Models of water cycle and continental crust formation on Earth during Hadean and Eo-Archean

Ocean on the Earth is a key feature, which is likely responsible for the onset and further operation of plate tectonics as well as for the origin of life. Geochemical data suggests that ocean existed on the Earth already since at least the middle Hadean time. Recent studies also infer that after the solidification of the magma ocean, mean concentration of water in the Earth’s mantle could have been up to few 1000 ppm and that extraction of part of it formed the surface ocean. However, a clear understanding of this process is still lacking.

Here, we report results of modelling of Earth’s evolution during its first 1.5 Gyr with a focus on water cycle and generation of the continental crust. We use geodynamic code StagYY in 2D spherical annulus geometry that generates both basaltic and felsic melts, includes cooling of the core and uses an advanced treatment of water. We also included the effect of water on density of crustal and mantle materials based on experimental data and thermodynamic calculations.

Our models start just after solidification of magma ocean with assumed initial mantle potential temperature of 1900K and core temperature of 5000K. We run models with different initial mean water content in the mantle reaching up to 1500 ppm. In all the models, most of the water is initially concentrated in the mantle transition zone (MTZ), because of its higher water storage capacity. Due to the lower density of the water-containing materials, this leads to Rayleigh–Taylor instabilities and hot and “wet” mantle plumes rapidly rise to the surface. As a result, a large amount of mantle water is outgassed forming the surface ocean in just a few million years. Simultaneously, a significant amount of continental crust is produced. Masses of the produced ocean and continental crust depend on the initial concentration of water in the mantle. For instance, for the initial mean water concentration of 1000 ppm, ocean mass of about 1.5 times recent ocean masses (OM) and continental crust of about 0.7 times present-day continental crust mass (CCM) is produced during 7 Myr. Water outgassing from the mantle dominates during the first 100 Myr till ocean mass reaches about 2 OM. Afterwards, the outgassing by plumes and in-gassing by subduction are mostly balanced with a tendency of the surface ocean mass to decrease with time during the 1.5 Gyr.

Interestingly, in all models, MTZ behaves as a buffer for water cycle and despite it’s high water storage capacity, it’s mean water content mostly remains below 400 ppm, rising to up to 1500 ppm only for the short time periods when a number of cold slabs are resting in MTZ. We will show results from a set of models and compare the model-predicted trace elements ratios with the recent geochemical data.