Mineralogical and isotopic record of biotic and abiotic diagenesis of the Callovian–Oxfordian clayey formation of Bure (France)

Abstract

The Callovian-Oxfordian (COx) clayey unit is being studied in the Eastern part of the Paris Basin at depths between 400 and 500 m depth to assess of its suitability for nuclear waste disposal. The present study combines new mineralogical and isotopic data to describe the sedimentary history of the COx unit. Petrologic study provided evidence of the following diagenetic mineral sequence: (1) framboidal pyrite and micritic calcite, (2) iron-rich euhedral carbonates (ankerite, sideroplesite) and glauconite (3) limpid calcite and dolomite and celestite infilling residual porosity in bioclasts and cracks, (4) chalcedony, (5) quartz/calcite. Pyrite in bioturbations shows a wide range of d 34 S (À38& to +34.5&), providing evidence of bacterial sulphate reduction processes in changing sedimentation conditions. The most negative values (À38& to À22&), measured in the lower part of the COx unit indicate precipitation of pyrite in a marine environment with a continuous sulphate supply. The most positive pyrite d 34 S values (À14& up to +34.5&) in the upper part of the COx unit indicate pyrite precipitation in a closed system. Celestite d 34 S values reflect the last evolutionary stage of the system when bacterial activity ended; however its deposition cannot be possible without sulphate supply due to carbonate bioclast dissolution. The 87 Sr/ 86 Sr ratio of celestite (0.706872-0.707040) is consistent with deposition from Jurassic marine-derived waters. Carbon and oxygen isotopic compositions of bulk calcite and dolomite are consistent with marine carbonates. Siderite, only present in the maximum clay zone, has chemical composition and d 18 O consistent with a marine environment. Its d 13 C is however lower than those of marine carbonates, suggesting a contribution of 13 C-depleted carbon from degradation of organic matter. d 18 O values of diagenetic chalcedony range between +27& and +31&, suggesting precipitation from marine-derived pore waters. Late calcite crosscutting a vein filled with chalcedony and celestite, and late euhedral quartz in a limestone from the top of the formation have lower d 18 O values ($+19&), suggesting that they precipitated from meteoric fluids, isotopically close to present-day pore waters of the formation. Finally, the study illustrates the transition from very active, biotic diagenesis to abiotic diagenesis. This transition appears to be driven by compaction of the sediment, which inhibited movement of bacterial cells by reduction of porosity and pore sizes, rather than a lack of inorganic carbon or sulphates.

Equilibrium partial pressure of CO2 in Callovian–Oxfordian argillite as a function of relative humidity: Experiments and modelling

“…The mineral assemblage of the Callovian-Oxfordian (COx) claystone, which traps fluids in its residual porosity, is represented by a dominant detrital fraction of clay minerals, quartz, feldspar and bioclasts, cemented by diagenetic carbonate, sulphide and sulphate minerals (Gaucher et al, 2004;Clauer et al, 2007;Tournassat et al, 2008;Lerouge et al, 2011). The formation fluids are slightly saline waters, with ionic strengths of about 80-100 meq/L (Vinsot et al, 2008), that contain small amounts of dissolved gases.…”

“…The mineralogy and main physical properties of the sample are summarized in Table 2 The detailed mineralogy of the COx argillite from the URL has been described and discussed by Gaucher et al (2004), Claret et al (2004) and Clauer et al (2007). The formation having undergone only limited burial (600 m) and diagenetic (T<50 °C) processes (Pellenard et al, 1999;ANDRA, 2005), the carbonate fraction is composed predominantly of calcite with minor dolomite, ankerite and siderite (Lerouge et al, 2011). It has a low organic matter content (<0.6%; Claret et al, 2005) that is immature in the petroleum sense (Elie et al, 2000).…”

A Deep Alteration and Oxidation Profile in a Shallow Clay Aquitard: Example of the Tégulines Clay, East Paris Basin, France

“…Celestite in association with pyrite was identified by SEM in only few samples ( Figure 5(h)). Pyrite associated with micrite and dolomite is common early diagenetic mineral formed in the bacterial sulfate reduction (BSR) zones of marine clay formations that evolved in reducing conditions [7,8]. Rare celestite associated with framboidal pyrite was probably formed from residual sulfates at the end of the bacterial activity.…”

“…Diagenetic minerals consisted of glauconite (8 ± 6%) with variable amounts of calcite and minor dolomite, siderite, pyrite, and phosphates. All these diagenetic minerals are commonly formed at early stage of diagenesis in the bacterial sulfate reduction (BSR) zones of marine clay formations that evolved in reducing conditions [7,8]. The microsparite and celestite, contemporaneous of the compaction, filled the residual porosity.…”

Inputs from in situ experiments to the understanding of the unsaturated behaviour of Callovo-Oxfordian claystone

“…Sediments of the Callovo-Oxfordian unit consist of a dominant clay fraction associated with carbonate, quartz with minor feldspars, and accessory minerals [5]. At the MHM URL main level (-490 m depth), the clay fraction is high (40-60%), and clay minerals consist of illite, ordered illite/smectite mixed layers, kaolinite, trioctahedral, iron-rich chlorite, and minor biotite.…”