Investigating melt inclusions: insights into elemental recycling and boron isotopic signature from the Italian post-collisional setting

This research investigates the role of melt inclusions in understanding elemental recycling and boron isotopic signatures within the Italian post-collisional geological context, particularly in continental subduction zones and their complex processes. Subduction zones, where one tectonic plate is thrust beneath another, are crucial for transporting surface materials into the mantle, affecting Earth's volatile and elemental cycles. This process contributes to mantle heterogeneity and influences the atmosphere through volcanic outgassing, which impacts global climate. Boron isotopes serve as effective tracers for subduction recycling due to their high fractionation at low temperatures and strong fluid mobility, providing significant insights into the fate of subducted materials in both oceanic and continental collision settings. Analysing melt inclusions (MIs) in minerals like olivine offers direct estimates of pre-eruptive volatile concentrations, but interpreting primary melt compositions is challenging due to the migration and mixing of melts and fluids. The study addresses two primary objectives. First, it explores crustal recycling contributions, investigating the primary magmas responsible for compositional diversity in Italy and the distribution of individual subducted components like fluids and sediments within the subduction system. Second, it examines boron isotope signatures, analysing their geographic and temporal variations and identifying the impact and sources of boron within subduction dynamics. Methodologically, the study employs carbon and oxygen stable isotope analysis using a continuous-flow technique that releases CO2 from fluid and melt inclusions for isotope ratio mass spectrometry (IRMS), which allows for precise isotope measurements and helps identify CO2 sources in geological samples. The research focuses on high- potassium magmatic provinces in Italy, using MIs in olivine to understand mantle heterogeneity and the two-stage melting model for boron. The findings reveal significant crustal contributions to elemental recycling in Italian volcanic rocks, particularly those high in potassium, with variations linked to mantle heterogeneity and past subduction events. Volatile components in volcanic inclusions indicate the role of recycled materials in the generation of volcanic 7 gases, with implications for understanding past climate impacts. The mantle beneath Italy shows heterogeneity influenced by both oceanic and continental subductions, as evidenced by variations in major elements, trace elements, and boron isotopic compositions in MIs. Future work should focus on refining techniques for analysing CO2 in MIs and further exploring the dynamics of volatile recycling, particularly in complex subduction settings like Italy. Recommendations include improving methods for analysing small-volume inclusions to better capture primary melt compositions, extending studies to other subduction zones to compare elemental and volatile recycling on a global scale, and investigating the long-term climate impacts of subduction-derived volcanic outgassing.