minerals as records of temperature conditions and duration of thermal anomalies

ABSTRACT: 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 Ixm) 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 ~ 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 6~80 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 8180 values and thus may have formed at somewhat lower temperatures. The 8180 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 hat the duration of the hydrothermal events at 190 Ma and 150 Ma were rather short, <1 Ma, whereas the youngest event was protracted over