ИСТИНА

Changbaishan Tianchi volcano (known as Paektusan in Korean) on the Chinese–North Korean border is composed of lavas and pyroclastic rocks of the trachybasalt–trachyte–pantellerite–comendite composition. We used our data on melt inclusions in minerals of the rocks to estimate the composition of the melts that produced the trachybasalts, trachybasaltic andesites, comendites, and pantellerites of Changbaishan Tianchi volcano. All of the analyzed melts are rich in alkalis and FeO. The sulfur content of the mafic melts varies from 0.08 to 0.13 wt %. The pantellerite melts are sulfur-free but the groundmass of the pantellerites contains pyrite crystals. Some of the mafic melts are dry (0.1–0.2 wt % H2O), while others contain H2O (1 wt %). The H2O content of the comendite melts varies from 3.1 to 4.8 wt %, and the pantellerite melts contain 0.2–0.6 wt % H2O. The estimated crystallization parameters of the melts as follows from our data of melt, fluid and crystalline inclusions studies correspond to broad temperature and pressure ranges of 1220–700ºC and 3100–1000 bar. The redox parameters ΔlogfO2 relative to the NNO buffer are +0.92 to +1.42 log units for the basalt melts, –1.61 to –2.09 log units for the trachybasaltic andesite melts, –2.63 to –1.89 log units for the comendite melts, and –2.09 to –3.15 log units for the pantellerite ones. The crystallization of the magma is proved to have been associated with dramatic changes in the redox parameters, which depended on the degassing of the magma. Degassing was responsible for variations in the contents of water and sulfur in the melts. As the pressure decreased, the gas phases became progressively enriched in H2O and S, and the S oxidation state was thereby principally changed. Sulfur degassing in the form of SO2 at decreasing pressure should have resulted in Fe3+ reduction to Fe2+. The redox regime was also controlled by the presence of H2 in the system, with hydrogen produced by water dissociation at the pressure decrease. Our evaluations of the oxygen fugacity and certain features in the behavior of volatile components in the magmatic system based on our data on crystalline, fluid, and melt inclusions in minerals of the rocks led us to conclude that the magma of Changbaishan Tianchi volcano evolved in relatively shallow chambers (which occurred at depths of 13 to 3.5 km), when the pressure decreased from 3100 to 1000 bar at temperatures of 1220–700ºC, and with melt degassing continuously changing the redox regime and, correspondingly, the redox conditions of magma crystallization from oxidized to reduced ones.