Hemispheric Geochemical Dichotomy of the Mantle Is a Legacy of Austral Supercontinent Assembly and Onset of Deep Continental Crust Subduction

Abstract

Ocean island basalts (OIB) erupted at hotspots, like Hawaii, reveal the presence of both primordial (Mukhopadhyay & Parai, 2019) domains and the influence of ancient subducted continental and oceanic crust -which generate EM (enriched mantle) and HIMU (high-μ, where μ = 238 U/ 204 Pb) geochemical reservoirs, respectively-in the deep mantle (Hofmann, 1997;White, 2015). Early work demonstrated that the distribution of compositional domains in the mantle is not random (Dupré & Allègre, 1983;Hedge, 1978). Hart (1984) then showed that a suite of hotspots located primarily in the southern hemisphere exhibit enriched mantle (EM) signatures, including anomalously high 87 Sr/ 86 Sr, that are largely absent in northern hemisphere hotspots. Using radiogenic isotopes, he suggested the presence of a large-scale EM reservoir centered in the southern hemispheric mantle beneath the Atlantic, Pacific and Indian oceans that supplies southern hemisphere hotspots. Called the Dupal (Dupré + Allègre) anomaly (Dupré & Allègre, 1983;Hart, 1984), this southern hemispheric feature is the most geographically widespread geochemical domain in

Lithospheric unzipping explaining hot orogenesis during continental subduction

“…This mode of orogenesis may have been the default until continents became strong enough so that their thinned margins became dragged down into subduction zones. This transition may have occurred at around 650 Ma, when metamorphic, geochemical, and plate tectonic lines of evidence suggest that deep continental recycling into the mantle initiated (Brown & Johnson, 2018;Brown et al, 2022;Jackson & Macdonald, 2022). The lithospheric unzipper concept may thus explain hot Proterozoic orogenesis as a geological expression of modern-style (continental) subduction in an Earth that was much hotter than today.…”

Zircon xenocrysts from Easter Island (Rapa Nui) reveal hotspot activity since the middle Jurassic

“…If these zircons resulted from subduction, they were either transported over large distances by mantle flow (Greenough et al, 2021), or they were introduced into the mantle when continents were still nearby. The geochemistry of central Pacific hotspots reveals geochemical traces of subducted continental crust that is widely thought to be present in the plume source at the core-mantle boundary and that subducted in the late Precambrian to early Paleozoic (e.g., Jackson & Macdonald, 2022) This would suggest that these xenocrysts may have been trapped in the upper mantle for hundreds of millions of years (Greenough et al, 2021;van Hinsbergen et al, 2020;, likely enclosed within mineral grains that provides a way to remain stable under asthenospheric conditions (Bea et al, 2020;Cambeses et al, 2023).…”

Extensive H2O degassing in deeply erupted submarine glasses inferred from Samoan melt inclusions: The EM2 mantle source is damp, not dry

“…Samoan lavas have unique geochemical signatures that extend to Enriched Mantle 2 (EM2) isotopic compositions (e.g., high 87 Sr/ 86 Sr, intermediate 206 Pb/ 204 Pb, and low 143 Nd/ 144 Nd). The EM2 mantle source has been suggested to be derived from recycled continental materials (e.g., White & Hofmann, 1982;White & Duncan, 1996;Jackson & Macdonald, 2022). Therefore, characterization of Samoan EM2 lavas can provide insights into the role that deep continental crust subduction plays in controlling the volatile budgets of the mantle.…”