Controls of Ca/Mg/Fe activity ratios in pore water chemistry models of the Callovian--Oxfordian Clay Formation
Résumé
The pore water chemistry model of the Callovian-Oxfordian clay formation considers that the divalent Ca, Mg and Fe cations are controlled by equilibrium reactions with pure carbonates: calcite for Ca, dolomite for Mg and siderite for Fe. Results of a petrological study and the Ca/Mg and Ca/Fe activity ratios computed based on natural pore water chemistry provide evidence that equilibrium with pure calcite and pure dolomite is a reasonable assumption for undisturbed pore waters; on the other hand siderite cannot be considered at equilibrium with pore waters at the formation scale.