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.