9730967 Sims Temporal constraints on mid-ocean ridge basalt (MORB) magmatism both on- and off-axis are critically needed to better understand problems associated with MORB petrogenesis and the processes of crustal accretion at the MOR. At present, several fundamental questions related to the temporal component of MORB magmatism remain unanswered, for example: 1) How does crustal residence time affect the degree of crystal fractionation of MORBs and what is the time scale of this residence? 2) Is MOR magmatism and crustal accretion focused at the axis or is there a significant component of off-axis volcanism? To better understand how the chemistry of MORB varies through space and time as a function of source chemistry, melt generation and transport, and crustal fractionation we will be measuring U-series disequilibria, major- and trace-element compositions and isotopic compositions (Nd, Sr and Pb) in a spatially and geologically well-characterized, two-dimensional grid of MORB samples from the 9 50'N area of the EPR. Knowing the eruption ages for these axial samples is unique with respect to most deep ocean basalt suites and eliminates a large uncertainty in the interpretation of the U-series measurements. This zero age suite also provides an important baseline from which to evaluate the off-axis samples whose ages are not known.
