6. Geochemical Cycling of Fluorine, Chlorine, Bromine, and Boron and Implications for Fluid-Rock Reactions in Mariana Forearc, South Chamorro Seamount, ODP Leg 195¹

Wei Wei², Miriam Kastner², Annette Deyhle², and Arthur J. Spivack³

ABSTRACT

At the South Chamorro Seamount in the Mariana subduction zone, geochemical data of pore fluids recovered from Ocean Drilling Program Leg 195 Site 1200 indicate that these fluids evolved from dehydration of the underthrusting Pacific plate and upwelling of fluids to the surface through serpentinite mud volcanoes as cold springs at their summits. Physical conditions of the fluid source at 27 km were inferred to be at 100°–250°C and 0.8 GPa. The upwelling of fluid is more active near the spring in Holes 1200E and 1200A and becomes less so with increasing distance toward Hole 1200D. These pore fluids are depleted in Cl and Br, enriched in F (except in Hole 1200D) and B (up to 3500 µM), have low ¹¹B (16–21), and have lower than seawater Br/Cl ratios. The mixing ratios between seawater and pore fluids is calculated to be ~2:1 at shallow depth. The F, Cl, and Br concentrations, together with B concentrations and B isotope ratios in the serpentinized igneous rocks and serpentine muds that include ultramafic clasts from Holes 1200A, 1200B, 1200D, 1200E, and 1200F, support the conclusion that the fluids involved in serpentinization originated from great depths; the dehydration of sediments and altered basalt at the top of the subducting Pacific plate released Cl, H₂O, and B with enriched ¹⁰B. Calculation from B concentrations and upwelling rates indicate that B is efficiently recycled through this nonaccretionary subduction zone, as through others, and may contribute the critical missing B of the oceanic cycle.

¹Wei, W., Kastner, M., Deyhle, A., and Spivack, A.J., 2005. Geochemical cycling of fluorine, chlorine, bromine, and boron and implications for fluid-rock reactions in Mariana forearc, South Chamorro Seamount, ODP Leg 195. In Shinohara, M., Salisbury, M.H., and Richter, C. (Eds.), Proc. ODP, Sci. Results, 195 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/195_SR/106/106.htm>. [Cited YYYY-MM-DD]

²Scripps Institution of Oceanography, University of California San Diego, La Jolla CA 92093-0212, USA. Correspondence author: wewei@ucsd.edu

³Graduate School of Oceanography, University of Rhode Island, Narragansett RI 02882, USA.

Initial receipt: 19 January 2004
Acceptance: 10 May 2005
Web publication: 7 September 2005
Ms 195SR-106