Interaction between magmatic fluid and meteoric water at an active volcano

1960年左右，在日本九州中部的久州-柳山火山，测得富马尔水蒸气具有较高的氚和氧-18同位素比值，被认为是岩浆水。在此之后，在1995年潜水喷发前的几十年间，富马林水蒸气的同位素比值逐渐下降，喷发前富马林地区富马林水蒸气和温泉水的同位素含量表明，部分大气降水在伴随水蒸气分离的情况下渗入火山深处，达到水的临界点。超临界大气水蒸气可与来自深部的岩浆水蒸气混合，富马尔水蒸气的氚含量表明，大气降水在火山蒸汽系统中的停留时间在5年左右较短，表明岩浆流体与大气水蒸气的混合速度较快。然而，富马林区温泉水中的氚含量表明，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，而液态水循环系统仅作为一个子系统非常缓慢。喷发前几十年来火山喷发蒸汽中岩浆水含量的下降可能是由于深部岩浆蒸汽管道渗透率的变化所致。渗透率降低导致浅层岩浆蒸汽上移压力下降。如果岩浆流体的压力低于水的临界压力，在岩浆流体管道周围形成的两相带中的大气水蒸气可以进入管道并与岩浆水混合。

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.