New mafic magma refilling a quiescent volcano: Evidence from He-Ne-Ar isotopes during the 2011-2012 unrest at Santorini, Greece

In 2011-2012, Santorini was characterized by seismic-geodetic-geochemical unrest, which was unprecedented since the most-recent eruption occurred in 1950 and led to fear an eruption was imminent. This unrest offered a chance for investigating the processes leading to volcanic reactivation and the compositional characteristics of involved magma. We have thus analyzed the He-Ne-Ar-isotope composition of fluid inclusions in olivines and clinopyroxenes from cumulate mafic enclaves hosted in cogenetic dacitic lavas of the 1570-1573 and 1925-1928 eruptions of Nea Kameni. These unique data on Aegean volcanism were compared with those of gases collected in quiescent periods and during the unrest. The He-3/He-4 ratios (3.1-4.0 Ra) are significantly lower than the typical arc-volcano values (R/Ra approximate to 7-8), suggesting the occurrence of magma contamination in Santorini plumbing system, which would further modify the He-3/He-4 ratio of parental magmas generated in the local metasomatized mantle. The He-3/He-4 values of enclaves (3.1-3.6 Ra) are comparable to those measured in gases during quiescent periods, confirming that enclaves reflect the He-isotope signature of magma residing at shallow depths and feeding passive degassing. A significant increase in soil CO2 flux from Nea Kameni and anomalous compositional variations in the fumaroles were identified during the unrest, accordingly with previous studies. Simultaneously, He-3/He-4 ratios up to 4.0 Ra were also measured, demonstrating that the unrest was due to the intrusion into the shallow plumbing system of a more-primitive He-3-rich magma, which is even volatile richer and less contaminated than mafic magma erupted as enclaves. This new intrusion did not however trigger an eruption.