Abstract: The micro features of the pumice and ash produced by the eruptions at 1000a BP and 5000a BP at the Tianchi volcano in the Changbaishan Mountains are studied in the paper. TEM and SEM-EDX analyses reveal the surface characters and thickness of the weathered layers of the volcanic glass and crystal feldspar in the pumice. The result shows that the average thickness of the weathered layers on the surface of the volcanic glass for the first eruption (1000a BP) is 0.98μm. The feldspar has fresh surface and sharp edge angles, some of which present approximately parallel fissures and circle vesicles. For the second eruption (5000a BP) the average thickness of the weathered layers on the surface of the volcanic glass is 3.74μm, and the feldspar has obtuse edge angles and morel basins with 1μm depth. These features indicate that the thickness of the weathered layers on the surface of the volcanic glass and the micro structure of the feldspar from the Tianchi volcano are related with the ages of volcanic eruptions. Therefore, they can be used as indicators for determination of a sequence of volcanic eruptions. The EDX analysis also shows that the weathered layers on the surface of the volcanic glass contain increased Al and reduced Si in chemical composition with respect to the volcanic glass itself. It is inferred that such a change should have occurred in a low-temperature setting and a nearly neutral to weak acid solution. Hence it is possible to identify the dates and sequences of volcanic eruptions as well as their environments based on a systematic study of micro characteristics of eruptive materials in the Tianchi caldera.

Key words: Tianchi volcano, volcanic glass, feldspar, weathered layers

摘要： 研究了长白山天池火山1000年和5000年前2次大喷发产生的浮岩和火山灰的微观特征,通过TEM-EDX和SEM-EDX分析了浮岩中的火山玻璃和长石晶屑的表面特征及风化层厚度。5000年前大喷发浮岩中的火山玻璃风化层平均厚度3.74μm,1000年前大喷发为0.98μm;5000年前大喷发浮岩中的长石表面风化溶蚀度大于1000年前大喷发物中的长石;两次大喷发浮岩中的火山玻璃风化层化学组成与火山玻璃相比富Al和Fe而Si减少。综合上述特征认为,天池火山喷发物中矿物的微观特征的差异与火山喷发年代和喷发环境有关。因此,系统研究天池火山喷发物的微观特征是十分必要的,并且具有一定的应用价值。

关键词: 天池火山, 火山玻璃, 长石, 风化层