Diffusive helium emissions as a precursory sign of volcanic unrest

Significant increases in helium emissions from the soil and ³He/⁴He ratios in groundwater on El Hierro Island (Canary Islands, Spain) were observed prior to the 2011–2012 submarine eruption off the coast of the island. The changes of diffusive helium emissions rate were observed one month prior to the submarine eruption onset (12 October 2011) and the major increase preceded increases in seismic energy release during the volcanic unrest. Measured ³He/⁴He ratios in groundwaters from a well in El Hierro Island increased from 2–3 R_A to 7.2 R_A (R_A = ³He/⁴He ratio in air) 1 month prior to the eruption onset, and reached a peak of 8.2 R_A, indicating a dominant magmatic contribution to the dissolved gases in ground waters. ³He/⁴He values and diffusive helium emission studies have been extremely important for forecasting the onset of the volcanic unrest and subsequent volcanic eruption. An aseismic exsolution of magmatic gases from magma bodies beneath El Hierro Island through fractures and vertical permeability structures increased the diffusive helium emission rate prior to episodes of seismic energy release associated with the volcanic unrest.