DISCOVERY OF HYDROTHERMAL ACTIVITIES IN THE FONUALEI SPREADING AND RIFT CENTER (FRSC) IN THE LAU BASIN

Abstract

Hydrothermal activities were discovered along the axis of Fonualei spreading and rift center (FRSC) and off-axis active caldera in the northeastern Lau basin during the hydrothermal expedition performed by KORDI in 2005. Chemical tracers in water column, such as light transparency, Mn, CH4, and ATP, indicate that at least three active hydrothermal vent sites exist along the FRSC. Barite-rich hydrothermal chimney fragments also sampled from one of these hydrothermal vent sites in the FRSC. The other hydrothermal venting was discovered near the neovolcanic cone of the off-axis caldera (MTJ-1) near the northern spreading axis of the FRSC at around 15o20’S. The volcanic rocks from the FRSC and MTJ-1 are basalt to basaltic andesite and dacite, respectively. These rocks are enriched in LILE and depleted in HFSE, which suggests incorporation of arc component during the magma genesis. In addition, volcanic rocks from northern part of the FRSC show E-MORB signature that might be related to hot spot activity of north of the Lau basin (i.e. Samoa). Hydrothermal chimney fragments mainly consist of barite and sphalertie with minor amount of pyrite and chalcopyrite, which indicate that these chimneys are products of low-temperature venting. These chimney fragments have high gold contents up to 18 ppm. The water column anomalies were highest on the off-axis seamount MTJ-1. The sediment core sampled near the two hydrothermal vent sites in the FRSC contain enriched layers of hydrothermal elements, such as Mn, Cu, Zn, Pb, As, Ga, Ba, and Tl. Our preliminary data on water column, sediment, and chimney fragments support active hydrothermal activities in the study area.