The source region of potassium-rich magmatism in central Italy: Geochemical constraints from melt inclusions in olivine

Subduction and volcanism play a pivotal role in recycling Earth’s materials. They shape the interior and exosphere of Earth, influencing the carbon cycle, long-term climate changes, and the evolution of life. A critical aspect of subduction is the release of melts and fluids from the down-going tectonic slab, which modifies the overlying mantle through metasomatism. This process produces diverse magma compositions, including potassium-rich varieties, as observed in the Italian peninsula over the last 10 million years. By employing state-of-the-art analytical techniques on melt inclusions (MIs) in olivine, this research aims to enhance our understanding of potassic magma genesis, its source characteristics, and mantle heterogeneity driven by subduction-related processes. Chapter 2 details a low-blank analytical method for precise and accurate Pb isotope determination from silicate samples with sub-nanogram quantities of Pb. Resolving Pb isotopes at low concentrations enables high-resolution studies of previously-inaccessible samples, including individual minerals and melt inclusions. Chapter 3 presents element and Sr-Nd-Pb isotope data from olivine-hosted melt inclusions in ultrapotassic volcanic rocks from the Quaternary Roman magmatic province. Diverse melt compositions reveal a chemically and lithologically heterogeneous mantle source region modified by multi-stage metasomatism, with metasomatic veins containing variable amounts of Ca-amphibole, phlogopite, clinopyroxene, and accessory apatite. Chapter 4 focuses on the orogenic lamproites and shoshonites of the Plio-Pleistocene Tuscan magmatic province, using element and Sr-Nd isotope data from melt inclusions in olivine. Italian lamproites are shown to result from the mixing of unusually potassium-rich melts with shoshonitic melts. The potassium-rich melts are inferred to derive from incongruent melting of a metasomatised source containing K-richterite, phlogopite, clinopyroxene, and accessory apatite and Ti oxide, generating peritectic olivine in the mantle source as it melts. For both provinces, the enrichment of rare-earth elements and high-field strength elements in the primitive magmas is governed by accessory minerals, such as apatite and Ti-oxide, in the source. Overall, this dissertation demonstrates the value of olivine-hosted MIs for reconstructing mantle source compositions and early magma evolution, as well as the potential of Sr-Nd-Pb isotopes in MIs for furthering our understanding of crustal recycling in complex geodynamic settings. It also uncovers new aspects of post-collisional and potassic magmatism, including previously unrecognised potassium-rich primary melts and olivines formed through incongruent melting—features that likely have broader relevance beyond the studied region. Future research should aim to further resolve mantle metasomes using MIs, establish clear links to the metasomatic agents and subducted materials from which they originate, and further elucidate the interactions between enriched primitive melts and other mantle components during magma evolution and ascent.