FY 2001

Upper-mantle dynamics revealed by helium isotope variations along the Southeast Indian Ridge

Graham, D.W., J.E. Lupton, F.J. Spera, and D.M. Christie

Nature, 409, 701–703, doi: 10.1038/35055529 (2001)

Helium isotope variations in igneous rocks are important for relating isotopic heterogeneity to convective mixing in the Earth's mantle. High ³He/⁴He 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. At finer scales, ³He/⁴He variations along mid-ocean ridges have been related to underlying mantle heterogeneity, but relationships between the scales of geochemical segmentation and mantle convection remain enigmatic. Here we present helium isotope data for MORB glasses recovered along ~5,800 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 ~150 and ~400 km that appears to be related to secondary convection in the underlying mantle.