MORPHOLOGICAL AND CHEMICAL CHARACTERIZATION OF NEUTRALLY BUOYANT PLUME-DERIVED PARTICLES AT THE EASTERN MANUS BASIN HYDROTHERMAL FIELD, PAPUA NEW GUINEA

Abstract

Neutrally buoyant particulate matter emitted from hydrothermal plumes in the eastern Manus Basin hydrothermal field, Papua New Guinea, was detected and sampled by CTD-transmissometer surveys during marine expeditions in 1991 and 1993. Characterization of suspended particulate matter has helped to prove that the main sources of venting are along a prominent 20km-long NE–SW-striking felsic volcanic ridge. Elemental composition and particle morphology were determined by scanning electron microscopy and X-ray energy-dispersion spectroscopy. The size distribution and relative abundance of particles were determined by image analysis on selected samples. By looking at the composition of particulate matter, we can infer the origin, maturity and proximity of the source that is producing the particles. Seven types of particles have been morphologically and chemically characterized: i) filaments of probable biogenic origin of a Mn–Fe–Si phase containing traces of Cu and Zn, ii) orblike particles of an Fe–Mn–Si phase containing minor Mg, Al, P, K and Ca, iii) barite, iv) anhydrite, v) colloidal-sized amorph ous iron oxide, vi) an unidentified Si–Fe-rich phase, and vii) widespread biogenic detritus (shrimps, diatoms, silicoflagellates, copepods and unidentified gelatinous organic matter). Most of the particles are <2 � m in size and range from <2 � m for amorphous iron oxide to 100 � m for anhydrite and the Si–Fe-rich phase. Studies on suspended particulate matter constitute an effective way to recognize hydrothermal plumes.