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(< 120 ka), have been investigated in eight cores adjacent to the Valu Fa Ridge (VFR) in the southern Lau Basin. The sedimen ts consist mainly of foraminiferal nannofossiliferous ooze with ash and abu ndant amorphous Fe and Mn oxyhydroxides. Plume fall-out from active hydroth ermal vents was collected in the immediate vicinity of hydrothermal outlets . It contains barite, sphalerite and feldspar. Solutions adjacent to known outlets exhibit a significant enrichment in Mn, ill, Ph, Ag, Co, and Zn; th ese elements may find their way into the sediments by precipitation and/or adsorption. Sea water at greater distances (> 3 km) from hydrothermally act ive areas still shows an enrichment of Mn, Ag, Ba and Zn when compared to r egional (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 mainl y associated with silicate phases. Principal components analysis (PCA) of g eochemical data in the detrital-free, CaCO3-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/weather ed detrital fraction. Iron/manganese ratios in the oxide phase, range betwe en 1.2 and 2.8. These values are below those found in sediments at mid-ocea n ridges. Peaks in the downcore distribution in Mn appear to be contemporan eous in the cores studied and are probably the result of hydrothermal disch arges on the VFR. Hemipelagic sedimentation rates throughout the area avera ge 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) dur ing the 30-120 ka B.P. period. Maximum Mn AR adjacent to the VFR are compar able to the highest values previously reported for Recent sediments proxima l 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 propagat ing tip of the VFR. Recent hydrothermal input has also occulted about 33 km behind the propagator. This is in accord with the suggested location of fu lly evolved 'steady state' magma chambers occurring some distance (tens of kilometers) behind propagating rift tips. (C) 2000 Elsevier Science B.V. Al l rights reserved.