Major ion concentrations and chlorine isotope composition of water-soluble chloride and structurally bound chloride in DSDP/ODP serpentinized ultramafic rocks

• Main Authors: Barnes, Jaime D, Sharp, Zachary D
• Format: Other/Unknown Material
• Language: English
• Published: PANGAEA 2006
• Subjects: 107-651A; 109-670A; 147-895E; 153-920D; 173-1068A; 82-558; 84-566; 84-570; Deep Sea Drilling Project; DRILL; Drilling/drill rig; DSDP; Glomar Challenger; Joides Resolution; Leg107; Leg109; Leg147; Leg153; Leg173; Leg82; Leg84; North Atlantic/RIDGE; North Atlantic Ocean; North Pacific/SLOPE; North Pacific Ocean; Ocean Drilling Program; ODP; Tirreno Sea; North Atlantic
• Online Access: https://doi.pangaea.de/10.1594/PANGAEA.704766; https://doi.org/10.1594/PANGAEA.704766

Eight DSDP/ODP cores were analyzed for major ion concentrations and d37Cl values of water-soluble chloride (d37Clwsc) and structurally bound chloride (d37Clsbc) 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 (Xwsc) and 0.09±0.09 wt.% as structurally bound Cl (Xsbc)). 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‰ to +1.16‰, averaging to +0.21‰. 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 d37Cl values (+0.05‰ to +0.36‰); the other two cores (173-1068A (Leg-Hole) and 84-570) have negative bulk d37Cl values (-1.26‰ and -0.54‰). The cores with negative d37Cl values also have variable Cl**-/SO4**2- 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 ~1‰ with depth for both the water-soluble and structurally bound Cl fractions. Non-zero bulk d37Cl values indicate Cl in serpentinites was incorporated during original hydration and is not an artifact of seawater infiltration during drilling. Cores with positive d37Cl values are most likely explained by open system fractionation during hydrothermal alteration, with preferential incorporation of 37Cl from seawater into the serpentinite and loss of residual light Cl back to the ...