Histograms of the 4He/3He ratios analyzed in MORB glasses from individual ridge segments show a linar correlation between the ridge spreading rate and the standard deviation of the corresponding averaged 4He/3He ratio. We interpret the form of these histograms in terms of stirring time (&tgr;stir) of the upper mantle. This stirring time corresponds to the time that perturbations brought in from outside into the upper mantle, in the form of oceanic island source material or slabs, were attenuated by a factor of 1/e through the effect of convection. Using 4He/3He ratios of oceanic basalts, we calculate a stirring time close to 250 Ma, distinct from the residence time of ~1 Ga. This may indicate the existence of two scale upper mantle convection, rapid convection being responsible for the homogenization of helium in the upper mantle, and the consequent uniformity of the 4He/3He ratio, and slower convection being responsible for mantle outgassing and plate tectonic motion. ¿ American Geophysical Union 1995