| dc.description.abstracteng | This study provides a multidisciplinary approach, based on separated single minerals from Neapolitan volcanic products aimed to extend the present knowledge on the evolution of the Neapolitan magma source(s). Of particular interest is therefore to study volcanic deposits with tight stratigraphic and thus temporal control through time. In order to achieve the proposed goal, have been carried out the following research activities:
1) A detail geochemical and isotopic (87Sr/86Sr and 18O/16O) investigations on phenocrysts of the last eruption occurred on Ischia island (Arso eruption, 1302 A.D.) in order to better understand the geochemical features of the mantle source(s) and the shallow processes that affected the magmas feeding the volcanism at Ischia over the past millennia. Advances in laser fluorination mass spectrometry allow to directly link Sr- and O-isotope measurements for small samples with high analytical precision. Indeed, the combined use of radiogenic (87Sr/86Sr) and stable isotopes (18O/16O) allows to discriminate between mantle enrichment and crustal assimilation processes. This activity has provided an improved knowledge of the mixing processes (one of the main trigger mechanisms of Neapolitan eruption) acted in the Ischia plumbing system over the past 3 ka. Results of this study are shown in chapter 2.
2) Isotopic investigations (87Sr/86Sr and 18O/16O - 17O/16O) on handpicked minerals (feldspar, pyroxene, olivine) of pyroclastic products of the Neapolitan volcanic area, aimed at better defining the evolution processes of Neapolitan magmas in a worldwide context, and particularly role and identification of possible assimilants. In this study, have been selected K-trachybasalts of the Solchiaro eruption occurred at Procida island (ca. 20 ka), representing the least evolved and least contaminated magmatic component feeding the Phlegrean Volcanic District activity (D’Antonio et al., 2007), some relevant eruptions occurred at Ischia in the last 10 ka (Vateliero, Cava Nocelle, Arso and Zaro) and in order to cover the entire history of the Campi Flegrei and Somma Vesuvius volcanoes, samples have been selected from all the main periods of activity. Starting from the oldest period, samples from Campi Flegrei have been grouped as follow: 1. Pre-Campanian Ignimbrite samples (>39 ka); 2. Campanian Ignimbrite samples (39 ka); 3. Post Campanian Ignimbrite/Pre Neapolitan Yellow Tuff samples (< 39 and > 15 ka); 4. Neapolitan Yellow Tuff samples (15 ka); 5. Post-Neapolitan Yellow Tuff samples (<15 ka). For the Somma Vesuvius volcanic complex, samples from three main periods have been selected: 1. Pre caldera activity including lavas older than 22 ka; 2. Caldera-forming phase spanning from 22 ka to 79 A.D.; 3. Post caldera activity for product younger than 79 A.D. From these samples, minerals have been separated and purified. No such data were available for Procida, Campi Flegrei and Somma Vesuvius samples. From Ischia’s volcanic activity a first combination between 87Sr/86Sr and 18O/16O has been done by D’Antonio et al. (2013) for latitic magmas of the past 3 kyr (from the Molara, Vateliero and Cava Nocelle centers). Δ17O ratios (δ18O-δ17O) have been analyzed during my PhD for the first time on volcanic samples with the aim to identify with greater certainty the possible assimilants for the Neapolitan magmas. In particular, hoping to solve the fundamental question of whether or not shallow crustal Mesozoic carbonates play a major role in the assimilation process, important issue to better constrain the “shallow” magma reservoirs. Results are presented in chapter 3.
3) Analyses of phenocrysts zonation profiles to assess time scales of magmatic processes at the Campi Flegrei caldera, using Ba diffusion chronometry. Over the past decade, diffusion chronometry in zoned magmatic crystals has become an indispensable tool for recovering the timescales over which magmatic processes occur (e.g., Costa et al., 2008). This method has been successfully used to determine crystal residence timescales and investigate magma recharge rates in various volcanic settings, using Sr, Mg and Li diffusion in plagioclase, Ba diffusion in sanidine, Ni diffusion in olivine and Fe–Mg diffusion in olivine, clinopyroxene and orthopyroxene phenocrysts (e.g. Ginibre et al., 2002; Morgan et al., 2004). Time scales of petrogenetic processes (such as fractional crystallisation, crystal growth and crystal residence times) are critical and important mechanisms for understanding magma emplacement, remobilisation, transport and eruption at active volcanoes as Campi Flegrei are. To this purpose, samples representative of the main eruptive phases of the Agnano-Monte Spina eruption (4.7 ka) have been selected. Feldspar crystals were hand-picked and embedded into epoxy for combined energy-dispersive and wavelength-dispersive electron microprobe analyses. Sanidine crystals from whole rock thin sections were also selected for BaO core-to-rim compositional profiling, focusing on compositional breaks near the crystal rims that possibly record magma mixing processes just prior to eruption. This is the first study at assessing timescales of pre-eruptive processes through diffusion chronometry for the Campi Flegrei products and for the first time have been compared and discussed results from three different analytical techniques: (1) quantitative BaO point-measurements at 10 μm spatial resolution, (2) gray-scale swath profiles from accumulated BSE images and (3) Ba X-ray scans. Results are in chapter 4. | de |
