Depth variation of carbon and oxygen isotopes of calcites in Archean altered upper oceanic crust: Implications for the CO 2 flux from ocean to oceanic crust in the Archean Takazo Shibuya a, ⁎, 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 a Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan b Subsurface Geobiology Advanced Research (SUGAR) project, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan c Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan d Department of Geoscience, University of Wisconsin-Madison, 1215 W Dayton St. Madison WI 53706, USA e Department of Earth Science and Astronomy, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan f Department of Safety Systems Construction Engineering, Kagawa University, Kagawa 761-0396, Japan g Department of Environmental Science and Technology, Tokyo Institute of Technology, G1-25, 4259 Nagatsuta, Yokohama, 226-8502, Japan h Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, G1-25, 4259 Nagatsuta, Yokohama, 226-8502, Japan abstract article info Article history: Received 31 May 2011 Received in revised form 2 November 2011 Accepted 22 December 2011 Available online xxxx Editor: G. Henderson Keywords: Archean oceanic crust hydrothermal carbonation CO 2 flux carbon and oxygen isotopes seawater composition 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 con- tained in the greenstones are partly or completely replaced by calcite and other secondary minerals. The degree of carbonation correlates with stratigraphy; the volume concentration of calcite in greenstones decreases with increasing depth below the chert/BIF horizon. Our results clearly indicate that the hydrother- mal 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 posi- tive to negative stratigraphically downward. A model reconstructing the δ 13 C of calcite suggests that the ob- served 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 7 mol/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. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Ancient hydrothermally altered seafloor basalts potentially pro- vide key information about the physical and chemical conditions of subseafloor hydrothermal systems and chemical fluxes between seawater and basaltic crust (e.g., Coggon et al., 2010; Holmden and Muehlenbachs, 1993). Well preserved Archean greenstones are com- monly characterized by pervasive carbonation, silicification, sericitiza- tion, albitization, and chloritization, and these provide important clues to the physical and chemical traits of ancient seawater and/or the hy- drothermal fluids that altered the greenstones (Duchač and Hanor, 1987; Hofmann and Harris, 2008; Kitajima et al., 2001; Nakamura and Kato, 2004; Shibuya et al., 2007a; Terabayashi et al., 2003). Among these alterations, the strong carbonation of Archean oceanic crust has been suggested to be evidence for CO 2 -rich seawater in the past (Kitajima et al., 2001; Nakamura and Kato, 2004; Shibuya Earth and Planetary Science Letters 321-322 (2012) 64–73 ⁎ Corresponding author at: Precambrian Ecosystem Laboratory, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan. Tel.: +81 46 867 9647; fax: +81 46 867 9645. E-mail address: takazos@jamstec.go.jp (T. Shibuya). 0012-821X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2011.12.034 Contents lists available at SciVerse ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl