Diagenetic carbonates in clay-rich marine formations

Diagenetic carbonate minerals reflect the evolution of the Ca/Fe/Mg ratios of porewaters in clay-rich formations over geological time. Existing models consider equilibrium reactions with pure carbonates: calcite (for Ca), dolomite (for Mg) and siderite (for Fe) (Pearson et al., 2003; Gaucher et al., 2009). However, studies provide evidence that the chemical compositions of carbonates is complex. Their understanding remains a challenge but is needed to improve geochemical porewater models over a range of temperatures. Here, we present petrological investigations on four marine, reducing clay formations: Callovian-Oxfordian (COx) Clay at Bure (France), Toarcian Clay at Tournemire (noted Tournemire Clay, France), Opalinus Clay at Mont Terri and Benken (Switzerland) and Rupelian Boom Clay at Mol (Belgium), to characterise the stability and composition of diagenetic carbonates over a range of temperatures. The four clay formations differ from one to another by their age, their maximum burial and their geological history (Table 1). Boom Clay and COx Clay underwent limited burial and diagenetic processes (T 0.5 were identified in rare grains of Opalinus Clay and Tournemire Clay. δ18O of bulk dolomite/ankerite measured in COx Clay (~+27.5 ‰SMOW) and in Tournemire Clay (+21 to 22 ‰SMOW) suggest that they formed from a marine fluid at temperatures corresponding to the maximal burials attained by the units (~45°C for COx Clay and ~80-120°C for Tournemire Clay). Siderite occurs in two textural types: concretions in Opalinus Clay and in Boom Clay, and disseminated grains in the matrix of COx Clay, Opalinus Clay and Tournemire Clay. Textures of concretions strongly suggest that siderite formed early but only once the bacterially mediated sulfate reduction ceased. δ18O values for disseminated siderite in COx Clay (+26 to +29 ‰SMOW) and in Tournemire Clay (+19 to +22 ‰SMOW) indicate that siderite, like dolomite/ankerite, formed at temperatures corresponding to the maximal burial attained by these units. These data indicate that siderite may precipitate at different stages during diagenesis. In summary, it is evident that calcite is the major authigenic carbonate mineral that formed at different stages during burial. Calcite that formed later than the end of bacterial sulfate reduction has a Fe/(Fe+Mg) ratio up to 0.7-0.8, irrespective of the temperature at which it formed. This suggests that its composition remained essentially constant with increasing temperature. Formation processes of dolomite and ankerite are not well understood. According to oxygen isotopes, dolomite/ankerite formed until the time of maximal burial. It appears that ankerite may precipitate over a large range of temperatures (~50-110°C). The essentially homogeneous composition of siderite in the four clay formations, irrespective of formation temperature (~20-110°C), indicates that 1) siderite saturation is attained at different times during the diagenesis of the four studied formations, and 2) its composition is stable over a large range of temperatures