New experimental approach to study aqueous alteration of amorphous silicates at low reaction rates

• A new experimental setup is presented to study alteration rates at low temperature.
• Combined analysis allows full characterization of the altered solid.
• Alteration of amorphous silicate of fayalite 55 composition at 90 and 190 °C
• Amorphous Fe3 +-serpentine forms at the reaction front and Mg-rich amorphous phyllosilicates grow at the interface with the fluid.
• Applied to chondritic environments, alteration rates might be extremely fast compared to asteroidal heating timescales.

Understanding the kinetics of silicate alteration in aqueous media is central to the study of oceanic hydrothermal systems, nuclear glass durability or carbonaceous asteroids from which chondrites are coming. We present a new experimental approach in combination with an integrated analytical protocol designed to study alteration processes at low temperatures (< 200 °C) and slow reaction rates. We used pulsed laser deposition (PLD) to produce micrometer thick films of amorphous silicate of controlled chemical composition. After reacting with water in sealed capsules, the films consist of a succession of compositionally different layers. The initial planar geometry of the film allows a complete characterization of the transformed materials at the nanometer scale. By combining Rutherford back-scattering (RBS), nuclear reaction analysis (NRA), transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM), it is possible to constrain the propagation rate of the reaction fronts, the thicknesses of individual layers, spatial variations in composition, the nature of the interface between the layers, the iron redox state, the water content along depth profiles, as well as the porosity and the density.We investigated the serpentinization of amorphous silicate films with stoichiometry close to olivine (~ Fe1.1Mg0.9SiO4.15H0.3) at 90 °C (2 weeks) and 200 °C (2 h). In both cases, ~ 500 nm of altered material is formed. At the reaction front, a hydrated, amorphous and oxidized Fe-rich layer forms. At the interface with the fluid, a more Mg-rich layer develops. The system evolves towards a biphasic assemblage of Fe-serpentine and Mg-saponite composition. Both layers remain amorphous. It is shown that water propagation is coupled to hydrolysis, iron oxidation (Fe3 +/∑ Fe ratio > 50%) and H2 formation, whose quantifications are crucial to understand terrestrial serpentinization processes. Interfacial precipitation–dissolution seems to be the rate controlling mechanisms. In addition, we investigated a crystallized film reacted at 190 °C (2 h), which transformation rate is ten times slower than that of the amorphous silicate but is nevertheless readily observable. This approach can be used to understand alteration in terrestrial and extraterrestrial samples. In particular, we reproduced several features observed in carbonaceous chondrites (amorphous and oxidized hydrated silicates) and show that, at 90 °C, alteration may be faster than usually considered. It should allow us in the future to constrain the temperatures and timescales of alteration in chondrites.