Crust-mantle interactions and connections with the geodynamic system in the origin and circulation of fluids in the Comoros Archipelago - Indian Ocean

Resumen

The role of fluids in crust-mantle interactions is of crucial importance in providing information regarding the definition of the origin of magmas, the identification of their depths of origin, and the nature of the interaction processes that determine their characteristics. Complex geodynamic systems may therefore have an articulated history of processes that can be reconstructed by studying the fluids emitted in such areas, and by analysing the fluids included in the rocks that characterise these geological regions. In short, we can cite the thoughtful metaphor of Sadao Matsuo, the first leader of the Commission on the Chemistry of Volcanic Gases (CCVG), who referred to volcanic gas emissions as a “telegram from the Earth’s interior”. This research represents an attempt to read and interpret as much as possible from such complex messages with regard to an area of extreme geological pertinence and interest to contemporary volcanic geochemistry. The chosen area of study for the present work focuses on two islands of the Comoros archipelago: Grande Comore and Mayotte, located within the Mozambique Channel within a complicated geodynamic system of great interest due to the currently existing volcanic and seismic activity, of which a complete descriptive picture is currently lacking. In particular, especially with regard to fluid geochemistry, very little knowledge of gas and fluid emissions yet exists. In this sense, it is now even more compelling to understand these characteristics, not only in consideration of the high level of activity of the Karthala volcano in Grande Comore, but also with regard to the volcanic and seismic activity recently recorded at Mayotte Island, which is very close to the recent submarine volcano formed only 50 km South-East off-shore of Mayotte, and by far the largest known submarine eruption until now (Feuillet et al., 2021; Berthod et al., 2021). The specific aim of this study is to investigate the existing outgassing conditions on both islands in order to include this knowledge within a broader and multidisciplinary framework that will facilitate the understanding of the volcanic dynamics of this particular area of the Indian Ocean. Karthala volcano, located on Grande Comore Island, is the most active volcano in the western Indian Ocean after Piton de la Fournaise at La Reunion. Karthala is a basaltic shield volcano which has erupted regularly in the last century; fourteen eruptions are listed from 1904 to today, where the last eruptive activity occurred in 2007 (Bachèlery et al., 2016). Owing to its remote location, it is still under-studied and, in particular, little is known about its diffuse outgassing. The study of the diffuse outgassing of the Karthala volcano, with particular attention to the emission of CO2 from the flanks of the volcano, is therefore crucial for the assessment of the state of activity of the volcano. In addition, as two significant persistent fumarolic fields are present in its summit crater area, both soil and fumarolic gas emission are investigated in terms of gas-geochemistry in this work. Geochronological data recognize Mayotte as the earliest starting point of magmatic activity among the islands of the archipelago, which dates back to at least 10.58 Ma ago. The islands of Mohéli and Anjouan then follow at around 3.9 and 5 Ma ago, respectively, and finally Grande Comore at around 0.13 Ma (Michon, 2016). No recent eruptions have been recorded at Mayotte since the last occurred around 2050 BCE ± 500 (Smithsonian Institution - https://volcano.si.edu). ; however, stable volcanic activity at Mayotte is still present in the form of a large area of subaerial and underwater outgassing near its south-eastern coast at the Dziani Lake, which is situated in the north part of the island on the small island of Petite Terre. Significantly, Petite Terre was recently affected by a seismic crisis that lasted for several months, and was accompanied by the formation of the largest underwater volcano in recent centuries, about 50 km from its coast (see Ship spies largest underwater eruption ever - https://www.sciencemag.org/news/2019/05/; Berthod et al., 2021a, 2021b; Cesca et al., 2020; Feuillet et al., 2021; Lemoine et al., 2020; REVOSIMA, 2019). The thesis is divided into two sections: the first will focus on the Karthala and Petite Terre gas emissions, considering fumarolic fields, soil emissions and bubbling areas, with the purpose of identifying the main characteristics, similarities and differences, of gas chemistry and isotopic variability in the two islands; the second section of the thesis focuses on the difference between the two known bubbling areas at Petite Terre, where I include the further study of the Lake Dziani gas emissions that have been investigated only in the most recent surveys. In this study, I attempt to address the existing gap in knowledge regarding gas geochemistry on the islands focusing specifically on gas emissions, using an approach that combines different research objectives, and fieldwork and laboratory techniques, in order to: 1) define the chemical and isotopic characteristics of magmatic fluids in terms of the major gas components (CO2, CH4, H2S, H2 and H2O) in free gases, noble gases (He-Ne-Ar) and C isotope ratios; 2) measure soil CO2 concentration and identifying their 13C isotope signatures; 3) identify the similarities and differences regarding the volatiles emitted from the main volcanic and geothermal areas in both Grande Comore and Mayotte Islands; 4) conceptualise a geochemical model of the complex geodynamic framework of the archipelago, integrating the gas geochemistry results obtained through the processes entailed in points 1-3; 5) evaluate the impact of volcanic emissions in the local area for monitoring purposes. The success of such an approach has been previously demonstrated with regard to other magmatic systems worldwide, such as Mount Etna (Paonita et al. 2012, 2021), but particularly in relation to La Reunion, which is located in the Indian Ocean in a comparable geodynamic system of intraplate volcanism (Liuzzo et al., 2015; Boudoire et al. 2016, 2018, 2020). In terms of gas geochemistry, the various findings of this thesis converge towards the recognition of some notable peculiarities with regard to the two target islands, which can be summarised according to the following four points: First, the soil CO2 emissions are spatially distributed along the main structural features of both Grande Comore and Petite Terre; however, the carbon isotopic signature of soil CO2 emissions highlights a low magmatic contribution at distal areas of Karthala volcano, and a significantly higher magmatic contribution in CO2 emissions at Petite Terre. This difference may be ascribed to the different states of volcanic activity on the two islands at the time of the surveys. Second, with regard to the helium isotopic signature, the 3He/4He data are within the range of measurements in fluid inclusions of Grande Comore (Class et al., 2005), indicating for the gas emissions a low level of 3He/4He values ( ~6 ≤ Rc/Ra ≤ ~7.5 Petite Terrre; ~4.6 ≤ Rc/Ra ≤ ~5.8 Karthala), if compared with La Reunion signature (~12≤ Rc/Ra ≤~15 Boudoire et al. 2020). Third, the bubbling area on the sea (BAS) and Lake Dziani are likely fed by a common source (about 17 km below Petite Terre); however, Dziani lake is significantly affected by secondary processes that are mainly related to biotic activities in the lake, which result in the higher variability of gas chemistry, 13C in methane and CO2 than BAS. Fourth, the increased value of Rc/Ra between 2008 and 2018-19, and a not-reached isotopic equilibrium of 13CCH4 from the hydrothermal fluid, may be ascribed to the volcanic activity that generated the new submarine volcano 50 km offshore from Petite Terre. This consideration is also consistent with the final interpretation of this work, where the input of heated CO2-rich fluid into the Petite Terre hydrothermal system is a consequence of the perturbation of the shallow plumbing system by the offshore submarine eruption, resulting in higher equilibrium temperatures in 2018 and subsequent cooling down during and after the seismo-volcanic activity. This work is expected to make a significant step forward in the current knowledge of the gas geochemistry of the Comores archipelago, and, in particular, results in a better knowledge of the main characteristics of the emitted volcanic fluids. More importantly, it will assist in the recognition of which geochemical markers may be of potential relevance for volcanic monitoring purposes, thereby improving the understanding of the present state of its volcano activity. This latter aspect is especially important for the Karthala volcano and with regard to the ongoing sub-marine volcanic activity close to Mayotte. This would be of great support for local observation infrastructures and contribute to the improvement of applications in volcanic and environmental monitoring of this populated area. Finally, this work would also provide a valuable case study that may be applicable to other similar contexts worldwide, allowing the definition of a comprehensive model considering volcanological effect and social impacts within the same framework.