Hydrothermal inputs at ODP Sites 836, 837, 838 and 839 in relation to Eastern Lau Spreading Centre propagation in the Lau Basin, southwest Pacific

Abstract

Abstract Lau Basin Ocean Drilling Program (ODP) Sites 836, 837, 838 and 839 were drilled in basins west of the axial rift zone of the Eastern Lau Spreading Centre (ELSC), south of the Central Lau Spreading Centre (CLSC). Based on the models of basin evolution, Site 836 is thought to be located on ELSC-generated crust while Site 838 is located on pre-existing extended arc terrain to the west of ELSC-generated crust. Sites 837 and 839 are postulated to be close to the boundary separating ELSC-generated and pre-existing crust. Sediments recovered at all sites comprise late Pliocene to Pleistocene clayey nannofossil ooze containing sparse calcareous turbidites overlying a thick sequence of redeposited volcaniclastic sediments interbedded with hemipelagic clayey nannofossil oozes. All calcareous ooze sections contain hydrothermal ferromanganese oxides. The hydrothermal flux to the sediments at each site has been assessed using non-detrital Mn+Fe accumulation rates for recalculated, hemipelagic sediment intervals alone in order to remove the effect of sediment redeposition. Site 836 shows a typical mid-ocean ridge up-hole pattern of decreasing accumulation rates with increasing crustal accretion, as expected from its predicted location on ELSC-generated crust. Site 837 shows a similar pattern suggesting its location on ELSC-generated rather than pre-existing western Lau Basin attenuated crust. At Site 839 non-detrital Mn+Fe accumulation rates show an increase up-hole from basement to a peak before decreasing towards surface, reflecting increasing hydrothermal flux to the sediments with the southward propagation of the ELSC past this site and suggesting its location on pre-existing crust west of ELSC-generated crust. For the time-equivalent sediment section at Site 838, non-detrital Mn+Fe accumulation rate patterns are similar to those at Site 839. Although a period of low non-detrital Mn+Fe accumulation rate reflecting lower plume fallout flux prior to ELSC propagation past the site is not seen at the base of the sediment section at Site 838 (as the hole did not penetrate to basement), the accumulation rate patterns seen are not at variance with its postulated location on pre-existing attenuated crust west of ELSC-generated crust. Mean non-detrital Mn+Fe accumulation rate values are highest at Site 836 with lower mean values at Sites 837, 838 and 839. These values are within the range of those for East Pacific Rise sediments but lower than those for ELSC-propagation-associated sediments at Sites 834 and 835 in the northwestern part of the basin, suggesting that hydrothermal plume fallout associated with the southern, more recently generated portion, of the ELSC was lower than that of the older portion in the north.