RECONSTRUCTION OF PALAEOTHERMAL CONDITIONS IN A PASSIVE MARGIN USING ILLITE-SMECTITE MIXED-LAYER SERIES (BA1 SCIENTIFIC DEEP DRILL-HOLE, ARDECHE, FRANCE)

The comparison between measured and predicted smectite percent (%Sm) i n an illite-smectite (I-S) mixed-layer series was used to estimate the thermal conditions of diagenesis in the Ardeche sedimentary series. M easurements were performed using the decomposition procedure (Lanson a nd Besson, 1992) of XRD patterns obtained from oriented preparations ( <0.2 mu m fractions). Three particle populations were identified: I-S, poorly crystallized illite (PCI), and well crystallized illite (WCI); the %Sm was measured in the I-S population by comparison with calcula ted XRD patterns using the NEWMOD software (Reynolds, 1985). The theor etical evolution of %Sm with depth obtained with the two first-order k inetic reaction processes described by Velde and Vasseur (1992) was co mpared to the measured one at Balazuc. The best fit was obtained far t he following conditions: 35 degrees C km(-1) and between 1500 and 2000 m for the thermal gradient and the eroded piles, respectively. The pr edicted values for I-S compositions in the 10-0% Sm range are overesti mated by the model because the reaction controlling the composition is no longer the dissolution of I-S but the ripening of illitic particle s (PCI and WCI). Two zones can be distinguished in the sedimentary ser ies: (I) down to 55 m, the %Sm of I-S particles is highly variable and seems to be controlled by local variations in permeability and chemic al composition; (2) down to 550 m, the %Sm of I-S particles is more st rictly dependent on the general time-temperature conditions. An anomal ously high %Sm value is located in the vicinity of faults at 1669 m du e to local change of the water/rock ratio.