Halogens in serpentinised-troctolites from the Atlantis Massif: implications for alteration and global volatile cycling

Abstract

The concentrations of halogens in serpentinised olivine-rich lithologies in the lower oceanic crust (e.g. troctolites and wehrlites) and altered-gabbros, recovered from IODP Hole U1309D on the Atlantis Massif of the Mid-Atlantic Ridge, are contrasted. The aims were to evaluate if serpentinisation of lower crustal lithologies could significantly contribute to the volatile budget of oceanic lithosphere and test if serpentinites formed from seawater preserve seawater-like halogen signatures. The olivine-rich lithologies are variably serpentinised by lizardite with minor chrysotile. The maximum concentrations of halogens in the most strongly serpentinised samples are 70 µg/g F, 2,100 µg/g Cl, 9,800 ng/g Br and 8 ng/g I. In comparison, the maxima in interlayered gabbros are 200 µg/g F, 130 µg/g Cl, 400 ng/g Br and 9 ng/g I. The Br/Cl ratios of the altered gabbros are strongly influenced by the presence of amphibole, which preferentially incorporates the smaller halides. The serpentinised lithologies have low F/Cl ratios, due to their strong enrichment in seawater-derived Cl, and they have Br/Cl and I/Cl ratios intermediate of unaltered oceanic crust and seawater-derived fluids. Br/Cl and I/Cl ratios similar to seawater are best preserved in the most Cl-rich samples consistent with these ratios fingerprinting the fluid responsible for serpentinisation. Serpentinites formed from seawater in the lower ocean crust and lithosphere are likely to have low I/Cl ratios. Serpentinsed lithologies in the lower crust (and mantle lithosphere) could, therefore, significantly contribute to halogen subduction helping to explain the range of I/Cl ratios in arc lavas and a proposed decrease of mantle I/Cl over time.