Illite K-Ar dating and crystal growth processes in diagenetic environments: a critical review

K-Ar dating of illitic minerals is commonly used in studies of diagenetic series applied to oil prospecting. In spite of a great number of specialized papers, some problems remain unresolved. These are mostly due to a misunderstanding of the argon accumulation process during illitization. Criteria for identifying detrital-authigenic mineral mixtures, crystal ripening, fast precipitation or continuous nucleation-growth processes are discussed using K-Ar data available in the literature. Using different parameters, such as Delta(age) (age(K-Ar)-age(strati)), Delta(cryst) (diagenetic age(K-Ar)-age(strati)) or Delta(frac) (age(K-Arfraction)-age(K-Arfinest)), it is shown that the K-Ar age significance depends on the illite nucleation-growth processes. A 'diagenetic age' is obtained when these processes are rapid (the K2O accumulation period is shorter than 2sigma). If lower than this value, the K-Ar ratio depends on the proportions of new and old particles, respectively, which are controlled by the relative rates of nucleation, crystal growth and ripening.