Smectite-water exchanges at the Ceres crust

Abstract

<p>Dawn mission sensors detected pervasive Mg and NH<sub>4</sub> phyllosilicates mixed with a dark mineral component, probably magnetite, on Ceres’ surface, and observed Na and Mg carbonates locally associated to impact structures [1-4]. Ceres’ crust is mainly composed by different phases of silicates, water and salts. Stephan et al. [5] suggest that the NH<sub>4</sub>-phyllosilicate is also one of the most representative components in the crust, while the distribution of water as ice or liquid is dependent on the depth. Recent models show that Ceres precursors and the differentiated crust have suffered aqueous alteration and porosity reduction during its evolution, in which silicates and water have physically and chemically interacted [6].</p> <p>To understand the exchanges between water and the rock particles we are performing a set of experiments simulating the thermal evolution of two systems: 1) montmorillonite clays in liquid water; 2) montmorillonite clays in brine solutions.</p> <p>NH<sub>4</sub>-montmorillonite is obtained in the laboratory by cation substitution method [7] from the montmorillonite (Gonzales County, Texas, USA) ((Na,Ca)<sub>0.33</sub>(Al,Mg)<sub>2</sub>(Si<sub>4</sub>O<sub>10</sub>)(OH)<sub>2</sub>·nH<sub>2</sub>O). The resulting smectite was checked and characterized by XRD, IR and Raman spectroscopy.</p> <p>In the first set of experiments 1.5 wt% of both, the original and the NH<sub>4</sub>-montmorillonite, were suspended in liquid water and placed into a pressure cell. In order to simulate the conditions in the ice-rich crust, systems were cooled down to 263 K for 24 hours. After that, the samples were heated up to room temperature.</p> <p>During the heating of our first tests with pure water, just when the ice started to melt at 272 K, we observed shifts from 1 to 2.8 bar in the case of the montmorillonite, and to 2.6 bar when working with the NH<sub>4</sub>-enriched clay.</p> <p>In the second set of experiments, the protocol was repeated, but the original montmorillonite was suspended in an eutectic solution of NaCl (23 wt %). It also showed a pressure shift near the eutectic temperature of the solution 251 K from 1 to 1.5 bar.</p> <p>We interpret these pressure shifts as the effect of a positive volume change of the system, in which the reduction of the water volume by melting is overcompensated by the smectite swelling, even at the low clay quantities we are using in these experiments. When the phyllosilicate freezes, the interlayer distance is reduced [8] and the molecules of water release. This effect is reversible if the clay is in an aqueous environment. The number of molecules inserted between layers depends on the cation in the clay. The Na<sup>+ </sup>present in the original montmorillonite has the capability to incorporate more than 12 molecules of water [8]. Experiments done so far with NH<sub>4</sub>-smectites suggest that its facility to swell is lower in the NH<sub>4</sub>-montmorillonite than in the original montmorillonite [9].</p> <p>From the laboratory results, we can argue that the interaction between water-smectite during thermal evolution of Ceres’ crust could yield interesting geological effects such as the clay dehydration by freezing, the precipitation of salts from brines when swelling occurs or the generation of stresses by the deformation of the materials.</p> <p><strong>