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Helium isotopic textures in Earth’s upper mantle Public Deposited

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https://ir.library.oregonstate.edu/concern/articles/d791sm75q

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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).
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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. weresupported by the Marine Geology &Geophysics program of the U.S.National Science Foundation. The OSUNoble Gas Lab was establishedthrough a Major ResearchInstrumentation grant from MG&G.The Keck Foundation contributed toestablishment of the IsotopeGeochemistry clean rooms at SDSU.J.B.T. and F.A. received support fromthe French Agence Nationale de laRecherche (grant ANR-10-BLAN-0603M&Ms—Mantle Melting—Measurements, Models, Mechanisms).C.H. acknowledges the French InstitutPaul Emile Victor (IPEV) for ship timeand staff onboard the ship, and theFrench Institut National des Sciencesde l’Univers for financial supportthrough the program Campagne á lamer. Figure 1 was producedusing GeoMapApp (http://www.marine-geo.org/geomapapp), which issupported by the National ScienceFoundation and internal funding fromLamont-Doherty Earth Observatory.
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