Depth variation of carbon and oxygen isotopes of calcites in Archean altered upper oceanic crust: Implications for the CO2 flux from ocean to oceanic crust in the Archean
Middle Archean greenstones with mid-ocean ridge basalt affinity and overlying bedded chert/banded ironformation (BIF) are exposed in the Cleaverville area, Pilbara Craton, Western Australia. On the basis of thehydrothermal carbonation of these Cleaverville greenstones, we estimated the potential CO2 flux fromocean to oceanic crust and the physical–chemical conditions of the subseafloor hydrothermal system forthe middle Archean. The greenstones exhibit various extents of carbonation, and the igneous minerals containedin the greenstones are partly or completely replaced by calcite and other secondary minerals. Thedegree of carbonation correlates with stratigraphy; the volume concentration of calcite in greenstonesdecreases with increasing depth below the chert/BIF horizon. Our results clearly indicate that the hydrothermalcarbonation occurred along the axial zones of a middle Archean mid-ocean ridge. Both δ13C and δ18Ovalues of calcite also change with the depth below the chert/BIF horizon. The δ18O value of calcite decreaseswith increasing depth, reflecting increasing temperature, while the δ13C value of calcite changes from positiveto negative stratigraphically downward. A model reconstructing the δ13C of calcite suggests that the observedisotopic variation reflects the thermal structure and that the carbon source for calcite was derivedfrom seawater. These mineralogical, geochemical, and geological features of calcite provide the amount ofCO2 in the whole oceanic crust per seafloor unit area (1.2×107 mol/m2). Based on this value, CO2 flux fromthe ocean to the oceanic crust in the middle Archean can be estimated to be 1.5×1014 mol/yr when thespreading rate (m2/yr) of Archean oceanic crust is assumed to have been three times higher than it istoday. The estimated CO2 flux into the oceanic crust is two orders of magnitude higher than the modernvalue, which points to the significance of sea-floor hydrothermal carbonation in the Archean carbon cycle.
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