Experimental approach to the direct interaction between the proto-atmosphere and rocky crust of the early earth and its implications to the early evolution of earth-like planets

Abstract

The first eon of the Earth, Hadean, is no doubt critical for the evolution of Earth towards a habitable planet and the origin of life. However, there is almost no petrologic record preserved for this piece of history because of the long-term geological reworking. In this study, batch experiments simulating the interaction between the early Earth's ultramafic crust and H2O-CO2 atmosphere were performed in order to gain some new insights into the direct interaction and its influence on the evolution of mineral, atmosphere, ocean and the prebiotic chemistry on the early Earth.

Electron microscopic observations (including SEM and TEM) show that the secondary minerals produced in the experiments mainly include phyllosilicates, carbonates and Fe-oxide. Phyllosilicates which are essential for biomonomer synthesis can be found in each of our experiments. Different rock-water-H2O systems result in different clay minerals with varied crystal habits. Carbonates can be found in experiments carried out at temperature below 400 ºC. With the experimental temperature decrease from 400 ºC to 200 ºC, the formed carbonates change from calcite, dolomite and magnesite accordingly. Energy dispersive spectroscopy reveals the incorporation of iron in all kinds of carbonates. Hexagonal magnetite nanoplates are observed in komatiite-H2O-CO2 experiment carried out at 450 ºC. The mineralogical compositions imply that the interaction between the early Earth's ultramafic crust and H2O-CO2 atmosphere were able to produce clay minerals, carbonates and oxides on the rocky planets such as Earth, which was corroborated by the recent discovery of layered clay minerals and carbonates assemblages on Mars. More importantly, these secondary minerals are effective in catalyzing the inorganic molecular to biomolecules that are essential in prebiotic chemical evolution. The GC measurement of the gaseous phases trapped in the capsule after experiments show that abiogenetic methane, ethane and propane as well as hydrogen were detected in most our experiments. The relative concentrations of these gases are higher in high temperature experiments, which indicate high productivity of CH4 and H2 during the interaction between the early Earth's ultramafic crust and H2O-CO2 atmosphere. The abiotic formation and accumulation of H2, methane, and short hydrocarbon would not only provide material basis for the chemical evolution towards life but also play essential roles in preventing the surface of the Earth from freezing in the Hadean eon while the Earth was suffering from extensive precipitation of atmospheric CO2 and the faint young sun.

Generally, the earliest interaction between the Earth's ultramafic crust and H2O-CO2 atmosphere could have changed the physicochemical condition of the Earth's surface that favored the prebiotic chemical evolution towards life.