IN-SITU OBSERVATIONS OF DISSOLVED IRON AND MANGANESE IN HYDROTHERMAL VENT PLUMES, JUAN-DE-FUCA RIDGE

In situ measurements of dissolved manganese and total dissolved iron w ere conducted in hydrothermal plumes over the Juan de Fuca Ridge using a submersible chemical analyzer (Scanner). The Scanner was deployed a s part of a CTD/transmissometer/rosette instrument package on both tow -yos and vertical casts during the VENTS Leg I cruise in 1989. Dissolv ed manganese and total dissolved iron concentrations, along with tempe rature and light attenuation anomalies, were determined over the ridge crest every 5 s. Discrete samples for laboratory analyses of dissolve d iron II, total dissolved iron II+III and manganese were also collect ed. Metal to heat ratios (Me:Q) measured in situ were extremely variab le in one steady state plume, while an event plume had constant Me:Q. Uniform values of Mn:Q in the event plume demonstrate that Mn behaves conservatively in the near-field plume. Variability in the Mn:Q ratios in a steady state plume indicated the presence of at least two hydrot hermal sources with distinct Me:Q values. A simple mixing model shows that the contribution of Mn from high Me:Q sources, with a composition characteristic of black smoker vents, varies between 1 % and 99 % wit hin the core of the steady state plume with an average value of 55 %. On average, over 50 % of the excess heat within the plume originates f rom low Me:Q ratio sources, with a composition characteristic of low-t emperature, diffuse flow vent fluids. Less than 4 % of the volume of h ydrothermal fluids in the plume originates from black smokers. The Fe II concentrations were used to provide an estimate of plume age on a t ransect across the ridge axis. Plume ages were about 2.5 days on axis and > 12 days off axis. These plume ages were modeled to provide estim ates of plume transport and horizontal diffusion and show excellent ag reement with ages determined using Rn-222.