CHEMICAL AND PHYSICAL DIVERSITY OF HYDROTHERMAL PLUMES ALONG THE EASTPACIFIC RISE, 8-DEGREES-45'N TO 11-DEGREES-50'N

We conducted a survey of water-column hydrothermal plumes along a 350- km long section of the East Pacific Rise (EPR) axis stretching from 8- degrees-45'N to 11-degrees-50'N, including the region at 9-degrees-45' -54'N where 1991 ALVIN dives found evidence for recent seafloor volcan ic eruptions. Our survey included measurements of temperature anomaly, light attenuation (suspended particles), methane, hydrogen, iron, man ganese, helium, and aluminum. We detected strong light attenuation plu mes in two main regions: south of the Clipperton Transform Fault (CTF) from 9-degrees-27'N to 9-degrees-57'N and north of the CTF from 11-de grees-05'N to 11-degrees-35'N. However, the plumes at these two region s had very different physical and chemical characteristics. South of t he CTF, where the ridge is thought to be magmatically robust, the plum es had very high ratios of He-3/heat, methane/Mn and S/Fe. Plumes nort h of the CTF had much lower ratios of He-3/heat, methane/Mn, and S/Fe. These striking differences in volatile/heat and volatile/metal ratios suggest that the plumes in the vicinity of 9-degrees-50'N were derive d from a young, evolving hydrothermal system, while the activity at ap proximately 11-degrees-10'N is from an older, stable system. This surv ey demonstrated that water-column plume measurements can be used not o nly to accurately locate regions of hydrothermal activity, but also to determine the first-order chemical and physical characteristics of th e hydrothermal fluids exiting from the seafloor.