Novel determination method of geochemical response time for volcanic activity monitoring

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Volcanic activities release elements from magma at depth into shallow underground fluids which seep as hot springs. Helium is an element highly enriched in magma, thus it can be easily detected as magmatic isotope signal in the hot spring water. The magmatic helium signal appears to respond to eruption cycles at some volcanos, but the association between its variation and observed volcanic activities is often indecisive, making it difficult to use as an eruption forecasting tool. In those studies, the temporal variations of the volcanic signals from hot springs were compared with the volcanic activities of time that the samples were collected. However, the volcanic signal may travel for substantial time period underground before seeping from the springs so that the volcanic signature we detect could reflect the magmatic activity of tens of years ago. In order to evaluate the time scale of the possible delay, the proposed project will determine the travel time of the shallow subsurface fluids that carry the magmatic helium signals using radioisotopes of Kr and Ar. Ontake and Unzen volcanos, Japan, were selected as candidate sites because of their relatively recent volcanic activities and abundant helium monitoring records. Radioisotopes of Kr and Ar will be analyzed by Atom Trap Trace Analysis (ATTA) at Argonne National Laboratory and by low level decay counting at University of Bern. The chronological information will be used for re-interpreting the relationship between local geotectonic activities and He isotope records as well as for assessing the possibility of recovering 'fossil' magmatic signatures. This is the first application of these novel tools to a volcanic aquifer system. This work will provide fundamental knowledge to evaluate the feasibility of forecasting volcanic eruptions using geochemical tracers including helium. Two students will be trained in the international fieldwork and laboratory work.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.