Jean Pierre Donval

Hydrothermal activity in the Lau back-arc basin:Sulfides and water chemistry

The submersible Nautile completed 22 dives during the Nautilau cruise (R/V Nadir, April 17-May 10, 1989) for a detailed investigation of the southern Lau basin near Tonga. The objective of the scientific team from France, Germany, and Tonga was to understand the process of sea-floor ore formation associated with hydrothermal circulation along the Valu Fa back-arc ridge behind the Tonga- Kermadec trench. The four diving areas, between lat 21°25′S and 22°40′S in water ∼2000 m deep, were selected on the basis of results from cruises of the R/V JeanCharcot and R/V Sonne. The Nadir cruise provided proof of hydrothermal activity—in all four areas, over more than 100 km—as indicated by the widespread occurrence of hydrothermal deposits and by heat flow, conductivity, and temperature measurements near the sea bottom. The most spectacular findings were high-temperature white and black smokers and associated fauna and ore deposits. Hydrothermal water chemistry and sulfide composition data presented here indicate that this hydrothermal field is very different from the hydrothermal fields in oceanic ridges. This difference is seen in the water chemistry of the hydrothermal fluid (pH = 2 and high metal content) and the chemical composition of sulfides (enrichment in Ba, As, and Pb).

Hydrothermal activity in the Lau Basin: First results from the NAUTILAU Cruise

The Lau Basin, a back arc spreading center, is one of the most active hydrothermal areas in the ocean. A scientific team from France, Germany, and Tonga investigated the southern Lau Basin near Tonga in 1989 to study the processes of seafloor ore-mineral formation associated with hydrothermal circulation along the volcanic Valu Fa ridge (Ride de Valu Fa in Figure 1), which lies in back of the Tonga-Kermadec trench. Between April 17 and May 10 scientists on the R/V Nadir used the submersible Nautile to make 22 dives in the southern Lau Basin. The cruise was called NAUTILAU, for Nautile in Lau Basin. In addition to the standard equipment of the submersible (video and photo cameras, and temperature probe), a CTD (conductivity-temperature-depth) instrument was integrated with a “mini rosette” water sampling device used for the first time on the Nautile to obtain correlations between the geological observations and the physical and chemical anomalies measured in the seawater.