Temperature induced phase transformations at Mt. Etna (Italy) volcano: implications for the mechanical weakening of the volcanic edifice under magmatic stresses

Mt. Etna (Sicily, Italy) is one of the most active on Earth. Instability of the eastern and southern flanks, in terms of slow spreading and seismic activity of the volcano, is a dominant dynamic processes and is considered as a triggering mechanism of the eruptive activity. Extrusive lava flows were the subject of a number of laboratory studies aimed to investigate the evolution of physical parameters under cyclic loading. However, no temperature effects were investigated and more importantly only the shallow layers (~ 1.5km of depth) is made up of volcanites; the volcanic pile rests upon thick Mesozoic to Middle Pleistocene sedimentary sequences imbricated at the front of the Apennian-Maghrebian thrust belt. A variety of lithologies spanning from poorly consolidated terrigenous sediments to overconsolidated clays makes this sedimentary sequence and enhances the slow spreading detected. The tickest sequences are given from Triassic to Cretaceous carbonatic rocks (limestones and dolomites) and Creataceous to Pleistocene clays. Here we present a new dataset of Uniaxial Compressive Strength (UCS) and P-wave velocity measurements carried out at temperatures up to 800°C performed on both extrusive lava flows from the volcanic pile, a clay-limestone melanges (75 wt% calcite, 15 wt% quartz and 10 wt% kaolinite) and two limestones (95wt% and 99wt% calcite), which well represent the complexity of the sedimentary basement below Mt. Etna volcano. Measurements were carried out in a new experimental servo-controlled uniaxial apparatus allowing to simultaneously load samples at constant strain rate/stress and apply elevated temperatures (up to 1200°C). Results show that UCS values and P-wave velocities measured on the lava flows do not show significant changes up to 800°C, with average values of ~ 160-170MPa. On the contrary, UCS values and P-wave velocities relative to the marly limestone decrease from ~170 to ~90 MPa and from ~5.5 to ~4.0 Km s-1, by increasing temperatures up to 760°C. Chemical and XRD analyses revealed that phase transformations took place over two different temperature intervals. i) From 25 to 600°C kaolinite disappears, as dehydroxylation of phyllosilicates takes place; ii) From 600 to 760°C decarbonation process occur, which shrink the calcite content from ~75 to ~11 wt%. Above 600°C, CaO produced during ii) reacts with silica and atmospheric water, forming ~37 wt% of larnite (Ca2SiO4) and ~27 wt% of portlandite (Ca(OH)2), respectively. Preliminary results on limestones reveal that drop of velocity from ~3.5 and ~4.5 km s-1 respectively to ~1.5 and ~2.5 km s-1 , by temperatures up to 800°C. The bulk of decarbonation occurs after 700°C. The main implication of this study is that temperature induced phase transformations trigger weakening of the sedimentary basement and can promote instability processes at stresses much below the ones expected.