Abstract
Fully characterising the exchange of volatile elements between the Earth’s interior and surface layers has been a longstanding challenge. Volatiles scavenged from seawater by hydrothermally altered oceanic crust have been transferred to the upper mantle during subduction of the oceanic crust, but whether these volatiles are carried deeper into the lower mantle is poorly understood. Here we present evidence of the deep-mantle Cl cycle recorded in melt inclusions in olivine crystals in ocean island basalts sourced from the lower mantle. We show that Cl-rich melt inclusions are associated with radiogenic Pb isotopes, indicating ancient subducted oceanic crust in basalt sources, together with lithophile elements characteristic of melts from a carbonated source. These signatures collectively indicate that seawater-altered and carbonated oceanic crust conveyed surface Cl downward to the lower mantle, forming a Cl-rich reservoir that accounts for 13–26% or an even greater proportion of the total Cl in the mantle.
Highlights
Characterising the exchange of volatile elements between the Earth’s interior and surface layers has been a longstanding challenge
Because seismic tomography demonstrates that the mantle plume relevant to the Austral–Cook Islands chain is sourced from the layer with thermochemical anomaly above the core–mantle boundary[22,23], the subducted oceanic crust stored in the lowermost mantle would be the source of the HIMU basalts at Raivavae and Mangaia[24]
The Rairua basalts contain a larger component from recycled ancient oceanic crust with the HIMU signature than do the Anatonu basalts, which have an isotopic proximity to the mid-ocean ridge basalts (MORB) mantle source
Summary
Characterising the exchange of volatile elements between the Earth’s interior and surface layers has been a longstanding challenge. We show that Cl-rich melt inclusions are associated with radiogenic Pb isotopes, indicating ancient subducted oceanic crust in basalt sources, together with lithophile elements characteristic of melts from a carbonated source. These signatures collectively indicate that seawater-altered and carbonated oceanic crust conveyed surface Cl downward to the lower mantle, forming a Cl-rich reservoir that accounts for 13–26% or an even greater proportion of the total Cl in the mantle. We show that Cl enrichment in melt inclusions is associated with radiogenic Pb isotopes, which are indices of subduction-modified ancient oceanic crust in basalt sources. Melt inclusions with radiogenic Pb isotopes display lithophile element signatures of melts from a carbonated mantle source
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