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Helium isotopic textures in Earth’s upper mantle

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  • We report ³He/⁴He for 150 mid-ocean ridge basalt (MORB) glasses from the Southeast Indian 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 MORB range away from ocean island hot spots. 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 R[subscript A] 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, multitaper method, and continuous wavelet transform. Long-wavelength lobes with prominent power at 1000 and 500 km are present in all treatments, similar to hot spot-type spectra in Atlantic periodograms. The densely sampled region of the SEIR considered separately shows significant power at ~100 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).
  • Keywords: MORB, Mantle heterogeneity, Helium isotopes, Southeast Indian Ridge, Mid-ocean ridges
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  • Graham, D. W., B. B. Hanan, C. Hémond, J. Blichert-Toft, and F. Albarède (2014), Helium isotopic textures in Earth's upper mantle. Geochemistry Geophysics Geosystems, 15, 2048–2074. doi:10.1002/2014GC005264
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  • 15
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  • 5
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  • D.G. and B.H. were supported by the Marine Geology & Geophysics program of the U.S.National Science Foundation. The OSU Noble Gas Lab was established through a Major Research Instrumentation grant from MG&G.The Keck Foundation contributed to establishment of the Isotope Geochemistry clean rooms at SDSU.J.B.T. and F.A. received support from the French Agence Nationale de la Recherche (grant ANR-10-BLAN-0603M&Ms—Mantle Melting—Measurements, Models, Mechanisms).C.H. acknowledges the French Institut Paul Emile Victor (IPEV) for ship time and staff onboard the ship, and the French Institut National des Sciences De l'Univers for financial support through the program Campagne á lamer. Figure 1 was produced using GeoMapApp (http://www.marine-geo.org/geomapapp), which is supported by the National Science Foundation and internal funding from Lamont-Doherty Earth Observatory.
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