Experimental study on weathering of basalt in the present atmospheric and the early Proterozoic low oxygen environments under surface conditions

In order to compare the mineral chemical effects of acid rain on surface materials in the present oxygen level and the Proterozoic low oxygen environments, artificial chemical weathering experiments using an improved Soxhlet extraction apparatus were conducted for basalt, which had been covered even on the early earth 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 degrees Celsius for a different period of time up to 950 days in an open system. In the experiment under the low oxygen environment (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, CPX, ilmenite and glass as a groundmass. The leached 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 Proterozoic low oxygen and the present environments, SEM images showed that the dissolution of olivine surface by the H₂SO₄, HNO₃ and HCl solutions is remarkable. 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 leached solutions, molar ratios of Mg, Mn, Fe and so on were high by three kinds of acidic solutionsat pH 4, and low by CO₂ saturated water and distilled water. The molar ratio is calculated from the cumulative total mole of released elements divided by the mole of individual elements in the unaltered basaltic rock. The ratio of Fe, Mg, Ni, Zn and Co near 70 pm in ionic radius is high, and reflect the dissolution from the octahedral coordination of olivine. The ratios of Ca, Na, Sm, Ce, La and Sr near 110 pm is high, and reflect the dissolution from the cavities within the framework of plagioclase. Under the low oxygen environment, major elements such as Fe, Mg, Mn, Ca, Na, and minor ones such as Ni, Co, Zn tend to dissolve easily. REE is soluble in pH 4 HCl, CO₂ saturated water and distilled water under the low oxygen environment except for HNO₃ solution, and does not change in H₂SO₄ under both environments. The leaching ion concentrations of Mn, Cu, Zn, Ni, Co and REE associated with marine resources such as manganese nodules and BIF under the low oxygen are higher than those in the present average seawater.