Geochemical Clues on the Processes Controlling the 2005-2014 Unrest at Campi Flegrei Caldera, Italy

Abstract:

The understanding of the mechanism which triggers unrests at active calderas is one of the most problematic issues of modern volcanology. In particular, magmatic intrusion vs. hydrothermal dynamics is one of the central questions to understand the signals of several restless calderas of the Earth, including, for example, Yellowstone, Long Valley, and Campi Flegrei. Here we focus on Campi Flegrei caldera, sited in the densely inhabited metropolitan area of Napoli, where an inflation stage showing an accelerating trend started in 2005 and reached a maximum vertical displacement of about 24 cm in July 2014.

Fumarolic compositions compared with ground deformation data suggests that this ten year’s accelerating uplift is mainly caused by the overlapping of two processes: (i) short time pulses caused by injection of magmatic fluids into the hydrothermal system, and (ii) a long time process of heating of the rocks.

The short pulses are highlighted by comparing the residuals of ground deformation, fitted with an accelerating curve, with the fumarolic CO₂/CH₄ and He/CH₄ ratios which are good indicators of the arrival of magmatic gases into the hydrothermal system. These two independent datasets show an impressive temporal correlation, with the same sequence of five peaks with a delay of ~ 200 days of the geochemical signal with respect to the geodetic one. The heating of the hydrothermal system is inferred by an evident increase in the fumarolic activity and by temperature-pressure gas-geoindicators. The accelerating ground deformation is paralleled in fact by an increase in the fumarolic CO/CO₂ ratio and by a general decrease of the CH₄/CO₂ratio, both being sign of increased equilibration temperatures. Comparing the observed fumarolic compositions with the thermodynamically derived equilibrium values we infer that the heating is caused by the condensation of increasing amounts of steam.

According to a recent interpretation of fumarolic inert gas species, which relates their relative variation to open system magma degassing, we infer that the heating process reflects both an enrichment in the water content of the magmatic fluids and an increment in their flux. Heating of the rocks caused by magmatic fluids can be a central factor in triggering unrest at calderas.

This work has been done in the frame of the EC project MED_SUV.