basalts: Fingerprints of Earth’s core?

The short-lived Hf-W isotope system (t1/2 = 9 Ma) left evidence in both ancient and modern terrestrial rock record of processes that took place during the earliest stages of Earth’s accretionary and differentiation history. We report mW values (the deviation of W/W of a sample from that of laboratory standards, in parts per million) and corresponding He/He ratios for rocks from 15 different hotspots. These rocks are characterized by mW values that range from 0 to as low as 23 ± 4.5. For each volcanic system that includes rocks with negative mW values, the values tend to be negatively correlated with He/He. The W-He isotopic characteristics of all samples can be successfully modeled via mixing involving at least three mantle source reservoirs with distinct mW-He/He characteristics. One reservoir has He/He 8 R/RA and lW 0, which is indistinguishable from the convecting upper mantle. Based on high He/He, the other two reservoirs are presumed to be relatively un-degassed and likely primordial. One reservoir is characterized by mW 0, while the other is characterized by mW 23. The former reservoir likely formed from a silicate differentiation process more than 60 Myr after the origin of the solar system, but has remained partially or wholly isolated from the rest of the mantle for most of Earth history. The latter reservoir most likely includes a component that formed while Hf was extant. Mass balance constraints on the isotopic composition of the core suggest it has a strongly negative mW value of 220. Thus, it is a candidate for the origin of the negative mW in the plume sources. Mixing models show that the direct addition of outer core metal into a plume rising from the core-mantle boundary would result in collateral geochemical effects, particularly in the abundances of highly siderophile elements, which are not observed in OIB. Instead, the reservoir characterized by negative mW most likely formed in the lowermost mantle as a result of core-mantle isotopic equilibration. The envisioned equilibration process would raise the W concentration and lower the mW of the resulting silicate reservoir, relative to the rest of the mantle. The small proportion (<0.3 %) of this putative core-mantle equilibrated reservoir required to account for the mW signatures observed in OIB is insufficient to result in observable effects on most other elemental and/or isotopic compositions. The presumed primordial reservoirs may be linked to seismically distinct regions in the lower mantle. Seismically imaged mantle plumes appear to preferentially ascend from the vicinity of large low-shear velocity provinces (LLSVPs), which have been interpreted as thermochemical piles. We associate the LLSVPs with the primordial reservoir characterized by high He/He and mW = 0. Smaller, ultra-low velocity zones (ULVZs) present at the core-mantle boundary have been interpreted to https://doi.org/10.1016/j.gca. 2019.12. 020 0016-7037/ 2019 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Department of Lithospheric Research, University of Vienna, Vienna, Austria. E-mail address: andrea.mundl@univie.ac. at (A. Mundl-Petermeier). A. Mundl-Petermeier et al. /Geochimica et Cosmochimica Acta 271 (2020) 194–211 195 consist of (partially) molten lower mantle material. The negative mW signatures observed in some plume-derived lavas may result from small contributions of ULVZ material that has inherited its negative mW signature through core-mantle equilibration. 2019 Elsevier Ltd. All rights reserved.