Hydrothermal sedimentation at ODP Sites 834 and 835 in relation to crustal evolution of the Lau Backarc Basin

Abstract

Abstract Hydrothermally metal-enriched sediments from ODP Leg 135 Sites 834 and 835 in the Lau Backarc Basin have been studied with respect to basin evolution. Multivariate statistics (factor analysis) indicate element associations in the non-detrital, carbonate-free sediment fraction characteristic of hydrothermal oxide plume fallout. Selective chemical leach, X-ray diffraction and lithological studies confirm that the hydrothermal enrichment in the sediments at both sites results from distal oxide plume fallout. No evidence of a sulphide/weathered sulphide component is seen, suggesting that the sites have never been proximal to a hydrothermal discharge source. Factor scores for the hydrothermal oxide phase of the sediments reflect the hydrothermal activity associated with the propagation of the Eastern Lau Spreading Centre (ELSC) into the basin. The subsequent propagation of the Central Lau Spreading Centre (CLSC) into the basin is not clearly recorded. Non-detrital element accumulation rates for Mn and Fe confirm this. However, the results suggest a ‘time lag’ between the closest passage of the ELSC past the latitude of Sites 834 and 835 and maximum hydrothermal plume fallout flux to the sediment section. This may be due to a delay between ridge formation and either the initiation of hydrothermal systems on the ridge and/or their full development. In addition, however, plume dispersion patterns may have been affected by palaeo-current directions and basin topography. Element accumulation rate values show that the hydrothermal flux from the ELSC was of much greater intensity than that from the CLSC. The latter was comparable with values in presently accumulating sediments proximal to the CLSC and within the range typical of East Pacific Rise (EPR) values. The ELSC accumulation rates are comparable with, or higher than, the highest reported values from the EPR. Compared with EPR sediments, the Lau Basin hydrothermal plume fallout is rich in Mn relative to Fe. In particular, plume fallout associated with the ELSC exhibits a two- to four-fold enrichment of Mn relative to Fe compared with the EPR.