The 1990’s thermal crisis at La Fossa volcano (Aeolian Islands, Italy): Systematics of magmatic vapor plume expansion and decay within an active volcano

The La Fossa cone on Vulcano island (Italy), like most active volcanoes, undergoes periodic phases of increased heat and gas discharge unrelated to surface magmatism. Variations in the major component chemistry of fu- maroles during such cycles record the expansion and contraction of a magmatic vapor plume beneath the volcano in response to cycles of mafic magma intrusion several km below the surface and consequent changes in its interaction with surrounding meteoric water/seawater. Using previously unpublished major and minor element fumarole data for the 1990’s thermal crisis at La Fossa in conjunction with a primary mafic magma vapor composition derived from melt inclusion data and silicate melt-vapor partition coefficients, a thermochemical equilibrium model is developed to explain variations in fumarole gas chemistry in terms of subsurface gas-solid reactions during magmatic vapor expansion and dilution from source to surface. These reactions control total S and H2 through the production of anhydrite (CaSO4) and sulfide minerals. Pyrite, bismuthinite and sphalerite deposition, control the minor element composition of fumarole discharges. Most of the Cu is deposited as CuFe sulfides in the core of the expanding vapor plume so that it is not observed at surface. As is observed in fumarole sublimates, Au is deposited at low pressure in response to removal of H2S by sulfide deposition. The thermochemical modelling results coupled with fumarole sublimate compositional data provide a metal zonation model for the interior of La Fossa that may be considered generic for other active volcanoes and their ancient equivalents.