Dating multistage paleofluid percolations:: A K-Ar and 18O/16O study of fracture illites from altered Hercynian plutonites at the basement/cover interface (Poitou High, France)

Isotopic age determination of fluid events in basement fractures is of prime importance for the reconstruction of paleo-hydraulic regimes. However, material from fracture or wall-rock is not generally easily dated. In the case of fractures filled by minerals such as phyllosilicates, it can be expected that subsequent fluid circulation may cause partial alteration or recrystallization of the previous phyllosilicates, disturbing the isotope signature. Using a specific methodology, including the extraction process without contamination of the finest ( < 0.2 mum) authigenic fractions, identification of clay fractions by X-ray diffraction (XRD), scanning electron microscopy, and electron microprobe, analysis of stable isotope composition (D/H, delta(18)O) and K-Ar dating, fluid event ages can be estimated. This methodology was applied to the northwestern margin of the French Massif Central, where basinal brines have interacted with the sedimentary cover (infra-Toarcian formations) and the fractured Hercynian crystalline basement. During periods of unknown age, precipitates from brines sealed most basement fractures as carbonates, and a recrystallization of early Hercynian phengite (+chlorite) into illite and illite/smectite minerals occurred in the earlier fractures. XRD patterns show that the clay fraction is dominated by illite and ordered mixed-layer minerals having a relatively high amount of illite (I-S R3). In the shallowest samples beneath the paleo-surface, a late assemblage is identified as a mixture of I-S minerals, a part of them being characterized by significant amounts of swelling layers (I-S R0). The deltaD values of the clay fractions are fairly constant around -50 +/- 10parts per thousand, but delta(18)O values of the same clay fractions display a wide range from 8parts per thousand to 18parts per thousand (SMOW). The study of the size fractions between 2 and 0.2 gm reveals a correlation between K-Ar ages and delta(18)O values, which cannot be related to a change in the relative amounts of clay minerals. Deep fractures (at depths of 570 and 923 m) are characterized by delta(18)O values around 10.3 +/- 0.4parts per thousand SMOW and old ages ranging from 253 to 272 Ma. ne finest fractions ( < 0.2 mum) of clays located at shallow level ( < 300 m) beneath the paleo-surface yield the highest delta(18)O values (15.7-18.2parts per thousand) and the youngest K-Ar dates (188 to 198 Ma). These dates are likely to record the extensional episode and thermal anomaly related to the rifting of central Atlantic Ocean (c. 190 Ma). The other important thermal/extensional episode which affected the Aquitaine Basin, namely the opening of the Gascogne gulf at c. 120 Ma, is not detected by the K-Ar method on the clay fractions. We believe that resetting of clay-type K-Ar ages linked to recrystallization was favoured in the upper part of the basement where the ancient (Hercynian) clay minerals were destabilized by subaerial low-temperature alteration during Permo-Triassic times. From a methodologic point of view, the analytical characterization of several size fractions of clays from reactivated fractures, using both stable isotope geochemistry and K-Ar isotope geochemistry, can be considered as a powerful tool for dating fluid events when no suitable material for conventional methods is available. Copyright (C) 2004 Elsevier Ltd.