CHLORINE STABLE ISOTOPIC COMPOSITION OF SERPENTINITES
Cl- concentration and d37Cl values of water-soluble (wsCl) and structurally-bound Cl- (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 d37Cl values of 0.0 to +0.5 , in which sbCl averages ~+0.21 heavier than wsCl. Two of the cores (173-1068A and 84-570) have negative d37Cl values: 1.49 and 0.55 (wsCl) and 0.98 and 0.55 (sbCl), respectively.
There are no correlations between Cl content or d37Cl values and tectonic setting and geographic location. Samples were analyzed at closely spaced intervals in three cores: d37Cl values decrease by 1 with depth (~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 d37Cl value. If mantle Cl contribution is ~+4.7 (Magenheim et al., 1995) and serpentinite Cl removal is ~0.0 , then the d37Cl value of the ocean should be increasing by ~3 /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 d37Cl values of ~0 .
An alternative explanation for low d37Cl 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 d37Cl values. The preserved negative isotope signature of serpentinites indicates low fluid/rock ratios.
If mantle d37Cl 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. d37Cl values of arc volcanic gases may provide constraints on Cl cycling in subduction zones.