Caldera unrest driven by CO2-induced drying of the deep hydrothermal system

C Troise,G De Natale,R Moretti,F Sarno

doi:10.1038/s41598-018-26610-2

Abstract

Interpreting volcanic unrest is a highly challenging and non-unique problem at calderas, since large hydrothermal systems may either hide or amplify the dynamics of buried magma(s). Here we use the exceptional ground displacement and geochemical datasets from the actively degassing Campi Flegrei caldera (Southern Italy) to show that ambiguities disappear when the thermal evolution of the deep hydrothermal system is accurately tracked. By using temperatures from the CO2-CH4 exchange of 13C and thermodynamic analysis of gas ascending in the crust, we demonstrate that after the last 1982–84 crisis the deep hydrothermal system evolved through supercritical conditions under the continuous isenthalpic inflow of hot CO2-rich gases released from the deep (~8 km) magma reservoir of regional size. This resulted in the drying of the base of the hot hydrothermal system, no more buffered along the liquid-vapour equilibrium, and excludes any shallow arrival of new magma, whose abundant steam degassing due to decompression would have restored liquid-vapour equilibrium. The consequent CO2-infiltration and progressive heating of the surrounding deforming rock volume cause the build-up of pore pressure in aquifers, and generate the striking temporal symmetry that characterizes the ongoing uplift and the post-1984 subsidence, both originated by the same but reversed deformation mechanism.

Highlights

Caldera unrest is a complex phenomenon consisting in a departure from the background or baseline behaviour of geophysical and geochemical indicators, such as changes in ground level, seismicity, gravity and degassing due to magma and/or hydrothermal system dynamics[1,2,3]
We discriminate uplift episodes driven by magma intrusion at shallow depth from those driven by deep fluid influx in the aquifers, taking advantage of the exceptional, more than 35-years long, Campi Flegrei caldera (CFc) geochemical record of CO2-rich gases discharged from fumaroles located within the Solfatara crater
Our approach is based on the reconstruction of the thermal evolution and phase changes of the deep hydrothermal system, independently recorded by carbon isotopic fractionation, and on thermodynamic analysis of CO2-rich ascending hydrothermal fluids

Summary

Introduction

Caldera unrest is a complex phenomenon consisting in a departure from the background or baseline behaviour of geophysical and geochemical indicators, such as changes in ground level, seismicity, gravity and degassing (total discharge, CO2 emissions and changes in gas chemistry) due to magma and/or hydrothermal system dynamics[1,2,3]. Large uncertainties in the knowledge of the links between hydrothermal and magmatic processes affect the forecasting of the unrest evolution Such uncertainties in determining the source and likely outcome of the unrest are even larger for calderas than for other volcanoes, essentially because of the stronger influence of hydrothermal systems at calderas[2,3,4]. We discriminate uplift episodes driven by magma intrusion at shallow depth from those driven by deep fluid influx in the aquifers, taking advantage of the exceptional, more than 35-years long, CFc geochemical record of CO2-rich gases discharged from fumaroles located within the Solfatara crater Combined with a model-independent description of ground-deformation patterns, and contrary to other recent model outcomes[25], this analysis unravels the non-magmatic nature of the ongoing CFc unrest, due to supercritical excursion of the deep hydrothermal system. Our model-independent approach can be applied widely to determine the source of unrest and its temporal evolution at other calderas

Methods

Results

Conclusion