The presence of adsorbed ions on calcite surfaces can significantly affect the adsorption and desorption of organic molecules, which is critical for oil recovery and biomineralization. In this study, the structure of calcite–artificial seawater interfaces from 25 to 80 °C was experimentally and theoretically investigated by surface X–ray scattering and molecular dynamics simulations, respectively. The small difference in the CTR scattering profiles at different temperatures could be attributed to the relaxed outermost calcite surface. The electron density profile of the NaCl solution (0.5 mol/kg) exhibits peaks near the calcite surfaces. The two peaks closest to the surface can be interpreted as adsorbed water molecules, inner–sphere Na+ complexes, and inner– and outer–sphere Cl− complexes. Thus, the adsorbed Cl− formed two peaks near the calcite’s surface, while Na+ formed a single peak as an inner–sphere complex. It should be noted that there was no strong covalent bond between these inner–sphere complexes and the calcite surface. These structural differences between adsorbed cations and anions could be explained by the balance of the interactions between the surface Ca2+ and CO32−, adsorbed ions, and the surrounding water molecules. The presence of inner–sphere Cl− complexes destabilizes surface Ca2+, whereas Na+ has an insignificant effect on the structure of surface CO32−. Adding a small amount (0.045 mol/kg) of Mg2+ and SO42− appears to enhance the relaxation of the interfacial structure.
Compositional variation of olivine in serpentinized peridotites provides a significant constraint on modeling the redox conditions of serpentinization and the tectonothermal history of ophiolites. Here I report the variations of Fe, Mg, Mn, and Ni contents of olivine from the Oeyama ophiolite, SW Japan and show textural and chemical evidence for compositional modification of olivine related to high–temperature (T) serpentinization. The Fe–enrichment of olivine adjacent to antigorite without significant magnetite formation indicates a reducing condition for high–T serpentinization. Systematic variations of forsterite (Fo) component with distance from antigorite suggest Mg–Fe volume diffusion took place in olivine porphyroclasts under the conditions of high–T serpentinization. In addition, a similar diffusion pattern of Mn to Fe results in a retrograde trend in MnO–Fo diagram, which could be a useful indicator of high–T serpentinization. Retrograde antigorite is different from prograde antigorite in having a shape of elongated blade, lacking a significant amount of magnetite inclusion, and being more ferrous than lizardite. The existence of retrograde antigorite provides another piece of evidence for high–T serpentinization even if olivine has been decomposed by intense low–T serpentinization. Approximate estimation of time required for the observed Mg–Fe diffusion profiles of olivine porphyroclasts reveals that a cooling duration under the conditions of high–T serpentinization was much longer than that of amphibolite–facies metasomatism previously reported. This suggests a long residence time of the forearc peridotites within the serpentinized mantle wedge following rapid exhumation immediately after the amphibolite–facies metasomatism.