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

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