HYDROTHERMAL PLUMES IN THE EASTERN MANUS BASIN, BISMARCK SEA - CH4, MN, AL AND PH ANOMALIES

We present evidence for strong hydrothermal activity in the eastern Ma nus Basin (depth: 1700-2100 m), the existence of large scale triple-la yered buoyant plumes at depths of similar to 1100 m (''shallow plume'' ), similar to 1700 m (''deep plume''), and similar to 1400 m (''middle plume'' with less extent than the other two plumes) that were reveale d from water column anomalies of CH4, Mn, Al and pH observed in Novemb er to December 1990. Judging from the horizontal distribution of these parameters, the deep plume seems to originate from two distinct hydro thermal sites (eastern and western sites) in the researched area, the eastern site being visually ascertained with deep-tow observations at the same lime. The CH4/Mn ratio (mol mol(-1)) of the deep plume (0.02- 0.05) is the lowest yet observed in hydrothermal plumes. The order of magnitude difference of CH4/Mn ratios between the shallow plume and th e deep plume suggests that different kinds of fluid-rock interaction o ccurred to make the hydrothermal end members for the deep and shallow plumes. The shallow plume, which had an areal extent of more than 50 k m, may be an episodic ''megaplume'', because it was not recognized in the previous CH, profiles in 1986, and because it has a similar CH4/Mn ratio as the megaplume observed in the North Fiji Basin. We found tha t the eastern deep plume is characterized by enormously high aluminium concentrations (0.6-1.5 mu mol kg(-1)), pH anomalies (similar to 0.1) and high Al/Mn ratios (10-17). The endmember fluid for the eastern de ep plume may have an unusually low pH value to dissolve this much alum inum during fluid-rock interaction, or this plume may originate from a n eruption-influenced fluid.