Basaltic dykes in evaporites: a natural analogue

Abstract

AbstractWe present rare earth element (REE) data of basalt and salt samples from central Germany where basaltic dykes of Tertiary age crosscut Upper Permian rock and potash salt. The glassy rims of the dykes can be considered as a natural analogue for the corrosion of nuclear waste glass in a salt repository, whereas the REE data from the salt can serve as an analogue for radionuclide migration in salt next to a leaking nuclear waste repository because the light rare earths (LREE) have a geochemical behavior similar to that of some actinides.Our basalt data demonstrate mobility and fractionation of the REE during postintrusive circulation of salt brines. The processes controlling this behavior of the REE were dissolution and reprecipitation of phosphate minerals. The salt data show that a small portion of the REE has left the basalt during postintrusive fluid circulation and migrated into the salt where a strong depletion of the LREE can be observed with increasing distance from the basalt contact. This fractionation is most probably due to precipitation of LREE-enriched accessory minerals such as apatite. In analogy to this, a similar behavior might be expected from actinides such as Am and Cm, which would in the case of a leaking salt nuclear waste repository probably be immobilized when phosphate minerals are present in the backfill material.