Abstract: Melt inclusions are the tiny silicate drops(usually <300μm)of magma trapped during crystals growing in the magma chamber.As phenocrysts completely enclose the melt,forming pressure vessels that prevent the included melt drop from outgassing during decompression and eruption,the melt inclusions have the potential to preserve the pre-eruptive composition of the melt.If unaltered,melt inclusions represent the major,trace and volatile element chemistries of pre-eruptive magma during the complicated effusive and explosive eruptions.Silicate-melt inclusions provide a unique possibility of reconstructing the chemical composition of a magma at a specific stage of its evolution.They are usually solid in the room temperature.It's the counterreaction of the actual procedure that heats melt inclusions and makes them homogenized.Theoretically,the homogenized melt inclusions are thought to keep magma characteristics when they were trapped.The temperature at which the daughter crystal disappears can be deemed as the temperature when magma was trapped.At present,the heating stage experiments provide the only access to the knowledge of magma before eruption.Based on previous works,it is believed there are at least three eruption periods in Holocene,for Tianchi volcano,i.e.the yellow comenditic pumice of～5000a BP(Eruption Ⅰ),the gray comenditic pumice and pyroclastic flow of～1000a BP(Eruption Ⅱ,i.e."the millennium great eruption"),and the black trachy pumice and tuff of～300a BP(Eruption Ⅲ).There are a large number of melt inclusions found in the alkali feldspars of the pumice and pyroclastic flow,which are different in size and color.We have made experimental studies on the homogenization temperatures of the melt inclusions in feldspars from these eruptions of Tianchi volcano in Holocene using Leitz 1350 heating stage.For Eruption Ⅰ,the homogenization temperatures are from 938℃ to 1014℃.There are two temperature intervals for Eruption Ⅱ:850～926℃ and 1005～1024℃ respectively.And 974～1062℃ are the measured homogenization temperatures for Eruption Ⅲ.There is not distinct difference between homogenization temperatures in Eruption I and Ⅲ,but for Eruption II,the situations are rather complicated,implying that the～1000 AD explosive eruption was triggered by injection and mixing of two kinds of magma with different temperatures.Our results also indicate that small melt inclusions are easy to get homogenized,while the large ones,especially those containing a large number of daughter crystals,are quite difficult to get homogenized.In addition,homogenization temperature well correlate with the size of melt inclusion in the host crystal,as well as with the time when a fast heating rate turns to a slow heating rate,and with whether it's the first time to get homogenized or not.We will get different homogenization temperatures for the same melt inclusion with repeated homogenization processes.Furthermore,the next homogenization temperature will be higher than the former one after repeated heating,indicating the water loss or hydrogen diffusion occurs during repeated heating.EMP analysis also testifies the chlorine loss occurring while the melt inclusions are heated.Generally the T_h is higher than magma temperature.The systematical experiments are aimed at choosing the proper size of melt inclusion and proper heating rate for the heating stage experiment.

Key words: Tianchi volcano, alkali feldspar, melt inclusion, heating stage, homogenization temperature

摘要： 长白山天池火山在全新世曾多次喷发,以往研究结果基本上认定至少有3期喷发物:距今约5000a前淡黄色碱流质浮岩、距今约1000a前灰白色碱流质浮岩和碎屑流以及距今约300a的黑色粗面质浮岩和熔结凝灰岩。喷发物斑晶矿物中含有众多熔体包裹体,形貌特征各有不同。经Leitz 1350高温热台对天池火山全新世3期喷发物长石中熔体包裹体的均一法测温实验,结果表明第1,3喷发期的岩浆温度相差不大,而千年大喷发期较为复杂,揭示当时有2个不同温度段的熔体存在,为大喷发可能由2种岩浆注入和混合而触发的论点提供了线索。实验还证实,个体小的熔体包裹体易于均一,而个体较大的、尤其是含有大量子晶的熔体包裹体则很难均一;均一温度与熔体包裹体大小、何时从较快升温速率变为较慢升温速率、是否为初次升温时测量得出有关。同一颗熔体包裹体多次数升温至均一,每次得到的均一温度不同,而且每升温一次得到的均一温度均比前一次高,证实了均一过程中的确有H扩散的现象。

关键词: 长白山天池火山, 碱性长石, 熔体包裹体, 高温热台, 均一温度