Serpentinization and Fluid Pathways in Tectonically Exhumed Peridotites from the Southwest Indian Ridge (62–65°E)

JOURNAL OF Journal of Petrology, 2015, Vol. 56, No. 4, 703–734 PETROLOGY Advance Access Publication Date: 27 April 2015 Original Article doi: 10.1093/petrology/egv014 Serpentinization and Fluid Pathways in Tectonically Exhumed Peridotites from the Southwest Indian Ridge (62–65E) Stéphane Rouméjon1*, Mathilde Cannat1, Pierre Agrinier1, Marguerite Godard2 and Muriel Andreani3 1 Institut de Physique du Globe de Paris, Sorbonne Paris Cité, CNRS-UMR7154, Paris, France, 2Géosciences Montpellier, Université Montpellier 2, CNRS-UMR5243, Montpellier, France and 3Laboratoire de Géologie de Lyon, CNRS-UMR5276, ENS-Université Lyon 1, France *Corresponding author. Present address: Centre for Geobiology, University of Bergen, Bergen, Norway. E-mail: Received October 9, 2014; Accepted March 16, 2015 ABSTRACT Peridotites exhumed in the footwall of axial detachment faults at slow-spreading ridges are highly serpentinized. Most mid-ocean ridge detachment settings are magmatically active and hydrous fluid circulation in and near the fault has been shown to be influenced by the presence of melt or magmatic lithologies. Our working area along the Southwest Indian Ridge (62–65 E) is nearly amagmatic and represents an end-member to study the hydrous alteration of exhumed peridotites without these magmatic influences. We use an integrated petrological approach combining microstructural, mineralogical and chemical observations to unravel the sequence of serpentinization in 272 dredged samples of variably serpentinized peridotites and to document the circulation of serpentinizing fluids in and near the exhumation faults. We find that serpentine recrystallization and veins overprint the initial serpentinite mesh texture in 25% of the samples. Oxygen isotope data suggest that this sequence developed at relatively high temperatures (271–336 C) and under increasing fluid–rock ratios, from near stoichiometry for mesh texture formation to >10 during recrystallization. Increasing fluid supersaturation relative to serpentine favors the replacement of mesh texture lizardite by chrysotile and polygonal or polyhedral serpentine. We attribute local recrystallization into antigorite to moderate Si-metasomatism, possibly following pyroxene serpentinization. We do not observe the more pronounced Si-metasomatism leading to talc replacing serpentine that is reported for the more magmatically active Mid-Atlantic Ridge detachment settings and is attributed to prior leaching of magmatic rocks. Scales of preferential fluid pathways in our samples evolved from pervasive and close-spaced (<500 mm) microfractures during the formation of the initial serpentine mesh texture, to centimeter-thick planar domains of enhanced fluid flux, spaced at 10 cm intervals and probably grouped in corridors that may be up to 100 m across. Serpentine minerals are enriched in some fluid-mobile elements (Cl, B, U) relative to the peridotite protolith, and several elements (Al, Fe, Si, Cu, As, Sb, REE) are redistributed at the millimeter to decimeter scale. Serpentinizing fluids were seawater-derived, probably mildly alkaline (small to no europium anomalies), reducing and H2-enriched (formation of magnetite). These fluids may have been similar to, though warmer than, those venting at the ultramafic-hosted Lost City hydrothermal fluid (30 N, Mid-Atlantic Ridge). Key words: serpentinization; slow-spreading ridges; fluid–rock interactions; fluid pathways C The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: V 703 704 INTRODUCTION Serpentinized peridotites, associated with variable amounts of intrusive gabbros, crop out extensively at slow-spreading ridges (Karson et al., 1987; Dick, 1989; Cannat et al., 1992, 1995b; Dick et al., 2003; Michael et al., 2003; Seyler et al., 2003; Kelemen et al., 2004; Sauter et al., 2013). These rocks are tectonically exhumed on-axis (Karson, 1990; Cannat et al., 1992) by large offset normal faults also called detachment faults (Cann et al., 1997; Tucholke et al., 1998; Lavier et al., 1999). These detachments uplift fresh peridotites from the base of the brittle lithosphere toward shallower levels where hydrothermal circulation is active, leading to intense fluid–rock interaction and abundant serpentinization. Compared with peridotites, serpentinites have a weaker rheology (Reinen et al., 1994; Escartı́n et al., 1997), and distinct magnetic properties (Toft et al., 1990; Oufi et al., 2002). Serpentinization reactions may be accompanied by 30% volume increase (Hostetler et al., 1966; Coleman, 1971; O’Hanley, 1992) and result in a significant decrease in density and seismic velocities (Christensen, 1972; Miller & Christensen, 1997). Serpentinization reactions are exothermic and may fuel low-temperature hydrothermal circulation (Kelley et al., 2001; Lowell & Rona, 2002; Früh-Green et al., 2003; Allen & Seyfried, 2004). Although serpentinization reactions are commonly described as isochemical with respect to major elements (Coleman & Keith, 1971), they release significant amounts of H2 into the water column (e.g. Charlou et al., 2002), possibly feeding microbial ecosystems on the seafloor (Shock & Holland, 2004). Serpentine minerals also incorporate fluid-mobile trace elements that are derived from seawater (e.g. B, Li, C; Bonatti et al., 1984; Decitre et al., 2002; Früh-Green et al., 2004; Boschi et al., 2008; Delacour et al., 2008b; Vils et al., 2008) and from prior mineral leaching by the serpentinizing fluids [e.g. S, As, Sb, light rare earth elements (LREE); Alt & Shanks, 2003; Paulick et al., 2006]. These elements are stored in the serpentinized portion of the oceanic lithosphere and recycled into the mantle at subduction zones (Vils et al., 2008; Deschamps et al., 2010, 2013). Fluid–rock interactions leading to serpentinization have been studied using both petrological investigations on serpentinized peridotite samples (Früh-Green et al., 1996; Agrinier & Cannat, 1997; Bach et al., 2004; Boschi et al., 2006; Paulick et al., 2006) and analyses of fluids escaping from ultramafic-hosted hydrothermal vents (Kelley et al., 2001, 2005; Douville et al., 2002; Schmidt et al., 2007), with contributions from experimental and theoretical works (e.g. Wetzel & Shock, 2000; Allen & Seyfried, 2003; Foustoukos et al., 2008). Altogether, these studies show that serpentinization reactions may occur under variable conditions (temperature, pH, redox, silica activity) and that prior interactions with magmatic lithologies may modify the temperature and the composition of the serpentinizing Journal of Petrology, 2015, Vol. 56, No. 4 fluids. End-member hydrothermal fluids venting at ultramafic-hosted black-smoker type vents result from interactions with hot magmatic rocks and are high-temperature (365 C), low-pH and Si-rich (Douville et al., 2002; Schmidt et al., 2007; Seyfried et al., 2007, 2011). So far, end-member mid-ocean ridge hydro (...truncated)