THE EFFECT OF MAGMATIC ACTIVITY ON HYDROTHERMAL VENTING ALONG THE SUPERFAST-SPREADING EAST PACIFIC RISE

A survey of hydrothermal activity along the superfast-spreading (appro ximately 150 millimeters per year) East Pacific Rise shows that hydrot hermal plumes overlay approximately 60 percent of the ridge crest betw een 13 degrees 50' and 18 degrees 40'S, a plume abundance nearly twice that known from any other ridge portion of comparable length. Plumes were most abundant where the axial cross section is inflated and an ax ial magma chamber is present. Plumes with high ratios of volatile (He- 3, CH4, and H2S) to nonvolatile (Mn and Fe) species marked where hydro thermal circulation has been perturbed by recent magmatic activity. Th e high proportion of volatile-rich plumes observed implies that such e pisodes are more frequent here than on slower spreading ridges.