The hydrothermal alteration of mantle rocks, referred as serpentinization, occurs when the mantle is exposed to aqueous fluids circulating below 400°C, leading to the formation of serpentine, hydrogen and other minerals. It is a process heavily involved in mass exchange between the mantle and the surface and influences geochemical cycling and fluid-mobile elements. It occurs in various submarine environments including mid-ocean ridges and subduction zones and it affects the physical and chemical properties of the oceanic lithosphere.
It is also pivotal to current theories on the origin of life. Serpentinization is likely to have provided the crucial chemical gradients required for life to being when the earth was simply rock, water and carbon dioxide.
Despite being a process vital to our understanding of the origin of life and the Earth´s lithospheric mantle activity, the rates and the environmental factors affecting serpentinization are poorly understood. A collaborative effort from Virginia Tech, The Free University of Berlin, Woods Hole Oceanographic Institution and The University of Texas used synthetic fluid inclusions as micro-reactors in olivine crystals as a model to study the rate of serpentinization. This method allowed them to study mineral precipitation and water activity in real time and in situ.
When discussing their work, Dr. Lamadrid said “We trapped synthetic fluid inclusions (tiny droplets of fluid) with a seawater-like composition in gem quality olivine crystals and then we set the samples to serpentinization conditions (~280ºC). Within a few days, serpentine crystals begin to precipitate inside the synthetic fluid inclusions. Since the inclusions are isolated any changes inside the inclusion can be observed and we can model them as chemical micro-reactors. The serpentinization reaction consumes H2O, so the original salinity starts to increase as more H2O leaves the fluid to form new serpentine crystals. As such, we were able to monitor the amounts of H2O leaving the fluid by measuring changes of the salinity inside the inclusion. The salinity of the fluid inclusions were measured with high precision by measuring changes in the freezing point depression of the fluid inclusions with the Linkam THMSG600 stage.”
Their technique allowed them to study the mineralogy and chemistry of the reaction products. After carrying out experiments with different salinities and fluid compositions, they found the reaction to be highly sensitive to the salinity and chemistry of the fluid. This poses interesting concepts of where serpentinization may occur in the earth’s mantle as well on other planetary bodies. Their novel micro-reactor technique could also be applied to many other minerals, reaction products, and fluid compositions to study fluid-rock reactions in real time and in situ.
By Tabassum Mujtaba
Lamadrid, H. M. et al. Effect of water activity on rates of serpentinization of olivine. Nat. Commun. 8, 16107 doi: 10.1038/ncomms16107 (2017).