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

In response to the 1996 magmatic intrusion on the Gorda Ridge near 42.68 degrees N, 126.78 degrees 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 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 20x10(6) moles of Fe, or similar to 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. (C) 1999 Elsevier Science Ltd. All rights reserved.