Cl/Br and δ³⁷Cl evolution in seawater expelled during the compaction of MX-80 smectite

Abstract : During a high pressure (up to 150 MPa) high temperature (up to 150°C) compaction experiment of MX-80 smectite equilibrated with ocean water, it was found that the Cl and Br concentrations in the expelled solution are progressively being decreased while simultaneously being increased in the compacted clay. This significant retention of Cl and Br in the pore water of the compacted clay is indicative of ultrafiltration by the clay. Despite the size difference between the two anions no significant chemical partitioning between Cl and Br is observed in the expelled water. The δ³⁷Cl value of the expelled water shows a sharp decrease (from -0.1 ‰ down to -0.5‰) during the first steps of compactions (between 45MPa - 21°C and 65MPa - 30°C) and then remains stable until 125MPa - 120°C. The largest impact on anion and stable isotope composition is seen during the earlier phases of the compaction. It is supposed that the surface chemistry of the clay in combination with decreasing porosity result in re-equilibration of compressed and overlapping diffuse double layers which in part, drive the anion and δ³⁷Cl evolution.