A Study On Magma Degassing Effect Of Holocene Explosive Eruptions Of The Tianchi Volcano, Changbai Mountains

The Changbai Mountains are located on the border between Jilin province, China and DPRK. The parts of the mountains in China territory belong to Antu county of Yanbian Chaoxian Autonomous State and Fusong county of Baishan, Jilin province. With its long eruption history over 2 Ma, the caldera is now filled with water and is called Tianchi ("the sky lake") volcano.This thesis discusses the characteristics of minerals and petrology-chemistry evolution of Tianchi volcanic rocks, magma degassing effect and its disaster effect on environment during Holocene explosive eruptions through heating stage experiments, electron microprobe analyses and Fourier transform infrared spectroscopy for melt inclusions hosted in alkaline feldspars.The studies of magma evolution and magnetotelluric sounding results show that there are two magma chambers beneath the Tianchi volcano, i.e., mantle reservoir and crust chamber. The Tianchi volcano experienced the magma evolution from parental potassic trachybasalt coming from mantle reservoir in the shield-forming stage, trachyte and lately comendite coming from crust chamber in the cone-forming stage after crystallization, to comenditic pumice and some trachytic welded tuff and pumice in Holocene. The magma evolution of the Tianchi volcano was deeply affected by crystallization differentiation, in which fractional crystallizations of olivines, pyroxenes and plagioclases played important roles. Fo values of olivines were between 54 and 70 in the shield-forming stage, and they quickly decreased their Fo values to almost the end member (fayalite) in the cone-forming stage and Holocene eruptions. Augite and plagioclase (labradorite) phenocrysts appeared in the shield-forming stage, while hedenbergite and alkaline feldspar (anorthoclase and sanidine) appeared in the cone-forming stage and Holocene eruptions. Chemical compositions of minerals and major element variations all indicate that the magma evolutionary trend is from MgO- and CaO-rich to SiO₂-, K₂O- and Na₂O-rich. These features, together with characteristics of REE, trace elements and Sr-Nd isotope ratios, suggest that the eruption materials of Holocene, cone-forming stage and shield-forming stage are the products evolved from a same magma reservoir. As the original magma fractionated and increased its alkali content gradually when evolved, therefore alkali phenocrysts appeared in the melt. The evolution of phenocrysts in three eruption stages is in conformity with the evolutionary trend of fractional crystallization.It is well known that there occurred several eruptions of the Tianchi volcano in Holocene. In this thesis, it is suggested that the Holocene eruptions of the Tianchi volcano include three eruption periods according to volcanic strata in Tianwen Peak, i.e., Eruption Period I, 5000 4000 a BP bright yellow comenditic pumice, Eruption Period II ("The millennium explosive eruption" hereafter), 1000 a BP gray comenditic pumice and pyroclastic flow, and Eruption Period III, 300～100 a BP black trachytic pumice and welded tuff. Vesicular pumice and pyroclastics imply the volatile degassing through magma eruptions and lava flows. The thesis aims to study the magma degassing effect of Holocene explosive eruptions of the Tianchi volcano, Changbai Mountains. Among them, Eruption Period II is the study focus for its widespread and characteristic pumice.Melt inclusions are largely found in alkaline feldspars from the millennium eruption of the Tianchi volcano. Most of them have more than one bubble and some of them have daughter crystals. These melt inclusions can be separated into two distinct groups (nominated as "C' and "Y" group in this thesis) according to their characteristics of shape, color and number of bubble. "C" group melt inclusions, collected on the crater rim, brown to yellow, have irregular shapes and lots of bubbles. Some of them devitrified and have daughter crystals, indicating the long cooling time of magma. While the "Y" group melt inclusions, transparent, with no color and daughter crystal, collected in Yuanchi, 30 km east of the caldre, have roughly round shape, implying the equilibrium with host crystal. Electron microprobe analysis data show the two groups of melt inclusions are trachyte and rhyolite respectively and composition of the "Y" group is more evolved than the "C" group according to CaO/Al₂O₃ ratio, which indicates the magma chamber has different compositions prior to the millennium explosive eruption.This work has have experimentally studied the homogenization temperatures of the melt inclusions in host feldspars from these eruptions of the Tianchi volcano in Holocene using Leitz 1350 heating stage performed at Laboratoire Pierre Sue, CNRS-CEA, France. For Eruption I, the homogenization temperatures (T_h) vary from 938℃to 1014℃. There are two T_h intevals for Eruption II: 850℃～926℃and 1005℃～1024℃respectively. And 974℃～1062℃are measured homogenization temperatures for Eruption III. There are not distinct differences between homogenization temperatures in Eruption I and III, but for Eruption II, the things are rather complicated, i.e. there exist two homogenization temperatures groups.Using the by-difference method, it was found that H₂O concentrations in two groups are the same situation as chlorine, i.e., the "Y" group melt inclusions have both high chlorine and water content. Analysed by Fourier Transform Infrared Spectroscopy Nicolet Magna-IR 550, these melt inclusions are found to have high water concentrations of 1.6 %～3.6 %, which are first published data for water contents in melt inclusions using FTIR in China. CO₂ hasn't been detected in these analyses, indicating CO₂ concentration is below the detection limit. Sulphur contents are somehow similar in two groups. It will degas continuously rather than eruption degassing. So the degassing quantity of sulpher for the millennium eruption may also be underestimated. The study indicates that melt inclusions in the "Y" group undergo more sulphur degassing than those in the "C" group, and S/Cl ratios in the "Y" group is much lower than those in the "C" group. According to the data, the alkaline rhyolitic magma concentrated large quantities of water and chlorine prior to the eruption. The strongly degassed volatile made the millennium eruption occur more explosively. Based on the data of total mass of eruption materials, volatile concentration in melt inclusion and matrix glass, calculated S and Cl contents degassed during that eruption reach the magnitude of several to tens of million tons, and degassed water was even higher than S and Cl. The quantity of halogen projected to the atmosphere was rather high and listed the second in documented history.The discover of "C" and "Y" group melt inclusions has important significance for the millennium explosive eruption of the Tianchi volcano and discloses the non-single composition of magma prior to the eruption. The melt inclusion in the "C" group indicates comparatively primitive composition with low volatile (H₂O and Cl), while that of the "Y" group implies the more evolved magma with high volatile. The two melts were trapped in alkaline feldspars when depositing at different levels of the crust magma chamber, and were erupted through different conduits when the millennium explosive eruption occurred.Crystallization differentiation and magma mixing are two important processes in magma evolution of the Tianchi volcano, the former of which determines the bimodal compositional distribution of volcanic rocks, and the latter became the triggering factor for the great eruption 1000 years ago. During the main eruption period of rhyolite magma from late Pleistocene to Holocene, some basaltic trachyandesite, trachyandesite or trachyte magma erupted episodicly throughout the cone-forming stage, which reveals the fact that there was a multi-layer magma chamber system beneath the Tianchi volcano. The trachybasalte influx from the mantle reservoir disturbed the magma layers and caused the magma mixing, then triggered the millennium explosive eruption.