Alteration and weartering processes in the early terrestrial crust

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The materials of the terrestrial crust have contineously been weathered and altered by water since its formation. These phenomena are the most characteristic features of material evolution on the earth in the present solar system. However, the process of the weathering/alteration can not easily be traced because of the following volcanic, tectonic activities on the earth. We can see some products of the proteozoic alteration/weathering activities in the archean anorthosite complex in Superior Province, Canada. In order to simulate the proteozoic environment, we carried out the dissolution experiments on the single crystals of olivine, pyroxene and plagioclase, where the oriented thin specimens were used. Changing the experimental conditions (temperature, ph, time and orientation), the dissolution rate was measured and the surface of the reacted crystal was precisely observed. It can be concluded that the microtextures such as twinning or cleavage contribute very much on the dissolution rate and that the anisotropy of the crystal structure plays essential roll on the dissolution process.