Chapter 14 A hydrothermal model for ground movements (bradyseism) at Campi Flegrei, Italy

Abstract

Ground movements (bradyseism) at Campi Flegrei, Italy, have been explained by a classical model that involves the intrusion of new magma to shallow depth, or by models which emphasize both the magmatic and aquifer effects. The authors describe a model for the ground deformations that involves only hydrothermal fluids, of magmatic or meteoric/marine origin, with no direct involvement of the magma, other than as a heat source. They explain the bradyseism at Campi Flegrei by a hydrothermal model, using the porphyry systems (Henley and McNabb, 1978; Burnham, 1979; Fournier, 1999) as an analogue of the Campi Flegrei system. In this view. Campi Flegrei might very well represent a modern analogue of a mineralized porphyry system, as has been demonstrated for White Island, New Zealand (Rapien et al., 2003). The authors used fluid and melt inclusion data from Campi Flegrei and other volcanoes of the Neapolitan area (Vesuvius, Ponza and Ventotene) to demonstrate the linkage with porphyry systems. Fluid inclusions in all the above volcanic systems show clear evidence of various stages of silicate melt/hydrosaline melt/aqueous fluid/CO2 immiscibility during the magmatic evolution and its transition from magmatic to hydrothermal stage, comparable to the plastic, lithostatic domain in porphyry systems. In contrast, convectively driven fluids are found only in the volcaniclastic sediments of the Campi Flegrei caldera (in the geothermal wells of San Vito and Mofete fields), and are representative of the brittle, hydrostatic domain. The coexistence of liquid-dominated and vapor-dominated inclusions in the same fluid inclusion assemblage is strong evidence of boiling conditions during inclusion trapping, whereas fluid inclusions with daughter crystals trapped in samples from deeper, hotter levels indicate a high concentration of solute (brines), as confirmed by drilling. The scenario suggested by fluid inclusion data indicates that the Campi Flegrei system receives an influx of saline water (magmatic+seawater), localized in aquifers at depths of ∼2.5–3 km. The fluids are heated by the underlying crystallizing magma and remain under lithostatic pressure for long periods. The pressure in the upper, apical part of the magma chamber increases as water exsolves from the magma and causes uplift of the overlying rocks (positive bradyseism). When the system ruptures, due to the increasing pressure, the regime changes from lithostatic to hydrostatic, resulting in boiling, hydraulic fracturing, volcanic tremors and finally pressure release leading to deflation of the ground. Afterward, the system begins to seal again due to the precipitation of newly formed minerals and a new phase of positive bradyseism will occur only after several years when the system “reloads” under new lithostatic pressure condition. In this scenario, a hydrothermal eruption can still occur, but only if the fluids pass from lithostatic to hydrostatic pressure when the overlying rocks have a thickness <500 m. If this happens, the hydrothermal eruption could trigger a magmatic eruption, as was probably the case of the Monte Nuovo eruption in 1538 ad.