Diagenetic carbonates in clay-rich marine formations
Résumé
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 <50°C), whereas Opalinus and Tournemire Clay were buried deeper and underwent higher-temperature diagenesis up to 100-120°C. Table 1: Ages, burial, current depths and maximal temperatures of the studied clay formations. Boom Clay (Belgium) COx Clay (France) Opalinus Clay (Switzerland) Tournemire Clay (France) Mol Bure Benken Mont Terri Tournemire Age (Ma) 32-29 165-158 176-172 176-172 186-176 Maximum burial (m) 265 600 1700 1100 1900 Current depth (m) 183-286 422-552 539-652 215-310 210-460 Max. temperature (°C) <30 45 85 85 80-120 The four clay formations contain a major clay fraction (illite, illite-smectite mixed layer, kaolinite and chlorite) associated with variable silty (dominant quartz, feldspar) and carbonate (bioclastic and diagenetic) fractions. Authigenic minerals include carbonates (dominant calcite, dolomite/ankerite and siderite) with minor sulfides (dominant pyrite, sphalerite and galena), Ba-Sr sulfates and glauconite. Based on petrological observations, diagenetic phases are infrequent but nevertheless contributed to sealing of the residual porosity visible under the microscope. Detailed studies of textural relationships between authigenic phases in COx Clay (Lerouge et al., 2011) and in Opalinus Clay, combined with previous works in Rupelian Clay (Laenen and Craen, 2004) and Tournemire Clay (Peyaud et al., 2005) provide evidence of numerous similarities and suggest that most authigenic phases formed during early diagenesis (eogenesis) in the still unconsolidated sediment (Figure 1). At this stage, a major cementation with micritic calcite and pyrite, plus minor euhedral calcite, occurred. Based on evidence from sulfur isotopes (Lerouge et al., 2011) and textural observations, framboidal pyrite was formed by bacterial reduction of marine sulfate (BSR) coupled with organic matter degradation close to the water/sediment interface. Calcite has low Mg and Fe contents consistent with a marine environment, and Fe/(Fe+Mg) ratios <0.4. Early dolomite is quite pure. The low Fe/(Fe+Mg) ratio of early carbonates formed is due to the consumption of iron by pyrite formation. Once the bacterially mediated sulfate reduction and Fe consumption ceases, pore water chemistry changes and is recorded by the later carbonates, of glauconite and of Ba-Sr sulfates. At this stage, the higher Fe availability in porewater results in an increase of the Fe content and of the Fe/(Fe+Mg) ratio in later calcite and in dolomite, but also in the precipitation of ankerite and siderite. Calcite filling residual porosity in bioclast-rich or silty beds in Opalinus Clay or replacing bioclasts has Fe/(Fe+Mg) ratios up to 0.7-0.8. Dolomite and ankerite generally occur as µm-sized euhedral grains. Their contents are <2 % in Opalinus Clay and Tournemire Clay, and about 2-7 % in COx Clay. The chemical compositions of dolomite/ankerite differ among the formations. Dolomite grains have Fe-rich rims with Fe/(Fe+Mg) ratios generally lower than 0.2 in the cores in all studied formations. Ankerite compositions with Fe/(Fe+Mg) >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.