Depth variation of carbon and oxygen isotopes of calcites in Archean altered upperoceanic crust: Implications for the CO₂ flux from ocean to oceanic crust in the Archean

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• 作者：Takazo Shibuya^a ; ^{takazos@jamstec.go.jp} ; Miyuki Tahata^c ; Kouki Kitajima^d ; Yuichiro Ueno^c ; Tsuyoshi Komiya^e ; Shinji Yamamoto^c ; Motoko Igisu^e ; Masaru Terabayashi^f ; Yusuke Sawaki^c ; Ken Takai^a ; ^b ; Naohiro Yoshida^g ; ^h ; Shigenori Maruyama^c
• 关键词：Archean ; oceanic crust ; hydrothermal carbonation ; CO2 flux ; carbon and oxygen isotopes ; seawater composition
• 刊名：Earth and Planetary Science Letters
• 出版年：2012
• 期刊代码：18_0012821x
• 类别：ph
• 出版时间：1 March, 2012
• 卷：321-322
• 期：Complete
• 页码：64-73
• 文件大小：5212 K

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 CO₂ 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 未¹³C and 未¹⁸O values of calcite also change with the depth below the chert/BIF horizon. The 未¹⁸O value of calcite decreases with increasing depth, reflecting increasing temperature, while the 未¹³C value of calcite changes from positive to negative stratigraphically downward. A model reconstructing the 未¹³C 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 CO₂ in the whole oceanic crust per seafloor unit area (1.2 脳 10⁷ mol/m²). Based on this value, CO₂ flux from the ocean to the oceanic crust in the middle Archean can be estimated to be 1.5 脳 10¹⁴ mol/yr when the spreading rate (m²/yr) of Archean oceanic crust is assumed to have been three times higher than it is today. The estimated CO₂ 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.