GrahamDavidCEOASHeliumIsotopicTextures_AuxiliaryMaterial.zip

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: Southeast Indian Ridge, Mid-ocean ridges, Mantle heterogeneity, MORB, Helium isotopesKeywords: Southeast Indian Ridge, Mid-ocean ridges, Mantle heterogeneity, MORB, Helium isotope

The redox state of arc mantle using Zn/Fe systematics

International audienceMany arc lavas are more oxidized than mid-ocean-ridge basalts and subduction introduces oxidized components into the mantle(1-4). As a consequence, the sub-arc mantle wedge is widely believed to be oxidized(3,5). The Fe oxidation state of sub-arc mantle is, however, difficult to determine directly, and debate persists as to whether this oxidation is intrinsic to the mantle source(6,7). Here we show that Zn/Fe-T (where Fe-T = Fe2+ + Fe3+) is redox-sensitive and retains a memory of the valence state of Fe in primary arc basalts and their mantle sources. During melting of mantle peridotite, Fe2+ and Zn behave similarly, but because Fe3+ is more incompatible than Fe2+, melts generated in oxidized environments have low Zn/Fe-T. Primitive arc magmas have identical Zn/Fe-T to mid-ocean-ridge basalts, suggesting that primary mantle melts in arcs and ridges have similar Fe oxidation states. The constancy of Zn/Fe-T during early differentiation involving olivine requires that Fe3+/Fe-T remains low in the magma. Only after progressive fractionation does Fe3+/Fe-T increase and stabilize magnetite as a fractionating phase. These results suggest that subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. Thus, the higher oxidation states of arc lavas must be in part a consequence of shallow-level differentiation processes, though such processes remain poorly understood