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Oceanic hotspots with extreme enriched mantle radiogenic isotopic signatures—including low¹⁴³Nd/¹⁴⁴Nd indicative of subducted continental crust—are linked to plume conduits sampling the southern hemispheric mantle. However, the mechanisms responsible for concentrating subducted continental crust in the austral mantle are unknown. We show that subduction of sediments and subduction eroded material, and lower continental crust delamination, cannot generate this spatially coherent austral geochemical domain. However, continental collisions—associated with the assembly of Gondwana‐Pangea—were positioned predominantly in the southern hemisphere during the late Neoproterozoic appearance of widespread continental ultra‐high‐pressure metamorphic terranes, which marked the onset of deep subduction of upper continental crust. We propose that deep subduction of upper continental crust at ancient rifted‐passive margins during ca. 650‐300 Ma austral supercontinent assembly resulted in enhanced upper continental crust delivery into the southern hemisphere mantle. Similarly enriched mantle domains are absent in the boreal mantle plume source, for two reasons. First, continental crust subducted after 300 Ma—when the continents drifted into the northern hemisphere—has had insufficient time to return to the surface in plumes sampling the northern hemisphere mantle. Second, before the first known appearance of continental ultra‐high‐pressure rocks at 650 Ma, deep subduction of upper continental crust was uncommon, limiting its subduction into the northern (and southern) hemisphere mantle earlier in Earth history. Our model implies a recent formation of the austral enriched mantle domain, explains the geochemical dichotomy between austral and boreal plume sources, and may explain why there are twice as many austral hotspots as boreal hotspots.

The Earth's upper mantle is isotopically heterogeneous over large lengthscales, but the lower limit of these heterogeneities is not well quantified. Grain scale trace elemental variability has been observed in mantle peridotites, which suggests that isotopic heterogeneity may be preserved as well. Recent advances in isotope ratio mass spectrometry enable isotopic analysis of very small samples (e.g., nanograms or less of analyte) while maintaining the precision necessary for meaningful interpretation. Here we examine four peridotite xenoliths—hosted in lavas from Savai'i (Samoa hotspot) and Tahiti (Societies hotspot) islands—that exhibit grain scale trace element heterogeneity likely related to trapped fluid and/or melt inclusions. To evaluate whether this heterogeneity is also reflected in grain scale isotopic heterogeneity, we separated clinopyroxene, orthopyroxene, and (in the most geochemically enriched xenolith) olivine for single‐grain⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd analyses. We find, in some xenoliths, extreme intra‐xenolith isotopic heterogeneity. For example, in one xenolith, different mineral grains range in⁸⁷Sr/⁸⁶Sr from 0.70987 to 0.71321, with corresponding variability in¹⁴³Nd/¹⁴⁴Nd from 0.512331 to 0.512462. However, not all peridotite xenoliths which display trace elemental heterogeneity exhibit isotopic heterogeneity. Based on coupled isotopic and trace element data (i.e., a negatively‐sloping trend in⁸⁷Sr/⁸⁶Sr vs. Ti/Eu), we suggest that carbonatitic metasomatism is responsible for creating the intra‐xenolith isotopic heterogeneities which we observe. This carbonatitic component falls off the array defined in⁸⁷Sr/⁸⁶Sr‐¹⁴³Nd/¹⁴⁴Nd space by Samoa hotspot basalts, which suggests a second, distinct EM2 (enriched mantle II) component is present in the Samoa hotspot that is not readily recognized in erupted products, but is instead seen only in mantle peridotite xenoliths.

The spatial distribution of the geochemical domains hosting recycled crust and primordial (high‐³He/⁴He) reservoirs, and how they are linked to mantle convection, are poorly understood. Two continent‐sized seismic anomalies located near the core‐mantle boundary—called the Large Low Shear Wave Velocity Provinces (LLSVPs)—are potential geochemical reservoir hosts. It has been suggested that high‐³He/⁴He hotspots are spatially confined to the LLSVPs, hotspots sampling recycled continental crust are associated with only one of the LLSVPs, and recycled continental crust shows no relationship with latitude. We reevaluate the links between LLSVPs and isotopic signatures of hotspot lavas using improved mantle flow models including plume conduit advection. While most hotspots with the highest‐³He/⁴He can indeed be traced to the LLSVP interiors, at least one high‐³He/⁴He hotspot, Yellowstone, is located outside of the LLSVPs. This suggests high‐³He/⁴He is not geographically confined to the LLSVPs. Instead, a positive correlation between hotspot buoyancy flux and maximum hotspot³He/⁴He suggests that it is plume dynamics (i.e., buoyancy), not geography, which determines whether a dense, deep, and possibly widespread high‐³He/⁴He reservoir is entrained. We also show that plume‐fed EM hotspots (enriched mantle, with low‐¹⁴³Nd/¹⁴⁴Nd), signaling recycled continental crust, are spatially linked to both LLSVPs, and located primarily in the southern hemisphere. Lastly, we confirm that hotspots sampling HIMU (“high‐μ,” or high²³⁸U/²⁰⁴Pb) domains are not spatially limited to the LLSVPs. These findings clarify and advance our understanding of deep mantle reservoir distributions, and we discuss how continental and oceanic crust subduction is consistent with the spatial decoupling of EM and HIMU.

Mangaia, an ocean island in the Cook‐Austral volcanic chain, is the type locality for the HIMU mantle reservoir and has also been shown to exhibit evidence for recycled sulfur with anomalous δ³⁴S and Δ³³S that has been attributed an Archean origin. Here we report bulk S‐isotope data from sulfide inclusions in olivine and pyroxene phenocrysts from one of the previously analyzed and four additional Mangaia basalts to further test for the prevalence of anomalous S in the HIMU mantle source feeding Mangaia. We document compositions that range from −5.13‰ to +0.21‰ (±0.3 2σ), +0.006‰ to +0.049‰ (±0.016 2σ), −0.81‰ to +0.69‰ (±0.3 2σ) for δ³⁴S, Δ³³S, and Δ³⁶S, respectively. These data extend the range of measured compositions and suggest S‐isotope heterogeneity in the HIMU mantle source at Mangaia. We show that S‐isotope compositions of bulk sulfide in olivine is not in isotopic equilibrium with bulk sulfide in pyroxene from the same samples and that samples from a confined area (M4, M10, M12, and M13) in the northern central part of the island show a distinct covariation for δ³⁴S and Δ³³S. This isotopic variation (forming an array) suggests mixing of sulfur from two sources that were captured at different stages of crystallization by phenocrysts in the Mangaia HIMU sulfur endmember.

Melt inclusions with large, positive Sr anomalies have been described in multiple tectonic settings, and the origins of this unusual geochemical feature are debated. Three origins have been proposed, all involving plagioclase as the source of the elevated Sr: (i) direct assimilation of plagioclase‐rich lithologies, (ii) recycled lower oceanic gabbro in the mantle source, and (iii) shallow‐level diffusive interaction between present day lower oceanic crust (i.e., plagioclase‐bearing lithologies) and the percolating melt. A “ghost plagioclase” signature (i.e., a large, positive Sr anomaly without associated high Al₂O₃) is present in melt inclusions from Mauna Loa. We present new⁸⁷Sr/⁸⁶Sr measurements of individual olivine‐hosted melt inclusions from three Hawaiian volcanoes, Mauna Loa, Loihi, and Koolau. The data set includes a Mauna Loa melt inclusion with the highest reported Sr anomaly (or highest (Sr/Ce)_N, which is 7.2) for Hawai'i. All melt inclusions have⁸⁷Sr/⁸⁶Sr values within the range reported previously for the lavas from each volcano. Critically, the⁸⁷Sr/⁸⁶Sr of the high (Sr/Ce)_Nmelt inclusion lies within the narrow range of⁸⁷Sr/⁸⁶Sr for Mauna Loa melts that lack high (Sr/Ce)_Nsignatures. Therefore, to explain the high (Sr/Ce)_Nratio of the ghost plagioclase signature using an ancient recycled gabbro, the gabbro‐infused mantle source would have had to evolve, by chance, to have the same⁸⁷Sr/⁸⁶Sr as the source of the Mauna Loa melts that lack a recycled gabbro (ghost plagioclase) signature. Alternatively, shallow‐level diffusive interactions between Mauna Loa plagioclase‐rich cumulates and a percolating mantle‐derived melt provides a simpler explanation for the presence of the high (Sr/Ce)_NMauna Loa melts.