Ductile flow in sub-volcanic carbonate basement as the main control for edifice

25 Limestone in volcanic basements has been identified as a hazard in terms of edifice stability 26 due to the propensity of calcite to decompose into lime and CO2 at high temperatures (>600 27 ̊C), causing a decrease in mechanical strength. To date, such hypotheses have been tested by 28 experiments performed at ambient pressure. The present work determines the mechanical 29 strength of limestone under sub-volcanic conditions of pressure and temperature and 30 evaluates the effect of calcite decomposition. To this end, we use Mt. Etna as a case study, 31 deforming sub-Etnean carbonate samples under tri-axial compression using a Paterson 32 deformation apparatus. We evaluate the effect of thermal decomposition of calcite on sample 33 strength by comparing closed and open systems and measuring the permeability evolution 34 under static conditions. Mechanical and micro-structural observations at a constant strain rate 35 of 10 5 s 1 and at a confining pressure of 50 MPa indicate that the rocks are brittle up to and 36 including 300 ̊C. At higher temperatures the deformation becomes macroscopically ductile, 37 i.e., deformation is distributed throughout the sample. The brittle to ductile transition is 38 accompanied by an irreversible permeability decrease from ~10 to ~10 m between 200 39 and 600 ̊C. We present new evidence that permanent change in permeability is due to ductile 40 processes closing the initial pore space. Samples deformed at temperatures up to 900 ̊C do 41 not contain any decarbonation products. At these temperatures, permeability is sufficiently 42 low to permit CO2 pore pressures to increase, thereby increasing local CO2 fugacity, which in 43 turn strongly limits the decarbonation reaction. We note that, for non-pure calcite rocks, 44 permeability might be sufficient to allow decarbonation reactions to occur. As such, 45 variability in lithologies may slightly influence the efficiency of decarbonation reactions. We 46 conclude that, in a closed system, the instability of Mt. Etna is related to high temperature 47 induced ductile flow of basement limestone rather than chemical / mineralogical changes. 48 This may have important implication for the stability of volcanoes within carbonate-rich 49 basement, as carbonates become significantly weak at high temperatures, which may increase 50 the risk of sector collapse. 51