Primitive Helium Is Sourced From Seismically Slow Regions in the Lowermost Mantle

C. D. Williams , S. Mukhopadhyay , M. L. Rudolph , B. Romanowicz (1, 2)

1 IPGP - Institut de Physique du Globe de Paris
2 Collège de France - Chaire Physique de l'intérieur de la Terre

A major goal in Earth Science has been to understand how geochemical characteristics of lavas at the Earth's surface relate to the location and formation history of specific regions in the Earth's interior. For example, some of the strongest evidence for the preservation of primitive material comes from low ⁴He/³He ratios in ocean island basalts, but the location of the primitive helium reservoir(s) remains unknown. Here we combine whole-mantle seismic tomography, simulations of mantle flow, and a global compilation of new and existing measurements of the ⁴He/³He ratios in ocean island basalts to constrain the source location of primitive ⁴He/³He material. Our geodynamic simulations predict the present-day surface expression of plumes to be laterally offset from their lower mantle source locations. When this lateral offset is accounted for, a strong relationship emerges between minimum ⁴He/³He ratios in oceanic basalts and seismically slow regions, which are generally located within the two large low shear-wave velocity provinces (LLSVPs). Conversely, no significant relationship is observed between maximum ²⁰⁸Pb^*/²⁰⁶Pb^* ratios and seismically slow regions in the lowermost mantle. These results indicate that primitive materials are geographically restricted to LLSVPs, while recycled materials are more broadly distributed across the lower mantle. The primitive nature of the LLSVPs indicates these regions are not composed entirely of recycled slabs, while complementary xenon and tungsten isotopic anomalies require the primitive portion of the LLSVPs to have formed during Earth's accretion, survived the Moon-forming giant impact, and remained relatively unmixed during the subsequent 4.5 billion years of mantle convection.

Mots clés: primitive helium; large low shear-velocity provinces; mantle flow; plume conduits

Domaines: Planète et Univers [physics]

C. D. Williams, S. Mukhopadhyay, M. L. Rudolph, B. Romanowicz. Primitive Helium Is Sourced From Seismically Slow Regions in the Lowermost Mantle. Geochemistry, Geophysics, Geosystems, 2019, 20, pp.4130-4145. ⟨10.1029/2019GC008437⟩. ⟨insu-03586634⟩