Halogens (F, Cl, Br, I) in altered ocean crust from the East Pacific Rise and SW Indian Ridge

Subduction-related melts are variably enriched in H₂O and halogens demonstrating that slab-derived fluids with variable salinity enrich the sub-arc mantle. In order to better understand the source of halogen-bearing slab fluids, we investigated the F, Cl, Br and I content of altered basalts and gabbros, which have previously been less well characterised for halogens than sediments and serpentinites.

Lavas and dykes from the 1500 m deep IODP Hole U1256D in the East Pacific Rise have concentrations of 130-300 ppm F, 106-1470 ppm Cl, 267 -3050 ppb Br and 5-22 ppb I. F shows little variation with depth, whereas Cl, Br and I, which preserve Br/Cl and I/Cl ratios surprisingly close to the mantle range, are more abundant in deep amphibolite facies alteration than at shallow levels.

Gabbros from the 800 m deep IODP Hole U1473A in the SW Indian Ridge have concentrations of <20-262 ppm F, 19-840 ppm Cl, 44-1240 ppb Br, 1-2490 ppb I, 0.2-8.0 wt. % H₂O and <0.05-2.7 wt. % CO₂. The concentrations of F, Cl and Br broadly track H₂O showing a slight decrease with depth. In contrast, the concentration of I tracks CO₂, with the maximums of 1500-2500 ppb I (that are similar to sedimentary rocks) between 200 and 400 mbsf where carbonate veins and clay-rich alteration have a high intensity. These samples have exceptionally high I/Cl ratios of 1000 times the mantle, but mantle-like Br/Cl. The association of I with red clay alteration and a strong correlation between CO₂ and I in felsic veins, implies that seawater-derived iodate might be retained in both clay and carbonate (as lauterite [Ca(IO₃)₂]) under oxidising conditions.

Together with previous studies, the new data show that dehydration of clay-rich alteration in the oceanic crust, organic-rich sediments or serpentinites can all generate slab fluids with high I/Cl ratios, but clay-rich alteration is distinguished from the other reservoirs by its low Br/Cl. Subducted components in back arc basins have variable signatures encompassing high I/Cl and low Br/Cl ratios that are similar to the clay-rich alteration (previously attributed to sediments or serpentinites); as well as mantle-like signatures that are similar to the strongly altered samples from Hole U1256D. Subduction of altered ocean crust could therefore be a major source of halogens in the mantle.