Carbon Cycle in the Archean Plate Tectonics: Hydrothermal Carbonation, Metamorphic Decarbonation and Storage in the Mantle

Geological studies in the Archean greenstone belt have shown that hydrothermal alteration at mid-oceanic ridge (MOR) had caused extensive carbonation in the oceanic crust. Hence, the MOR hydrothermal carbonation and subsequent subduction-zone metamorphism had an important role in the Archean carbon cycle. The aim of this study is phase-petrological characterization of carbonation-decarbonation processes in the Archean ocean floor and subduction zone, and incorporate these processes to the Archean carbon cycle model. Basic parameters for bulk rock-composition and physicochemical conditions of the hydrothermal process are taken from those of greenstones in the 3.5Ga North Pole area, Pilbara Craton, Australia. In the North Pole area, carbonate-bearing mineral assemblage is restricted within upper 1 km from the bottom of bedded chert which represents ancient ocean floor. The mean modal abundance of carbonate mineral reaches to roughly 30 vol. percent in the carbonated-zone. A pressure condition of the hydrothermal carbonation at the MOR is estimated to be about 30 MPa, and the temperature of the hydrothermal fluid was 350 degC. Amount of the carbonate depends on XCO2 of the source hydrothermal fluid and rock/fluid ratio, at constant P-T condition. The amount of the carbonate changes intermittently as a function of XCO2, because the stability of carbonate is constrained by discontinuous reactions. Stability of carbonate minerals in the oceanic crust and hanging wall peridotite in the subduction zone is estimated including transfer of fluid from the oceanic crust to the wedge mantle. During the subduction-zone metamorphism along the Archean subduction-zone geotherm, the carbonated oceanic crust releases H2O-CO2 fluids with various XCO2 to the wedge mantle, and the fluid reacts with hanging-wall peridotite to form carbonates again. Although the oceanic crust could not keep CO2 as carbonate in the Archean high-T subduction, the carbonate was still stable in the peridotite. The peridotite could store about 6 wt percent of CO2. Assuming that the hanging-wall peridotite was dragged down to deep mantle with the subducting plate, the subduction flux of CO2 into the deep mantle is estimated to be 1E12 kg/y. A flux-reservoir model calculation showed that the Archean plate tectonic sub-system worked as a CO2 remover from the Archean atmosphere. This process compensated increasing solar luminosity and kept the ocean planet Earth.