Helium isotope variations in igneous rocks are important for relating isotopic heterogeneity to convective mixing in the Earth's mantle. High He-3/He-4 ratios at many ocean islands, along with lower and relatively uniform values in mid-ocean-ridge basalts (MORBs), are thought to result from a well mixed upper-mantle source for MORB and a distinct deeper-mantle source for ocean island basalts(1). At finer scales, He-3/He-4 variations along mid-ocean ridges have been related to underlying mantle heterogeneity(2,3), but relationships between the scales of geochemical segmentation and mantle convection remain enigmatic. Here we present helium isotope data for MORB glasses recovered along similar to5,800 km of the southeast Indian ridge, and develop an approach to quantitatively relate spatial variations in geochemical and geophysical parameters at the Earth's surface. A point-to-point correlation analysis reveals structure in the helium isotope data at length scales of similar to 150 and similar to 400 km that appears to be related to secondary convection in the underlying mantle.