Illite <scp> ⁴⁰ Ar </scp> / <scp> ³⁹ Ar </scp> low‐temperature mini‐plateaux: Enlightenment on hydrocarbon accumulation

ABSTRACT Authigenic illite samples from the Baiyun Sag of the Pearl River Mouth Basin, South China Sea, were analysed by 40 Ar/ 39 Ar laser stepwise heating. Novel techniques were applied to distinguish authigenic illite signal from detrital contamination: (1) a fine‐step heating technique was elaborately applied to gently release argon from authigenic illite at low laser power steps and avoid the argon from coarse‐grained detrital minerals and (2) a powerful purification system including a temperature‐controlled cryotrap was employed to remove most organic matter. In this study, sample BY39 yielded a well‐defined low‐temperature mini‐plateau with a mini‐plateau age of 15.0 ± 0.7 Ma (2σ, 12 consecutive steps). Sample BY53 exhibited a low‐temperature segment with a weighted mean age of 16.5 ± 0.3 Ma (2σ, 8 consecutive steps), which, despite a subtle ascending trend, aligns with the age limit of BY39. These results represent the first reported instance of authigenic illite mini‐plateaux obtained from complex mineral mixtures. The robust nature of the BY39 mini‐plateau suggests that the youngest illite generation is not a trace component but represents a significant volumetric pulse of mineral growth, providing a sufficient argon budget to sustain multiple concordant steps. This rapid illitisation might be triggered by transient fluid pulses laden with CO 2 and organic acids derived from kerogen and oil cracking. The brief acceleration instead of an immediate cessation of illite formation during hydrocarbon accumulation provides a solid foundation for dating petroleum accumulation by the authigenic illite 40 Ar/ 39 Ar fine‐step heating technique. Additionally, high‐temperature heating steps yielded detrital ages (76 to 217 Ma) that reflect the provenance contribution of Mesozoic granites from South China to the Baiyun Sag.