GNGTS 2013 - Atti del 32° Convegno Nazionale

Belly” before the mud runs over the shale shakers. A slight vacuum was established to improve gas extraction and to pump the gas into a nearby container for real-time gas analysis. Delay time between gas extraction and analysis was determined with 2 minutes. All data were corrected for this delay. In the container, condensed water was removed from the gas phase. The gas was then analyzed with a quadrupole mass spectrometer (QMS, Balzers OmniStar) for N 2 , O 2 , CH 4 , CO 2 , H 2 , He and Ar. Results and conclusions . The analyzed gas phases mainly consist of CO 2 with lower amounts of CH 4 and local outflow of H 2 S. Highest gas concentrations were observed in the lowermost 30 m. The temperature at 410 m was ~60°C and the thermal gradient was ~0.1°C/m. The total gamma ray emission varies in the range of 80-220 gAPI, with an average at 162 gAPI. The Vp/Vs ratio is similar to the value at the surface and displays an evident reduction between 170 and 210 m reaching values close to 2 below 210 m. Compared to the mean value of Vp/Vs found by Battaglia et al. (2008) that is close to 1.8, the measured values in the hole are very high. The resistivity logs indicate a stratified subsurface. Macroscopic and microscopic investigations together with diffraction data on mud samples allows: a) defining the primary sample lithology; 2) describing the relationships among texture, mineralogy and depth of the drilled rocks; 3) examining the character of the secondary minerals. SEM-EDS analyses of the two cored rocks showed a widespread hydrothermal alteration due circulation of thermal fluids. Sr-isotope data provide constraints to stratigraphic correlations. In particular, the drilled rocks are generally made of mostly pumices and subordinately scoriae with variable crystals and vesicles, and include dense lithic-type grains such as gray lavas and hydrothermal altered clasts. Crystals are up to several mm in size and include plagioclase, pyroxene, biotite, and rare magnetite. We can, therefore, reconstruct a complex pyroclastic sequence emplaced during several episodes of explosive eruptions and through secondary sedimentation in both subaerial and submarine environments. The sequence includes (top downward) i) pyroclastic deposits composed of variably vesicular and porphyritic fragments, ii) a succession of pyroclastic and volcanoclastic beds made up of sub-rounded or rounded vesicular to dense, heterogeneous pyroclastic fragments containing a variable amount of siliceous fossils, carbon, wood fragments and peat, iii) a ~60 m-thick level dominated by brown dense to vesicular glass fragments, iv) a low crystalline greenish tuffs between -270 to -470 m, and v) a basal gray pumice- and -scoriae bearing tuff. The amount of primary crystals is higher in the shallow 260 m and very low porphyricity was detected in the 260-470 m depth range. The marine paleoenvironment is testified by exclusively siliceous fossil remains. Secondary minerals appear and increase in abundance from 320 m; they include pyrite, carbonate and adularia. The secondary paragenesis of cores is more complicated consisting mostly of illite/montimorillonite and glauconite as replacement of primary volcanic glass (Fig. 2a) or dispersed within pumice pipes (Fig. 2b). The cement matrix is made of dolomite in isolated cuspate grains (Fig. 2c) and filling the veins (Fig. 2d). EDS analyses showed a pyrite mineralization surrounding rhombohedral carbonates. Albite is widespread, while sulphides and sulphates are generally rare (< 5% by area). Sr-isotope data are in the range of values detected in Campi Flegrei rocks (Di Renzo et al. , 2011; Pabst et al. , 2011), allowing correlation with outcropping deposits. Textural, mineralogical, petrographic and isotopic results will be corroborated by Ar-dating to reconstruct the stratigraphyic sequence and the history of caldera in this sector. Acknowledgements. This work has been supported by ICDP-CFDDP and PON-MON.I.C.A. projects. The VULCAMED project provided funds to install the XRD laboratory at the Istituto Nazionale di Geofisica e Vulcanologia, in Naples. Thanks are due to key partner Bagnolifutura S.p.A.. We thank the staff of the Laboratorio of Mass Spectrometry at the Istituto Nazionale di Geofisica e Vulcanologia - Osservatorio Vesuviano and in particular I. Arienzo for the kindly laboratory assistance. 267 GNGTS 2013 S essione 1.3