天池天文峰全新世火山喷发物长石表面硅膜特征及意义

Features of Silica Glazes on Feldspar from the Holocene Volcanic Eruptive Material at the Tianwen Peak,Tianchi,Changbai Shan and Their Implications

利用扫描电镜和能谱分析研究天池火山天文峰剖面全新世喷发物中长石表面硅膜的结构状态和化学组成,结果显示:天文峰剖面从顶部黑色浮岩向下到暗灰色浮岩中,长石表面发育有不同结构状态的硅膜。硅膜的结构特征有随火山喷发时代越早,长石颗粒表面硅膜越厚,结构越复杂;喷发时代越晚,长石颗粒表面硅膜越薄,其结构越简单的变化趋势,即长石表面硅膜的形态和厚度与火山喷发的时代具有一定的相关性。硅膜的主要化学组成是Si(33.3%～26.6%)Al(9.28%～6.22%),Na(6.08%～4.07%),K(3.84%～2.35%)和Fe(3.82%～1.64%)。化学组成特征范围指示硅膜是硅酸盐溶解沉积而成。硅膜元素之间有随着硅元素含量的增加,铝、钠含量减少,而钾和铁含量变化微弱的关系。天文峰剖面全新世喷发物长石表面硅膜的化学组成和元素之间的相关性类似于Dorn(1998)划分的富碎屑硅膜。硅膜特殊的形貌和化学组成变化反映了它是岩石天然风化、迁移,最后沉淀的客观成因。而Fe元素则可能与微生物作用有关。岩石风化、磨损的粒子和溶解的离子为硅膜的形成提供了物质来源。详细研究硅膜的特征,它将有可能成为从岩石微观方面探讨火山喷发后环境变化和火山喷发期次的一种新方法。

The structures and chemical compositions of feldspar surface silica glazes from the Holocene volcanic eruptive material at the Tianwen peak,Tianchi,Changbai Shan was studied using SEM and EDX.The result shows that silica glazes of varied structures developed on feldspar surfaces in the pumices on the Tianwenfeng cross section,which change in color from black at top downward to dark-grey at bottom.Their structural features vary with the ages of volcanic eruption.Early eruption of volcanic rocks resulted in thicker silica glazes and more complicated structure on the surface of feldspar grains;late eruption formed thinner glaze and simpler structure,i.e.the morphology and thickness of silica glazes on the feldspars are correlated with the time of volcanic eruption.These glazes are made up of Si（33.3%～26.6%）,Al（9.28%～6.22%）,Na（6.08%～4.07%）,K（3.84%～2.35%） and Fe（3.82%～1.64%）.With increasing Si content,the content of Al and Na tend to decrease,while contents of K and Fe have few change.Such a composition indicates that the glaze resulted from deposits of dissolved silicate.And the correlation between the composition of silica glaze and the elements is analogue to that of the glaze rich in fragment determined by Dorn in 1998.The features of these structure and variety of the compositions reflect the processes of rock weathering,migration and final deposit.Fe is probably related to microorganism.Rock weathering,wore grains and dissolved ion provide the glaze formation with material sources.The detailed observing glazes may be a new approach to study post-volcanic eruption settings and temporal process of volcanic eruptions from microstructures of rocks.