A multi-siderophile element connection between volcanic hotspots and Earth's core

The existence of resolvable 182W/184W deficits in modern ocean island basalts (OIB) relative to the bulk silicate Earth has raised questions about the relationship of these rocks to Earth's core. However, because the core is expected to host high abundances of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re), it would be expected that such heterogeneity is accompanied by correlating variability in HSE abundances among OIB, but this has not been observed. We report instead a relationship between the W isotopic compositions and Ru/Ir ratios of Hawai‘i and Iceland OIB, which represent two of Earth's primary mantle plumes. Previous studies have highlighted the unique behavior of Ru relative to Os and Ir during metal-silicate fractionation, particularly when sulfide phases are segregated with metal. Using the information from these studies, we construct models predicting the consequences for HSE fractionation of various scenarios in which 182W/184W deficits can be created. These models show that the observed trends are likely inconsistent with modern, active core-mantle interaction at the CMB, and instead the observed low-Ru/Ir, low-182W/184W OIB are best explained by metal-silicate interaction that happened at significantly lower pressures. Such conditions may reflect what is expected for metal-silicate equilibration during the process of core formation itself, meaning that the deep mantle sources of OIB, such as ultra-low velocity zones, may instead reflect preserved relics of core formation. An ancient origin for distinct domains now residing at the core-mantle boundary is consistent with geophysical and petrological observations, for example that the Mg/Fe ratio of ferropericlase in the D″ layer is in significant disequilibrium with the modern core. Additional work is required to constrain the behavior of HSE during silicate differentiation processes that may also generate low 182W/184W ratios. However, if modern OIB represent a direct link to the ancient processes of core formation, future geochemical studies may be able to unlock new information about the formation and evolution of the Earth using OIB.ISSN:0012-821XISSN:1385-013

Earth's geodynamic evolution constrained by W-182 in Archean seawater

Banded iron formations, precipitates of Precambrian seawater, record global W-182 isotope signatures derived from continental weathering and hydrothermal mantle fluxes into ancient oceans, tracking Earth's geodynamic evolution through deep time. Radiogenic isotope systems are important geochemical tools to unravel geodynamic processes on Earth. Applied to ancient marine chemical sediments such as banded iron formations, the short-lived Hf-182-W-182 isotope system can serve as key instrument to decipher Earth's geodynamic evolution. Here we show high-precision W-182 isotope data of the 2.7 Ga old banded iron formation from the Temagami Greenstone Belt, NE Canada, that reveal distinct W-182 differences in alternating Si-rich (7.9 ppm enrichment) and Fe-rich (5.3 ppm enrichment) bands reflecting variable flux of W from continental and hydrothermal mantle sources into ambient seawater, respectively. Greater W-182 excesses in Si-rich layers relative to associated shales (5.9 ppm enrichment), representing regional upper continental crust composition, suggest that the Si-rich bands record the global rather than the local seawater W-182 signature. The distinct intra-band differences highlight the potential of W-182 isotope signatures in banded iron formations to simultaneously track the evolution of crust and upper mantle through deep time

Ancient helium and tungsten isotopic signatures preserved in mantle domains least modified by crustal recycling

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