Boron isotopic variations in fumarolic condensates and thermal waters from Vulcano Island, Italy: Implications for evolution of volcanic fluids

Temporal variation in the isotopic composition of boron has been monitored in fumarolic condensates collected over an extended time period (1970-1996) from La Fossa crater, Volcano Island. We also report comparative boron isotopic data for representative Vulcano lavas and for shallow hydrologic samples (seawater, wells, thermal springs) from the north flank of La Fossa. Combined with concurrent chemical and isotopic (δ¹⁸O, δD) data for the fumaroles, these results indicate that the fumarolic fluids record mixing relations between three distinct fluid end members: (1) a dominantly magmatic fluid (EM1); (2) a mixture of modified seawater with magmatic fluid (EM2); and (3) an aqueous fluid produced from seawater by extensive wall-rock reaction, evaporation, and boiling (AF). Differences between the latter two end members are most clearly accentuated on the basis of the boron isotopic data. Long-term compositional variations for crater fumaroles were dominated by EM1-AF mixing between 1979-88, with progressive decrease in EM1 contribution with time, and by EM2-AF mixing between 1988-96. The exact spatial distribution of these fluid reservoirs remains unclear, but all must have been present throughout the monitoring period to account for the observed variations. Moreover, the combined B-O-H data seem to preclude important contributions from shallow meteoric reservoirs. Marked short-term variations in δ¹¹B closely coincide with episodes of local seismicity, which presumably triggered reorganization of hydrothermal circulation patterns; gradual variations over periods up to 3-4 years are associated with relatively low seismicity during which fluid circulation was likely influenced by effects of mineral precipitation on permeability of the hydrologic system.