A chlorine isotope study of DSDP/ODP serpentinized ultramafic rocks: Insights into the serpentinization process

Eight DSDP/ODP cores were analyzed for major ion concentrations and delta Cl-37 values of water-soluble chloride (delta Cl-37(WSC)) and structurally bound chloride (delta Cl-37(SBC)) in serpentinized ultramafic rocks. This diverse set of cores spans a wide range in age, temperature of serpentinization, tectonic setting, and geographic location of drilled serpentinized oceanic crust. Three of the cores were sampled at closely spaced intervals to investigate downhole variation in Cl concentration and chlorine isotope composition. The average total Cl content of all 86 samples is 0.26 +/- 0.16 wt.% (0.19 +/- 0.10 wt.% as water-soluble Cl (X-WSC) and 0.09 +/- 0.09 wt.% as structurally bound Cl (X-SBC)). Structurally bound Cl concentration nearly doubles with depth in all cores; there is no consistent trend in water-soluble Cl content among the cores. Chlorine isotope fractionation between the structurally bound Cl- site and the water-soluble Cl- site varies from -1.08 parts per thousand to + 1.16 parts per thousand, averaging to +0.21 parts per thousand. Samples with negative fractionations may be related to reequilibration of the water-soluble chloride with seawater post-serpentinite formation. Six of the cores have positive bulk delta Cl-37 values (+0.05 parts per thousand to +0.36 parts per thousand); the other two cores (173-1068A (Leg-Hole) and 84-570) have delta Cl-37 delta Cl-37 Cl-/SO42 negative bulk values (-1.26 parts per thousand and -0.54 parts per thousand). The cores with negative values also have variable 4 ratios, in contrast to all other cores. The isotopically positive cores (153-920D and 147-895E) show no isotopic variation with depth; the isotopically negative core (173-1068A) decreases by similar to 1 parts per thousand with depth for both the water-soluble and structurally bound Cl fractions. Non-zero bulk delta Cl-37 values indicate Cl in serpentinites was incorporated during original hydration and is not an artifact of seawater infiltration during drilling. Cores with positive delta Cl-37 values are most likely explained by open system fractionation during hydrothermal alteration, with preferential incorporation of Cl-37 from seawater into the serpentinite and loss of residual light Cl back to the ocean. Fluid/rock ratios were probably low as evidenced by the presence of water-soluble salts. The two isotopically negative cores are characterized by a thick overlying sedimentary package that was in place prior to serpentinization. We believe the low delta Cl-37 values of these cores are a result of hydration of ultramafic rock by infiltrating aqueous pore fluids from the overlying sediments. The resulting serpentinites inherit the characteristic negative delta Cl-37 values of the pore waters. Chlorine stable isotopes can be used to identify the source of the serpentinizing fluid and ultimately discern chemical and tectonic processes involved in serpentinization. (c) 2005 Elsevier B.V. All rights reserved.