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Lavas erupted at hotspot volcanoes provide evidence of mantle heterogeneity. Samoan Island lavas with high 87Sr/86Sr (>0.706) typify a mantle source incorporating ancient subducted sediments. To further characterize this source, we target a single high 87Sr/86Sr lava from Savai'i Island, Samoa for detailed analyses of 87Sr/86Sr and 143Nd/144Nd isotopes and major and trace elements on individual magmatic clinopyroxenes. We show the clinopyroxenes exhibit a remarkable range of 87Sr/86Sr-including the highest observed in an oceanic hotspot lava-encompassing ~30% of the oceanic mantle's total variability. These new isotopic data, data from other Samoan lavas, and magma mixing calculations are consistent with clinopyroxene 87Sr/86Sr variability resulting from magma mixing between a high silica, high 87Sr/86Sr (up to 0.7316) magma, and a low silica, low 87Sr/86Sr magma. Results provide insight into the composition of magmas derived from a sediment-infiltrated mantle source and document the fate of sediment recycled into Earth's mantle.

Ocean island basalts (OIB) show variable 182W deficits that have been attributed to either early differentiation of the mantle or core‐mantle interaction. However, 182W variations may also reflect nucleosynthetic isotope heterogeneity inherited from Earth's building material, which would be evident from correlated 182W and 183W anomalies. Some datasets for OIB indeed show hints for such correlated variations, meaning that a nucleosynthetic origin of W isotope anomalies in OIB cannot be excluded. We report high‐precision W isotope data for OIB from Samoa and Hawaii, which confirm previously reported 182W deficits for these samples, but also demonstrate that none of these samples have resolvable 183W anomalies. These data therefore rule out a nucleosynthetic origin of the 182W deficits in OIB, which most likely reflect the entrainment of either core material or an overabundance of late‐accreted materials within OIB mantle sources. If these processes occurred over Earth's history, they may have also been responsible for shifting the 182W composition of the bulk mantle to its modern‐day value. We also report Mo isotope data for some Hawaiian OIB, which reveal no resolved nucleosynthetic Mo isotopic anomalies. This is consistent with inheritance of 182W deficits in OIB from the addition of either core or late‐accreted material, but only if these materials have a non‐carbonaceous (NC) meteorite‐like heritage. As such, these data rule out significant contributions of carbonaceous chondrite (CC)‐like materials to either Earth's core or late accretion. Plain Language Summary: Some ocean island basalts (OIB) may contain a record of processes and characteristics of the deepest parts of Earth's mantle, including at the boundary between the iron‐rich core and mantle. Like some prior studies, we measured tungsten isotopes within OIB from Hawaii and Samoa, and report that tungsten isotopes in these OIB differ in their characteristics compared to what is observed in modern rocks that are most representative of the upper part of Earth's mantle. One explanation for these tungsten isotope anomalies is that they are a signature of chemical interaction between the core and lower mantle, suggesting that the core 'leaks' into the lower mantle. Another possibility proposed here is that these tungsten isotope anomalies reflect ancient crust that contained dense, meteorite‐like materials, which sank to the bottom of the mantle during Earth's early history. Using isotopes of another element, molybdenum, we show that the source(s) of these tungsten isotope anomalies do not contain a significant number of materials that originated from the outer Solar System before being added to Earth during its formation. Key Points: 182W deficits in ocean island basalts are confirmed, but correlated 182W–183W anomalies present in prior datasets are not confirmed182W deficits may reflect core‐mantle interaction or an overabundance of late‐accreted materials, but not nucleosynthetic effectsMo isotope data similar to BSE estimate; W‐Mo data rule out significant contribution of CC‐like material to Earth's core or late accretion [ABSTRACT FROM AUTHOR]

Basaltic lavas erupted at some oceanic intraplate hotspot volcanoes are thought to sample ancient subducted crustal materials (1,2). However, the residence time of these subducted materials in the mantle is [...]

Several nickel-rich and helium-rich lava samples from ocean islands and large igneous provinces suggest that mantle plume material formed by core-mantle interaction during the crystallization of a melt-rich layer or basal magma ocean. Nickel-rich peridotites in the deep mantle Claude Herzberg and co-authors examine lavas from Baffin Island, West Greenland, the Ontong Java Plateau, Isla Gorgona, and Fernandina in the Galapagos Islands. The basalts at these locations, thought to have derived from deep mantle (mantle plume) sources, are shown to have formed from peridotite with a nickel content about 20% higher than those that form modern oceanic crust at mid-ocean ridges. The authors propose that such nickel-rich mantle plumes may be sampling a less-degassed nickel-rich source formed by core-mantle interaction. High .sup.3He/.sup.4He ratios in some basalts have generally been interpreted as originating in an incompletely degassed lower-mantle source.sup.1,2,3,4,5,6,7,8,9. This helium source may have been isolated at the core-mantle boundary region since Earth's accretion.sup.4,5,6. Alternatively, it may have taken part in whole-mantle convection and crust production over the age of the Earth.sup.7,8,9; if so, it is now either a primitive refugium at the core-mantle boundary.sup.8 or is distributed throughout the lower mantle.sup.7,9. Here we constrain the problem using lavas from Baffin Island, West Greenland, the Ontong Java Plateau, Isla Gorgona and Fernandina (Galapagos). Olivine phenocryst compositions show that these lavas originated from a peridotite source that was about 20 per cent higher in nickel content than in the modern mid-ocean-ridge basalt source. Where data are available, these lavas also have high .sup.3He/.sup.4He. We propose that a less-degassed nickel-rich source formed by core-mantle interaction during the crystallization of a melt-rich layer or basal magma ocean.sup.5,6, and that this source continues to be sampled by mantle plumes. The spatial distribution of this source may be constrained by nickel partitioning experiments at the pressures of the core-mantle boundary., Author(s): Claude Herzberg [sup.1] , Paul D. Asimow [sup.2] , Dmitri A. Ionov [sup.3] [sup.4] , Chris Vidito [sup.1] , Matthew G. Jackson [sup.5] , Dennis Geist [sup.6] Author Affiliations: [...]

Large outpourings of basaltic lava have punctuated geological time, but the mechanisms responsible for the generation of such extraordinary volumes of melt are not well known (1). Recent geochemical evidence [...]

Helium is a powerful tracer of primitive material in Earth's mantle. Extremely high ³He/[sup.4]He ratios in some ocean-island basalts suggest the presence of relatively undegassed and undifferentiated material preserved in Earth's mantle. However, terrestrial lavas with high ³He/[sup.4]He ratios have never been observed to host the primitive lead-isotopic compositions that are required for an early (roughly 4.5 Gyr ago) formation age (1,2). Here we show that Cenozoic-era Baffin Island and West Greenland lavas, previously found to host the highest terrestrial-mantle ³He/[sup.4]He ratios (3-5), exhibit primitive lead-isotope ratios that are consistent with an ancient mantle source age of 4.55-4.45 Gyr. The Baffin Island and West Greenland lavas also exhibit [sup.143]Nd/[sup.144]Nd ratios similar to values recently proposed for an early-formed (roughly 4.5 Gyr ago) terrestrial mantle reservoir (6,7). The combined helium-, lead- and Nd-isotopic compositions in Baffin Island and West Greenland lavas therefore suggest that their source is the most ancient accessible reservoir in the Earth's mantle, and it may be parental to all mantle reservoirs that give rise to modern volcanism., Relative to terrestrial-mantle He, there are extremely high ³He/[sup.4]He ratios in the solar wind (roughly 310 [R.sub.a], where [R.sub.a] is the present-day ³He/[sup.4]He ratio in the atmosphere, 1.38 x 10 [...]

Lavas from the Samoan volcanic chain show the most enriched geochemical signatures ever documented in oceanic lavas ([sup.87]Sr/[sup.86]Sr as high as 0.7205). In order to test the hypothesis that their source contains a component of recycled upper continental crust, we measured oxygen isotope compositions of olivine phenocrysts from these lavas. Correlations between [delta][sup.18]O of olivines (5.11[per thousand]-5.70[per thousand]) and [sup.87]Sr/[sup.86]Sr and [sup.207]Pb/[sup.204]Pb of whole rocks, as well as Ce/Pb and Nb/Th ratios of whole rocks, indicate that (1) measured [delta][sup.18]O are primary, mantle-derived values, and (2) the enriched mantle source of these lavas contains continental crust or its derivative sediments. The observed trend between [delta][sup.18]O and [sup.87]Sr/[sup.86]Sr can be fit using either clastic marine sediment or continental crust values of [delta][sup.18]O, Sr concentration, and [sup.87]Sr/[sup.86]Sr, but only those for clastic marine sediments are compatible with trace element modeling. We conclude that the enriched source for Samoan basalts was created by sedimentation of continent-derived material into a marine environment, followed by subduction and mixing with ambient mantle. Keywords: oxygen isotopes, Samoa, enriched mantle, EM2, ocean island basalts.

The classical model for the generation of hotspot tracks maintains that stationary and deep-seated mantle plumes impinge on overriding tectonic plates, thereby generating age-progressive trails of volcanic islands and seamounts. Samoa has played a key role in discrediting this model and the very existence of mantle plumes, because early geochronological work failed to demonstrate a linear age progression along this chain of islands. Specifically on Savai'i Island, the bulk of the subaerial volcanics is younger than 0.39 Ma, much younger than the 5.1 Ma age predicted from the classical hotspot model and a constant 7.1 cm/yr Pacific plate motion. This discrepancy led to alternative magma-producing mechanisms that involve the cracking of the lithosphere beneath the Samoan islands, as a result of the extensional regime generated by the nearby Tonga Trench. Here we report [sup.40]Ar/[sup.39]Ar ages from the submarine flanks of Savai'i Island showing that its volcanic construction began as early as 5.0 Ma and in a true intraplate setting. This reinstates Samoa as a primary hotspot trail associated with a deep mantle plume and a linear age progression. Keywords: primary hotspots, [sup.40]Ar[sup.39]Ar geochronology, Pacific plate, plate extension, seamounts, Samoa.