Variability of fluid composition in hydrothermal systems of the Manus Basin

As a part of the Japan-France cooperative New STARMER project, various hydrothermal fluids were collected in the Manus Basin betwween 1995 and 2000 from the active vents called Vienna Woods, PacManus, Desmos, SuSu Knolls, Suzette). The Manus Basin is a backarc basin which exhibits a various range of active spreading and rifting tectonics. Hydrothermal venting is found both on basaltic spreading axis (Manus spreading Center) and on dacitic domes in rifting zones (South-Eastern Ridge). High CH4 and nephel anomalies are observed in the seawater column. The fluid endmembers at these sites exhibit very different chemical characteristics in terms of temperature (100 to >300^oC), pH (2.6 to 4.5), H2S (<2 to 9 mM), major, trace, REE elements and gas content, and isotopes. These results indicate significant magmatic input into the hydrothermal fluids of the Manus basin, reflecting variety in modes of occurrence of hydrothermal systems and confirm the variability in the chemistry of fluids issued from backarc basins and oceanic ridges. The variations in fluid composition are linked to the nature of seawater-rock interaction (basalt, andesite, dacite,...), influenced by phase-separation process, and controlled by tectonic and/or volcanic activity. Results from the Manusflux (1995) and Manaut (2000) diving cruises are discussed.