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Hydrothermal paroxysm at the Colli Albani Volcano

andrea parossismoA group of geologists including Andrea Billi (IGAG) has recently worked on understanding the process of degassing and hydrothermal paroxysm in the volcanically-active area of Colli Albani, Rome .

The main issue addressed in this work is the process leading to fluid subsurface entrapment and pressure increase up to hydro-fracturing and, possibly, to paroxysm in a hydrothermal setting, so to envisage such processes and mitigate their effects in the volcanically-active study area and elsewhere. A field-and-laboratory multidisciplinary approach is used in the fossil (late Pleistocene) portion of an active hydrothermal system (Colli Albani Volcano, Rome, Italy). In this area, sulfate and sulfide mineralizations and strongly altered ignimbrites are exposed. The alteration acme occurs on top of a buried normal fault, where abundant degassing is still active, and fades away in 2-3 km. Based on pervasive vs. discrete alteration styles, mineral assemblages, and further evidence, proximal and distal alteration domains are recognized. Both domains underwent steam-heated advanced argillic alteration with likely temperatures up to c. 400 °C in the proximal domain and less than 150 °C in the distal domain.
The process of hydrothermal alteration progressively and severely depleted many elements from the most permeable rock units, whereas the lowest-permeable unit (Tufo Lionato) underwent fracture and porosity healing accompanied by both mass and volume gain. In the proximal domain, the advanced argillic hydrothermal alteration eventually formed a substantial barrier to fluids.
The hydrothermal fluids accumulated in and below this barrier, which was then suddenly hydro-fractured when heat-driven hydraulic pressure overcame the effective stress, thus possibly leading to hydrothermal paroxysm. The decompression associated with hydro-fracturing enhanced gas exsolution and mineral precipitation from the entrapped.
 
Results on Geological Society of America Bulletin: 

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