A central problem in geodynamics is how patterns of isotopic variability in lavas erupted at the Earth's surface are linked to convective mixing in the underlying mantle. In an attempt to better quantify the variability, we have adopted an approach from mixture theory, by defining the scale of segregation (L). The scale of segregation concept provides a quantitative estimate of size which is precisely definable from spatially referenced geochemical data. We have computed L from the spatial self-correlation for 3He/4He, 87Sr/86Sr, 143Nd/144Nd and 206,207,208Pb/204Pb in "zero age" lavas sampled along the globe-encircling mid-ocean ridge system. Our working hypothesis is that small scale convective patterns in the upper mantle play a significant role in the dispersion of these isotopic tracers. Differences in L between the respective ocean basins may then be quantitatively related to unsteadiness of small-scale mantle convection, due to thickening of the oceanic lithosphere, mantle plume impingement, or lateral temperature differences between continental and oceanic lithosphere. Only a narrow range of L values is observed within each ocean basin. In general, the isotopic tracers have L between 300-500 km for the North Atlantic, South Atlantic. and Indian Oceans. Helium appears to have a value for L that is about twice the value determined for the other tracers in the Atlantic and Indian Ocean mantle. For the Pacific Ocean, L appears to be significantly less, between 200-300 km for all tracers, indicating that mantle "blobs" in this region are smaller. Based on these observations, dispersion appears to be more efficient in the Pacific upper mantle than elsewhere.