Volcanic Gases and Hot Springs

A long-standing geochemical problem is the source of water in volcanic eruptions and geothermal systems how much is derived from the magma itself and how much is recycled meteoric water One of the principal and unequivocal conclusions drawn from stable isotope studies of fluids in volcanic hydrothermal systems is that most hot spring waters are meteoric waters derived from local precipitation (Craig et al. 1956 Clayton et al. 1968 Clayton and Steiner 1975 Truesdell and Hulston 1980, and others). 

Most hot spring waters have deuterium contents similar to those of local precipitation, but are usually emiched in as a result of isotopic exchange with the country rock at elevated temperatures. The magnitude of the oxygen isotope shift depends on the O-isotope composition of both water and rock, the mineralogy of the rock, temperature, water/rock ratio, and the time of interaction. 

There is increasing evidence, however, that a magmatic water component cannot be excluded in some volcanic systems. As more and more data have become available from volcanoes around the world, especially from those at very high latitudes, Giggenbach (1992) demonstrated that horizontal shifts are actually the excep- 

CO2 is the second most abundant gas species in magmatic systems. In a survey of CO2 emanations from tectonically active areas worldwide, Barnes et al. (1978) attributed 8 C-values between -8 and -4%c to a mantle source. This is, however, problematic, because average crustal and mantle isotope compositions are more or less identical and surflcial processes that can modify the carbon isotope composition are numerous. A more promising approach may be to analyze the C-content of CO2 collected directly from magmas at high temperatures. 

It is well documented that carbon dioxide in vesicles of MORE is derived from the upper mantle. In island arcs and subduction-related volcanism major portions of carbon may derive from limestones and organic carbon. Sano and Marty (1995) demonstrated that the C02/ He ratio in combination with the 8 C-value can be used to distinguish between sedimentary organic, limestone, and MORE carbon. Using this approach Nishio et al. (1998) and Fischer et al. (1998) concluded 

---

The isotope composition of magmatic volatiles and related isotope fractionation processes can be deduced by analyses of glasses, volcanic gases, and hot springs. The main process that can cause isotope fractionation of volatile compounds is degassing... 

Shinohara H, Giggenbach WF, Kazahaya K, Hedenquist JW (1993) Geochemistry of volcanic gases and hot springs of Satsuma-Iwojima, Japan. Geochem J 4/5 271-286. 

Volcanic gases and hot springs Sedimentary basins Commercial tank gas... 

Hydrogen sulfide occurs in natural gas. It also is found in many sewer gases. It is a by-product of many industrial processes. Trace amounts of dissolved H2S are found in wastewaters in equilibrium with dissolved sulfides and hydrosulfides. It also is found in volcanic eruptions, hot springs and in troposphere. The average concentration of H2S in the air is about 0.05 ppb. 

Sulphur dioxide and sulphurous gases attack basic rocks and glasses superficially at high temperatures (900° C.) with the formation of water-soluble sulphates, chiefly sodium sulphate.3 It is probable that such reaction and the solution of the products in hot springs during the early post-volcanic period explain the origin of alkaline sulphated thermal waters. 

Baskov Y, Vetshteyn V, Surikov S, Tolstikhin I, Malyuk G, Mishina T (1973) Isotopic composition of H, O, C, Ar, and He in hot springs and gases in the Kurile-Kamchatka volcanic region as indicators of formation conditions. Geochem Inti 10 130-138... 

---