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

Abstract

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 H2O. The abundances of hydrothermal Cl, Br and I are consistent with alteration at a seawater/rock ratio of <1. However, hydrothermal F is more enriched than expected and some amphiboles have high F/Cl ratios of 10-30 that provide evidence for the minor additional involvement of F-rich magmatic fluids. The abundant late-stage felsic veins that transect the gabbros and account for 1.5 vol.% of Hole U1473A lithologies are suggested as the most likely source of F-rich magmatic fluids.Downhole variations show F, Cl, Br and H2O are most abundant in amphibole- and clay-rich alteration zones at the top of the hole. The highest I concentrations of 1-2.5 ppm delineate an oxidised CO2-rich alteration zone in which seawater iodate was incorporated into carbonate and Fe-oxyhydroxide alteration. The role of iodate, which is more reactive than other halides, in generating I-rich alteration can be distinguished from alteration by I-rich sedimentary pore waters because the oxidised alteration is characterised by high I/Cl together with low Br/Cl, whereas organic matter in pore waters is enriched in both Br/Cl and I/Cl.The halogens have inferred compatibilities of F− > IO3− > 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 of <4. In comparison, low temperature limonite (FeOOH.nH2O) has 160-8500 ppm Cl (median ∼840 ppm Cl). Amphibole is the dominant host of Cl and F in amphibolite facies alteration, but chlorite and limonite are important at lower grades. The samples have fairly constant Br/Cl ratios suggesting that Br excluded from the amphibole lattice is retained in minerals such as chlorite and/or non-structural sites. High I concentrations of 10's of ppm are inferred for some carbonate and limonite. Overall amphibole is a less dominant host of halogens than has been suggested previously, which has important implications for the eventual release and availability of halogens during subduction zone metamorphism.