Clay Minerals (1996) 31,203-208 CLAY MINERALS AS RECORDS OF TEMPERATURE CONDITIONS AND DURATION OF THERMAL ANOMALIES IN THE PARIS BASIN, FRANCE- DISCUSSION C. SPOTL, R. H. WORDEN* AND F. WALGENWITZ'~ lnstitut.fiir Geologie und Palilontologie, Universitilt Innsbruck, Innrain 52, 6020 lnnsbruck, Austria, * School of Geosciences, The Queen's University o fBelfast, Belfast, BT71NN, UK, and t Elf Aquitaine Production, CSTJF avenue Larribau, 64018 Pau Cedex, France (Received 31 October 1995; revised 28 November 1995) ABSTRACT: A highly illitic phase containing <5-10% interstratified smectite was described by Mossmann et al. (1992) and, more recently, by Clauer et al. (1995) from Upper Triassic sandstones of the Paris Basin (France). This phase is present in all size-fractions of all samples and was interpreted as authigenic. The K/Ar model ages of this illite range from 189-200 Ma and, combined with oxygen isotope data, were cited as evidence of high-temperature (220-250~ hydrothermal precipitation from highly ~SO-enriched fluids at burial depths of only 500 m. We suggest that this type of illite is more likely to be diagenetically altered detrital illitic material and unlike authigenic illite-smectite which is also present in many of their samples. This reassessment of their petrographic observations leads to a much more realistic diagenetic interpretation consistent with previous studies of the basin and avoids the need to invoke basin- wide hydrothermal activity during shallow burial. Clauer et al. (1995) suggested that widespread hydrothermal fluid flow and high temperature rock- water interaction occurred in the Paris Basin at burial depths of 500 m. Their hypothesis is based on K/Ar and oxygen isotope data from illitic clay minerals and authigenic quartz in Upper Triassic sandstones. The authors calculated equilibrium isotope temperatures of 220-250~ for cogenetic quartz and illite-smectite (I-S). They also concluded that the formation water responsible for the quartz- illite growth was extremely enriched in 180 (g~80 of +9 to +13.5%o SMOW). These interpretations are at odds with data on the diagenetic, thermal and hydrological evolution of the basin. These data show no evidence of hydrothermal alteration and suggest that maximum burial temperatures did not exceed ~140~ even in areas buried to />3 km (Guilhaumou, 1993; Walgenwitz & Worden, 1993; Demars & Pagel, 1994). The purpose of this comment is to demonstrate that the chain of argument in the Clauer et aI. (1995) paper and an earlier paper from the same laboratory on the same material (Mossmann et al., 1992) is flawed and based on several questionable assumptions. We show that data presented in these papers can be interpreted in an alternative way without the need to invoke unusual hydrothermal fluid flow along faults in this basin. THE HIGH-TEMPERATURE MODEL Clauer et al. (1995) studied Rhaetian (Upper Triassic) sandstones from four wells and two outcrops in the basin. Most of the subsurface samples are from intermediate burial depths (~900-1200 m), while one sample is from 2692 m. The Rhaetian sandstones are compositionally immature, containing abundant matrix clay and chemically unstable lithic fragments. These are highly altered and sericitized in deeply buried samples (Mossmann et al., 1992, p. 214). Based on powder X-ray diffraction (XRD) analyses of various grain-size fractions, Mossmann et al. (1992) identified three distinct types of I-S clay minerals, referred to as I-S phases 1, 2 and 3. Whilst most grain-size fractions consist of two or three phases, three samples from well MN06 and the sample from the deep well, V, are exclusively I-S phase 1. The K/Ar model ages (<0.2 gm size-fraction) of 9 1996 The Mineralogical Society