Recarbonation of metamorphosed marls, Jordan

Abstract

The Maqarin area, northern Jordan, hosts some unusual, hyperalkaline (pH= 12.5) groundwaters 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 biomic 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 CO 2 from adjacent combustion zones to access reaction sites. Low δ 18O CaCO 3 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 14 activities show that soil CO 2 was a component of the later recarbonating atmosphere. Once saturated conditions prevailed in the alteration zone, recarbonation ended and alteration evolved to hydroxide and sulphate dissolution reactions. The recarbonation reactions are a field-scale analogue of recarbonation and 14C attenuation in cementitious barriers for radioactive waste repositories.