Volatiles in Magmatic Systems
The physical and chemical setting of geothermal systems is dominated by waters of surfi- cial origin; nevertheless, the heat sources are believed to be magma-s, and there is also a high probability that magmatic fluids contribute heat and some dissolved components H2s, SO2, CO2, … ) to the modern hydrothermal systems that are tapped for energy. By the same token, epithermal and other fossil hydrothermal deposits may well have received contributions of metals from magmas. Because subsurface zones of magma influence are never directly observed during magmatic activity, the magmatic story is an after-the-fact interpretation of mineral assemblages; and it is an emerging story with many chapters still unwritten. This chapter will develop the basis necessary to deal with the magmatic equilibria responsible for some of the magmatic gases.
For the most part we shall deal with silicic magmas because they are much more commonly associated with both geothermal activity and hydrothermal ore deposits than are mafic ones, but the general calculations described here are relatively insensitive to rock type. It is the minor minerals — especially the titanium-iron oxides and pyrrhotite — that are most critical in this discussion.
At magmatic temperatures many species are associated as neutral complexes rather than ionic compounds. So, for example, HCl, NaCl, and KCl are not ionic, but neutral molecules at temperatures above about 500°C or so (Franck, 1956; Quist and Marshall, 1968). Similarly, the sulfurous gases are present as H2S and SO2 rather than ionized species. As a consequence, reactions at magmatic