Filippo Mundula

A simplified scheme for the description of textural features in Welded Ignimbrites: the example of San Pietro Island (Sardinia, Italy)

Welded ignimbrites show a large variability of welding intensity and of post-depositional crystallization. According to existing literature,a complete characterization of welded ignimbritic units involves: 1) a classification scheme which includes up to 5-6 categoriesof welding intensity; 2) the definition of the style of post depositional crystallization. This classification can be very time-consumingand well beyond the standards required for a general description of ignimbrite units for regional geological mapping purposes.During the geological survey of the San Pietro Island (Sardinia, Italy) in the framework of the new geological map of Italy on a scale1:50000 (CARG Project), a large number of samples from different ignimbritic units has been collected. The geology of the area is characterizedby Oligocene-Miocene volcanic deposits, which testify for the intense explosive activity with the deposition of many variablywelded ignimbritic units and, subordinately, local effusive activity, mainly characterized by the emplacement of lava coulees. All theproducts derive from the eruption of highly evolved magmas of both calc-alkaline and peralkaline affinity.We present here a new simplified scheme for the facies classification of these ignimbritic deposits. The scheme is based on threemain textural features: the aspect ratio of the fiammae, the degree of glass shards alignment and the style of crystallization of the matrix.All these features can be used to describe the ignimbrite units for the scope of their lithological characterization in mapping projects.We suggest that the scheme can have a general interest for the geological mapping of areas dominated by the presence of weldedignimbritic units.

Geochemical and textural constraints on degassing processes in sub-Plinian eruptions: case-study of the Greenish Pumice eruption of Mount Somma-Vesuvius

Plinian eruptions are characterized by high intensity and an overall steady character, and result in a stable convective column. The main processes controlling the dynamics of such steady and stable plume systems have been extensively investigated. Conversely, sub-Plinian eruptions are unsteady, as recorded by the large variability of the products and deposits. Our knowledge of the processes creating this unsteadiness on various timescales remains limited, and still requires more observations as well as theoretical and experimental investigation. Here, we focus on the sub-Plinian eruption of the Greenish Pumice (GP, 19,265 ± 105 BP), Mt. Somma-Vesuvius (Italy). On the basis of coupled geochemical and textural analyses of samples from the well-established stratigraphy of the GP deposits, we investigate volatiles (H2O, CO2, F, Cl) to better constrain the unsteady sub-Plinian eruptive style. This allows us to carry out a detailed study of the degassing processes in relation to the eruption dynamics. We find that degassing by open-system processes generally dominates throughout the entire eruption, but alternates with episodes of closed-system degassing. The fluctuating degassing regimes, responsible for the variable magma ascent rate within the conduit, are also responsible for the eruptive column instability. Volatile behavior is well correlated with textural heterogeneities of the eruptive products. Both reflect higher conduit heterogeneity than for Plinian eruptions, where we find a higher horizontal gradient in magma ascent velocity due to a smaller conduit diameter.