Nicolas Rividi's PhD | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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Early and present Geobiosphere

  Nicolas Rividi's PhD

Diagenesis and hydrothermalism of archean oceanic sediments (3.5 Gyr, Dresser Formation, Pilbara Craton, Australia): A window on early terrestrial ecosystems
Encadrant (et co-encadrant) : 
Résumé: 

The characterization of Archean environments and ancient life is still controversial. To address these questions, two stratigraphic drill cores were performed thought the 3.495 Gyr Dresser Formation (Australia) that consist of metakomatiitic oceanic floor, over-layered by hydrothermal barite–sulphides early deposits and quartz– ankerite sedimentary sequence. All these rocks were widely affected by hydrothermal circulations (100–200°C, pH ~5.5–6; Fe–Mg) associated with Si, K, Al & Ba metasomatism. However, some organic matter (OM), iron oxides and tiny inclusions of quartz, calcite and siderite have been preserved within ankerite core from sedimentary layers and could reflect marine Archean environments. Siderite precipitates early whereas calcite is an exsolution. Siderite composition reflects a partial dissolution associated to iron-oxides precipitation before to be trapped inside ankerite. These equilibriums are closely linked to ƒO₂(g) and carbonates could register redox state during their precipitation. Isotopic study (macro- and micro-scale) of OM show two δ13Corg pools: one at -35‰ (methanogenesis or Fischer-Tropsch-type reaction) the other at -15‰ (anoxygenic photosynthesis). The study of Dresser carbonates is crucial to understand the occurrence of some extraterrestrial carbonates. Raman spectroscopy is a powerful device able to analyze in situ, chemically and structurally, carbonates–OM–iron oxides assemblages on planetary surfaces. 

Date de soutenance: 
Wednesday 29 September 2010 - 14:00
Jury: 
Roger Buick (Washington University)- Reviewer; Olivier Beyssac (UPMC) - Reviewer; François Guyot (Paris Diderot University); Laurent Richard (AMPHOS 21)