A multiple-hotspot origin has been proposed for the Cook-Austral volcanic lineament in the southwest (SW) Pacific. One candidate, the Rarotonga hotspot, is associated with volcanism on only two islands: Rarotonga (1.157 +/- 0.003 to 1.697 +/- 0.055 Ma) and Aitutaki (youngest stage: 1.382 +/- 0.058 to 1.941 +/- 0.035 Ma), located similar to 250 km apart in a north-south (N-S) direction. Their overlapping eruption ages and lack of clear age progression preclude a robust evaluation of a hotspot origin. This study identifies and investigates a new volcanic feature, Tama Seamount-located similar to 80 km east-southeast (ESE) of Rarotonga Island along a Pacific plate flowline-potentially associated with the Rarotonga hotspot. New ages from two lavas (Ar-40/Ar-39 groundmass ages: 0.664 +/- 0.027 and 0.816 +/- 0.020 Ma) and from a nepheline syenite xenolith-including zircon U-Pb dating (weighted mean age: 0.74 +/- 0.02 Ma) and a biotite Ar-40/Ar-39 age (0.730 +/- 0.007 Ma)-constrain Tama's age. These ages align with a Rarotonga hotspot age progression based on Pacific plate motion, and support a Rarotonga hotspot origin for Tama. We also report on the first lava samples dredged from the submarine flanks of Rarotonga Island, which exhibit geochemical fingerprints and an age (1.493 +/- 0.007 Ma) consistent with those of previous Rarotonga samples. Furthermore, radiogenic isotopes of Tama samples in this study overlap with existing Rarotonga Island data, supporting a genetic relationship between Tama Seamount and Rarotonga Island. Together, these new results suggest Tama Seamount represents the youngest known expression of the Rarotonga hotspot.

AbstractThe Samoan hotspot generated an age‐progressive volcanic track that can be traced back to 24 Ma at Alexa Bank, but the trace of the older portion (>24 Ma) of the hotspot track is unclear. We show that six seamounts located in and around the Magellan Seamount chain—north of the Ontong‐Java Plateau (OJP)—have ages (87–106 Ma), geochemistry, and locations consistent with absolute plate motion model reconstructions of the Samoan hotspot track in the late Cretaceous, and three additional seamounts have geochemistry and locations consistent with a Samoan origin. However, a large segment of the Samoan hotspot (24–87 Ma) remains unidentified. Absolute plate motion models show that, from ∼60 to 30 Ma, the OJP passed over the Samoan plume. The exceptional thickness of the OJP lithosphere may have largely suppressed Samoan plume melting because the inferred volcanic trace of the Samoan hotspot wanes, and then disappears, on the OJP. Fortunately, 44 Ma volcanism at Malaita Island, located on the southern margin of the OJP, has a location, age, and geochemistry consistent with a Samoan plume origin, and provides a “missing link” bridging the younger and older segments of the Samoan hotspot. Our synthesis of geochemical, geochronological, and plate motion model evidence reveals that Samoa exhibits a clear hotspot age progression for over 100 Myr. Passage of ancient plateaus over young plumes—here called “plume‐plateau” interaction—may be relatively common: the OJP also passed over the putative Rarotonga hotspot, and the Society and Pitcairn hotspots were overtopped by the Manihiki Plateau.

Long-lived radioactive isotopes provide valuable information on the evolution of Earth's geological reservoirs. Coupled measurements of the 138La-138Ce and 147Sm-143Nd systems have attracted much interest, but some critical reservoirs, such as the deep continental crust, have yet to be investigated. To address this gap, we report Ce-Nd isotope measurements of 35 crustal samples from Canada, western Europe, and Siberia, with Archean to Phanerozoic ages. Most samples from western Europe and Siberia are interpreted to represent the deepest continental crust because of low SiO2 content (< 55 wt%) and generally mafic compositions. In contrast, Precambrian composites from the Canadian shield have highly felsic compositions (61–71 wt% SiO2) and represent the old upper continental crust. Xenoliths from the Massif Central (France) and samples from uplifted massifs in the Ivrea-Verbano zone (Italy) plot on the Ce-Nd mantle array. Precambrian Canadian composites and xenoliths from Udachnaya (Siberian Craton) plot to the left of the mantle array, with the Siberian samples farthest from the array. The La-Ce system yields a ca. 1.8 Ga age for the Siberian xenoliths, distinct from the ca. 2.7 Ga Sm-Nd errorchron age. The La-Ce system was most likely reset during a large-scale episode of delamination and rejuvenation of the Archean lower lithosphere established in Siberia. A change in the La/Ce ratio during this event explains an increasing deviation with time of the Ce-Nd isotopic composition of the Siberian deep crust from the mantle array. We introduce a new parameter (θ) to quantify the likelihood of any rock evolving away from the mantle array. It represents the angle between the evolution vector of a sample and the mantle array, and it is calculated from measured La/Ce and Sm/Nd ratios. Samples that plot to the right of the mantle array have θ > 0, whereas samples that plot to the left of the mantle array have θ < 0. The angle θ was used to compare our samples to a worldwide granulite compilation (1581 samples) because it is independent of the age and the initial isotopic composition of the samples. The majority of lower crust rocks have negative θ, suggesting they are prone to evolve to the left of the mantle array, but to a lesser extent than the Siberian samples that are characterized by the most negative θ values. Thus, the Siberian samples with the most unusual Ce-Nd isotopic compositions are the only group to plot close to the deep crust end-member calculated from classical mass-balance budget of the bulk silicate Earth. These results suggest that the lower crust sampled by Siberian xenoliths is only a minor component in the terrestrial deep crust. We use this database to estimate new parent/daughter ratios for the lower continental crust: 138La/142Ce = 0.00380 ±0.00007 and 147Sm/144Nd = 0.128 ±0.007. Measurements and models are finally reconciled when considering massive recycling of lower continental crust through Earth's history, totaling 3 to 4 present-day, continental crust masses.

Volcanic hotspots are thought to initially form by melting in an upwelling mantle plume head followed by melting of the plume tail. Plate motion then generates an age progressive volcanic track originating from a large igneous province that connects to an active hotspot. However, the most voluminous large igneous province, the ~120 Ma Ontong-Java Nui Plateau (OJP-Nui) in the mid-Pacific, appears to lack such a volcanic track. Although the Louisville hotspot track was originally proposed as a candidate, limited constraints for Pacific absolute plate and plume motion prior to 80 Ma suggest a mismatch[1]. Existing Pacific models rely on age-distance data from the continuous Hawaii-Emperor and Louisville volcanic tracks, but their seamounts older than ~80 Ma are now subducted, and elsewhere on the Pacific plate only discontinuous and sparse seamount tracks can be found that formed prior to 80 Ma[2–7]. These existing models require ~1,200 km of latitudinal motion for the Louisville plume to also erupt the OJP-Nui1, yet paleolatitude estimates from to ~70 Ma to today remain within error of its present location[8,9] and suggest that any major amount of Louisville plume motion should precede that time. Here we provide evidence from geochemistry and eruption ages[9–14] demonstrating that Samoa and Rurutu-Arago are the longest-lived Pacific hotspots that can be traced back to ~120 Ma (and older) in the West Pacific where they subduct into the Mariana Trench. These newly defined tracks provide for an alternative Pacific absolute plate motion model, with better constraints for a plate rotation between 80-100 Ma, and allow us to establish Louisville as the missing volcanic track for OJP-Nui without requiring major plume motion.

Models for hotspot volcanism suggest a rising plume head forming large igneous provinces (LIP) followed by a timetransgressive volcanic track.However, the Ontong Java Plateau (OJP)-the most voluminous LIP in the geologic record-lacks such a volcanic track and most current absolute plate motion (APM) models do not extend 'robustly' prior to 80 Ma as these APM models are confined by the age of subduction of Louisville and Hawaii hotspots.Movement of the Hawaiian hotspot independent from Louisville and Rurutu-Arago during the Emperor Stage between 50-80 Ma further restricts APM modelling beyond 80 Ma.Early paleomagnetic data suggests Louisville as a candidate for OJP's associated hotspot track, though current APM models would require significant plume motion for this to be accurate and rely on atypical structures with unclear relationships to typical hotspot chains.During the KM2201 expedition in the West Pacific Seamount Province dredges were recovered from 42 seamounts along the hypothesized extensions of the Rurutu-Arago and Samoa hotspots to update existing APM models for the Pacific Plate extending up to 120 Ma.By combining Sr-Nd-Pb-Hf isotope fingerprinting and 40 Ar/ 39 Ar dating of the oldest portion of Rurutu-Arago and Samoa, these hotspot tracks can be adequately defined as long-lived, intraplate anchor points for APM modelling beyond 80 Ma and up to possibly 120 Ma.Rurutu and neighboring Macdonald Hotspot have overlapping unique HIMU composition endmember with a ~10 Myr offset in age progressions.Similarly, late-stage Samoa and Rarotonga and the Cook-Austral Islands have overlapping EM1 isotopic signatures but a ~15 Myr difference in age progressions. 40 Ar/ 39 Ar dating of recovered basalt phases of hornblende, biotite, plagioclase, clinopyroxene and/or groundmass will be conducted to develop a high-resolution age analysis of these time-transgressive features.We hypothesize that newly defined Rurutu will have a westward age progression from ~80-120 Ma approaching the Mariana's trench.Similarly, Samoa-Magellan will likely continue a northward age progression from ~80-100 Ma.Utilizing 40 Ar/ 39 Ar dating and Sr-Nd-Pb-Hf isotope geochemistry to develop a highresolution age vs. composition dataset, we will extend understanding of Pacific plate motion into the Cretaceous and assess the association between OJP and the Louisville Hotspot Trail.

Oceanic hotspots with extreme enriched mantle radiogenic isotopic signatures—including high 87Sr/86Sr and low 143Nd/144Nd indicative of ancient subduction of continental crust—are restricted to the southern hemispheric mantle. However, the mechanisms responsible for concentrating subducted continental crust in the austral mantle are unknown. We show subduction of sediments and subduction eroded material, and lower continental crust delamination, cannot generate this spatially coherent austral domain. However, late Neoproterozoic to Paleozoic continental collisions—associated with the assembly of Gondwana and Pangea—were positioned predominantly in the southern hemisphere during the late Neoproterozoic appearance of widespread continental ultra-high-pressure (UHP, >2.7 gigapascals) 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 austral supercontinent assembly, from 650-300 Ma, resulted in enhanced upper continental crust delivery into the southern hemisphere mantle. In contrast, EM domains are absent in boreal hotspots, for two reasons. First, continental crust subducted after 300—when the continents drifted into the northern hemisphere—has had insufficient time to return to the surface in plumes feeding northern hemisphere hotspots. Second, before the appearance of continental UHP rocks at 650 Ma, upper continental crust was not subducted to great depths, thus precluding its subduction into the northern hemisphere mantle during the Precambrian when continents may have been located in the northern hemisphere. Our model implies a recent formation of the austral EM domain, explains the geochemical dichotomy between austral and boreal hotspots, and may explain why austral hotspots outnumber boreal hotspots.

The radiogenic Pb isotope compositions of basalts from the Samoan hotspot suggest various mantle endmembers contribute compositionally distinct material to lavas erupted at different islands [1]. Basalts from the Samoan islands sample contributions from all of the classical mantle endmembers, including extreme EM II and high 3He/4He components, as well as dilute contributions from the HIMU, EM I, and DM components. Here, we present multiple sulfur isotope data on sulfide extracted from subaerial and submarine whole rocks associated with several Samoan volcanoes—Malumalu, Malutut, Upolu, Savaii, and Tutuila—that sample the full range of geochemical heterogeneity at Samoa and allow for an assessment of the S-isotope compositions associated with the different mantle components sampled by the Samoan hotspot. We observe variable S concentrations (10-1000 ppm) and δ34S values (-0.29‰ to +4.84‰ ± 0.3, 2σ). The variable S concentrations likely reflect weathering, sulfide segregation and degassing processes. The range in δ34S reflects mixing between the primitive mantle and recycled components, and isotope fractionations associated with degassing. The majority of samples reveal Δ33S within uncertainty of Δ33S=0 ‰ ± 0.008, suggesting Δ33S is relatively well mixed within the Samoan mantle plume. Important exceptions to this observation include: (1) a negative Δ33S (-0.018‰ ±0.008, 2σ) from a rejuvenated basalt on Upolu island (associated with a diluted EM I component) and (2) a previously documented small (but resolvable) Δ33S values (up to +0.027±0.016) associated with the Vai Trend (associated with a diluted HIMU component) [2]. The variability we observed in Δ33S is interpreted to reflect contributions of sulfur of different origins and likely multiple crustal protoliths. Δ36S vs. Δ33S relationships suggest all recycled S is of post-Archean origin. The heterogeneous S isotope values and distinct isotopic compositions associated with the various compositional trends confirms a prior hypothesis; unique crustal materials are heterogeneously delivered to the Samoan mantle plume and compositionally influence the individual groups of islands. [1] Jackson et al. (2014), Nature; [2] Dottin et al. (2020), EPSL

The Cook-Austral volcanic lineament extends from Macdonald Seamount (east) to Aitutaki Island (west) in the South Pacific Ocean and consists of hotspot-related volcanic islands, seamounts, and atolls. The Cook-Austral volcanic lineament has been characterized as multiple overlapping, age-progressive hotspot tracks generated by at least two mantle plumes, including the Arago and Macdonald plumes, which have fed volcano construction for ∼20 m.y. The Arago and Macdonald hotspot tracks are argued to have been active for at least 70 m.y. and to extend northwest of the Cook-Austral volcanic lineament into the Cretaceous-aged Tuvalu-Gilbert and Tokelau Island chains, respectively. Large gaps in sampling exist along the predicted hotspot tracks, complicating efforts seeking to show that the Arago and Macdonald hotspots have been continuous, long-lived sources of hotspot volcanism back into the Cretaceous. We present new major- and trace-element concentrations and radiogenic isotopes for three seamounts (Moki, Malulu, Dino) and one atoll (Rose), and new clinopyroxene 40Ar/39Ar ages for Rose (24.81 ± 1.02 Ma) and Moki (44.53 ± 10.05 Ma). All volcanoes are located in the poorly sampled region between the younger Cook-Austral and the older, Cretaceous portions of the Arago and Macdonald hotspot tracks. Absolute plate motion modeling indicates that the Rose and Moki volcanoes lie on or near the reconstructed traces of the Arago and Macdonald hotspots, respectively, and the 40Ar/39Ar ages for Rose and Moki align with the predicted age progression for the Arago (Rose) and Macdonald (Moki) hotspots, thereby linking the younger Cook-Austral and older Cretaceous portions of the long-lived (>70 m.y.) Arago and Macdonald hotspot tracks.

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‐grain 87 Sr/ 86 Sr and 143 Nd/ 144 Nd analyses. We find, in some xenoliths, extreme intra‐xenolith isotopic heterogeneity. For example, in one xenolith, different mineral grains range in 87 Sr/ 86 Sr from 0.70987 to 0.71321, with corresponding variability in 143 Nd/ 144 Nd 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 in 87 Sr/ 86 Sr 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 in 87 Sr/ 86 Sr‐ 143 Nd/ 144 Nd 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.

Abstract 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 hotspot 3He/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 high 238U/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.

Abstract We present major and trace element data on clinopyroxene‐hosted, glassy melt inclusions (MIs) from a highly enriched (high 87Sr/86Sr = 0.71856) lava (ALIA‐D115‐18) from the Samoan hot spot. Following correction for post‐entrapment crystallization, the MIs are trachytic (62.6–65.4 wt% SiO2, 1.5–3.0 wt% MgO, and 7.2–10.6 wt% total alkalis (Na2O + K2O)) ALIA‐D115 lavas dredged off the submarine flanks of Savai'i Island, western Samoa have previously been suggested to exhibit binary endmember magma mixing between a mafic, low 87Sr/86Sr magma and an evolved, high 87Sr/86Sr magma (the Enriched Mantle 2 endmember; EM2). In major and trace element space, these ALIA‐D115 lavas form a mixing trend between more primitive Samoan shield lavas and the MIs of this study, suggesting the MIs could represent the EM2‐derived mixing endmember. Recent efforts to constrain the EM2 mixing endmember composition shows remarkable overlap with the MI compositions, supporting the argument they could represent the 87Sr/86Sr mixing endmember to the ALIA‐D115 lavas and are derived from the EM2 endmember. Additionally, one melt inclusion contains Fo73 olivines not in equilibrium with the trachytic melt, suggesting the melt interacted with a mafic magma prior to entrapment within host clinopyroxene. Projecting the ALIA‐D115 mixing trend in 87Sr/86Sr versus SiO2 and K2O space shows that the trend intersects an average MI composition at 87Sr/86Sr values of ∼0.725, thereby offering a window into more extreme compositions associated with the EM2 endmember.

Abstract 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 new 87Sr/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 have 87Sr/86Sr values within the range reported previously for the lavas from each volcano. Critically, the 87Sr/86Sr of the high (Sr/Ce)N melt inclusion lies within the narrow range of 87Sr/86Sr for Mauna Loa melts that lack high (Sr/Ce)N signatures. Therefore, to explain the high (Sr/Ce)N ratio 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 same 87Sr/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)N Mauna Loa melts.

The short-lived Hf-182-W-182 isotope system (t(1/2) = 9 Ma) left evidence in both ancient and modern terrestrial rock record of processes that took place during the earliest stages of Earth's accretionary and differentiation history. We report mu W-182 values (the deviation of W-182/W-184 of a sample from that of laboratory standards, in parts per million) and corresponding He-3/He-4 ratios for rocks from 15 different hotspots. These rocks are characterized by mu W-182 values that range from similar to 0 to as low as -23 +/- 4.5. For each volcanic system that includes rocks with negative mu W-182 values, the values tend to be negatively correlated with He-3/He-4. The W-He isotopic characteristics of all samples can be successfully modeled via mixing involving at least three mantle source reservoirs with distinct mu W-182-He-3/He-4 characteristics. One reservoir has He-3/He-4 approximate to 8 R/R-A and mu W-182 approximate to 0, which is indistinguishable from the convecting upper mantle. Based on high He-3/He-4, the other two reservoirs are presumed to be relatively un-degassed and likely primordial. One reservoir is characterized by mu W-182 approximate to 0, while the other is characterized by mu W-182 <= -23. The former reservoir likely formed from a silicate differentiation process more than 60 Myr after the origin of the solar system, but has remained partially or wholly isolated from the rest of the mantle for most of Earth history. The latter reservoir most likely includes a component that formed while Hf-182 was extant. Mass balance constraints on the isotopic composition of the core suggest it has a strongly negative mu W-182 value of similar to-220. Thus, it is a candidate for the origin of the negative mu W-182 in the plume sources. Mixing models show that the direct addition of outer core metal into a plume rising from the core-mantle boundary would result in collateral geochemical effects, particularly in the abundances of highly siderophile elements, which are not observed in OIB. Instead, the reservoir characterized by negative mu W-182 most likely formed in the lowermost mantle as a result of core-mantle isotopic equilibration. The envisioned equilibration process would raise the W concentration and lower the mu W-182 of the resulting silicate reservoir, relative to the rest of the mantle. The small proportion (<0.3 %) of this putative core-mantle equilibrated reservoir required to account for the mu W-182 signatures observed in OIB is insufficient to result in observable effects on most other elemental and/or isotopic compositions. The presumed primordial reservoirs may be linked to seismically distinct regions in the lower mantle. Seismically imaged mantle plumes appear to preferentially ascend from the vicinity of large low-shear velocity provinces (LLSVPs), which have been interpreted as thermochemical piles. We associate the LLSVPs with the primordial reservoir characterized by high He-3/He-4 and mu W-182 = 0. Smaller, ultra-low velocity zones (ULVZs) present at the core-mantle boundary have been interpreted to consist of (partially) molten lower mantle material. The negative mu W-182 signatures observed in some plume-derived lavas may result from small contributions of ULVZ material that has inherited its negative mu W-182 signature through core-mantle equilibration. (C) 2019 Elsevier Ltd. All rights reserved.