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

Abstract

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 3 He/ 4 He ratios (3.1-4.0 Ra) are significantly lower than the typical arc-volcano values (R/Ra $ 7-8), suggesting the occurrence of magma contamination in Santorini plumbing system, which would further modify the 3 He/ 4 He ratio of parental magmas generated in the local metasomatized mantle. The 3 He/ 4 He 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 CO 2 flux from Nea Kameni and anomalous compositional variations in the fumaroles were identified during the unrest, accordingly with previous studies. Simultaneously, 3 He/ 4 He 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 3 He-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.

Chemical and isotopic features of cold and thermal fluids discharged in the Southern Volcanic Zone between 32.5°S and 36°S: Insights into the physical and chemical processes controlling fluid geochemistry in geothermal systems of Central Chile

“…The analytical error and the reproducibility were ±0.05‰ and ±0.1‰, respectively. The He isotopic ratios (expressed as R/Ra, where R is the Ne ratio were analyzed by separately introducing He and Ne into a split-flight-tube mass spectrometer (GVI Helix SFT) after performing standard purification procedures (e.g., Rizzo et al, 2015). The analytical error was ±1%.…”

The Geothermal Resource in the Guanacaste Region (Costa Rica): New Hints From the Geochemistry of Naturally Discharging Fluids

“…The R/Ra (where R is the 3 He/ 4 He measured ratio and Ra is the 3 He/ 4 He ratio in the air: 1.39 × 10 −6 ; Mamyrin and Tolstikhin, 1984) and 4 He/ 20 Ne ratios were determined at the INGV laboratories in Palermo (Italy) by separately introducing He and Ne into a split-flight-tube mass spectrometer (GVI Helix SFT) after performing standard purification procedures (Rizzo et al, 2015). The analytical error was ±1%.…”

Volcanic Hazard Assessment at Santorini Volcano: A Review and a Synthesis in the Light of the 2011-2012 Santorini Unrest

“…The changes in the chemical compositions of fumarolic gases on Nea Kameni were interpreted to reflect temperature increases in hydrothermal fluids, due to increased magmatic gas supply at depth (Tassi et al, 2013;Parks et al, 2013;Rizzo et al, 2015). The volume increase associated with deformation was attributed to intrusion of magma and associated fluids (Newman et al, 2012;Parks et al, 2012).…”

“…Both volume and gas composition changes may also have been in part tectonic in origin due to stress accumulation on regional faults (Feullet, 2013) caused by flexuring of the caldera block and increasing rock permeability and hence gas emissions (Tassi et al, 2013). Input of new magma has been also proposed on the basis of He isotope ratios (Rizzo et al, 2015). Parks et al (2015), used a 20-year record of GPS and Insar data to reveal a slow (~6 mm/y) subsidence of southern Nea Kameni between 1993 and 2010, followed by unrest-related inflation in 2011-2012.…”

“…2). The ISMOSAV monitoring network recorded changes in the monitored parameters (seismicity, topographic changes, thermal state, chemistry of the hot fluids and soil gases; Tassi et al, 2013), and campaign measurements also detected changes in soil gas fluxes and compositions (Parks et al, 2013;Rizzo et al, 2015). In addition, GPS networks (Newman et al, 2012) and satellite radar observations detected ground uplift at rates approaching 1 cm/month on parts of the Kameni islands (Parks et al, 2012Foumelis et al, 2013;Lagios et al, 2013), with higher (but unmeasured) uplift rates possibly occurring under water.…”