Halogens in altered ocean crust from the East Pacific Rise (ODP/IODP Hole 1256D)

The halogens (F, Cl, Br and I), HO and CO were investigated in eighteen representative lavas, dykes and a gabbro spanning depths 750–1450 mbsf in the International Ocean Discovery Project (IODP) Hole 1256D. Whole rock analyses of halogens, HO and CO, and in situ F and Cl electron microprobe analyses were combined to provide new information about hydrothermal alteration of the ocean crust and the mineral controls on the abundances of all four halogens in altered ocean crust that is subducted into the mantle. Whole rock concentrations of halogens, HO and CO are heterogeneous at all crustal levels with maxima in highly altered zones of fluid infiltration. Nonetheless, Cl and Br show a general increase down the hole from minima of 130 ppm Cl and 330 ppb Br in the lavas, with dominant saponite-chlorite alteration, to maxima of 1620 ppm Cl and 2720 ppb Br in amphibolite facies granoblastic dykes and the gabbros near the base of the hole. In contrast, HO concentrations of 1.5–2 wt% were common in clay-rich (saponite) alteration with lower values in the deeper amphibole-rich samples. The concentrations of F (130–300 ppm), I (6–25 ppb) and CO (0.13–0.43 wt.%) do not show any obvious relationship to depth. The altered lavas, dykes and gabbros are enriched in Cl, Br, I and HO by ∼2–20 times, but have F concentrations similar to uncontaminated fresh N-MORB glasses from the East Pacific Rise. The majority of amphiboles analysed in this and previous studies have low F/Cl ratios that are typical of hydrothermal amphiboles formed from F-poor seawater-derived fluids. The most Cl-rich amphiboles in the deepest gabbro typically contain ∼2000–4000 ppm Cl, but a patch of amphibole with 1.6 wt.% Cl was found close to a chlorapatite. Together the Cl-rich amphibole and hydrothermal chlorapatite, with up to 5.4 wt.% Cl, provide evidence for high salinity seawater-derived brines (∼50 wt.% salts) at the base of the hole. In addition a population of F-rich amphiboles (>1000 ppm F) with elevated F/Cl ratios (F/Cl ∼ 6–8) provide evidence for the passage of F-rich magmatic fluids in the granoblastic dykes and gabbros. Hydrous silicates including saponite-chlorite at the top of the hole and amphibole below 1300 mbsf, typically accommodate most of the F and Cl (e.g. 20–60%) in whole rock samples. Apatite accommodates a furthe