摘要
长白山天池火山是一座巨型活火山,在近代至少发生过数次喷发.虽然近年来没有再喷发,但是有关天池火山再次喷发的争论引起学界广泛关注.岩浆系统的研究是火山灾害和预测的基础,主要包括地球物理和岩石学两方面.综合近年来天池火山岩浆房地球物理探测发现,普遍认为在火山区中、上地壳存在分布范围较广的岩浆房,而在中地壳至下地壳也存在范围较为集中的岩浆存储区.鉴于地球物理方法具有多解性和不能给出岩浆成分信息等的局限性,本文用岩石热力学模拟来约束岩浆房的成分和深度.结果显示:(1)天池火山玄武岩在岩石圈地幔或更深处不能经历斜长石的分离结晶,可能只经历少量橄榄石的分离结晶;(2)造盾低MgO玄武岩是原始母岩浆在地壳(约18~27 km,0.5~0.9 GPa)中演化后喷出的,而不是先前认为的从地幔岩浆房中演化后直接喷出形成的;(3)粗面质岩浆是由碱性玄武岩在15~18 km(0.5~0.6GPa,中上地壳)结晶分异形成;(4)天池火山岩浆系统可能要比先前认为的壳幔双层岩浆房模式复杂,地壳岩浆系统应该主体为玄武质岩浆,酸性岩浆只占很小的比例.这些信息对火山岩浆系统和喷发预测的研究有重要意义.
The depths of magma storage regions are critical for understanding the magma plumbing system and magma evolving processes,and are studied and constrained mainly using geophysical and petrological techniques. In this study,we review the geophysical studies that in recent years,focused on the Tianchi magma reservoirs and find that several studies consistently reported the existence of a wide magma storage region at about 10–15 km depth that narrows down to 15–40 km,under the Tianchi volcano. To provide an independent constraint,we used petrological thermodynamics simulation(phase equilibria) to reconstruct the depth and composition of the Tianchi volcano magma chamber. The results show that:(1) the Tianchi primitive magma may not have plagioclase fractional crystallized in the lithospheric mantle or deeper,and may only experienced a small degree of olivine fractional crystallization;(2) the lower Mg O shield basalts erupted from the magma chamber that is located in the earth's crust(18–27 km,0.5–0.9 GPa),instead of being directly ejected from the lithospheric mantle as it was assumed previously;(3) the trachytic magma is stored at depth of about 15–18 km(0.5–0.6 GPa,in the upper crust) and evolved from alkali basalt;(4) the Tianchi volcanic magmatic system may be more complicated than previously thought. The basaltic magma should occupy the main crustal magma reservoirs,from the lower to the mid-upper crust,while the acidic magma accounts for only a very small proportion and is concentrated in the upper crust.