Noble gas isotopes at Deception Island (Antarctica) reveal degassing‑derived eruptions and cosmogenic helium signatures: implications for the current high levels of volcanic activity and geochronology of its eruptive history

Deception Island is one of the most active volcanoes in Antarctica with more than twenty explosive eruptions in the past two centuries. Any future volcanic eruption(s) is a serious concern for scientists, tourists, marine ecosystems and even for global oceanographic processes. Currently, it is highly challenging to carry-out -at least- low frequency volcanic gas monitoring at Deception Island because of the arduous climatic conditions and its remote location. We managed to do it for connecting the information of He, Ne, and Ar isotopes measured in olivine samples of the main past eruptive events with their on-going bubbling emissions at surface. Results reveal (i) ascending primitive magmas outgassed volatiles with a MORB-like He isotopic signature as at present day data; (ii) intensive degassing (fractionated 4He/40Ar*), before the beginning of the main eruptive episodes, as nowadays. All this demonstrates how the pre-eruptive noble gas signals of volcanic activity is an important step toward a better understanding of the magmatic dynamics and has the potential to improve eruption forecasting. We also describe the cosmogenic signal (i.e. high 3He/4He ratios of 56-910 RA) in the crystal lattice of olivine phenocrysts (total fusion in crushed poweder) much higher than the magmatic values previously obtained in the inclusions of the same olivines. The cosmogenic details reveal age ranges of c. 4-6 Ma and c. 4 ka for the pre-caldera and syn-caldera deposits, respectively, thus being the first quantitative geochronological approach to date the formation of Deception Island.