| Serpentinites contain significant amounts of chlorine (up to 1 wt% Cl), as well as 13 wt% H2O, and are stable to depths of 150-200 km in subduction zones, thereby making serpentinites a major carrier of chlorine and water into the deep mantle. Incorporation of chlorine from the serpentinite into the slab-derived fluid will alter the resulting fluid chemistry by generating a high-salinity fluid with a low water activity. Serpentinite dehydration is likely responsible for intermediate to deep-focus earthquakes and serpentinite-derived high-salinity fluid can drastically alter the trace element transport efficiency from slab-derived fluids to the mantle wedge, ultimately affecting metasomatic properties and water activities in metamorphic systems. Not only does serpentinite chemistry affect geodynamics, subduction zone geochemistry, and continental metasomatism, but is also a major component in the global chlorine cycle, potentially balancing the cycle.; This dissertation comprises six chapters, each investigating the role of serpentinite-hosted chlorine on subduction zone geochemistry, metasomatism, tectonics, and the global chlorine cycle. Serpentinites are shown to host Cl within multiple sites, including water-soluble salts, iowaite, fine-grained amorphous serpentine along bastite and relict olivine-serpentine mesh interfaces, and within bastite lamellae and chrysotile tubes. Chlorine concentration and chlorine isotope composition analyses on both watersoluble and structurally bound Cl serpentinites from a series of ODP/DSDP seafloor serpentinites and obducted serpentinites from Italy and Costa Rica show that delta 37Cl values in serpentinites are a sensitive indicator of the source of the serpentinizing fluid and can be used to decipher tectonic processes involved in serpentinization. The chlorine isotope and concentration data from serpentinites, as well as the additional delta37Cl measurements from meteorites, mantle-derived materials, and early Earth evaporites, are then used to model the secular global chlorine cycle. Serpentinites are shown to be a major contributor of Cl to the mantle, but isotopic work on volcanic gases is necessary to fully evaluate the model. |
