Helium isotopic textures in Earth's upper mantle

Abstract

We report 3 He/ 4 He for 150 mid-ocean ridge basalt (MORB) glasses from the Southeast Indian Ridge (SEIR). Between 81 E and 101 E 3 He/ 4 He varies from 7.5 to 10.2 R 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 3 He/ 4 He, while 3 He/ 4 He above 9 R A likely indicates melting of pyroxenitefree 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 3 He/ 4 He varies in-phase with axial depth along the SEIR at 1000 km length scale, suggesting a coupling between melt production, 3 He/ 4 He and regional variations in mantle temperature. Collectively, our results show that the length scales of MORB 3 He/ 4 He 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).

Kolumbo submarine volcano (Greece): An active window into the Aegean subduction system

“…Substantial spatial variations of the 3 He/ 4 He ratios over a 10-km distance have been recently observed at the mid oceanic ridge (7.5–10.2 Ra45). However, to the best of our knowledge, it is rare that two distinct and active volcanic systems located only a few km apart from one another display at the same time such a wide variation in 3 He/ 4 He ratio.…”

Lithospheric mantle evolution in the Afro-Arabian domain: Insights from Bir Ali mantle xenoliths (Yemen)

“…The helium isotopic compositions ( 3 He/ 4 He) of samples from Bir Ali show a restricted range from 7.2 to 7.9 R A ( Table 5). The mean 3 He/ 4 He ratio, weighted inversely to the analytical error, is very well defined to be 7.74 ± 0.03 R A and within the range of values for asthenospherederived MORB (Graham et al, 2014). Helium concentrations in the olivines (for peridotites) and bulk rocks (for pyroxenites) range between 2.4 and 580 nano-ccSTP/g.…”

“…Helium isotope analyses were performed at Oregon State University (OSU) following methods described in Graham et al (2014) and using a Nu Instruments® noble gas mass spectrometer. Briefly,~450-600 mg of olivine was handpicked to be free of surface alteration, ultrasonically cleaned and dried.…”

Variable Crustal Production Originating From Mantle Source Heterogeneity Beneath the South East Indian Ridge and Amsterdam‐St. Paul Plateau

“…The presence of high 3 He/ 4 He ratios along segment I2 suggests that the ASP mantle plume is derived from a high 3 He/ 4 He source region. This plume mantle source has 3 He/ 4 He ratios significantly higher than the shallow Indian Ocean MORB mantle ( 3 He/ 4 He= 7 – 10 Ra; Graham et al, ). Hot spots having high 3 He/ 4 He ratios are generally taken to be sourced at significantly greater depths than the MORB mantle source (Kurz et al, ).…”