Abstract:
Cl (super -) concentration and delta (super 37) Cl values of water-soluble (wsCl) and structurally-bound Cl (super -) (sbCl) were determined for serpentinites from eight ODP/DSDP cores varying in tectonic setting, geographic location, depth in core and age. The average total Cl content is 0.27 wt% (0.19 wt% wsCl; 0.08 wt% sbCl; n=86). The majority of samples have delta (super 37) Cl values of 0.0 to +0.5ppm, in which sbCl averages approximately +0.21ppm heavier than wsCl. Two of the cores (173-1068A and 84-570) have negative delta (super 37) Cl values: -1.49ppm and -0.55ppm (wsCl) and -0.98ppm and -0.55ppm (sbCl), respectively. There are no correlations between Cl content or delta (super 37) Cl values and tectonic setting and geographic location. Samples were analyzed at closely spaced intervals in three cores: delta (super 37) Cl values decrease by 1ppm with depth ( approximately 60 m) in one core and remain constant in the other two; Cl content increases, decreases, and remains constant in different cores. Most serpentinites have a near-seawater (super 37) Cl value. If mantle Cl contribution is approximately +4.7ppm (Magenheim et al., 1995) and serpentinite Cl removal is approximately 0.0ppm, then the delta (super 37) Cl value of the ocean should be increasing by approximately 3ppm/Ga. The isotopically negative cores are two of the oldest analyzed, consistent with the hypothesis of oceanic Cl isotope temporal evolution, but not with our observations that old evaporites have delta (super 37) Cl values of approximately 0ppm. An alternative explanation for low (super 37) Cl values in the two cores is that serpentinization occurred by influx of sedimentary pore waters, rather than seawater. The light cores are distinguished by a thick overlying sedimentary package in direct contact with ultramafics. Rifting/faulting postdate sediment deposition exposing the underlying ultramafics to sedimentary pore waters, previously shown to have negative delta (super 37) Cl values. The preserved negative isotope signature of serpentinites indicates low fluid/rock ratios. If mantle (super 37) Cl values are non-zero, then the near-zero value of serpentinites (excepting the light cores) can be used to identify seawater subduction and interaction with subduction-related metamorphism and volcanoes. For example, chlorine isotopic analysis of highly-saline fluid inclusions in Alpine eclogites may confirm deep recycling of seawater-derived fluids in these rocks. delta (super 37) Cl values of arc volcanic gases may provide constraints on Cl cycling in subduction zones.