Clay minerals as records of temperature conditions and duration of thermal anomalies in the Paris Basin, France

Upper Triassic sandy horizons in the Paris Basin were sampled at depths ranging from outcrop in the northeast to 2,700 m in the centre of the basin. The smallest clay sub-fractions (<0.2 μm) from the deepest central samples consist mainly of illite and chlorite whose K-Ar age is ∼ 190 Ma. These minerals formed at a relatively high temperature of 220-250°C, as determined from the oxygen isotope fractionation between authigenic illite and associated quartz overgrowths, but at a burial depth of only 500 m. In a nearby drillhole that crosscuts a fault zone reactivated during the 190 Ma event, illite is less well-crystallized and has a higher δ18O value suggesting different physical and chemical conditions of formation during the same hydrothermal episode. Two other generations of illite-smectite mixed-layer clays formed in the same Triassic horizon: one at ∼150 Ma and the other at ∼80 Ma. These younger clays have higher δ18O values and thus may have formed at somewhat lower temperatures. The δ18O values of the fluids from which the different illite-smectite mixed-layer minerals form range from +9 to +13.5 per mil (SMOW). An Ar diffusion code was used to estimate, on the basis of the Ar loss of the clay-type material, the duration of these events. The results suggest that the duration of the hydrothermal events at 190 Ma and 150 Ma were rather short, <1 Ma, whereas the youngest event was protracted over a much longer period of ∼37 Ma. Comparison between K-Ar ages of the different mixed-layer minerals and sedimentation rates of the sediments since the Palaeozoic shows significant accelerations of the rates at ∼ 190-200 Ma and 150 Ma, and a less important one at ∼80 Ma. These observations provide additional evidence that the first two events were promoted by basement-related tectono-thermal activities. The third event is considered to be of diagenetic origin.