Interaction between magmatic fluid and meteoric water at an active volcano

1960年前后，在日本九州中部的久州-伊沃山火山测量到了氘和氧-18的高同位素比的富玛罗蒸汽，被认为是岩浆水。此后，在1995年火山喷发前的几十年里，富马罗蒸汽的同位素比值逐渐下降。喷发前火山喷发区富马罗蒸汽和温泉水的同位素含量表明，部分大气水深入火山下，并伴有蒸汽分离，达到水的临界点。超临界大气蒸汽可与深部岩浆蒸汽混合。富马罗蒸汽的氚含量表明，火山蒸汽系统中大气水的停留时间相对较短，大约在5年左右，表明岩浆流体与大气蒸汽的混合速度较快。然而，富马罗区温泉水的氚含量表明，液态大气水的停留时间很长，超过一个世纪。这表明火山内的蒸汽系统是大气水循环的主要系统，而液态水循环系统只是一个非常缓慢的子系统。火山喷发前几十年间，火山喷发蒸汽中岩浆水含量的减少可能与深部岩浆蒸汽管道渗透率的变化有关。渗透率的降低导致岩浆蒸汽在浅层向上运动的压力降低。当岩浆流体压力低于水的临界压力时，岩浆流体导管周围形成的两相带的大气蒸汽可以进入导管并与岩浆水混合。

Fumarolic steam of high isotopic ratios of deuterium and oxygen-18, which is regarded as magmatic water, had been measured around 1960 at Kuju-lwoyama volcano, central Kyushu, Japan. After that, the isotopic ratios of fumarolic steam had been gradually failing during several decades before the 1995 phreatic eruption.Isotopic contents of fumarolic steam and hot spring water in the fumarolic area before the eruption suggest that part of meteoric water infiltrates deeply under the volcano with accompanied steam separation and reaches to the critical point of water. The super-critical meteoric steam could mix with magmatic steam originated from deep.Tritium contents of fumarolic steam show that a relatively short residence time of meteoric water within around 5 years in the steam system in the volcano, indicating a rapid mixing between magmatic fluids and meteoric steam. However, tritium contents of hot spring water in the fumarolic area suggest a very long residence time of liquid meteoric water over one century. These indicate that the steam system in the volcano is the main system of meteoric water circulation, while the liquid water circulation system is very slow only as a sub-system.The decrease of magmatic water content in fumarolic steam during several decades before the eruption may be owing to the change of conduit permeability of magmatic steam at deep zones. The permeability reduction causes the pressure depression of magmatic steam moving upward in the shallow zones. If the pressure of magmatic fluids falls lower than the critical pressure of water, meteoric steam in two-phase zones formed around the magmatic fluids conduits can enter the conduits and mix with magmatic water.