Key new pieces of the HIMU puzzle from olivines and diamond inclusions.

Mantle melting, which leads to the formation of oceanic and continental crust, together with crust recycling through plate tectonics, are the primary processes that drive the chemical differentiation of the silicate Earth. The present-day mantle, as sampled by oceanic basalts, shows large chemical and isotopic variability bounded by a few end-member compositions. Among these, the HIMU end-member (having a high U/Pb ratio, µ) has been generally considered to represent subducted/recycled basaltic oceanic crust. However, this concept has been challenged by recent studies of the mantle source of HIMU magmas. For example, analyses of olivine phenocrysts in HIMU lavas indicate derivation from the partial melting of peridotite, rather than from the pyroxenitic remnants of recycled oceanic basalt. Here we report data that elucidate the source of these lavas: high-precision trace-element analyses of olivine phenocrysts point to peridotite that has been metasomatized by carbonatite fluids. Moreover, similarities in the trace-element patterns of carbonatitic melt inclusions in diamonds and HIMU lavas indicate that the metasomatism occurred in the subcontinental lithospheric mantle, fused to the base of the continental crust and isolated from mantle convection. Taking into account evidence from sulfur isotope data for Archean to early Proterozoic surface material in the deep HIMU mantle source, a multi-stage evolution is revealed for the HIMU end-member, spanning more than half of Earth's history. Before entrainment in the convecting mantle, storage in a boundary layer, upwelling as a mantle plume and partial melting to become ocean island basalt, the HIMU source formed as Archean-early Proterozoic subduction-related carbonatite-metasomatized subcontinental lithospheric mantle.

Recycled ancient ghost carbonate in the Pitcairn mantle plume.

The extreme Sr, Nd, Hf, and Pb isotopic compositions found in Pitcairn Island basalts have been labeled enriched mantle 1 (EM1), characterizing them as one of the isotopic mantle end members. The EM1 origin has been vigorously debated for over 25 years, with interpretations ranging from delaminated subcontinental lithosphere, to recycled lower continental crust, to recycled oceanic crust carrying ancient pelagic sediments, all of which may potentially generate the requisite radiogenic isotopic composition. Here we find that δ26Mg ratios in Pitcairn EM1 basalts are significantly lower than in normal mantle and are the lowest values so far recorded in oceanic basalts. A global survey of Mg isotopic compositions of potentially recycled components shows that marine carbonates constitute the most common and typical reservoir invariably characterized by extremely low δ26Mg values. We therefore infer that the subnormal δ26Mg of the Pitcairn EM1 component originates from subducted marine carbonates. This, combined with previously published evidence showing exceptionally unradiogenic Pb as well as sulfur isotopes affected by mass-independent fractionation, suggests that the Pitcairn EM1 component is most likely derived from late Archean subducted carbonate-bearing sediments. However, the low Ca/Al ratios of Pitcairn lavas are inconsistent with experimental evidence showing high Ca/Al ratios in melts derived from carbonate-bearing mantle sources. We suggest that carbonate-silicate reactions in the late Archean subducted sediments exhausted the carbonates, but the isotopically light magnesium of the carbonate was incorporated in the silicates, which then entered the lower mantle and ultimately became the Pitcairn plume source.

A younger and protracted emplacement of the Ontong Java Plateau.

The submarine volcanic emplacement of the Ontong Java Plateau (OJP) is the suggested cause of Ocean Anoxic Event 1a (OAE 1a). However, no precise timing and duration exists for the formation of OJP, and its connection to OAE1a relies mainly on proxies in the sedimentary record. We provide high-precision 40Ar/39Ar data from OJP drill and dredge sites that considerably improve OJP's eruptive history. The ages determined from this work are as much as 10 million years younger than previous dates and indicate a protracted formation over at least 6 million years. OJP now appears too young to have caused OAE1a, but we suggest that it may have had a role in the later OAE1b. The protracted eruptive sequence has implications for the emplacement dynamics of OJP and other large igneous provinces.

Zinc isotopic evidence for recycled carbonate in the deep mantle.

Carbonate, the major carbon reservoir on Earth's surface, can enter into the mantle by subduction. However, evidence for recycled surficial carbonates in the deep mantle is still scarce. Ocean island basalts from Cook-Austral islands and St. Helena Island, widely called HIMU basalts because of their high µ = 238U/204Pb sources, are thought to be fed by mantle plumes originating in the lower mantle. Here we report exceptionally high δ66Zn values (δ66Zn = 0.38 ± 0.03‰) of these HIMU lavas relative to most published data for oceanic basalts (δ66Zn = 0.31 ± 0.10‰), which requires a source contributed by isotopically heavy recycled surficial carbonates. During subduction of the oceanic lithosphere, melting of mixed surficial carbonates and basaltic crust in the deep mantle generates carbonatite melts, which metasomatizes the nearby mantle and the resultant carbonated mantle ultimately evolves into a high-δ66Zn HIMU source. High-δ66Zn signatures of HIMU basalts, therefore, demonstrate that carbonates can be transported into Earth's deep mantle.

Tiny droplets of ocean island basalts unveil Earth's deep chlorine cycle.

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.