ALTERATION EXPERIMENTS OF SILICATE MINERALS AND THE EARLY EARTH ATMOSPHERE

Surface environment of the early Earth is totally different from the present. It is assumed that the total pressure of the early atmosphere and the surface temperature are 26MPa (H₂O 20MPa, CO₂ 6MPa) and 130-330 degrees C, respectively. In this study, interactions between minerals of the early Earth and atmosphere, in which the very first rain of the Earth has just started to fall, are investigated experimentally. The experiments are carried out at constant pressure of 26MPa and under temperatures between 200 and 450 degrees C, covering a dew point of the fluid with the H₂O-CO₂ composition of 20:6. Run durations are 2-14 days. Starting materials are powdered natural olivine and mixture of olivine, orthopyroxene, diopside and plagioclase. The composition of the mixture is determined to resemble unfractionated early crust composition. Approximately 10 mg of starting materials are sealed in gold capsules with distilled water and silver oxalate that releases CO₂ over 250 degrees C. Weight ratio of the starting material and the H₂O-CO₂ fluid is unity. In the run products from the olivine starting material, magnesite (Mg100-75 Fe0-25) occurs between 240 and 330 degrees C. From the mixture starting material, dolomite-like carbonate mineral with a composition of Ca60-15 Mg35-70 Fe1-15 was formed at around 250 degrees C. Hydrous amorphous silicate materials are observed over 300 degrees C in the run products with both starting materials. Calcite or magnesite does not occur in the run products of the mixture starting material. The early Earth atmosphere should be extremely reactive with all of main mineral phases of the early crust, and formed dolomite and hydrous silicates.