火山灰全岩与原位分析差异:以四海龙湾记录的1600年前金龙顶子火山喷发为例
Differences between Whole Rock and Insitu Analysis on Tephra: Evidence from the 1600 a cal. BP Eruption of Jinlongdingzi Volcano

火山灰地层学主要依靠火山玻璃的成分进行源区的识别，然而，当火山玻璃成分不均一时就会对源区识别造成影响.四海龙湾玛珥湖位于东北新生代龙岗火山区内，其湖泊沉积物可以记录周边的火山喷发，电子探针等方法得到的火山玻璃成分可以揭示其喷发来源.同时，玛珥湖的纹层年龄可以限定其喷发年龄.在四海龙湾沉积物(钻孔2008)的78~79cm岩芯处出现一层火山灰层，其纹层年龄为AD 308a，电子探针结果显示火山玻璃具有不均一的主量元素组成，为玄武质粗安岩到碱玄质响岩.FeO、TiO2及Al2O3等与MgO协变图解更清楚地显示了此次火山喷发的火山玻璃成分的不均一性.火山玻璃与火山灰全岩对比分析显示，全岩分析在火山灰地层学应用中容易造成对比偏差.
Tephra layers with unknown ages are usually identified and fingerprinted by their glass compositions.But,such correlations may be problematic when they own heterogeneous glass compositions.Lake Sihailongwan locates in the Longgang volcanic field and its varved-lacustrine sediments can preserve the volcanic products from surrounding areas.Additionally,its varve-chronology can provide an advantage chronological framework for recent geological events which are difficult to date by other dating methods.A core drilled from Lake Sihailongwan (core 2008) shows that there is a tephra layer at 78-79 cm and an age of AD 308 was assigned to it by varve-chronology.The volcanic glass exhibits a heterogeneous composition,ranging from basaltic trachyandesite to tephriphonolite.Such a heterogeneous characteristic of glass shards from this tephra layer can be clearly revealed by the biplots of FeO, TiO2 and Al2O3 vs. MgO.For the applications to the tephrostratigraphy,whole rock analysis of tephra may be wrong and could not reflect the heterougeneous of primary magma while composition of glass is a better choice