditions in Changbaishan Tianchi volcano are favourable for the formation of high-temperature hydrothermal systems. The presence of high-temperature upward-moving magma in the crust is evidenced by modern volcanic activity and shallow residual magma chambers serve as an ideal heat source for the formation of high-temperature geothermal systems. Although the Changbaishan Tianchi volcano did not form a high-temperature hydrothermal system on the surface, it is possible that a latent high-temperature geothermal system could form in the subsurface. With the support of the project, we analysed the crustal thermal structure and upward magmatic heat state of the region using geothermal methods. We also investigated the possible existence of a hidden shallow high-temperature geothermal system in the subsurface based on data from the field geothermal geological survey of Changbaishan area and the research results of previous researchers.

The shallow thermal structure was calculated by using the Fourier heat conduction equation, based on the two geothermal wells of CR₁ and CR₂ thermophysical parameters data and the regional heat flow data. For depths between 5 km and the depth of the Curie point （D_C）, we calculated the depth of the Curie point （D_C） based on aeromagnetic anomolies data and used geothermal heat flow constraints to calculate the temperature of the Curie point （T_C） , then we calculate the geothermal temperature gradient （dT/dZ）. At last, we obtained the thermal structure between 5 km and the Curie points, combining the temperature at 5 km depth with the calculated values. The shallow density structure of the Tianchi volcano in Changbaishan area was obtained by using the gravitational anomaly genetic-finite cell method. This was achieved through parametric substitution of thermal and density parameters and inversion of the density difference at the stratigraphic interface. Poisson's equation and steady-state Fourier heat conduction equation were found to have formal similarities. The obtained results are as follows:

The temperature at a depth of 2 km below the Changbaishan Tianchi volcanic zone ranges from 66 to 110℃, with an average of 78℃. At a depth of 12 km, the temperature ranges from 313 to 417℃, with an average of 372℃. The depth of the area is shallow, with an average of 12.7 km, and the temperature at this depth is 375℃. The eruption center of Changbaishan Tianchi volcanic and Wangtiane volcano is the uplift area of Curie geothermal surface. The ground temperatures in the shallow and middle layers of the crust are centred on Changbaishan volcano and Wangtianwan volcano on the southwest side, forming a connected high-temperature area in the northeast-southwest direction. The north-south temperature profile reveals a deep crustal-mantle heat source beneath the eruption centre of Changbaishan volcano. As heat transfers from downward to upward, the curie surface slowly uplifts from south to north, forming a low and slow high-temperature uplift at a depth of 12−15 km below the Tianchi volcano. This uplift continuously provides heat upward to the magma chambers.

Gravity inversion was used to study sedimentary strata based on artificial seismic velocity data. The results show that around the eruption centre of Tianchi volcano, the residual densities of the layers at different depth are positively and negatively interspersed. This indicates that the geological bodies of each stratum and the surrounding rocks have a structural pattern of interpenetration in a transversal direction. The average surface density is 1.90 g/cm³, which is consistent with the low-density characteristics of the accumulation rock based on pyroclastic flow formed by multiple eruptions overlying the surface. The average density values of the subsurface layers at various depths are 1.96 g/cm³, 2.21 g/cm³, 2.63 g/cm³, and 2.66 g/cm³, respectively. There is a large jump in the density value between the second and third layers. The density jump zone corresponds to a depth of approximately 3.5−5.5 km （including surface elevation） on the profile. The difference in density between the upper and lower layers is approximately 0.42 g/cm³, which is supposed to be a layer of density step high pressure zone formed during rapid deposition caused by volcanic eruption.

Based on the above calculations, we have analysed that the depth of the Changbaishan Tianchi volcanic area is approximately 8.6 km, shallower than the average depth of the global crust. This indicates that the heat source in this area is shallow and located in a shallow layer of the crust. This finding is consistent with the geological fact that the Changbaishan Tianchi volcano is an active volcano that has had eruptive activities since the Holocene. The upper uplift zone of the Changbaishan Tianchi volcanic area, along the eruption centres of Changbaishan and Wangtiane, is oriented in a northeast-southwest direction. This reflects the influence of deep geodynamic processes on the thermal state of the shallow crust. Due to the scarcity of heat flow measurement points in the Changbaishan Tianchi volcanic area, the curie surface is crucial for studying the shallow thermal structure of the region. It also provides a theoretical basis for investigating the trajectory of underground magma source storage and transport in the Changbaishan crater.

Furthermore, we consider the shallow surface density step high pressure zones to be a crucial factor in the development of the deep latent high-temperature geothermal system of the Changbaishan Tianchi volcano. The pressure and material in the deep part of the eruption centre are instantly released during the eruption of volcanic material on the surface, causing the volcanic neck to instability and collapse. This forms rapid sedimentary tectonic conditions, creating a special density step high pressure zone at a depth of 3.5−5.5 km. The sedimentary fluid between the density step high pressure zone and the high-temperature gas-liquid channel experiences abnormally high fluid stress due to the high pressure of the overlying high-pressure zone and the high temperature of the underlying high-temperature gas-liquid channel, causing the high-temperature and high-pressure thermal fluid to converge along the favourable area at the bottom boundary of the density step high pressure zones, forming a hidden high-temperature geothermal system. The hydrothermal activity of the hot springs beneath the surface is not directly linked to the magma chambers. Instead, it is connected to the high-temperature geothermal fluid through the top interface of the density step high pressure zones.

Main conclusions: ① The geothermal geological and hydro-geological conditions of Changbaishan Tianchi volcano are favourable. Although there is no high-temperature hydrothermal activity on the surface, it is assumed that a high-temperature geothermal system is concealed underground. There is an obvious density step high pressure zone at a depth of 3.5−5.5 km in the area around the eruption centre of Tianchi volcano, which is a favourable area for the formation of hidden high-temperature geothermal resources. This area is favourable for the formation of concealed high-temperature geothermal resources. ② The high-pressure zone is crucial to the formation of geopressured–geothermal resources. Anomalous high-pressure zone may only appear during rapid deposition. During the Late Cenozoic era, the volcanic neck of Changbaishan Tianchi volcano area collapsed due to the instantaneous release of deep pressure and material from the eruption centre. This event created the necessary dynamic condition for rapid deposition and the formation of density step high pressure zones. The high-pressure zone caused by the density step is also a significant factor contributing to the absence of high-temperature hydrothermal activity on the surface of Changbaishan Tianchi volcano.