Geochemical and petrological investigation of Large Igneous Provinces: implications for the redox state of the Earth’s interior through time and its role on catastrophic volcanic events

The Earth’s interior oxygenation (i.e., its redox state, fo2) is an important issue in geosciences due to its primary role in many processes occurring at depth like diamond formation, metasomatism, partial (redox) melting and consequent mobilization of volatile elements (C, O, H, S) in the form of CO2, H2O, SO2 followed by the scavenging of trace elements (e.g., Sc, V, Hg). The composition of the released volcanic gases, and in turn, the chemistry of the terrestrial atmosphere, reflects changes in the mantle redox state throughout the Earth’s history. These variations are recorded by the Fe oxidation state (Fe2+ or Fe3+ determined by Mössbauer spectroscopy) in redox-sensitive mantle rock-forming minerals like spinel, garnet, clinopyroxene from peridotites and eclogites or trapped as inclusions in lithospheric diamonds. There is evidence that Earth’s mantle and atmosphere experienced simultaneous increasing in O2 level along the geological time, linked by the onset of plate tectonics and magmatism about 3 Ga ago. The eruption of CO2-rich melts like kimberlite, formed through an unusual influx of O2 at the mantle source coincided with the emplacement of large igneous magmatic provinces (LIPs). These events characterized by wide-scale outgassing of CO2 and SO2 related to extensive (redox) melting at mantle depth, had a strong impact on Earth’s habitability, coinciding with the major mass extinction and oceanic anoxic events (OAEs) of the Phanerozoic. The triggered perturbations of the C and S cycles can be recorded at global and local scale in coeval sediments along with anomalous concentration of mantle-derived trace elements like Hg, a geochemical proxy largely used to track LIPs signature in the sedimentary record. The objective of this thesis is to provide a link between the redox evolution of the Earth's mantle and the onset of large magmatic events that led to dramatic climate changes and mass extinction events. In order to have a comprehensive understanding of how Earth’s interior fo2 affects the speciation and extraction of volatiles, natural mantle samples like spinel-peridotites, Archean eclogites and E-type diamonds, representative of different geological setting (continental lithospheric mantle and ancient subduction zones) were investigated. Once established the mechanisms of upper mantle oxidation and modeled the speciation of volatiles like CO2 and SO2 with respect to C and S, I investigated a well-known boundary represented by the Bonarelli level (94 Ma, OAE2), the diagenesis of which is synchronous to the large-scale volcanic activity of Caribbean, High Arctic and Madagascar LIPs. Here, I focused on detecting elements carried by gaseous species, such as Hg, which is a well-known global marker of LIP, but lacks a robust geochemical record and mineralogical assertation to support its mantle origin. The mantle spinel-peridotites from the Hyblean Plateau (Italy) are an example of a strongly oxidized continental lithospheric mantle (up to 1.2 log units ∆FMQ) resulting from extensive interaction with metasomatic subduction-related S-bearing CO2-rich silicate melts. This study reveals that the speciation of the S-bearing fluid crystallized as sulfate or sulfide/native sulfur in olivine fluid inclusions, which are derived from the exsolution of the deeply originated metasomatic agent upon decompression/cooling, is controlled by the local fo2. The use of in situ synchrotron Mössbauer spectroscopy applied for the first time on tiny spinel inclusions in olivine from a Mt. Vulture (Italy) wehrlitic sample has opened new possibilities to retrieve redox conditions of mantle xenoliths by looking at the Fe3+/∑Fe and chemical composition of spinel inclusions. The study of eclogitic diamonds from the Udachnaya kimberlite pipe (Siberia) shed light on the mechanisms of diamond formation and indicates that metasomatism has an insignificant effect on the redox conditions of the eclogites, which can act as an efficient redox buffer over time. In addition, eclogite xenoliths from the V. Grib pipe are the product of an oxidized Archean protolith and suggest that the redox heterogeneities in the eclogitic mantle are likely caused by variations in the composition and Fe3+/∑Fe ratios of the subducted oceanic protolith, the latter inherited by primordial redox heterogeneities at the convecting mantle source. The multidisciplinary investigation of Bonarelli level coeval to LIP emplacement has revealed anomalous Hg concentrations with a deep primitive mantle isotopic signature, suggesting its mobilization through volatile-rich melts during large volcanic eruptions. The results of this multidisciplinary investigation have revealed novel information about the heterogeneous redox conditions of upper mantle over time and space, as well as the crucial role of redox-driven processes in mobilizing volatiles, which have implications for sedimentary and biological processes on the Earth’s surface. Furthermore, this study has raised new questions, some of which have been preliminarily addressed. Given the evidence of a mantle origin for the Hg released in the frame of LIPs, preliminary (and in progress) in situ synchrotron high-pressure and -temperature stability and liquid structure experiments were conducted on the sulfide Hg end-member, i.e., cinnabar (HgS), to understand the mechanism of Hg mobilization from the mantle source. These experiments laid the groundwork for future solubility experiments of Hg in oxidized LIP-related melts, which are crucial for modelling the deep Hg cycle.