Fri, Nov 13
14:00
Amphi
Visio
The Earth’s mantle has recorded more than 4.5 billion years of evolution, from accretion to mantle convection and the recycling of surface materials. This complex history has produced a heterogeneous mantle, expressed notably through the isotopic diversity of terrestrial magmatism. Noble gases, chemically inert and atmophile, are privileged tracers of these heterogeneities and of mantle evolution. This thesis investigates the nature of the mantle using noble gases across three intraplate magmatic settings: the Icelandic plume, the oceanic lithosphere of the Comoros, and the subcontinental lithosphere of the Massif Central.
The Icelandic plume preserves a primordial mantle component inherited from the early stages following accretion, providing insights into early processes such as the origin of volatile elements. Single-vesicle analyses of an Icelandic subglacial pillow-lava yield a mantle source 20Ne/22Ne ratio of 12.63 ± 0.14, indistinguishable from that of the Galápagos plume. Both signatures are also indistinguishable from that expected for implanted solar wind (ISW), supporting this model for the origin of terrestrial light volatiles.
The Fani Maoré basanites (2018-2021 eruption, Mayotte) are particularly valuable samples because they are pristine and gas rich. Their He-Ne-Ar signatures are compatible with a DMM-type upper mantle. Together with the absence of detectable plutonium-fission-derived xenon, these signatures argue against a deep mantle plume contribution despite the lithophile isotope signatures. Low-degree melting of ancient oceanic lithosphere, metasomatized by recycled carbonate-rich melts, is proposed to explain the regional volcanism.
In the Massif Central, carbo-gaseous springs from Vichy to Mont-Dore display He-Ne signatures compatible with a subcontinental mantle source, with an increasing crustal contribution northward and no typical plume signature. Radon and thoron measurements further reveal distinct regimes of crustal fluid circulation and storage, associated with extreme variability in noble gas signatures at small spatial scales.