A new paper from Dr. Barry Hanan and others that shows show that the MORB He variability are dominantly controlled by folding and stretching of heterogeneities during regional and mesoscale mantle flow, and by sampling during the partial melting process.
Helium isotopic textures in Earth’s upper mantle
David W. Graham1, Barry B. Hanan2, Christophe Hémond3, Janne Blichert-Toft4 and Francis Albarède4
1College of Earth, Ocean, & Atmospheric Sciences, Oregon State University, Corvallis, OR, United States
2Department of Geological Sciences, San Diego State University, San Diego, CA, United States
3Domaines Océaniques, Institut Universitaire Européen de la Mer, place Nicolas Copernic, Plouzané, France
4Laboratoire de Géologie de Lyon, Ecole Normale Supérieure de Lyon and Université Claude Bernard Lyon 1, CNRS UMR 5276, 46 Allée d’Italie, Lyon, France
We report 3He/4He for 150 mid-ocean ridge basalt (MORB) glasses from the Southeast Indian Ridge (SEIR). Between 81°-101°E 3He/4He varies from 7.5 to 10.2 RA, encompassing more than half the MORB range away from ocean island hotspots. Abrupt transitions are present and in one case the full range occurs over ~10 km. Melting of lithologically heterogeneous mantle containing a few percent garnet pyroxenite or eclogite leads to lower 3He/4He, while 3He/4He above ~9 RA likely indicates melting of pyroxenite-free or eclogite-free mantle. Patterns in the length scales of variability represent a description of helium isotopic texture. We utilize four complementary methods of spectral analysis to evaluate this texture, including Periodogram, Redfit, Multi-Taper Method and Continuous Wavelet Transform. Long-wavelength lobes with prominent power at 1000 km and 500 km are present in all treatments, similar to hotspot-type spectra in Atlantic periodograms. The densely sampled region of the SEIR considered separately shows significant power at ~100 km and ~30-40 km, the latter scale resembling heterogeneity in the bimodal distribution of Hf and Pb isotopes in the same sample suite. Wavelet transform coherence reveals that 3He/4He varies in-phase with axial depth along the SEIR at ~1000 km length scale, suggesting a coupling between melt production, 3He/4He and regional variations in mantle temperature. Collectively, our results show that the length scales of MORB 3He/4He variability are dominantly controlled by folding and stretching of heterogeneities during regional (~1000 km) and mesoscale (~100 km) mantle flow, and by sampling during the partial melting process (~30 km).