Primitive Helium Is Sourced From Seismically Slow Regions in the Lowermost Mantle. (19th August 2019)

• Record Type: Journal Article
• Authors: Williams, C. D.; Mukhopadhyay, S.; Rudolph, M. L.; Romanowicz, B.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 20:Number 8(2019)
• Page Start: 4130
• Page End: 4145
• Publication Date: 2019-08-19
• Subjects: primitive helium -- large low shear‐velocity provinces -- mantle flow -- plume conduits; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2019GC008437
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: 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 4 He/ 3 He 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 4 He/ 3 He ratios in ocean island basalts to constrain the source location of primitive 4 He/ 3 He 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 4 He/ 3 He 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 208 Pb * / 206 Pb * 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. Plain Language Summary: Geochemical variations in volcanic rocks erupted at Earth's surface indicate differences in mantle composition, but our understanding of the location, formation, and history of compositionally distinct mantle domains remains incomplete. In particular, some hotspot lavas contain signatures of primitive regions within the mantle that have remained relatively isolated and unprocessed throughout Earth's history. Here we use models of mantle flow to predict the locations within the mantle that are sampled by the mantle plumes associated with hotspot volcanism. Combining these models of mantle flow with state‐of‐the‐art seismic images and a comprehensive catalog of hotspot lava geochemistry, we find that hotspots with a more primitive geochemical signature (as indicated by the isotopes of helium) sample the two large low shear‐velocity provinces in the lowermost mantle. Complementary constraints from xenon and tungsten isotope ratios associated with primitive materials then require these continent‐sized provinces in Earth's deep interior formed early in Earth's history, survived the violent Moon‐forming giant impact, and remained relatively unmixed with the rest of the solid Earth over the past 4.5 billion years. Key Points: Geodynamic models predict the lateral offset of plume source locations in the lower mantle from their present‐day surface expressions These models predict a strong relationship between primitive (low) 4 He/ 3 He ratios and seismically slow regions of the lower mantle These models predict no significant relationship between recycled, high 208 Pb * / 206 Pb * ratios, and seismically slow regions of the lower mantle