Carbonatization of oceanic crust by the submarine hydrothermal activity and its significance as a CO2 sink in the early Archean (3.5Ga): I. Mineral description.

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Early Archean (3.5 Ga) hydrothermally altered greenstones from the Marble Bar area in the Pilbara craton, Western Australia, were investigated. Several successions of greenstone and hydrothermal bedded chert are exposed in this area. Greenstones in the study area are overlain by hydrothermal bedded cherts with a various thickness. The extensively altered greenstones are crosscut by massive chert veins that are considered as hydrothermal feeders. The greenstones are classified into two types based on its protolith; basalt and dolerite. The alteration mineral assemblage of dolerite (epidote+actinolite+chlorite+quartz+albite) indicates that the alteration condition was typical greenschist facies. On the other hand, the mineral assemblage of alteration minerals in basalts (carbonate minerals+K-mica+quartz+chlorite) implies that basalt was altered by the hydrothermal solution with high PCO2 and K+ concentration. Carbonate minerals of basalt (calcite+ankerite+siderite) were developed by the seafloor hydrothermal alteration. Geological and mineralogical features of these. carbonatized greenstones lead us to propose that large amounts of carbonate minerals were precipitated by the submarine hydrothermal alteration and that carbonatization in the oceanic crust played an important role as a CO2 sink in the early Archean atmosphere and ocean.
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