OBSERVATIONS OF MANGANESE AND IRON AT THE COAXIAL SEA-FLOOR ERUPTION SITE, JUAN-DE-FUCA RIDGE

Magmatic injection during June/July 1993 on the CoAxial segment of the Juan de Fuca Ridge resulted in an axial lava eruption, an overlying chronic-style hydrothermal plume, and at least three megaplume-like event plumes. A comprehensive view of Mn and Fe liberated as hydrothermal discharge during this volcano-hydrothermal event was obtained as part of a rapid field response that included continuous in situ measurements of Mn and Fe. Distribution maps for Mn, Fe and temperature anomaly within the chronic and event plumes reveal significant thermochemical diversity that we ascribe to variable extraction efficiencies for heat and metals by source fluids, and to plume age differences. The Mn/heat data suggest that diffuse fluids could be the primary source of the chronic plume at northern CoAxial segment but make only a minor contribution to event plumes. Measurements of ephemeral Fe(II) were used to estimate that the event plumes were similar to 2, similar to 4, and >6-8 days old when first sampled, corresponding to formation times during peak seismic activity. The inventories of Mn and Fe within the three CoAxial event plumes were similar to values previously reported for megaplumes.