CHEMICAL AND PHYSICAL DIVERSITY OF HYDROTHERMAL PLUMES ALONG THE EAST PACIFIC 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 volcanic eruptions. Our survey included measurements of temperature anomaly, light attenuation (suspended particles), methane, hydrogen, iron, manganese, helium, and aluminum. We detected strong light attenuation plumes 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-degrees-05'N to 11-degrees-35'N. However, the plumes at these two regions had very different physical and chemical characteristics. South of the CTF, where the ridge is thought to be magmatically robust, the plumes had very high ratios of He-3/heat, methane/Mn and S/Fe. Plumes north 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 derived from a young, evolving hydrothermal system, while the activity at approximately 11-degrees-10'N is from an older, stable system. This survey demonstrated that water-column plume measurements can be used not only to accurately locate regions of hydrothermal activity, but also to determine the first-order chemical and physical characteristics of the hydrothermal fluids exiting from the seafloor.