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

Abstract

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 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-At 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 basementrelated tectono-thermal activities. The third event is considered to be of diagenetic origin.The prime factors affecting burial and/or episodic thermal diagenesis of sedimentary rocks and formational fluids can be classified into those characterizing the sediments and fluids such as mineralogy, petrography, petrophysics and fluid chemistry, and those imposed on the sediments such as pressure, temperature and duration of the events. Most of these parameters can be measured and/or estimated, but making a precise determination of the timing (epoch and duration) and of the maximum temperature reached is very difficult.

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

“…Their earlier publication on the same samples separately showed scanning electron microscope (SEM) photomicrographs of both I-S and quartz cements but did not demonstrate that they were cogenetic (Mossmann et al, 1992). Even though no photographic evidence was supplied by Clauer et al (1995), there must always be doubt concerning the veracity of textural relations, even those observed using an SEM. Existence of an intergrowth of quartz and I-S does not necessarily prove that they grew together.…”

“…In their most recent paper, Clauer et al (1995) presented oxygen isotope data of the same clay mineral samples and of a few quartz separates and calculated equilibrium isotope temperatures for the quartz-illite pair. Mineral-mineral isotope geothermometers will give erroneous results unless a number of prerequisites are fulfilled.…”

“…Rhaetian sandstones) present difficulties during isotopic analysis. However, despite this, Clauer et al (1995) managed to separate the quartz fractions and define the ratios of detrital to diagenetic quartz. The results, based on four data points from three different samples, suggest a 6180 value of 17.3 + 1.3%o for authigenic quartz.…”

“…1. Extrapolation method used by Clauer et al (1995) to calculate the 8180 value of pure I-S phase 1 in samples from well SG16 based on a K/Ar model age of 190 Ma (left diagram). SG16-2 is an "anomalous" sample from the same well that was discarded by the authors.…”

“…We are not convinced that quartz and I-S are truly cogenetic. Clauer et al (1995) stated that Triassic sandstones in the Paris Basin are cemented with quartz, I-S and different carbonate phases. They stated that the genetic link between quartz cement and the first I-S phase "has been clearly established" (p. 3).…”