Upper-mantle dynamics revealed by helium isotope variations along the southeast Indian ridge

Helium isotope variations in igneous rocks are important for relating isoto pic 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 val ues in mid-ocean-ridge basalts (MORBs), are thought to result from a well m ixed upper-mantle source for MORB and a distinct deeper-mantle source for o cean island basalts(1). At finer scales, He-3/He-4 variations along mid-oce an ridges have been related to underlying mantle heterogeneity(2,3), but re lationships between the scales of geochemical segmentation and mantle conve ction remain enigmatic. Here we present helium isotope data for MORB glasse s recovered along similar to5,800 km of the southeast Indian ridge, and dev elop an approach to quantitatively relate spatial variations in geochemical and geophysical parameters at the Earth's surface. A point-to-point correl ation analysis reveals structure in the helium isotope data at length scale s of similar to 150 and similar to 400 km that appears to be related to sec ondary convection in the underlying mantle.