Volatiles in Magmas, Volcanic Gases, and Thermal Waters

Abstract

Behavior of volatiles such as H₂O, CO₂, SO₂, and halogens during degassing of basaltic magmas has been reviewed based on their abundances in volcanic rocks and high temperature volcanic gases. Degassing is controlled mainly by the solubilities of gases in magmas. Analyses of the volatiles in glass inclusions in phenocrysts of erupted rocks can afford important information on the role of volatiles in the pre-eruptive magmas. However, the volatiles are partially or even almost completely degassed from the magmas during their ascent and shallow emplacement, depending on the solubilities. Thus care must be taken to interpret the chemical and isotopic data on the volatiles in subaerially erupted rocks. On the contrary, volatile materials trapped under high pressure in glassy parts of submarine basalts can give a clue to estimate the chemical and isotopic compositions of volatiles in the mantle. Budgets of volatiles between the crust and mantle in the current platetectonic regime suggest an important view that water is now being carried into the mantle from the crust, if the present production rates of volcanic rocks at ridges, hot spots and island arcs are balanced with the subduction rates of the oceanic crust, because of the increased water content of the oceanic crust due to hydration during submarine hydrothermal alteration. Volcanic gases from island-arc andesitic volcanoes, especially those from Japan, are generally rich in H₂O, indicating secondary incorporation of meteoric groundwater during magmatic and post-magmatic stages. It is essential to remove the secondary modification in order to deduce the primary information from the volcanic gas studies. Stable isotopic studies of the volcanic gases and associated thermal waters are powerful tool in this respect. High temperature end-members of the andesitic exhalations have been characterized by the discharge of gases with the isotopic compositions (δD, δ¹³C, δ³⁴S) that are significantly higher than those from the primary basaltic magmas. The δD values of water in high temperature volcanic gases from many andesitic and dacitic volcanoes fall in a narrow range bewteen -15 and -35‰ irrespective of their localities. These characteristics may be related to genesis of andesitic magmas.