| Volcanic products recorded abundant information about the physical and chemical processes of magma before eruption, the explosion degree of the eruption, the relationship between magma and groundwater, the emplacement process on the surface (such as lava flow, tephra fall and pyroclastic flow), and the post-eruption weathering process. Therefore, the detailed microstructural research of volcanic products may provide some insights into the magmatic processes before, during and after eruption, which can not be obtained from ordinary macroscopic study. In this thesis, I applied the microstructural analysis, and especially quantitative analysis methods, to the study of volcanology. The microstructures of volcanic products from the Heikongshan volcano, Dayingshan volcano and Ma'anshan volcano of Tengchong volcanic cluster in Yunnan province, Nanlongwan volcano of Longgang volcanic cluster in Jilin province, and Changbaishan Tianchi volcano in Jilin province were studied to reveal their eruption sequences, types, mechanisms as well as the kinematics and dynamics of volcanic eruption of these volcanoes. The results may provide a scientific basis for early warning of volcanic disasters of these volcanoes.1 Research contents and methods of this thesis1.1 Tengchong volcanic lavasThe research contents of Tengchong volcanic lavas involve the analyses of phenocrysts, matrix and xenoliths of the lavas. The procedures are as follows: (1) The collected samples were cut and polished into thin sections, and then were observed under the optical microscope to qualitatively describe their textures and mineral composition. (2) Bulk rock analyses of the lava samples and microprobe analyses of the matrix and typical phenocrysts were carried out to determine the composition types of lavas and phenocrysts. (3) The contents and sizes of lava phenocrysts were measured, and their crystal size distribution (CSD) and CSD parameters were calculated based on the crystal size distribution theory. And then the kinematics and dynamics of magmatic process were discussed based on the obtained results. (4) Statistic analyses of the microlites in the lavas were carried out to get the crystal size distribution (CSD) and CSD parameters of the microlites, and then the characteristics of magmatic process reflected by the results were discussed. (5) The microstructures of the xenoliths in the lavas were observed, and their mineral compositions were analyzed by means of the electron microprobe. And then the equilibrium temperature and pressure of the xenoliths were calculated. Finally,the origin and geological significance of the xenoliths were discussed. 1.2 Pyroclastics from the Longgang Nanlongwan volcanoThe research on the pyroclastics from Longgang Nanlongwan volcano involves the analyses of particle size and morphology. The procedures are as follows: (1) Sieving the pyroclastics by using oscillating sieve, and calculating the distribution, parameters and fractal dimension of the grain sizes. (2) Thin sections of the particles of each size class were made for microscopic observation of their microstructures and mineral compositions. (3) Observing the morphology and mineral composition of particles under a stereoscope, and selecting the representative grains for SEM observation. (4) Scanning electron microscopic (SEM) analysis and spectroscopic (EDX) analysis of the selected particles were carried out to reveal the surface texture and composition of particles. (5) Finally,the characteristics of volcanic eruption were discussed on the basis of particle size distribution, microstructures, surface textures and composition of the pyroclastics.1.3 Tephra fallout from the Millennium eruption of the Changbaishan Tianchi volcanoThe research contents of the tephra fallout from the Millennium eruption of the Changbaishan Tianchi volcano include: (1) Sieving the pyroclastic particles. (2) Photographing the morphologic images of particles of different size classes, and processing the images using the Photoshop software. (3) Using the NIH Image software to calculate the statistical parameters of pyroclastic morphology and discussing the variation of shape parameters with different sizes. (4) Calculating the final settling velocity and the spreading scope of the pyroclastics by using different shape parameters.2 Research results of this thesis2.1 Tengchong volcanic lavasThe following important conclusions can be drawn from the results of chemical composition analysis, microstructural analysis and crystal size distribution (CSD) analysis of the plagioclase phenocrysts of the lavas from the Heikongshan, Ma'anshan and Dayingshan volcanoes in the Tengchong volcanic cluster.2.1.1 Microstructural analysis of Heikongshan lavas(1) According to the bulk rock chemical analysis, the lavas of flow Unit I in the Heikongshan volcano can be assigned to basaltic trachyandesite in composition, while the lavas of Units II~IV are trachyandesite. In addition, some lavas were found to be trachyte in composition, which might be considered to belong to the youngest unit (Unit V) based on the normal sequence of magma evolution of this volcano. These three types of lavas have three different types of microstructures that are represented by different contents and compositions of phenocrysts and groundmasses, which might reflect the changes of physical environment in the magma chamber. (2) The crystal size distribution (CSD) analysis of plagioclase phenocrysts in the lavas shows that the CSD parameters (slope, intercept, characteristic length of plagioclase crystals and residence time) exhibit an undulating variation within the different units, and this may indicate multi-periods of magma replenishment. The mixing of new magma with the residual magma caused the concave up of the CSD curves of plagioclase phenocrysts.(3) According to the aforementioned data, the Heikongshan volcanic eruption can be divided into four periods, and each period had experienced the processes of magma injection, mixing, repose, crystallization and eruption.2.1.2 Comparison among Heikongshan, Dayingshan and Ma'anshan trachyandesite The following conclusions can be drawn from the results of the comparison of the compositions, textures and microstructures among the Heikongshan, Dayingshan and Ma'anshan lavas:(1) Bulk rock analyses of the lava samples show that the compositions of these thress lavas can be referred to as trachy basalt, basaltic trachyandesite, trachyandesite and trachyte, but are mainly trachyandesite.(2) The microstructural analysis shows that the Heikongshan trachyandesite has similar contents of pyroxene and plagioclase phenocrysts, but the Ma'anshan and Dayingshan trachyandesite have very low content of pyroxene phenocrysts. The plagioclase phenocrysts in Ma'anshan and Dayingshan trachyandesite have reaction rims that are rarely observed in Heikongshan trachyandesite. Olivine phenocrysts are ubiquitous in the Ma'anshan trachyandesite, but are rarely found in the Heikongshan and Dayingshan trachyandesite.(3) The results above show that Heikongshan trachyandesite and Ma'anshan and Dayingshan trachyandesite have different types of microstructures, different bulk compositions and different contents of phenocrysts, which reflect the different magmatic environments, and might suggest that they were from different magma chambers.(4) The CSD analyses of micro-phenocrysts in Ma'anshan and Dayingshan trachyandesite show that the CSD curves have a good linear shape, indicating that the formation environment of the micro-phenocrysts was relatively stable. It is postulated that the micro-phenocrysts were formed by the cooling effect of wall rocks when the magma rose and stayed somewhere in the crust. The differences of the calculated residence time and the nucleation rates of micro-phenocrysts might reflect the different undercooling degrees of the magma, probably because of the different rise speed of the magma.2.1.3 Xenoliths in Heikongshan lavasThe following conclusions can be drawn from the results of the study on the microstructures, mineral compositions and equilibrium temperature of the xenoliths in Heikongshan lavas: (1) The trachyandesite lavas from the Heikongshan volcano of the Tengchong volcanic cluster, Yunnan Province contain relatively abundant mafic and ultramafic small xenoliths comprising mainly gabbro, pyroxenite and rare lherzolites, all less than 2.5 cm in size.(2) Gabbro xenoliths are characterized by "open" texture represented by relatively abundant vesicles and ground-mass fillings, while pyroxenite xenoliths are characterized by equigranular and poikilitic texture. Their mineral compositions (clinopyroxene±orthopyroxene±plagioclase) are similar to the compositions of phenocrysts in lavas, and their equilibration temperature (1000-1125oC) is consistent with the crystallization temperature of the phenocrysts (998-1108oC). Thus, the textures and compositions of these xenoliths are different from those of lower crustal and mantle xenoliths, and were derived from the magma chamber. Among them, the gabbroic xenoliths came from the crystal-bubble-liquid zone at the top of the magma reservoir, while the pyroxenite xenoliths came from the cumulates in the bottom of magma reservoir.(3) The studied single lherzolite xenolith with porphyroclastic texture has similar mineral compositions to the mantle xenoliths from eastern China, and is considered to be of upper mantle origin. It was brought into the magma reservoir by replenishment magma, and might have experienced a cooling event before eruption.(4) The equilibration temperature and pressure of gabbroic and pyroxenite xenoliths are 1000-1125oC and 0.49-0.99 GPa, respectively, which reflect the conditions of the magma reservoir.(5) The different degrees of fractional crystallization in magma reservoir might cause the formation of local composition layering, while strong convection caused by the entry of replenishment magma might cause the mixing of the layers, resulting in the complexity of the groundmass in the lavas.2.2 Pyroclastics from the Longgang Nanlongwan volcanoThe main results of the microscopic observation of pyroclastic morphology, analysis of particle size, and scanning electron microscopic (SEM) observation of pyroclastic surface textures of the samples from a profile of Nanlongwan volcano, Longgang volcanic cluster are as follows:(1) Microscopic observation of the pyroclastics of the Nanlongwan volcano makes sure that the pyroclasts produced by phreatic, phreatomagmatic and magmatic explosive eruptions have different compositions and morphology.(2) Particle size analysis for the pyroclastics of the Nanlongwan volcano indicates that the particles produced by phreatomagmatic eruption have smaller grain size, poor sorting, and greater fractal dimension. The pyroclasts produced by magmatic explosive eruption, however, have larger grain size, well sorting, and relatively small fractal dimension. The particle size distribution feature may bear a relation to the explosive degree of magma. (3) The SEM analysis for the pyroclastics of the Nanlongwan volcano shows clearly that the degree of magma-water reaction was different at different eruption stages. Exsolved gas and water might all caused an explosive eruption. The angular and porous volcanic particles indicate that the eruption might be caused by gas exsolution. The surface textures of particles, such as cracking, bread-crust structure, cauliflower shape surface and step-like fracture indicate that a strong reaction between the magma and water occurred during the eruption, and that the water vapor was the main driving force for the explosive eruption.(4) The eruption of the Nanlongwan volcano can be attributed to explosive volcanic eruption, which includes the phreatomagmatic eruption advancing to magmatic eruption at the early stage, and the late phreatomagmatic eruption, but dominated by phreatic eruption.(5) The EDS analysis shows that the Mg values tended to increase with the eruption process, but the Si values changed unobviously in the particles, except for some strongly weathered particles, in which the Si values were leached out. The Na values in the particles from the earlier eruption were lost severely. Iron oxide and bauxite had produced some particles owing to weathering.2.3 Tephra particles erupted during the Millennium eruption of the Changbaishan Tianchi volcanoThe quantitative analysis of morphology for the tephra particles erupted during the Millennium eruption of Changbaishan Tianchi volcano yields the following results:(1) The tephra particles erupted during the Millennium eruption of the Changbaishan Tianchi volcano, consist mainly of pumice, which have irregular shapes, varying from very equant to elongate and from subrounded to angular. With decreasing of the particle size, the elongation of particles becomes significant, and the proportion of elongated particles increases from 15.02% to 47.5%. Moreover, the smaller the size of particles is, the more angular the shape of particles becomes.(2) An average shape parameter of F = 0.72 has been obtained through the morphological characterization of the pumice particles. The terminal velocity of the pumice particles erupted during the Millennium eruption of the Changbaishan Tianchi volcano that were assumed to be spherical (F=1) is about 1.52 times the terminal velocity of the pumice particles of F = 0.72, and the terminal velocity of the pumice particles that were assumed to be ellipsoidal (F=0.5) is about 0.89 times the terminal velocity of the pumice particles of F = 0.72.(3) When the wind speed is taken to be 10 m/s, the isopach maps of tephra particles with different shape parameters show that the dispersal range expands with the decrease of the shape parameter F.(4) The results of this study show that the morphology of tephra particles has a great effect on the calculation of their final settling velocity, and hence on the modeling of the dispersion range of the tephra particles. Therefore, great attention should be paid to the morphology of tephra particles in the further research.3 Main innovations of this thesisThe main innovations of this thesis can be described as follows:(1) This study is the first one in China to comprehensively apply microstructural analysis to volcanologic research. In this study, various microstructural analysis methods, such as microscopic observation, SEM and EDX analysis, CSD analysis, grain size analysis, surface texture analysis, and chemical composition analysis have been applied to study the physical and chemical processes of volcanic eruption, and to explore the kinematics and dynamics of magmatic process.(2) This thesis is the first case in our country to calculate the crystal residence time and nucleation rate and to discuss the kinematics and dynamics of magmatic process of active volcanoes through the quantificative microstructural analysis of phenocrysts.(3) The occurrence of mafic and ultralmafic xenoliths in Tengchong lavas has been reported long before, but the characteristics and origin of these xenoliths have never been studied in detail. Therefore, this thesis is the first case to study in detail the microstructures, compositions, equilibration temperature and pressure, as well as the source and origin of the xenoliths in Tengchong Heikongshan lavas. Moreover, it is the first to propose that most of these xenoliths should not be of lower crust or upper mantle origin.(4) It is the first case to study quantitatively the morphology of tephra erupted from Changbaishan Tianchi volcano, to calculate shape parameters (AR, FF, CC and RT) of the particles, and to discuss the effect of the particles morphology on the final settling velocity and diffusion range of volcanic tephra.
|
PDF Full Text Request