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ABSTRACT: Hydrothermal Activity in the Lau Back Arc Basin: Plumes and Hot Fluids Chemistry

J. L. Charlou, J. P. Donval, M. P. Caprais, J. Erzinger, Y. Fouquet, U. Von Stackelberg

During the French-German cruise Nautile (April-May 1989), 22 dives have been completed to understand the processes of seafloor arc formation associated with hydrothermal circulation along the volcanic Valu FA ridge. The geological environment and sulfide deposits are described elsewhere. Other main objectives were to look for active sites by temperature and geochemical (CH4, Mn, 3He) anomalies, to study the plumes, and to collect hot fluids for mineral and gases composition, and compare them to hot fluids previously sampled in other geodynamical contexts (EPR, MAR). A CTD instrument associated with water sampling devices has been used with success, showing to be a good exploration tool.

The CTD recordings, obtained in real time inside the Nautile, show the narrow relation between the geological structure and the temperature anomalies. The anomalies' amplitude and intensity permit the precise localization of hot hydrothermal discharges (Vai Lili site), diffuse (Hine Hina field) and nonperceptible inputs.

Buoyant plumes producing entrainment and vertical transport up to 200 m above the seafloor are clearly identified with high CH4 (up to 4.4 µl/L) and Mn (up to 90 µmol/Kg) concentrations.

For the first time, black smokers (240° to 334°C) were collected in a back arc environment. The samples (more than 90% pure hydrothermal fluid) have end member pH (1.8), among the lowest ever measured in oceanic hydrothermal fluids. The calculated end-member concentrations are enriched for Cl (0.65-0.75 mol/kg), Na (0.52-0.58 mol/Kg), Ca (30 mmol/Kg), K (55-67 mmol/Kg), Sr (123 µmol/Kg), Rb (72-92 µmol/kg), Li (690 µmol/Kg). Compared with other hydrothermal waters, Si is slightly depleted (12-14 mmol/Kg), Fe (1.1-2.5 mmol/Kg), Mn (5.8-6.9 mmol/Kg), Cu (16-43 µmol/Kg), Zn (1.2-3 mmol/Kg) concentrations are high. The Vai Lili site fluid concentrations in B (twice seawater), Ba (up to 40 µmol/Kg), Zn (up

to 3 mmol/Kg), Pb (up to 7 µmol/kg), As (up to 11 µmol/Kg) as well as the molar Cs/Rb and Fe/Mn ratios of respectively 0.024 and 0.2 are unexpected.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990