GEOCHEMISTRY OF HYDROTHERMAL SEDIMENTS FROM ODP SITES 834 AND 835 IN THE LAU BASIN, SOUTHWEST PACIFIC

Geochemical investigations have been carried out on hydrothermally enr iched metalliferous sediments collected at ODP Sites 834 and 835 in th e Lau Basin. Enhanced hydrothermal inputs of both Mn and Fe occur in a n acid-reducible oxide phase, unlike mid-ocean ridge sediments where F e is mainly present in more resistant HCl soluble phases. Barium, V, C o, Ni, Cu, Zn, Pb and REEs mainly follow Mn and Fe in their behaviour and associations and are thought to be largely scavenged from seawater by Mn- and Fe-rich plume particulates, although there is a significan t HCl soluble non-hydrothermal component of Ba present. Compositional differences between Eastern Lau Spreading Centre and Central Lau Sprea ding Centre hydrothermal sediments, and between them and hydrothermal sediments on mid-ocean ridges, are thought to be due to a combination of factors including temporal and areal variation in the composition o f hydrothermal solutions, variation in hydrothermal flux to the sedime nts related to their proximity to propagator or spreading centre, and variable containment of the Lau Basin hydrothermal plume consequent on changing basin morphology through time. Our results are consistent wi th published models for the evolution of the Lau Basin and confirm tha t a time lag occurs between propagator passage and maximum hydrotherma l flux to associated sediments. (C) 1997 Elsevier Science B.V.