Sample preparation, technique development and data collection were performed to achieve objective 1-3. In total olivine from 88 scoria and lavas from Italy and the Marianas arc was prepared to be ready for geochemical analyses.
We invested to develop an in-house build heating-quenching stage for quenching the homogenised glass in melt inclusions when the appropriate temperature was reached (1400 °C)(Nikogosian et al., Goldschmidt 2021). For P1 (Objective 1) we have refined precise and accurate Pb isotope analyses as part of the ultra-low blank clean lab procedures and Thermal Ionisation Mass Spectrometry (TIMS) techniques and have evaluated the effect of blank contributions (Bracco-Gartner et al.,2020 Goldschmidt conference presentation).
To identify individual subducted components (Objective 1) we have performed combined Sr-Nd-Pb isotope analyses of melt inclusions from the Italian continental subduction setting and from the Marianas oceanic arc (project 1). For Italy previously characterised individual melt inclusions from the Roccamonfina, Ernici, Latera, Vulsini, Sabatini and Alban Hills volcanoes were analysed (See Koornneef et al., 2019 Nature Communications and Bracco-Gartner et al., 2019 Ecrofi conference and Goldschmidt 2020 Conference abstracts). In addition, we characterised new olivine and melt-inclusions from Torre Alfina, Radicofani, Cupaello, Vulture, Vesuvius, Campi Flegrei and Etna (Bracco-Gartner et al., Goldschmidt 2021 & 2022; Luciani et al., Goldschmidt 2022; Bracco Gartner et al., GCA 2023).
For the Marianas oceanic subduction setting olivine and melt inclusions (major and trace elements and Sr-Nd and Pb isotopes) from 4 islands were analysed (Koornneef et al., 2020 Goldschmidt conference abstract). As expected the melt inclusions data shows significantly more variability in isotope compositions compared to the host lavas especially for Sr allowing us to reveal recycled components. For Nd the analytical error is such that it is more difficult to resolve the variability.
As part P2 (Objective 1-3), we have i) further evaluated the analytical techniques to determine combined C and O isotopes is CO2 inclusions in olivine and quartz and (Luciani et al., Goldschmidt 2020 & Luciani et al., Chemical Geology 2021) ii) we have applied Raman spectroscopy to identify volatile types and quantify the CO2 concentrations in melt inclusion glass (Cordeiro 2020 BSC thesis), iii) we have performed high pressure piston cylinder experiments in combination with SIMS CO2 determination to evaluate melt inclusion homogenisation efficiency as a function of pressure (Nikogosian et al., Goldschmidt 2020 conference presentation), and iv) we analysed Boron isotope compositions and concentrations to look at volatile and sediment recycling in Italy. The Boron concentration are exceptionally high and d11B are low indicating an important role for phengite melting deep in the subduction system. We suggest a two stage process where deep phengite/sediment melts metasomatise the mantle wedge to create veins that melt again to contribute to the volcanism (Luciani et al., 2021 & 2022 Goldschmidt; Luciani et al., 2023, EPSL; Luciani et al., 2024 in prep).
For P3 (objectives 1&3) we complemented the data on the volcanic output within the Italian post-collisional subduction setting with data on sediments from the Apennine accretionary prism collected through fieldwork in May 2021. The sediments have been analysed for major and trace element compositions as well as their Sr-Nd and Pb isotope compositions to evaluate the potential sediment input. Deep sea clay samples that cover ophiolitic seafloor complexes were found to have compositons required to explain the volcanic variability. We have developed a comprehensive two-stage melting and mixing model using the sediment compositions to derive subduction fluxes and quantify the contributing components. Melt and mixing conditions were evaluated as part of objective 1. The effect of changing subduction dynamics from north to south (objective 3) were assessed by using different sediment compositions and melting conditions from north to south (Koornneef et al., Goldschmidt 2022; Schriever et al., Chemical Geology in prep).n) (Nikogosian et al., 2020, Goldschmidt conference presentation).
