Major, trace, and rare-earth element geochemistry of Early Archean (3.5 Ga) hydrothermally altered greenstone are reported in order to clarify geochemical features of seafloor hydrothermal alteration including carbonatization and silicification and to estimate CO₂ flux sunk into the altered oceanic crust by the hydrothermal carbonatization. The greenstone collected from the Marble Bar area, eastern Pilbara Craton, can be divided into five types on the basis of microscopic characteristics; dolerite, basalt, unidentified rock, highly silicified rock, and interpillow material. The dolerite is subdivided into relatively fresh and altered ones based on the presence or absence of primary clinopyroxene. The basalt and unidentified rock are subdivisible into carbonatized and non-carbonatized ones.
The whole-rock chemical composition of the relatively fresh dolerite is essentially similar to that of modern midocean ridge basalt (MORB) except for enrichments of large-ion-lithophile elements such as K₂O, Rb, and Ba. Compared to the relatively fresh dolerite, other altered rocks are enriched in K₂O, Rb, Ba and is depleted in Na₂O, reflecting the presence of K-mica that replaces the primary plagioclase. The CO₂ enrichment in the carbonatized rocks is attributed to the formation of carbonate minerals, indicating that significant amounts of CO₂ in the circulating seawater was added to the oceanic crust during the hydrothermal alteration. Moreover, relationships between CO₂ and CaO, MgO, Fe₂O₃* (total iron as Fe₂O₃), and MnO in the altered greenstone imply that there was essentially neither gain nor loss of Ca, Mg, Fe, and Mn during the carbonatization. This suggests that the oceanic crust trapped CO₂ in the hydrothermal solution by using Ca, Mg, Fe, and Mn in the oceanic crust.
Based on our results, a net sink of CO₂ in the oceanic crust by the Early Archean seafloor hydrothermal alteration is estimated to be (0.27-2.7)×10¹³mol/yr. This carbon flux can be comparable to the present total carbon flux by carbonate precipitation, carbon burial, and seafloor hydrothermal alteration, suggesting that the carbonatization of oceanic crust by the seafloor hydrothermal activity played an important role as a sink of CO₂ in the Early Archean atmosphere and ocean.

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