Depth variation of carbon and oxygen isotopes of calcites in Archean altered upperoceanic crust: Implications for the CO _2 flux from ocean to oceanic crust in the Archean

摘要

Middle Archean greenstones with mid-ocean ridge basalt affinity and overlying bedded chert/banded iron formation (BIF) are exposed in the Cleaverville area, Pilbara Craton, Western Australia. On the basis of the hydrothermal carbonation of these Cleaverville greenstones, we estimated the potential CO _2 flux from ocean to oceanic crust and the physical-chemical conditions of the subseafloor hydrothermal system for the middle Archean. The greenstones exhibit various extents of carbonation, and the igneous minerals contained in the greenstones are partly or completely replaced by calcite and other secondary minerals. The degreeof carbonation correlates with stratigraphy; the volume concentration of calcite in greenstones decreaseswith increasing depth below the chert/BIF horizon. Our results clearly indicate that the hydrothermal carbonation occurred along the axial zones of a middle Archean mid-ocean ridge. Both δ ~(13)C and δ ~(18)O values of calcite also change with the depth below the chert/BIF horizon. The δ ~(18)O value of calcite decreases with increasing depth, reflecting increasing temperature, while the δ ~(13)C value of calcite changes from positive to negative stratigraphically downward. A model reconstructing the δ ~(13)C of calcite suggests that the observed isotopic variation reflects the thermal structure and that the carbon source for calcite was derived from seawater. These mineralogical, geochemical, and geological features of calcite provide the amount of CO _2 in the whole oceanic crust per seafloor unit area (1.2×10 ~7mol/m ~2). Based on this value, CO _2 flux from the ocean to the oceanic crust in the middle Archean can be estimated to be 1.5×10 ~(14)mol/yr when the spreading rate (m ~2/yr) of Archean oceanic crust is assumed to have been three times higher than it is today. The estimated CO _2 flux into the oceanic crust is two orders of magnitude higher than the modern value, which points to the significance of sea-floor hydrothermal carbonation in the Archean carbon cycle.