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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
No full textThe 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
Revealing redox variability in the lithospheric mantle: insights from Mg-chromite inclusions in diamonds from the Udachnaya kimberlite pipes
No full textThe study of mineral inclusions trapped within lithospheric diamonds stands as one of the fundamental approaches to directly study the Earth’s interior, enhancing our comprehension of its chemistry and deep geological processes. The inclusions provide crucial insights into the pressure (P), temperature (T), and redox conditions (fO2) occurring during the diamond nucleation and growth. Among the various mineral inclusions observed in lithospheric diamonds, those characterized by a peridotitic mineral assemblage (P-type), frequently exhibit the presence of Mg-chromite (Stachel et al., 2022) that suggests their involvement in redox-driven diamond formation from CO2-bearing melts. Since the current understanding of the Earth’s interior redox state primarily relies on peridotite samples from the shallow upper mantle based on the Fe3+ content of spinels (Ballhaus et al., 1991), investigating the chemistry of spinel mineral inclusions in diamonds would extend the knowledge of the mantle redox state to greater depths.
In this study, we focused on a suite of nine diamonds extracted from the Udachnaya kimberlite pipes, located within the Siberian craton. These diamonds show multiple dark and transparent inclusions with sizes between 30 and 200 μm in diameter. The selected diamonds are polished to expose some of the inclusions. The chemical composition was determined by electron microprobe while textural features were observed by scanning electron microscopy. The Fe3+/ΣFe ratios of both encapsulated and polished inclusions were measured by in situ synchrotron Mössbauer spectroscopy at the ID18 beamline of the ESRF synchrotron (Grenoble, France), employing a 6 x 15 μm2 focused beam.
The analyzed Mg-chromites exhibit FeO contents of 15-17 wt% and Cr# and Mg# of 0.85-0.92 and 0.54-0.61, respectively. The Mössbauer data collected on the Mg-chromites show a large variability with Fe3+/ΣFe ratios ranging from 0.07 to 0.28. Interestingly, variable chemical composition for the Mg-chromites among inclusions trapped in the same diamond along with fO2 ranging within 2 log units below the fayalite-magnetite-quartz buffer suggest possible changes in the chemistry of the diamond growth medium providing, therefore, evidence of redox heterogeneities in the lithospheric mantle underneath the Siberian platform
The oxidation state of iron in Mg-chromite inclusions from lithospheric diamonds: implications for the redox heterogeneities in the upper mantle
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Redox state of Hyblean mantle xenoliths investigated by crystal chemistry, noble gases and fluid inclusions
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Mobility of volatile-bearing magmas in oxidised planetesimals: implications for CO2 loss and storage during accretion
No full textFerric Iron in eclogitic Garnet and Clinopyroxene from the V. Grib Kimberlite Pipe (NW Russia). Evidence of a highly oxidized subducted slab
No full textEstimates of oxygen fugacity of eclogitic rocks are linked to the redox evolution of the oceanic protolith during subduction and its residence in the lithospheric mantle, and, based on knowledge of pressures and temperatures, allow modelling of the speciation of volatile elements and diamond (or graphite) versus carbonate stability. To date, the oxygen fugacity of mantle eclogites has been shown to vary between −6 (Kasai, Congo and Udachnaya, Siberia) and −0.1 (Udachnaya, Siberia) log units (relative to the fayalite–magnetite–quartz buffer, FMQ), linked to the low Fe3+ contents of garnets. In this study, we investigated the Fe oxidation state of coexisting garnet and clinopyroxene hand-picked out of 17 diamond-free high-MgO and low-MgO mantle eclogites (dated at 2.84 Ga) from the Grib kimberlite pipe (East-European platform). Measured Fe3+/∑Fe values range between 0.03 and 0.19 for garnet and 0.18–0.38 for clinopyroxene, the former being higher than what was measured previously in garnets equilibrated at mantle conditions. The Fe3+/∑Fe of the reconstructed bulk rock ranges between 0.10 and 0.15 for high-MgO eclogites and 0.10 and 0.24 for low-MgO eclogites (with uncertainties of ± 0.02 and ± 0.03 in both cases). Thermobarometric calculations result in equilibration pressures and temperatures of 3.0–5.2 (± 0.4) GPa and 720–1050 (± 60) °C for both high-MgO and low-MgO eclogites, slightly lower than previous P–T estimates of mantle eclogites from the Udachnaya kimberlite pipe (Siberian craton). At these conditions, ∆logfo2 (FMQ) calculated using the available oxythermobarometric model varies from −1.7 to −0.6 log units for high-MgO eclogites and from −2.9 to 0.9 log units for low-MgO eclogites. Samples recording ∆logfo2 (FMQ) ≤ −1 log units overlap with North Slave, West Africa and Udachnaya eclogites, with no difference among eclogite types. The average values of −1.2 (± 0.4) log units for high-MgO and −0.6 (± 1.1) log units for low-MgO eclogites suggest different redox conditions of basaltic protoliths during subduction worldwide. Previous geochemical studies on the same rock samples reported evidence of cryptic metasomatism in both garnet and clinopyroxene that we demonstrate being not recorded by their major elements, while modal metasomatism evidenced by the presence of phlogopite as a product of interaction with a kimberlitic melt only affects the MgO of the bulk rock. Therefore, we suggest that high Fe3+/∑Fe ratios for garnet (> 0.10) and for reconstructed bulk rocks in the case of both low-MgO and high-MgO samples cannot be due to metasomatic interaction with an oxidized fluid, but rather are the consequence of Fe3+ redistribution in an unusually oxidized mafic protolith upon metamorphism. Our results highlight the redox variability of eclogites of Archaean age at conditions more oxidized than present-day mid-ocean ridge basalts (MORBs) and imply an oxidizing nature of the convective mantle source where magma was formed with consequent speciation of C in the form of carbonate fluid explaining, therefore, the lack of eclogitic diamonds in V. Grib kimberlite pipe
An updated database of mercury emissions based on remote sensing, geochemical surveys and laboratory experiments
No full textVariations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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