Abstract
Mantle rocks exposed to subsurface conditions or hydrothermal fluids react in response to disequilibrium. In nature serpentinization and carbonation are the two prevalent fluid-rock reactions affecting mantle rocks. Listvenites form by the complete conversion of peridotites into Mg-carbonates-quartz rocks by interaction with reactive CO2-bearing fluids. The geochemical and mineralogical complexity of listvenites led to the establishment of different genetic models, however the processes and conditions favouring listvenite formation are poorly documented. Our study focusses on listvenite exposures along the base of the Semail ophiolite mantle in Oman, which have been the focus of drilling Oman Drilling Project Hole BT1B.The present PhD thesis benefited from high-resolution core sampling from Hole BT1B, further complemented by regional sampling. Aiming to better understand the processes of peridotite carbonation, our study combines a multi-scale and multi-technique approach including: fieldwork, microstructural characterization (optical and scanning electron microscopy, Raman spectroscopy, EBSD), whole rock (XRF, ICP-QMS) and mineral geochemistry (EPMA, LA-ICPMS), and isotopic studies (bulk C-Fe-Zn isotopes, C-O isotopes on carbonates).In the Fanja region, listvenites occur as kilometre-scale tabular sheets or smaller lenses interlayering the ophiolite mantle close to or along the basal thrust of the ophiolite. On field, peridotite carbonation defined reactions fronts from serpentinized peridotites, to intensively foliated and carbonated serpentinites at the transition with listvenite. The mantle section mainly consists of (Cpx-)harzburgites, lherzolites and rare dunites. Protoliths show selective enrichments in fluid-mobile Li, Rb, Sr, Cs, Ba and Pb. This compositional signature is typical of basal peridotites in Oman. Peridotite carbonation proceeded by the infiltration of fluids rich in As, Sr, Sb, Ba and Pb, as evidenced by bulk rock and mineral trace element compositions. Spatial disparity in the occurrence of magnesite-rich and dolomite-rich listvenites suggests their formation during different metasomatic events and/or from multiple sources of fluids.The petrological study of carbonated serpentinites and listvenites indicate that serpentinization underwent completion prior to the onset of carbonation. Carbonation progressed through distinctive stages of pervasive rock matrix replacement and veining. Incipient carbonation is texturally ascribed to pseudomorphic replacement of serpentine veins and the generation of antitaxial carbonate veins. The onset of pervasive carbonation is characterized by the formation of chemically zoned Fe-rich magnesite spheroids and aggregates in the vicinity of antitaxial Fe-rich magnesite veins. This transient stage was followed by extended matrix replacement by Mg-rich magnesite, prior to silicification of the remaining background serpentine. Variations in magnesite compositions indicate changes in fluid compositions, redox conditions and/or fluid access during carbonation.The bulk carbon isotopes compositions of carbonated peridotites in Hole BT1B indicate a strong inorganic component in reactive fluids. This is compatible with fluids sourced from the devolatilzation of clastic sediments underlying the ophiolite triggered by burial during regional convergence. The distribution of oxygen isotope signatures of carbonates across the well suggest carbonation at low temperature (<200 °C) by infiltration of repeated fluid batches at different structural levels over time. The bulk iron and zinc isotope compositions of BT1B listvenites series are among the most variable for mantle rocks considering a single lithological suite. Fe isotopes compositions remained relatively unaffected during successive serpentinization and carbonation. In contrast, a clear correlation between δ13CTC and δ66Zn indicate the preferential mobility and transport of Zn in reactive fluids.