The implication of melt inclusion for the millennium eruption of Changbaishan Tianchi volcano.

Changbaishan Tianchi volcano has several eruptions in Holocene,one of which occurred in～1000 AD was the Plinian eruption of large scale(i.e.The Millennium Eruption),and its eruption products,gray comenditic pumice and pyroclastic flows formed by the eruption plume collapse,fell down and distributed in a wide range in Changbaishan area,also in Korean peninsula and north Japan.Through systematic field sampling,thin slices observing under microscope and analyzing,two melt inclusion groups in hosted alkali feldspars from that explosive eruption,theCrater group and the Yuanchi group,were found with quite different colors,shapes and chemical compositions,which has an important significance for disclosing the mechanism of the Millennium eruption.On the basis of EMPA data,the compositions of Crater group are dacite and trachy-dacite,and their host crystals are sanidines,while those of the Yuanchi group are trachy-dacite and rhyolite,and the host crystals are anorthoclases.Compared with the melt inclusions of Crater group,those of the Yuanchi group have the higher SiO_2,H_2O and cl contents,and they are more evolved as well.The Yuanchi group are perhaps the magma fractionation products at the late stage prior to the Millennium Eruption. The two melt inclusion groups provide the evidence for the layered crustal magma chamber and testify the non-monocomposition within it.They were probably trapped and erupted in the same eruption,but in different eruption cycles.The injection of magma from mantle reservoir to crustal chamber resulted in the magma disturbing and mixing from different layers.The volatile exsolved and formed the gradient in the uppermost magma chamber,and the final volatile oversaturation triggered the millennium explosive eruption of Tianchi volcano,which ever had the huge impact for the climate at that time.