Artificial chemical weathering of basaltic rock under the earth surface conditions of the present and the Proterozoic era

In order to compare the mineral chemical effects of acid rain on surface materials under the present oxygen level and the early Proterozoic or late Archean low oxygen (before the GOE) environmental conditions, artificial chemical weathering experiments using an improved Soxhlet extraction apparatus were conducted for basalt, which had already been covered on the early earth’s surface. Some dozens of polished basalt plates put in the extraction chamber were reacted to HCI, H₂S0₄ and HN0₃ solutions at pH 4, and CO₂ saturated water, and distilled water at 50℃ for a different period of time up to 950 days in an open system. In the experiment under the low oxygen condition (5×10⁻⁴ PAL), the whole extraction apparatus was placed in the acrylic glove box, and oxygen was removed by the deoxidizer, and it was carried out in the nitrogen gas flow. The basalt was composed mainly of olivine as a phenocryst, and plagioclase, clinopyroxene, ilmenite and glass as a groundmass. The extracted sample solutions were collected, and analyzed using ICP-MS. Morphological, chemistry and altered product of each mineral surface were studied by SEM, EPMA, XRD and microscopy techniques.

Under both the low oxygen before the GOE and the present oxygen concentration conditions, SEM images showed remarkable dissolution of olivine surface by the H₂SO₄, HNO₃ and HCl solutions. The (Mg + Fe)/Si on the olivine surface and (Na + Ca + K)/ (Al + Si) on the plagioclase surface decreased significantly with increasing experimental period. In chemistry of the extracted solutions, molar ratios of many elements such as Mg, K and Zn tend to be high in the three acidic solutions at pH 4, and low by the CO₂ saturated water and distilled water. The molar ratio is calculated by dividing the cumulative total mole of each extracted element by the mole of individual element in the unaltered basaltic rock. The ratios of Fe, Mg, Ni, Zn and Co near 70 pm in ionic radius are high, and reflect the dissolution from the octahedral coordination of olivine. The ratios of Ca, Na, Sm, Ce, La and Sr near 110 pm are high, and reflect the dissolution from the cavities within the framework of plagioclase. Under the low oxygen condition, major elements such as Fe and Mn, and minor ones such as Zn tend to dissolve easily in all extraction solutions. Ce and Eu in REE, and Nb, Ti, Y and Zr in HFS elements are soluble in pH 4 HCl and H₂SO₄, CO₂ saturated water and distilled water under the low oxygen condition. The results suggest that easily extracted elements under the low-oxygen condition of the early Proterozoic or late Archean influenced the evolution of continental crust, land and ocean, and may have contributed to the formation of the early Earth's surface environment.