HR: 1330h
AN: B52B-1043    [PDF]
TI: Subduction of the Carbonated Archean Oceanic-Crust and its Implication to Evolution of Earth's Early Atmosphere
AU: * Omori, S
EM: omori@geo.titech.ac.jp
AF: Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo, 1528551 Japan
AU: Kitajima, K
EM: saburo@geo.titech.ac.jp
AF: Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo, 1528551 Japan
AU: Maruyama, S
EM: smaruyam@geo.titech.ac.jp
AF: Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo, 1528551 Japan
AB: Geological studies in the Archean greenstone belt have shown that plate tectonics had already started at 3.8Ga, and the ocean floor hydrothermal alteration had formed much carbonate in oceanic crust before subduction [Ohta et al. 1996; Komiya et al. 1999; Nakamura 1999; Hayashi et al. 2000; Kitajima 2003; Shibuya 2003MS]. These observations suggest that the significant amount of carbon had carried into the mantle via the Archean subduction zone. In the present study, we investigated a mode of carbonation of oceanic metabasite in 3.5 Ga North Pole area (East Pilbara greenstone belt, W. Austraria) and estimated the amount of carbon dioxide fixed in the metabasite. Then, subduction flux of carbon dioxide carried into the deep mantle was estimated using phase diagrams for carbonated metabasite and peridotite with the geologically estimated thermal-structure model of the Archean subduction zone. In the North Pole area, carbonate-bearing mineral assemblage is restricted within upper 1000 m from the bottom of bedded chert indicating ancient ocean floor. The highly-carbonated sample consists of carbonate minerals of ca. 60-80 vol%, and mean content of the carbonate mineral in the carbonated zone is roughly 30 vol%. Under the Archean subduction-zone geotherm, the oceanic crust released CO$_{2}$ bearing fluid to the wedge mantle, and carbonate minerals such as magnesite were fixed in the peridotite wedge-mantle. The peridotite could contain about 2.6 wt% of CO$_{2}$ as carbonate, and the carbonates were stable in the dragged down-going peridotite during subduction. The subduction flux of CO$_{2}$ is estimated to be ca. 4.3E11 kg/y, if the production rate of MORB crust in the Archean was as same as in Meso-Cenozoic time. The balance of carbon flux shows that subduction of the carbonated crust reduced surface CO$_{2}$ by 3.4E11 kg/y. If this process continued for 1 billion year during the Archean, 3.4E20 kg of CO$_{2}$ was removed from the surface into the solid Earth. Using this value, we can constrain the primary CO$_{2}$ content in the atmosphere of the earliest Archean or the end of Hadean. The Hadean Earth may have not been covered by an ocean, due to the high XCO$_{2}$ in primary atmosphere. Removal of CO$_{2}$ from the atmosphere by the subduction of carbonated oceanic-crust may have worked much effectively in smaller planet, such as Mars. This process is possibly a critical factor to determine the fate of terrestrial planet after formation of the ocean.
DE: 5705 Atmospheres--evolution
DE: 6225 Mars
DE: 8125 Evolution of the Earth
DE: 9619 Precambrian
SC: Biogeosciences [B]
MN: 2003 Fall Meeting