Evidence of Hydrothermal Venting Recorded in Sediment Cores along the Fonualei Rift and Spreading Center in the NE Lau Basin

In order to understand the history of hydrothermal venting activities in the Fonualei Rift and Spreading Center (FRSC) located in the northeast Lau Basin, partition geochemistry and hydrothermal metal accumulation rates were investigated in two sediment cores. The cores consist of two alternating sediment units: dark grayish foraminiferal sandy mud (Unit A) and black volcanic glass-rich sands (Unit B). Unit A is characterized by high contents of hydrothermally associated elements such as Mn, Ba, Ni, Cu, As, Pb, and REEs. Unit B, enriched in Fe, V, Al, Ti, and Sc, was deposited under the effect of sporadic volcanic eruption, mainly occurred at 8,000-10,000 and 3,000-4,000 years ago, over the background input of hydrothermal components. Hydrothermally-driven Cu, Ni, V, Co and P were extracted with Mn dominantly by the acid-reducing agent while most of hydrothermal Ba and Al were extracted with Fe by 6N HCl, indicative of high crystalline nature of metal-bearing phases. About 20% of Cu was extracted with 6N HCl, indicating that Cu is also present as a sulfide mineral form. This possibility is supported by the proximity of sampling sites to the vents, suggested by high Fe/Mg ratios of hydrothermal components up to 43. Hydrothermal metal accumulation rates are between 1.8 and 37.1 mg/cm²/ky for Mn, 39.7 and 148.5 mg/cm²/ky for Fe, and 78 and 402 µg/cm²/ky for Cu. These estimated metal accumulation rates are comparable to those reported for sediments in the Central Lau Spreading Center and Valu Fa Ridge where active hydrothermal vents have been reported in many places. This indicates that active venting sites have also been existed in the FRSC at least for the last 10,000 years.