The corrosion of basaltic dykes in evaporites: Ar-Sr-Nd isotope and rare earth elements evidence

The corrosion of basaltic dykes from central Germany which intruded during the Tertiary into rock and potash salts of Upper Permian age (Zechstein) ha s been studied. The corrosion behavior of these glassy basalts can serve as a natural analogue for the long-term corrosion expected for nuclear waste glasses in a salt repository. Our data demonstrate mobility and fractionati on of the rare earth elements (REE) during a postintrusive circulation of s alt brines. The processes controlling this behavior of the REE were dissolu tion and reprecipitation of phosphates. The K-Ar data document the synintru sive assimilation of a salt phase followed by a postintrusive fluid circula tion in the peripheral parts. This circulation removed the assimilated salt and imported highly radiogenic salt Ar which was integrated into newly for med sheet silicates. The central part of the dyke was not affected by this circulation and could retain a mantle gas phase. The Sr isotope data docume nt a synintrusive assimilation of salt Sr and a strong exchange with salt f luids during the postintrusive corrosion which was strongest in a zone next to the chilled dyke margins. This locally stronger alteration is probably due to different cooling and shrinking rates of the basalt at the margins a nd in the core of the dyke which led to the formation of a highly permeable fractured zone. (C) 1999 Elsevier Science B.V. All rights reserved.