Hydrothermal sedimentation adjacent to the propagating Valu Fa Ridge, Lau Basin, SW Pacific

The partition geochemistry, and hydrothermal metal accumulation rates (AR) during the Late Quaternary (3 km) from hydrothermally active areas still shows an enrichment of Mn, Ag, Ba and Zn when compared to regional (500 m depth) Lau Basin sea water. The elements, Ni, Zn, Cu, Co and Mo, are associated with Mn oxides in the sediment and are thought to have been scavenged by hydrothermal oxides during the transport of hydrothermal oxides away from the spreading center. Only a low proportion of Fe is bound to the hydrothermal oxide phase in the sediments and this element is mainly associated with silicate phases. Principal components analysis (PCA) of geochemical data in the detrital-free, CaCO 3-free fraction of the sediments supports a geochemical “hydrothermal oxide” association between Mn, Ni, Mo, Cu, Zn, Pb, P, Be, Ba and La, with Fe associated with the silicate/weathered detrital fraction. Iron/manganese ratios in the oxide phase, range between 1.2 and 2.8. These values are below those found in sediments at mid-ocean ridges. Peaks in the downcore distribution in Mn appear to be contemporaneous in the cores studied and are probably the result of hydrothermal discharges on the VFR. Hemipelagic sedimentation rates throughout the area average 2.3 cm ka −1. Hydrothermal Mn AR range between 3 and 82 mg cm −2 ka −1 during the 0–30 ka B.P. period, and between 5 and 65 mg cm −2 ka −1 during the 30–120 ka B.P. period. Maximum Mn AR adjacent to the VFR are comparable to the highest values previously reported for Recent sediments proximal to the East Pacific Rise (EPR). During the entire period (0–120 ka B.P.), Mn AR are highest in core SO48/101 located about 90 km behind the propagating tip of the VFR. Recent hydrothermal input has also occurred about 33 km behind the propagator. This is in accord with the suggested location of fully evolved `steady state' magm