Halogens in Atlantis Bank gabbros, SW Indian Ridge: implications for styles of seafloor alteration

The processes controlling halogen (F, Cl, Br, I) abundances in gabbroic ocean crust recovered from the 809-m deep Hole U1473A drilled on the Atlantis Bank during International Ocean Discovery Program (IODP) Expedition 360 were investigated. The aims were to provide new constraints on hydrothermal alteration and the abundances of halogens potentially transported to subduction zones in oceanic crust produced on a slow spreading ridge. Halogens in 51 gabbros and felsic veins have concentrations of ∼20-260 ppm F, 15-840 ppm Cl, 44-1230 ppb Br and 1-2490 ppb I. On average the gabbros retain a melt-derived magmatic F component of 58 ± 26% but are dominated by ∼96% hydrothermal Cl, Br, I and H O. The abundances of hydrothermal Cl, Br and I are consistent with alteration at a seawater/rock ratio of IO > OH > Cl ≥ Br ∼ ≥ I in the investigated alteration assemblage. EPMA and SHRIMP analyses indicate amphibole contains 1000-3000 ppm Cl in amphibolite facies alteration at the top of the Hole. Amphibole, chlorite and talc in greenschist facies alteration have much lower concentrations, typically in the range of 20-800 ppm Cl (median ∼100 ppm Cl), and F/Cl ratios o