RECENT EXPERIMENTAL DEVELOPMENTS IN THE STUDY OF HYDROTHERMAL SYSTEMS: SERPENTINIZATION, CARBONATION AND DEHYDRATION REACTIONS

The hydrothermal alteration of ultramafic rocks is an important geologic process that occurs in a variety of geologic settings and under a wide range of environmental conditions like temperature, pressure, rock and fluid compositions. This alteration, usually known as serpentinization, encompasses a series of disequilibrium reactions that affects the physical and chemical properties of oceanic lithosphere, represents one of the major mechanisms driving mass exchange between the mantle and the Earth’s surface, and is central to current origin of life hypotheses as well as the search for microbial life on the icy moons of Jupiter and Saturn. Here we show results of a recently developed experimental methodology that uses synthetic fluid inclusions (SFI) trapped in ultramafic minerals as micro-reactors to monitor and quantify, in situ and in real time, rates of serpentinization and carbonation of olivine and pyroxene (olivine + H₂O = serpentine + brucite; pyroxene + H₂O = serpentine + talc; olivine + H₂O + CO₂ = magnesite + SiO₂). We trapped SFI at different pressure, temperature, fluid densities, and fluid compositions, to study how environmental factors like temperature and fluid compositions affect the kinetics of the serpentinization and carbonations reactions. Our results show that the serpentinization rates in olivine and pyroxene decrease as salinity, Mg and CO₂ in the fluid increases. The serpentinization rates of pyroxene are more than an order of magnitude faster than those of olivine at higher temperatures (~350ºC) where serpentinization of olivine becomes sluggish. After the serpentinization experiments, some of the samples were put at higher temperatures >450 ºC and we were able to monitor and quantify the dehydration reactions (serpentine + talc = pyroxene + H₂O; serpentine = talc + olivine + H₂O). The micro-reactor technique used in this study is a powerful tool to monitor fluid-rock disequilibrium and equilibrium reactions in situ and in real time and can be applied to a wide variety of host minerals, reaction products, temperatures and different starting fluid compositions.