Noble gas geochemistry in seismic (Umbria, Italy) and volcanic (Grand Comore Island, Indian Ocean) regions: New methodologies and implications

The noble gases are among the most powerful geochemical tools in different geological settings. For this reason, they represent one of the most valuable tracers of geochemical processes whose variations can be straightforwardly ascribed to magmatic/crustal dynamics. In this dissertation, noble gas (He, Ne, Ar) geochemistry applied in both seismic and volcanic regions are presented. The PhD research has firstly been devoted to the test on the field of an auto-sampler for high-frequency collection of the water samples in order to be analysed for their dissolved noble gases. Then, I developed a lab-based methodology for the in-vacuum extraction of noble gases from waters collected in the copper tubes. The auto-sampler, named S.P.A.R.T.A.H. (Syringe Pump Apparatus for the Retrieval and Temporal Analysis of Helium), has been installed for a period of 2 months in a seismically active sector in the Umbria region (central Apennines, Italy), and collected fluids from a natural spring for the entire duration of the deployment with the aim of obtaining short-term noble gas data. The proper functioning and successful operation of the extraction system has been tested and verified in the Noble Gas Laboratory at the Istituto Nazionale di Geofisica e Vulcanologia (INGV) of Palermo. In second part of the PhD research, I focused my attention on the high-flux CO2–rich gas emissions localized in the central sector of the Apennines. The sampled gases have been analyzed for their chemical and isotopic composition (e.g., noble gases). Then, I developed a background geochemical model of fluid circulation and secondary chemical processes that occur during the transfer of fluid in the shallow crustal layers. Moreover I recognize a progressive northward decrease of the mantle-derived He degassing at regional scale along the Apennines. Furthermore, this study highlighted that the variable compositions of the gas manifestations discharged across the Umbria region can be best interpreted as a result of the combination of two different chemical processes which are not mutually exclusive: 1) a mixing between a magmatic end-member (VCVD) and a shallow-sedimentary sources, and 2) solubility-controlled fractionation mechanisms taking place upon interaction with shallow subsurface waters. Finally, the project has been also addressed to the investigation of noble gases in fluids of an active volcanic system, Grande Comore Island (Indian Ocean). The noble gases (He, Ne, Ar) from fluid inclusions in peridotite mantle xenoliths coupled to radiogenic components (Sr, Nd, Pb) have been analysed to resolve the mantle source feeding the volcanism. Here I recognized a MORB-type mantle reservoir. In particular, the 3He/4He isotope compositions (up to 7.3Ra) fall in a range that overlaps the MORB mantle signature and the SCLM. The 20Ne/22Ne, 21Ne/22Ne and 40Ar/36Ar isotope ratios plot along a mixing between air and a typical MORB-type reservoir. The Sr-Nd-Pb isotope systematics shows a mixing line between Depleted MORB and Enriched Mantle reservoirs, but for two samples whose higher Sr isotope signatures point towards an EM2 source, showing isotopic similarities with carbonatite rocks from the East African Rift System and central-northern Madagascar alkaline rocks.