RECARBONATION OF METAMORPHOSED MARLS, JORDAN

The Maqarin area, northern Jordan, hosts some unusual, hyperalkaline ( pH = 12.5) ground-waters discharging from thermally metamorphosed bitu minous marls which formed through spontaneous, in situ combustion of t he bitumen. The groundwaters have evolved geochemically through hydrat ion, recarbonation and sulphatization of high temperature minerals. Mi neralogical relations of the carbonate phases were examined by XRD and cathodoluminescence in conjunction with a detailed investigation of s table isotope ratios by Nd-YAG laser microsampling. Carbon-13 contents trace the sequence of alteration reactions, involving high temperatur e decarbonation of host biomicritic marl, followed by in situ recarbon ation of secondary calcium hydroxide and calcium-silicate-hydrates (CS H). Carbonation took place shortly after thermal metamorphism, when no n-saturated conditions allowed an atmosphere rich in CO2 from adjacent combustion zones to access reaction sites. Low delta-18(CaCO3), value s suggest that the earliest phase of recarbonation took place by react ion with hydroxide at elevated temperatures while later phases formed at cooler temperatures. Variable C-14 activities show that soil CO2 wa s a component of the later recarbonating atmosphere. Once saturated co nditions prevailed in the alteration zones, recarbonation ended and al teration evolved to hydroxide and sulphate dissolution reactions. The recarbonation reactions are a field-scale analogue of recarbonation an d C-14 attenuation in cementitious barriers for radioactive waste repo sitories.