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Mantle plumes contain heterogenous chemical components and sample variable depths of the mantle, enabling glimpses into the compositional structure of Earth's interior. In this study, we evaluated ocean island basalts (OIB) from nine plume locations to provide a global and systematic assessment of the relationship between fO2 and He‐Sr‐Nd‐Pb‐W‐Os isotopic compositions. Ocean island basalts from the Pacific (Austral Islands, Hawaii, Mangaia, Samoa, Pitcairn), Atlantic (Azores, Canary Islands, St. Helena), and Indian Oceans (La Réunion) reveal that fO2 in OIB is heterogeneous both within and among hotspots. Taken together with previous studies, global OIB have elevated and heterogenous fO2 (average = +0.5 ∆FMQ; 2SD = 1.5) relative to prior estimates of global mid‐ocean ridge basalts (MORB; average = −0.1 ∆FMQ; 2SD = 0.6), though many individual OIB overlap MORB. Specific mantle components, such as HIMU and enriched mantle 2 (EM2), defined by radiogenic Pb and Sr isotopic compositions compared to other OIB, respectively, have distinctly high fO2 based on statistical analysis. Elevated fO2 in OIB samples of these components is associated with higher whole‐rock CaO/Al2O3 and olivine CaO content, which may be linked to recycled carbonated oceanic crust. EM1‐type and geochemically depleted OIB are generally not as oxidized, possibly due to limited oxidizing potential of the recycled material in the enriched mantle 1 (EM1) component (e.g., sediment) or lack of recycled materials in geochemically depleted OIB. Despite systematic offset of the fO2 among EM1‐, EM2‐, and HIMU‐type OIB, geochemical indices of lithospheric recycling, such as Sr‐Nd‐Pb‐Os isotopic systems, generally do not correlate with fO2. Plain Language Summary: Rocks from Earth's surface are mixed back into the interior during crustal recycling as a result of plate tectonics and subduction. For example, plate tectonics results in subduction of oceanic crust back into the mantle. Recycling of surface materials might oxidize the interior of the Earth. Mantle plumes, which are buoyantly rising portions of the mantle that create ocean islands such as Hawaii, Iceland, and Samoa, have the chemical and isotopic characteristics associated with recycled materials in their sources. Here we investigate rocks from mantle plumes that have heterogeneous isotopic compositions as a result of incorporating different types of recycled material to test whether their oxygen fugacity varies systematically with the type of recycled crust in their source. We show that some types of mantle plume‐derived rocks, called HIMU and enriched mantle 2 characterized by their extreme isotopic compositions, are more oxidized than the enriched mantle 1 or typical geochemically depleted rocks from mantle plumes and from spreading centers in the oceans. These results link certain recycled materials to oxidation of Earth's mantle. Key Points: Oxygen fugacity generally does not correlate with radiogenic isotopic compositions that trace recycled material in mantle‐derived rocksHIMU and enriched mantle 2 ocean island basalts are more oxidized than enriched mantle 1 or geochemically depleted ocean island basalts and mid‐ocean ridge basalts [ABSTRACT FROM AUTHOR]

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]

The formerly continuous Vitiaz Arc broke into its Vanuatu and Fijian portions during a reversal of subduction polarity in the Miocene. Basaltic volcanism in Fiji that accompanied the breakup ranged from shoshonitic to low‐K and boninitic with increasing distance from the broken edge of the arc that, presumably, marks the broken edge of the slab. The Sr‐Pb‐Nd isotope ratios of the slab‐derived component in the breakup basalts most closely match those of the isotopically most depleted part of the Samoan seamount chain on the Pacific Plate that was adjacent to the site of breakup at 4–8 Ma, and differ from those of subsequent basalts in spreading segments of the surrounding backarc North Fiji and Lau Basins. Subduction of the Samoan Chain along the Vitiaz Trench Lineament may have controlled the limit of polarity reversal and, hence, where the double saloon doors (Martin, 2013) opened. Prior to breakup, Fijian volcanics were more similar isotopically to the Louisville Seamount Chain. Plain Language Summary: The subduction zone that included Tonga and Fiji was once connected to Vanuatu. We attribute the arc breakup to subduction of the Samoan Seamount Chain. Volcanism in Fiji accompanying breakup ranges from shoshonitic closest the tear in the arc, to low‐K and boninitic farthest from it. The ambient mantle source of magma during breakup was the same as earlier in arc history but the slab‐derived component changed during breakup. Post‐breakup volcanism came from different mantle unaffected by subduction and derived from beneath the Pacific Plate. Key Points: The breakup between Fiji and Vanuatu may have been triggered by subduction of Samoan seamountsShoshonitic to low‐K and boninitic volcanism accompanied breakup with increasing distance from the breakThe mantle source of later basalts in surrounding backarc basins and islands came from beneath the Pacific Plate north of the breakup site [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.

Author(s): Matthew G. Jackson (corresponding author) [1]; Stanley R. Hart [2]; Anthony A. P. Koppers [3, 4]; Hubert Staudigel [3]; Jasper Konter [3]; Jerzy Blusztajn [2]; Mark Kurz [2]; Jamie [...]

We present the first detailed record of Holocene climate variation from Icelandic eolian soil deposits. Seven cold and windy episodes occurred in Iceland during the past 10 k.y., including the well-documented Little Ice Age (0.6-0.1 ka) and the 8.2 ka event. These windy events are associated with enhanced drift-ice discharge into the North Atlantic, and several are associated with evidence for cold and windy climate in central Greenland and diminution of deep-water formation in the North Atlantic. Although the Arctic Oscillation-North Atlantic Oscillation (AO-NAO) has been invoked to explain other climate teleconnections in the North Atlantic, our paleorecord of windiness in Iceland is not consistent with a persistent negative phase of the AO-NAO. Keywords: Holocene, Iceland, wind transport, grain-size analysis, loess, paleoclimate.

To deconvolve contributions from the four overlapping hotspots that form the "hotspot highway" on the Pacific plate—Samoa, Rarotonga, Arago-Rurutu, and Macdonald—we geochemically characterize and/or date (by the 40Ar/39Ar method) a suite of lavas sampled from the eastern region of the Samoan hotspot and the region "downstream" of the Samoan hotspot track. We find that Papatua seamount, located ~60 km south of the axis of the Samoan hotspot track, has lavas with both a HIMU (high μ = 238U/204Pb) composition (206Pb/204Pb = 20.0), previously linked to one of the Cook-Austral hotspots, and an enriched mantle I (EM1) composition, which we interpret to be rejuvenated and Samoan in origin. We show that these EM1 rejuvenated lavas at Papatua are geochemically similar to rejuvenated volcanism on Samoan volcanoes and suggest that flexural uplift, caused by tectonic forces associated with the nearby Tonga trench, triggered a new episode of melting of Samoan mantle material that had previously flattened and spread laterally along the base of the Pacific plate under Papatua, resulting in volcanism that capped the previous HIMU edifice. We argue that this process generated Samoan rejuvenated volcanism on the older Cook-Austral volcano of Papatua. We also study Waterwitch seamount, located ~820 km WNW of the Samoan hotspot, and provide an age (10.49 ± 0.09 Ma) that places it on the Samoan hotspot trend, showing that it is genetically Samoan and not related to the Cook-Austral hotspots as previously suggested. Consequently, with the possible exception of the HIMU stage of Papatua seamount, there are currently no known Arago-Rurutu plume-derived lava flows sampled along the swath of Pacific seafloor that stretches between Rose seamount (~25 Ma) and East Niulakita seamount (~45 Ma), located 1400 km to the west. The "missing" ~20-million-year segment of the Arago-Rurutu hotspot track may have been subducted into the northern Tonga trench, or perhaps was covered by subsequent volcanism from the overlapping Samoan hotspot, and has thus eluded sampling. Finally, we explore tectonic reactivation as a cause for anomalously young volcanism present within the western end of the Samoan hotspot track. [ABSTRACT FROM AUTHOR]

The element mercury (Hg) can develop large mass‐independent fractionation (MIF) (Δ199Hg) due to photo‐chemical reactions at Earth's surface. This results in globally negative Δ199Hg for terrestrial sub‐aerially‐derived materials and positive Δ199Hg for sub‐aqueously‐derived marine sediments. The mantle composition least affected by crustal recycling is estimated from high‐3He/4He lavas from Samoa and Iceland, providing an average of Δ199Hg = 0.00 ± 0.10, Δ201Hg = −0.02 ± 0.0.09, δ202Hg = −1.7 ± 1.2; 2SD, N = 11. By comparison, a HIMU‐type lava from Tubuai exhibits positive Δ199Hg, consistent with altered oceanic crust in its mantle source. A Samoan (EM2) lava has negative Δ199Hg reflecting incorporation of continental crust materials into its source. Three Pitcairn lavas exhibit positive Δ199Hg which correlate with 87Sr/86Sr, consistent with variable proportions of continental (low Δ199Hg and high 87Sr/86Sr) and oceanic (high Δ199Hg and low 87Sr/86Sr) crustal material in their mantle sources. These observations indicate that MIF signatures offer a powerful tool for examining atmosphere‐deep Earth interactions. Plain Language Summary: While Earth's mantle is continuously chemically and isotopically stirred by convection, some ocean island lavas preserve isotopic anomalies. Their most likely origin is the recycling of crustal material into Earth's mantle by subduction. A question is then whether these crustal materials originate from the ocean or the continents. By using mercury stable isotopic compositions, which have specific signatures in ocean and continent materials, we identify whether these anomalies are due to continental or oceanic crustal material in various ocean island basalts. Key Points: The Hg isotopic composition of the primitive mantle was determined by analyzing lavas from the Samoa and Iceland hotspotsKey samples from the canonical mantle end member were analyzed to track crustal recycling in the mantleWe demonstrate the presence of recycled oceanic and continental materials in the source of ocean island basalts [ABSTRACT FROM AUTHOR]

Basalts from the Samoan volcanoes 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 (N = 16) associated with several Samoan volcanoes—Vailulu‘u, Malumalu, Malutut, Upolu, Savai‘i, and Tutuila—that sample the full range of geochemical heterogeneity at Samoa and upon exhaustive compilation of S‐isotope data for Samoan lavas, 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 (10–1,000 ppm) and δ34S values (−0.29‰ ± 0.30 to +4.84‰ ± 0.30, 2σ). The observed variable S concentrations are likely due to sulfide segregation and degassing processes. The range in δ34S reflects mixing between the mantle origin and recycled components, and isotope fractionations associated with degassing. The majority of samples reveal Δ33S within uncertainty of Δ33S = 0‰ ± 0.008. Important exceptions to this observation include: (a) a negative Δ33S (−0.018‰ ± 0.008, 2σ) from a rejuvenated basalt on Upolu island (associated with a diluted EM I component) and (b) previously documented small (but resolvable) Δ33S values (up to +0.027 ± 0.016) associated with the Vai Trend (associated with a diluted HIMU component). The variability we observed in Δ33S is interpreted to reflect contributions of sulfur of different origins and likely multiple crustal protoliths. Δ36S versus Δ33S relationships suggest all recycled S is of post‐Archean origin.Plain Language Summary: Samoa is a group of volcanoes as islands and seamounts that reside in the south Pacific Ocean. This group of volcanoes are unique in that they erupt material from the deep mantle that was once at the surface, suggesting large scale crustal recycling was at play. Furthermore, distinct groupings of volcanoes and seamounts appear to erupt distinct recycled components. However, the nature and relative age of these components are not well understood. We use sulfur isotopes to gain insights into the materials erupted at Samoa. We provide evidence that the mantle plume beneath Samoa is geochemically zoned, as S‐isotopes are distinct at different groups of islands. We also argue that the identified recycled materials are best linked to Proterozoic sulfur and in some cases may represent recycled S reservoirs that are distributed among multiple Pacific Ocean island basalts.Key Points: Basalts from Samoan volcanoes preserve heterogeneous sulfur isotope signaturesWe observe unique S‐isotope compositions associated with distinct groups of Samoan volcanoesSulfur isotope variability is associated with multiple distinct recycled components [ABSTRACT FROM AUTHOR]

The Icelandic hotspot has erupted basaltic magma with the highest mantle‐derived 3He/4He over a period spanning much of the Cenozoic, from the early‐Cenozoic Baffin Island‐West Greenland flood basalt province (49.8 RA), to mid‐Miocene lavas in northwest Iceland (40.2 to 47.5 RA), to Pleistocene lavas in Iceland's neovolcanic zone (34.3 RA). The Baffin Island lavas transited through and potentially assimilated variable amounts of Precambrian continental basement. We use geochemical indicators sensitive to continental crust assimilation (Nb/Th, Ce/Pb, MgO) to identify the least crustally contaminated lavas. Four lavas, identified as "least crustally contaminated," have high MgO (>15 wt.%), and Nb/Th and Ce/Pb that fall within the mantle range (Nb/Th = 15.6 ± 2.6, Ce/Pb = 24.3 ± 4.3). These lavas have 87Sr/86Sr = 0.703008–0.703021, 143Nd/144Nd = 0.513094–0.513128, 176Hf/177Hf = 0.283265–0.283284, 206Pb/204Pb = 17.7560–17.9375, 3He/4He up to 39.9 RA, and mantle‐like δ18O of 5.03–5.21‰. The radiogenic isotopic compositions of the least crustally contaminated lavas are more geochemically depleted than Iceland high‐3He/4He lavas, a shift that cannot be explained by continental crust assimilation in the Baffin suite. Thus, we argue for the presence of two geochemically distinct high‐3He/4He components within the Iceland plume. Additionally, the least crustally contaminated primary melts from Baffin Island‐West Greenland have higher mantle potential temperatures (1510 to 1630 °C) than Siqueiros mid‐ocean ridge basalts (1300 to 1410 °C), which attests to a hot, buoyant plume origin for early Iceland plume lavas. These observations support the contention that the geochemically heterogeneous high‐3He/4He domain is dense, located in the deep mantle, and sampled by only the hottest plumes. Key Points: Baffin Island‐West Greenland high‐3He/4He lavas are more geochemically depleted than any other high‐3He/4He lavas globallyThe isotopic composition of the high‐3He/4He mantle source in the Iceland plume has evolved through timeBaffin Island and West Greenland primary melts record hotter temperatures than high‐MgO MORB, consistent with a deep, dense plume source [ABSTRACT FROM AUTHOR]

Highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), major and trace element abundances, and 187Re–187Os systematics are reported for xenoliths and lavas from Aitutaki (Cook Islands), to investigate the composition of Pacific lithosphere. The xenolith suite comprises spinel-bearing lherzolites, dunite, and harzburgite, along with olivine websterite and pyroxenite. The xenoliths are hosted within nephelinite and alkali basalt volcanic rocks (187Os/188Os ∼0·1363 ± 13; 2SD; ΣHSE = 3–4 ppb). The volcanic host rocks are low-degree (2–5%) partial melts from the garnet stability field and an enriched mantle (EM) source. Pyroxenites have similar HSE abundances and Os isotope compositions (Al2O3 = 5·7–8·3 wt %; ΣHSE = 2–4 ppb; 187Os/187Os = 0·1263–0·1469) to the lavas. The pyroxenite and olivine websterite xenoliths directly formed from—or experienced extensive melt–rock interaction with—melts similar in composition to the volcanic rocks that host the xenoliths. Conversely, the Aitutaki lherzolites, harzburgites and dunites are similar in composition to abyssal peridotites with respect to their 187Os/188Os ratios (0·1264 ± 82), total HSE abundances (ΣHSE = 8–28 ppb) and major element abundances, forsterite contents (Fo89·9±1·2), and estimated extents of melt depletion (<10 to >15%). These peridotites are interpreted to sample relatively shallow Pacific mantle lithosphere that experienced limited melt–rock reaction and melting during ridge processes at ∼90 Ma. A survey of maximum time of rhenium depletion ages of Pacific mantle lithosphere from the Cook (Aitutaki ∼1·5 Ga), Austral (Tubuai'i ∼1·8 Ga), Samoan (Savai'i ∼1·5 Ga) and Hawaiian (Oa'hu ∼2 Ga) island groups shows that Mesoproterozoic to Neoproterozoic depletion ages are preserved in the xenolith suites. The variable timing and extent of mantle depletion preserved by the peridotites is, in some instances, superimposed by extensive and recent melt depletion as well as melt refertilization. Collectively, Pacific Ocean island mantle xenolith suites have similar distributions and variations of 187Os/188Os and HSE abundances to global abyssal peridotites. These observations indicate that Pacific mantle lithosphere is typical of oceanic lithosphere in general, and that this lithosphere is composed of peridotites that have experienced both recent melt depletion at ridges and prior and sometimes extensive melt depletion across several Wilson cycles spanning periods in excess of two billion years. [ABSTRACT FROM AUTHOR]

In 2015 a geothermal exploration well was drilled on the island of Tutuila, American Samoa. The sample suite from the drill core provides 645 m of volcanic stratigraphy from a Samoan volcano, spanning 1.45 million years of volcanic history. In the Tutuila drill core, shield lavas with an EM2 (enriched mantle 2) signature are observed at depth, spanning 1.46 to 1.44 Ma. These are overlain by younger (1.35 to 1.17 Ma) shield lavas with a primordial "common" (focus zone) component interlayered with lavas that sample a depleted mantle component. Following ~1.15 Myr of volcanic quiescence, rejuvenated volcanism initiated at 24.3 ka and samples an EM1 (enriched mantle 1) component. The timing of the initiation of rejuvenated volcanism on Tutuila suggests that rejuvenated volcanism may be tectonically driven, as Samoan hotspot volcanoes approach the northern terminus of the Tonga Trench. This is consistent with a model where the timing of rejuvenated volcanism at Tutuila and at other Samoan volcanoes relates to their distance from the Tonga Trench. Notably, the Samoan rejuvenated lavas have EM1 isotopic compositions distinct from shield lavas that are geochemically similar to "petit spot" lavas erupted outboard of the Japan Trench and late stage lavas erupted at Christmas Island located outboard of the Sunda Trench. Therefore, like the Samoan rejuvenated lavas, petit spot volcanism in general appears to be related to tectonic uplift outboard of subduction zones, and existing geochemical data suggest that petit spots share similar EM1 isotopic signatures. Key Points: Within the 645‐m Tutuila drill core we find isotopically heterogeneous lavas as well as several abrupt temporal and geochemical boundariesThe proximity of Samoan volcanoes to the Tonga Trench and geochronology are consistent with a tectonic influence on rejuvenated volcanismThe tectonic setting and isotopic signatures of the Samoan rejuvenated lavas link them to "petit spots" outboard of the Japan Trench [ABSTRACT FROM AUTHOR]

We estimate the mantle source compositions for mantle plumes and, by implication, Earth’s lower mantle by: (1) measuring trace (e.g, Sc, V, Cu) and minor (e.g., Ca, Mn, Ni) element concentrations of high-forsterite olivine grains from several plume localities, (2) estimating the parent liquid compositions from which they crystallized, (3) calculating mantle potential temperatures and degrees of partial melting, and (4) estimating trace element compositions of depleted and enriched mantle sources. Our sample set includes two continental flood basalt provinces (Emeishan and Deccan), a flood basalt that erupted in a continental rift setting (Baffin Island), our type example of a thermal mantle plume (Hawaii), and lavas from the Siqueiros Transform at the East Pacific Rise, which represent the mid-ocean ridge system. We also present olivine (Ol) compositions for peridotite xenoliths from Kilbourne Hole, New Mexico, U.S.A., which are commonly used as primary and secondary analytical standards. We find that trace elements in lava-hosted olivine grains are too far removed from their mantle source to provide anything but greatly hindered views of such. Olivine compositions reflect not only evolving liquid compositions (including partial melting conditions and later fractionation), but also evolving O1+liq partition coefficients, which mostly increase with decreasing T during crystallization. Mantle compositions, delimited by maximum forsterite contents and estimates of parental magmas (and experimentally determined partition coefficients) indicate that our selected plumes reflect some combination of (1) a depleted mantle source that is quite similar to that obtained by other methods and (2) a variably enriched plume source that is more enriched than current estimates of pyrolite. The enriched plume mantle sources can be explained remarkably well as a mixture of subducted mid-ocean ridge basalt (MORB; Gale et al. 2013) and depleted MORB mantle (DM; Salters and Stracke 2004), with MORB:DMM ratios of 1:5 to 1:4. These ratios are most sensitive to estimates of melt fraction where plume parental magmas are last equilibrated with their mantle source, but are nonetheless consistent across a wide range of chemically very different elements, and estimates of MORB and DM obtained by very different means. Baffin Island is of particular interest. Like prior studies, we verify a high mantle potential temperature (Tp) of 1630 °C (compared to Tp = 1320–1420 °C for MORB from Cottrell and Kelley 2011 for O1 of Fo89.3–91.4). The Baffin source is also within error the same as DM with respect to trace elements, although still isotopically distinct; Baffin appears to be sourced in something that is akin to DMM that lies at the base of the mantle, where plumes acquire their excess heat. Thus while part of our analysis supports the concept of a “slab graveyard” at the bottom of the lower mantle (e.g., Wyession 1996), that cemetery is by no means ubiquitous at the CMB: subducted slabs are either unevenly interred, or efficiently excavated by later upwellings. [ABSTRACT FROM AUTHOR]

Mantle plumes upwelling beneath moving tectonic plates generate age-progressive chains of volcanos (hotspot chains) used to reconstruct plate motion. However, these hotspots appear to move relative to each other, implying that plumes are not laterally fixed. The lack of age constraints on long-lived, coeval hotspot chains hinders attempts to reconstruct plate motion and quantify relative plume motions. Here we provide s40Ar/39Ar ages for a newly identified long-lived mantle plume, which formed the Rurutu hotspot chain. By comparing the interhotspot distances between three Pacific hotspots, we show that Hawaii is unique in its strong, rapid southward motion from 60 to 50 Myrs ago, consistent with paleomagnetic observations. Conversely, the Rurutu and Louisville chains show little motion. Current geodynamic plume motion models can reproduce the first-order motions for these plumes, but only when each plume is rooted in the lowermost mantle. [ABSTRACT FROM AUTHOR]

Helium isotopes are used as a tracer for primitive reservoirs that have persisted in the Earth's mantle. Basalts erupted at several intraplate oceanic islands, including Hawaii, Iceland, Galapagos, and Samoa, have hosted the highest 3He/4He ratios (>30 Ra, where Ra is atmospheric 3He/4He ratio) globally that are far in excess of the 3He/4He typical of the upper mantle sampled at mid-ocean ridges (8 Ra). These lavas have been suggested to be melts of a primitive, or possibly slightly depleted, mantle reservoir, i.e., either fertile or a depleted peridotite. Here we report evidence for geochemical enrichment in the high-3He/4He mantle sampled by lavas with the highest 3He/4He from Hawaii, Samoa, and possibly Galapagos. The titanium concentrations in high-3He/4He lavas from Samoa are too high to be explained by melts of a mantle peridotite, even at infinitesimally small degrees of melting, and the elevated Ti corresponds to elevated Pb-isotopic ratios. The highest 3He/4He lavas from Loihi, Hawaii, also have Ti concentrations that are too high to be melts of primitive mantle peridotite at the degrees of melt extraction proposed for this ocean island. Thus, Ti-rich material must have been added to the high-3He/4He mantle reservoir, and this material is likely to be recycled mafic crust similar to MORBlike eclogite, which is consistent with the elevated Pb-isotopic ratios. We show that fractionation corrected, major element compositions of high-3He/4He alkalic lavas can be satisfactorily modeled by melting and melt-rock interaction scenario in a fertile peridotite-MORB-eclogite hybrid system. Primitive peridotitic and recycled eclogitic reservoirs are suggested to be intimately associated in the deepest mantle and high-3He/4He lavas from several localities may sample a mantle source that hosts a component of recycled oceanic crust. [ABSTRACT FROM AUTHOR]

The Samoan mantle plume samples two or more mantle components including an extreme EM2 composition with 87 Sr/ 86 Sr > 0.720 and a primitive component with high 3 He/ 4 He. The high 87 Sr/ 86 Sr melts have a unique potential to constrain the composition of the EM2 mantle end-member that is commonly attributed to subduction recycling. However, a previous study of H 2 O, CO 2 , S, F and Cl in Samoan glasses was hampered by the presence of unresolved assimilated sea water. The current study builds on the earlier work by extending the volatile database to include the trace halogens Br and I, and reporting new volatile data for additional glasses with 87 Sr/ 86 Sr up to 0.7125 and 3 He/ 4 He up to 15 Ra (Ra is the atmospheric 3 He/ 4 He ratio of 1.39 × 10 − 6 ). The selected glasses with MgO of 4.0 to 6.5 wt.% have CO 2 concentrations of 4 to 200 ppm that reflect degassing of CO 2 on the seafloor. The glasses contain 0.7–1.9 wt.% H 2 O, 880–1870 ppm F, 490–1790 ppm Cl, 1.9–7.2 ppm Br, 19–130 ppb I and 0.6–1.7 wt.% K. Correlations between the concentrations of these elements suggest the melts retain H 2 O concentrations close to pre-degassing values and demonstrate the melts have been variably affected by assimilation of seawater-derived brines. The brines are indicated to have had salinities of 55 ± 15 wt.% salt, F/Cl ratios close to seawater (e.g. <0.0001), Br/Cl ratios ∼ 40 % higher than seawater, and I/Cl ratios ten times the seawater value. It is calculated the melts assimilated brine fractions of 0 to 0.45 wt.%, which contributed up to ∼30% of the total H 2 O and up to ∼70% of the total Cl in the melts. After accounting for the effects of brine assimilation, the Samoan melts are suggested to have a fairly constant magmatic Cl/K of 0.05 ± 0.2 , which is lower than the median MORB value. Assimilation-corrected H 2 O/Ce values are not correlated with 87 Sr/ 86 Sr, but the sample with the highest 87 Sr/ 86 Sr of 0.7125 has an assimilation-corrected H 2 O/Ce of 83 ± 3 , that is significantly lower than typical MORB values of 150–300. These data confirm that the Samoan EM2 source is depleted in H 2 O, and perhaps Cl, relative to lithophile elements of similar compatibility. The glasses deemed free of seawater components, including the least enriched sample with 3 He/ 4 He of 15 Ra and the most enriched sample with 87 Sr/ 86 Sr of 0.7125, have indistinguishable Br/Cl and I/Cl that are within the narrow range determined for EM1 and EM2 glasses from Pitcairn and Society seamounts and mid-ocean ridge basalt glasses (e.g. Br/Cl = 0.0028 ± 0.0006 ; I/Cl = 0.00006 ± 0.00003 ; 2 σ ). The uniformity of mantle Br/Cl and I/Cl could be explained if halogens are not significantly subducted into the mantle. However, an alternative possibility is that the subduction zone ‘filter’ controls the abundance ratios of halogens recycled into the mantle, and recycled halogens with associated H 2 O, have been circulated throughout the mantle. [ABSTRACT FROM AUTHOR]

Abstract: In order to improve characterisation of volatiles in the EM1 and EM2 mantle sources, which are interpreted to contain subducted sedimentary or lithospheric components, we report electron microprobe, FTIR and SIMS CO2, H2O, S, F and Cl concentrations of variably enriched glasses from Pitcairn and Society seamounts in Polynesia. The analyses complement previously published Cl, Br and I data for some of the same glasses and all the techniques show reasonable agreement of better than 5–9% for S, Cl and H2O. The concentrations of H2O and all the halogens increase as a function of melt evolution with the highest values of 1.6wt.% H2O, 2100ppm F, 1500ppm Cl, 3.7ppm Br and 80ppb I in melts with <2wt.% MgO. In contrast, CO2 and S are strongly influenced by degassing and it is the least evolved melts that preserve the highest CO2 concentrations, that indicate CO2 oversaturation of some lavas. Comparison of volatile and non-volatile elements of similar compatibility (e.g. H2O/Ce, Cl/K) as a function of equilibration depth and 87Sr/86Sr, suggests H2O and halogens were not significantly degassed from melts equilibrated at >100bars and most of the variation in H2O/Ce reflects source composition. Despite their overall enrichment in volatiles, both the Pitcairn and Society EM1 and EM2 sources are depleted in H2O as well as Cl, Br, and I relative to Ce and K. Slightly different behaviour is suggested for F and a correlation between F/Cl and K/Cl in Pitcairn melts, could be explained by preferential subduction of F relative to Cl into the EM sources. The relative abundances of H2O, halogens and lithophile elements in melts from different tectonic settings indicate subduction-related H2O and Cl loss efficiencies are similar at ~90–96%; F is suggested to have a lower loss efficiency; whereas Br and I are lost more efficiently. Nonetheless, dehydrated lithosphere containing 5–10% of its original volatile content is interpreted as the most likely source of volatile enrichment in the Pitcairn and Society mantle sources. [Copyright &y& Elsevier]