EFFECTS OF EXPERIMENTAL LEACHING ON RB-SR AND K-AR ISOTOPIC SYSTEMS AND REE CONTENTS OF DIAGENETIC ILLITE

Leaching with 1 N HCl, 1 N NH4Cl, acetone, humic acid extracts, cation-exchange resin and NH4-EDTA have negligible effects on the Rb-Sr and K-Ar systems of different size fractions (<0.4 and 0.8-2 mum) of a 1 M diagenetic illite. None of these treatments, including the 1 N HCl treatment, induced preferential removal of radiogenic Sr-87 and Ar-40 from mineral structures. However, the 1 N HCl treatment removed significant amounts of Sr and REE, as compared to the treatments by NH4-EDTA and cation-exchange resin. However, much of these HCl-derived elements resided in non-clay minerals such as carbonate and Fe-oxide phases. The K-Ar and Rb-Sr ages of the samples were not affected by the HCl treatment any more than by the NH4-EDTA or the cation-exchange resin. Because of potential loss of solid material during elutriation of differentially treated samples, the HCl treatment is preferred over other methods allowing sufficient controls on the isotopic and mass balances between the leachate and the corresponding residue of an untreated sample. For Rb-Sr isotopic dating purposes of clay minerals from sedimentary and metasedimentary rocks, leaching experiments give information about the ambient environment of crystallization. Analysis of both the leachate and residue under carefully controlled conditions is recommended.