GNGTS 2017 - 36° Convegno Nazionale

188 GNGTS 2017 S essione 1.3 water and magma properties (e.g. density, viscosity) abruptly change, favoring magma eruptability. The marked increment of  87 Sr/ 86 Sr ratios from trachyte (0.707050) to latite (0.70762), strongly suggests the involvement of crustal contamination processes. Particularly the highest values are observed in separated matrix-glasses respect to sanidine and clinopyroxene crystals implying that contamination is a later process occurred mainly after minerals precipitation. The potential influence of assimilation in the petrogenesis of the analyzed Pomici di Base rocks was tested using the EC-AFC (Energy-ConstrainedAssimilation and Fractional Crystallization) model. Best fit was obtained considering a magma contamination by limestone rocks at an ambient temperature of 300 °C (depth 6 km, De Lorenzo et al. , 2006). The results show that the observed Sr and Nd isotopic variation is justified by the ingestion of 2-4 % of carbonate rocks by a magma, which has crystallized for about 55 % of its initial mass. The carbonatic contamination hypothesis is also supported by the abundance of carbonatic metamorphosed clasts found in the Pomici di Base fallout and PDCs deposits (Bertagnini et al. , 1998 and Landi et al. ,1999). Syn-eruptive processes. During the sustained-column phase of the Pomici di Base Plinian eruption, a large volume of trachytic to latitic products (4.4 km 3 ) was expulsed in some tens of hours with a mass discharge rate of 2-2.5 x 10 7 kg/s. Despite the observed chemical variation towards gas-poor mafic composition, flow rate as well as column height remained stable at high value (Bertagnini et al. ., 1998), thus requiring that additional mechanisms may have acted at sub-volcanic level during magma ascent and magnified the eruption intensity. Our 3D quantitative textural data suggest contrasting ascent histories between trachytic and latitic melts. Particularly, trachytic pumices have high degree of vesiculation (70-80 %) and are characterized by large coalescent bubbles separated by glass walls with thickness near to critical rupture threshold; thus suggesting that bubble growth has occurred for exsolution during decompression up to the achievement of a vesicularity threshold that depends on magma characteristics (expansion rate, melt viscosity, shear stresses, presence of different phases), above which experimental data indicate the occurrence of an abrupt increase in permeability. High throats number density and power law exponents > 3 in CVSDs distributions also support extended bubble coalescence during decompression. Measured VND (Vesicle Number Density) values, and the absence of microlites, indicate that trachytic magma ascent occurred at high decompression rate (4 – 8 MPa/s, on the basis of Toramaru, 2006) in closed degassing regime up to magma fragmentation occurred at a depth of < 40 MPa. On the contrary, the latitic scoriae erupted in the second part of sustained-column phase show low degree of vesiculation (40-50%) and a dominant fraction of small vesicles separated by thicker microlite-rich glass walls. These textural features suggest slow ascent rate favoring outgassing and in turn crystallization of microlites (15-30 vol.% of clinopyroxene and feldspar) and bubble collapse (low porosity value, associated to high bubble interconnectivity). Despite low-porosity and high-crystallinity nature of scoriae, their CVSDs show high VND values and multiple patterns not typical of Plinian rocks but previously recognized exclusively in experimental samples subjected to limestone interaction, interpreted as due to multiple vesiculation pulses triggered by fast release of CO 2 -rich fluids (Blythe et al. , 2015). As our Sr and Nd isotopic data indicate a later limestone contamination process in the petrogenesis of the hotter latitic melts, we suggest that the CVSDs trends can be justified by a delayed bubbles nucleation stage during an ongoing decarbonation process. Skarn recycling is the most plausible contamination mechanism, possibly triggered by the fast evacuation of the more evolved trachytic liquids from the cap of the reservoir, that caused the inception of the collapse of the deeper part of the plumbing system and the digestion/assimilation of stoped skarn blocks into the remnant hotter latitic liquids. In this condition CO 2 bubble formation and growth have to be very fast and vigorous (Deegan et al. , 2011) due to the low solubility of CO 2