Chlorine isotopic composition in seafloor serpentinites and high-pressure metaperidotites. Insights into oceanic serpentinization and subduction processes
Introduction
Volatile elements exert a strong influence on the chemical and physical properties of the Earth’s mantle. The study of their geodynamic cycles generally helps constrain the past and current evolution of the Earth. Chlorine is especially valuable in understanding the processes involved during exchanges between Earth’s reservoirs (e.g., melting, recycling, degassing, differentiation) since it behaves as an incompatible, soluble, and volatile element. Chloride is the major anionic species in seawater and Cl is present as a trace element in the upper mantle (less than 10 ppm; e.g., Michael and Schilling, 1989, Jambon et al., 1995, Michael and Cornell, 1998, Saal et al., 2002, Le Fèvre and Ottolini, 2005). Exchanges between mantle and surface reservoirs over time may, for example, have induced drastic modifications in ocean salinity. In spite of this importance, mantle degassing and recycling of altered oceanic lithosphere still remain poorly constrained, limiting the possibility to reach conclusive statements on the chlorine cycle.
The oceanic crust is a sink for seawater chlorine. Chlorine is generally weakly concentrated in fresh oceanic lithosphere (usually <200 ppm Cl in MORB glasses; e.g., Jambon et al., 1995, Michael and Cornell, 1998; and <10 ppm for fresh residual peridotites). In contrast, secondary minerals formed after seawater–rock interactions (e.g., amphibole, smectite, serpentine) can contain high amounts of chlorine (usually >500 ppm Cl; e.g., Rucklidge and Patterson, 1977, Ito et al., 1983, Vanko, 1986; for serpentine see review in Mével, 2003). Chlorine may be substituted for hydroxyl groups in mineral structures (e.g., Volfinger et al., 1985, Oberti et al., 1993, Anselmi et al., 2000) or occur in a weakly bound position, easily removable by water leaching (e.g., Rucklidge and Patterson, 1977, Seyfried et al., 1986, Sharp and Barnes, 2004, Bonifacie et al., 2005a).
The mechanisms of Cl incorporation in minerals and the behavior of water-soluble and structurally-bound chlorine during devolatilization of the subducted slab are poorly understood. Based on Cl measurements in high-pressure subducted rocks, Scambelluri et al., 1997, Scambelluri et al., 2004, Philippot et al., 1998 suggested that significant amounts of the subducting Cl may be recycled down to the deep mantle. The quantification of Cl fluxes between surface reservoirs and the Earth’s interior nevertheless remains highly uncertain. Estimates of the Cl flux output from the mantle (ridge-axis, intra-plate, arc, and back-arc magmatism) range between 2.7 and 43.8 × 1012 g yr−1 depending on assumed Cl content in the considered lithologies (e.g., Schilling et al., 1978, Jambon et al., 1995, Jarrard, 2003). Excluding arc volcano sources, where the majority of Cl likely derives from subducting altered oceanic lithosphere, the Cl flux output from the mantle ranges from ∼3.3 to ∼20 × 1012 g yr−1 (Ito et al., 1983, Jarrard, 2003). Estimates of the Cl flux input to subduction zones range from ∼2.5 × 1012 g yr−1 (Ito et al., 1983, Jarrard, 2003) to ∼12 × 1012 g yr−1 if both structurally bound and water-soluble Cl from serpentinites are included (Sharp and Barnes, 2004). These large uncertainties in Cl fluxes considerably limit the possibility to reach conclusive statements relative to the Cl geochemistry on Earth but could be overcome by systematic studies in these various geodynamic settings.
Chlorine stable isotope compositions (δ37Cl) may provide further insight into Cl geodynamics if isotopic compositions of Cl output from and input to the mantle are compared. Contrasting with approaches based on the quantification of Cl fluxes, the comparison of isotopic compositions is relatively easy because it deals with an intensive property of Cl. The isotopic approach requires, however, knowledge of the isotopic fractionations that may occur between the chlorine-bearing phases involved. Previous studies used Cl isotopes to characterize Cl geochemical behavior and fate in various geodynamic settings (e.g., Ransom et al., 1995, Magenheim et al., 1995, Spivack et al., 2002, Bonifacie et al., 2004, Bonifacie et al., 2005b, Bonifacie et al., 2007b, Godon et al., 2004a, Sharp and Barnes, 2004, Bonifacie, 2005). These studies provide preliminary estimates of Cl isotopic compositions in Earth’s reservoirs (e.g., mantle, oceanic crust, seawater) and Cl isotope fractionations during exchange of chlorine between those reservoirs (e.g., alteration, subduction). Chlorine isotopes have also been suggested as tracers for reconstructing seafloor tectonic settings of serpentinization (Barnes and Sharp, 2006). Most serpentinites overlain by sediments supposed to be in place before serpentinization show negative δ37Cl values interpreted as resulting from interaction with sediment pore-waters (Barnes and Sharp, 2006) that have negative δ37Cl values (e.g., Ransom et al., 1995, Hesse et al., 2000, Godon et al., 2004a, Bonifacie et al., 2007b). In contrast, serpentinites with positive δ37Cl values are suggested to result from direct interaction with seawater (Barnes and Sharp, 2006) that has δ37Cl of 0‰ (Kaufmann et al., 1984, Godon et al., 2004b). A comparable pattern to that recorded in seafloor serpentinites was recently observed in serpentinites metamorphosed to low-pressure (LP) and high-temperature (HT) conditions (greenschist facies) and was used to suggest that low-grade metamorphism does not fractionate Cl isotopes (Barnes et al., 2006). Chlorine isotope data on rocks subducted to high-pressure (HP) and low-temperature (LT) conditions are still lacking although they may represent the best way to assess the composition of Cl recycled to the mantle.
In order to provide constraints on the global chlorine cycle, our study focuses on serpentinized peridotites. These rocks represent an ideal candidate for Cl transfer to the mantle because (i) they are the main Cl carrier in oceanic lithosphere and (ii) serpentine minerals can be stable down to 200 km subduction depth, depending on the geothermal gradient (Ulmer and Trommdsdorf, 1995). Two HP metasediments were also analyzed in order to give preliminary insights on the potential contribution of sediments to the recycling of Cl to the mantle. This paper reports in situ analyses of insoluble Cl content in oceanic serpentinites together with whole-rock Cl content and δ37Cl data on both oceanic serpentinites and HP metaperidotites. The data are used to determine the mechanisms of Cl incorporation in serpentinites and to investigate potential Cl isotope fractionations associated with seafloor serpentinization and serpentine dehydration during subduction.
Section snippets
Sample description
Table 1 summarizes the mineralogical assemblages of the samples investigated in the present study.
The studied oceanic serpentinites were collected from two locations (Fig. 1). A first suite was collected with the submersible Nautile from the western wall of the Mid-Atlantic Ridge South of the Kane fracture zone (MARK area, Mével et al., 1991). The second suite was dredged from the wall of the ultraslow spreading South West Indian Ridge (Decitre et al., 2002). The oceanic samples contain between
Methods
The analytical procedure used for measuring bulk-rock Cl content and isotopic composition in silicate samples has been described and validated in Bonifacie et al. (2007a) and is only summarized herein. The main steps consist of extraction of bulk chlorine (both soluble and insoluble fractions) from whole rocks by pyrohydrolysis of sample powders (granulometry <160 μm) and chloride transformation into CH3Cl gas for isotope ratio determination by gas-source, dual-inlet mass spectrometry (
Results
Table 1 reports bulk-rock Cl content and isotope composition of the investigated samples. Water contents are also reported in order to compare the behavior of Cl to the main volatile component. Table 2 shows major elements and Cl contents in various occurrences of serpentine (i.e., meshes replacing olivines, bastites replacing orthopyroxenes and hydrothermal veins) from oceanic serpentinites DR23-2-8 and DR69-1-14.
In this study, oceanic serpentinites show relatively large variations in both
Cl distribution in seafloor serpentinites and HP metaperidotites
Because Cl behaves as an incompatible element during partial melting (Schilling et al., 1980), fresh residual peridotites should have a lower Cl content than that estimated for the pristine mantle (<10 ppm Cl; e.g., Michael and Schilling, 1989, Jambon et al., 1995, Michael and Cornell, 1998, Saal et al., 2002). Bulk-rock Cl contents of serpentinized peridotites (average value 1105 ± 596 ppm Cl, 1σ; this study) are thus considerably higher than that of fresh oceanic peridotites. This concentration
Conclusions
The present data provide preliminary constraints on insoluble Cl speciation in oceanic serpentinites. Although the mechanism of Cl incorporation into serpentine remains unclear, this study shows that Cl distribution is heterogeneous at both sample and mineral scales and that Cl contents are higher in bastites than in meshes. This likely reflects the distortion of the serpentine structure in bastites (where Al is substituted for Si) and that insoluble Cl is present substituting for hydroxyl in
Acknowledgments
We thank S. Decitre for discussions and providing us with pre-characterized oceanic serpentinites. We thank F. Pineau, O. Oufi and M. Ader for helpful suggestions and constructive comments on this work. We are also grateful to J. Alt and three anonymous reviewers for their fruitful comments and suggestions on this manuscript. We wish to thank the captains and crews of the cruises who helped to recover samples; M. Girard and J.J. Bourrand for mass spectrometry/technical assistance; M. Evrard for
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