RECARBONATION OF METAMORPHOSED MARLS, JORDAN

The Maqarin area, northern Jordan, hosts some unusual, hyperalkaline (pH = 12.5) ground-waters discharging from thermally metamorphosed bituminous marls which formed through spontaneous, in situ combustion of the bitumen. The groundwaters have evolved geochemically through hydration, recarbonation and sulphatization of high temperature minerals. Mineralogical relations of the carbonate phases were examined by XRD and cathodoluminescence in conjunction with a detailed investigation of stable isotope ratios by Nd-YAG laser microsampling. Carbon-13 contents trace the sequence of alteration reactions, involving high temperature decarbonation of host biomicritic marl, followed by in situ recarbonation of secondary calcium hydroxide and calcium-silicate-hydrates (CSH). Carbonation took place shortly after thermal metamorphism, when non-saturated conditions allowed an atmosphere rich in CO2 from adjacent combustion zones to access reaction sites. Low delta-18(CaCO3), values suggest that the earliest phase of recarbonation took place by reaction with hydroxide at elevated temperatures while later phases formed at cooler temperatures. Variable C-14 activities show that soil CO2 was a component of the later recarbonating atmosphere. Once saturated conditions prevailed in the alteration zones, recarbonation ended and alteration evolved to hydroxide and sulphate dissolution reactions. The recarbonation reactions are a field-scale analogue of recarbonation and C-14 attenuation in cementitious barriers for radioactive waste repositories.