Evolution of talc- and carbonate-bearing alterations in ultramafic rocks on Leka (central Norway)
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The thesis focuses on several low-angle fracture zones within the ultramafic section of the Leka Ophiolite Complex, central Norway, along which the original lithology has been completely serpentinized and carbonated. The alteration zones have a core made up of talc-carbonate bearing rocks surrounded by serpentine-carbonate bearing rocks with a sharp contact towards the country rock peridotites. Mineral assemblages of the alterations are controlled by the temperature, pressure, XCO2 and the chemical composition of the protolith. The study is based on petrographic and geochemical analysis of samples from three different alteration zones. To assess the metamorphic evolution of the study area, forward modeling has been carried out using the thermodynamic software Perple_X. Modeling of mineral equilibrium in the SiO2 - MgO - FeO - Fe2O3 - CaO - H2O - CO2 system is used to constrain the conditions during complete serpentinization and carbonation of partly altered peridotites. Conditions during alteration of the country rock peridotites was constrained in the SiO2 - MgO - FeO - Fe2O3 - CaO - H2O system.
The partly altered peridotites consist of the mineral assemblage olivine - clinopyroxene - serpentine - magnetite - brucite and formed at temperatures < 400°C by infiltration of pure H2O fluids. Completely serpentinized rocks with the mineral assemblage serpentine - magnesite - magnetite - dolomite formed at temperatures < 510°C and low XCO2 ( 0.05) by the breakdown of the minerals in the partly altered peridotites. Talc-carbonate rocks formed at static conditions by the breakdown of the serpentine in the previously formed serpentinite rock to form the assemblage talc - magnesite - magnetite - dolomite at temperatures < 550°C and higher concentrations of XCO2 . Carbon isotope values determined for dolomite from carbonate lenses within the talc-carbonate rock yield d13C values of ~5, indicative of a mantle source for the carbon required for the carbonation. Oxygen isotope values dSMOW 18O of ~10.8 - 11.3% together with initial 87Sr/86Sr values between 0.7029 - 0.7063, suggest dehydration of rocks with mantle affinity as a source for the fluids.
The combination of radiogenic- and stable isotopes leads to the interpretation that the source of fluids for the hydration and carbonation of the peridotites is the dehydration of partly hydrated ultramafic rocks. The dehydration most likely occurred during the post-Caledonian extension in the Devonian. Assuming that the temperature conditions estimated for the formation of the talc-carbonate rocks represent minimum temperatures of the fluids at their origin, it is shown that the temperatures are high enough to trigger dehydration reactions of hydrous peridotites occurring in deeper parts of the ophiolite complex. The buoyant fluids could have moved up through the fracture network and reacted with the rocks at shallower depths. High pressure gradients during fluid flow may have resulted in enhanced permeability through hydrofracturing.