9811136 Duncan Institution: Oregon State University Title: Radiometric dating of near-ridge seamounts by the 40Ar-39Ar method This research will continue development of 40Ar-39Ar radiometric dating methods for volcanic rocks from the ocean floor, in the age range 100,000 years and older. We will apply these new methods specifically to clusters and lineations of undersea volcanoes near the spreading ridges. Such individual volcanoes provide a sampling of the flow and melting of the mantle beneath ridge systems that is broader than the focused, often homogenized sampling presented by the spreading ridge axis. The 40Ar-39Ar incremental heating method is superior to others because of the common occurrence of K in ocean floor volcanic rocks, analytical methods are well-tested and described, and in optimum circumstances measurements can be made to analytical uncertainties of <1%. We will address several first-order questions about crustal accretion processes near spreading ridges by determining the age relations of a large collection of volcano chains and clusters in the Rano Rahi field, East Pacific Rise, collected by the MOAI program from the "superfast" spreading ridge section (15-19#161#S). Our principal goals are: (1) to determine whether the near-ridge volcanoes present predominantly a spatial or temporal recording of off-ridge, shallow mantle melting, (2) to define the width of the accretionary zone, and determine whether this has a predominantly 2- or 3-dimensional geometry, and (3) to assess whether localized sources for melting that produce seamounts persist for long periods.
