K-Ar dating of illitic fractions of Estonian "blue clay" treated with alkylammonium cations

Two clay fractions of a Cambrian claystone from Estonia, consisting essenti ally of illite and 20% expandable illite-smectite, (I-S) were treated with C-12 and C-18 alkylammonium cations for K-exchange. Both the untreated and treated samples were dated by the K-Ar method. The treated clays lost sever al percent of their original K2O, with greater losses for longer-chain cati ons and for longer reaction time, in accordance with previously published s tudies. The dates of the treated clay fractions were 20-30 Ma lower than th ose of the untreated clays. The decrease in the dares suggests preferential opening of older, detrital clays. The K-Ar dates of the illite layers susc eptible to K extraction by the various treatments were calculated by subtra cting K2O and radiogenic Ar-40 values of the consecutive step products, and they were plotted against the total % K2O removed, used as an indicator of the reaction progress. Extrapolation of the plot revealed a detrital (1550 Ma) and a diagenetic (380 Ma) age for the 2 illitic minerals present in th e investigated shale sample. The inferred Devonian age of diagenesis of the Estonian clay corresponds to the period of massive dolomitization in the a rea. Both alteration processes can be related to a Devonian incursion of ho t or alkaline fluids, which helps to explain the occurrence of 20% expandab le I-S in claystones that have never been buried more than 1000 m. Extrapolated K-Ar ages and K2O contents of the illitic minerals, estimated from the X-ray diffraction (XRD) data, were used to model the experimental data. A good agreement was reached when dilution effects (chlorite and expa nded illite) were taken into account.