Carbonate veins trace seawater circulation during exhumation and uplift of mantle rock: Results from ODP Leg 209 Wolfgang Bach a, ⁎, Martin Rosner a, b, 1 , Niels Jöns c , Svenja Rausch a , Laura F. Robinson d , Holger Paulick e, 2 , Jörg Erzinger b a Geoscience Department, University of Bremen, 28359 Bremen, Germany b Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum GFZ, Telegrafenberg, D-14473 Potsdam, Germany c Geoscience Department and MARUM, University of Bremen, 28359 Bremen, Germany d Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA e Steinmann Institut, University of Bonn, Universität Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany abstract article info Article history: Received 10 February 2011 Received in revised form 7 September 2011 Accepted 13 September 2011 Available online 20 October 2011 Edited by R.W. Carlson Keywords: hydrothermal processes seawater circulation carbonate veining ocean-crust exchange Li isotopes age dating Carbonate veins hosted in ultramafic basement drilled at two sites in the Mid Atlantic Ridge 15°N area record two different stages of fluid-basement interaction. A first generation of carbonate veins consists of calcite and dolomite that formed syn- to postkinematically in tremolite–chlorite schists and serpentine schists that rep- resent gently dipping large-offset faults. These veins formed at temperatures between 90 and 170 °C (oxygen isotope thermometry) and from fluids that show intense exchange of Sr and Li with the basement ( 87 Sr/ 86 Sr = 0.70387 to 0.70641, δ 7 Li L-SVEC =+3.3 to +8.6‰). Carbon isotopic compositions range to high δ 13 C PDB values (+8.7‰), indicating that methanogenesis took place at depth. The Sr–Li–C isotopic composi- tion suggests temperatures of fluid-rock interaction that are much higher (T N 350–400 °C) than the temper- atures of vein mineral precipitation inferred from oxygen isotopes. A possible explanation for this discrepancy is that fluids cooled conductively during upflow within the presumed detachment fault. Aragonite veins were formed during the last 130 kyrs at low-temperatures within the uplifted serpentinized peridotites. Chemical and isotopic data suggest that the aragonites precipitated from cold seawater, which underwent overall little exchange with the basement. Oxygen isotope compositions indicate an increase in formation temperature of the veins by 8–12 °C within the uppermost ~80 m of the subseafloor. This increase corresponds to a high regional geothermal gradient of 100–150 °C/km, characteristic of young lithosphere undergoing rapid uplift. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Alteration of peridotite by circulation of seawater at slow-spreading mid-ocean ridges (MOR) has profound consequences for the thermal structure and rheology of the oceanic lithosphere, geochemical budgets of the ocean and the atmosphere and microbial processes within and at the seafloor. Carbonate veins are a common low- to moderate- temperature feature in all lithologies of altered ocean lithosphere. Previous studies have shown that the chemical and isotopic composi- tion of these carbonate veins can be used to gain information about the composition and physico-chemical properties of the precipitating fluid and the age of formation (e.g., Bonatti et al., 1980; Coggon et al., 2004; Eickmann et al., 2009; Früh-Green et al., 2003). Most previous studies of carbonates from altered ocean litho- sphere were focused on volcanic sections of crust generated at inter- mediate and fast spreading MORs (Alt and Teagle, 1999; Alt and Teagle, 2003; Staudigel et al., 1996). In these studies, it was proposed that the oceanic crust is an important sink for CO 2 due to the CO 2 -uptake during aging, with uptake rates on the order of 2·10 12 moles/yr (e.g., Alt and Teagle, 1999). Carbonate veining is about an order of magnitude less abundant in tectonically exhumed lower oceanic crust (Bach et al., 2001). The impact of vein carbonates hosted in ultramafic rocks on the global carbon budget, however, has not yet been assessed. This knowledge gap is critical, as 20–25% of the seafloor created along slow-spreading ridges is ultramafic in composition (Cannat et al., 2010), and ultramafic rocks are potentially important sites of CO 2 sequestration (Kelemen and Matter, 2008). To estimate carbonate vein abundance and reconstruct the physico- chemical conditions and timing of carbonate precipitation in ultramafic basement we studied carbonate veins from the slow-spreading Mid- Earth and Planetary Science Letters 311 (2011) 242–252 ⁎ Corresponding author. Tel.: + 49 421 218 65400. E-mail addresses: wbach@uni-bremen.de (W. Bach), martin.rosner@bam.de (M. Rosner), njoens@uni-bremen.de (N. Jöns), srausch@uni-bremen.de (S. Rausch), lrobinson@whoi.edu (L.F. Robinson), Holger.Paulick@Boliden.com (H. Paulick). 1 Now at: BAM Federal Institute for Materials Research and Testing, Department I. Analytical chemistry; reference materials, Unter den Eichen 87, 12205 Berlin, Germany. 2 Now at: Boliden Mineral AB, 93681 Boliden, Sweden. 0012-821X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2011.09.021 Contents lists available at SciVerse ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl