Helium isotopic textures in Earth’s upper mantle

This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://onlinelibrary.wiley.com/journal/10.1002/%28ISSN%291525-2027.We report ³He/⁴He for 150 mid-ocean ridge basalt (MORB) glasses from the Southeast Indian\ud Ridge (SEIR). Between 81°E and 101°E ³He/⁴He varies from 7.5 to 10.2 R[subscript A], encompassing more than half the\ud MORB range away from ocean island hot spots. Abrupt transitions are present and in one case the full range\ud occurs over ~10 km. Melting of lithologically heterogeneous mantle containing a few percent garnet pyroxenite\ud or eclogite leads to lower ³He/⁴He, while ³He/⁴He above ~9 R[subscript A] likely indicates melting of pyroxenite-free\ud or eclogite-free mantle. Patterns in the length scales of variability represent a description of helium isotopic\ud texture. We utilize four complementary methods of spectral analysis to evaluate this texture, including\ud periodogram, redfit, multitaper method, and continuous wavelet transform. Long-wavelength lobes with\ud prominent power at 1000 and 500 km are present in all treatments, similar to hot spot-type spectra in Atlantic\ud periodograms. The densely sampled region of the SEIR considered separately shows significant power at\ud ~100 and ~30–40 km, the latter scale resembling heterogeneity in the bimodal distribution of Hf and Pb\ud isotopes in the same sample suite. Wavelet transform coherence reveals that ³He/⁴He varies in-phase with\ud axial depth along the SEIR at ~1000 km length scale, suggesting a coupling between melt production,\ud ³He/⁴He and regional variations in mantle temperature. Collectively, our results show that the length scales\ud of MORB ³He/⁴He variability are dominantly controlled by folding and stretching of heterogeneities during\ud regional (~1000 km) and mesoscale (~100 km) mantle flow, and by sampling during the partial melting\ud process (~30 km)