Some basaltic and andesitic gases

Abstract

Igneous gases escape from igneous rocks and magmas. They may originate by effervescence or they may bubble through the melt from an external source. In this paper I review data (mostly post‐1963) on H2O, B, CO2, N2, O2, Ne, sulfur gases, Cl, Ar36, Ar40, Br, Kr, I, Xe, Hg, and Rn. The compositions of volcanic gases are compatible with solubility and melt composition for H2O, Cl, and possibly sulfur gases. For other gases there are either inadequate data or conflicting data (CO2). Volcanic gas emitted at surface pressure is generally very rich in H2O because H2O is the principal gas dissolved in most melts. CO2‐rich volcanic gases can be explained by separation of gas from melt at pressures of a few tens of atmospheres and by effervescence of magmas with high ratios of dissolved CO2/H2O (mostly alkaline basalts). There is no single magmatic gas phase. Glassy basalts from the sea floor have substantial concentrations of both active and noble gases. The concentrations of H2O and Cl correlate with the alkali concentration (especially K2O). The recorded concentrations of H2O and CO2 in sea floor basalts are far below saturation at 500 atm. Yet the basalts contain tiny vesicles. It appears likely that magmatic gas of basalts at sea floor pressure is 60% SO2, 20% H2O, 10% H2S, and 10% CO2 at 1200°C. The concentrations of Cl and H2O in basaltic and andesitic glasses trapped in large crystals in pumices from continental margins are about 10 times those of deep sea basalts. Detailed relationships suggest that at least some andesitic magmas develop in vapor‐saturated environments. The restored or potential concentrations of H2O and Cl in andesitic magmas I estimate by allowing for vapor loss during production of andesite from a basaltic precursor. I use the restored concentrations as a basis for estimating the rate of igneous outgassing in island arcs and continental margins. The inferred rates of outgassing are such that the present quantities of gaseous elements in seawater and sediment would be exhaled in 109 to 1010 years for H2O, Cl, Br, S, I, and Hg. The rates of outgassing of CO2 from Kilauea volcano and of SO2 from Pacaya exceed the rate which could be maintained by outgassing of erupted lava. These relations imply the existence of a larger source of these gases than that of the volcanic rocks. Apparently, large subsurface reservoirs of melt discharge gas out of the volcanic vents. It seems probable that volcanic rocks are generally volumetrically small portions of subsurface reservoirs which may be vapor‐saturated. Such a condition has major implications for crustal evolution, the origins of volcanic rocks, eruption mechanisms, and hydrothermal ore deposits.