FY 1999

Chemical variations of hydrothermal particles in the 1996 Gorda Ridge Event and chronic plumes

Feely, R.A., E.T. Baker, G.T. Lebon, J.F. Gendron, G.J. Massoth, and C.W. Mordy

Deep-Sea Res. Pt. II, 45(12), 2637–2664, doi: 10.1016/S0967-0645(98)00087-3 (1998)

In response to the 1996 magmatic intrusion on the Gorda Ridge near 42.68°N, 126.78°W in late February, we conducted three cruises to the region in March, April, and June. On all three cruises particulate samples were collected, along with CTD/nephelometer data and total dissolved Fe and Mn samples. During each cruise, we collected samples from large oblate spheroid-shaped event plumes. These event plumes had long axis diameters of about 10-15 km and ranged in depth from about 1800 m to 2700 m. A strong linear correlation between nephelometer voltage and particulate Fe allowed us to estimate the total amount of particulate Fe in the event plumes at approximately 21 × 10⁶ moles of Fe, or ~28% of the Fe in the 1986 megaplume on the Cleft Segment of the Juan de Fuca Ridge. We observed significant decreases in particulate Cu and Zn concentrations (>100% decrease in Cu/Fe and Zn/Fe ratios) between the Gorda Ridge event plumes. These results suggest that each of the two event plumes originated from a chemically distinct source fluid. Fe ferrihydrite particles maintained a constant ratio of coprecipitated oxyanion species in the two event plumes. Based upon the chemical inventories for particulate Fe, P, and V, we suggest that event plumes might play a small role in the geochemical budgets for these elements.