New tungsten isotope data for modern ocean island basalts (OIB) from Hawaii, Samoa, and Iceland reveal variable 182W/184W, ranging from that of the ambient upper mantle to ratios as much as 18 parts ...per million lower. The tungsten isotopic data negatively correlate with ³He/⁴He. These data indicate that each OIB system accesses domains within Earth that formed within the first 60 million years of solar system history. Combined isotopic and chemical characteristics projected for these ancient domains indicate that they contain metal and are repositories of noble gases. We suggest that the most likely source candidates are mega–ultralow-velocity zones, which lie beneath Hawaii, Samoa, and Iceland but not beneath hot spots whose OIB yield normal 182W and homogeneously low ³He/⁴He.
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BFBNIB, NMLJ, NUK, ODKLJ, PNG, SAZU, UL, UM, UPUK
We report new noble gas isotopic compositions and abundance ratios (helium, neon, and argon) extracted from six lherzolite xenoliths on Oahu Island, Hawaii, by vacuum crushing. Considering the ...equilibrium temperature of the xenoliths and the fluid density of their fluid inclusions, they originated from a pressure of 0.98–1.04 GPa, corresponding to 33–34 km depth. The Moho depth beneath Oahu Island is around 15 km, suggesting that the xenoliths are part of the sub-oceanic lithospheric mantle (SOLM), 90 million years old, after generation of mid-oceanic ridge basalts (MORB). The samples have 3He/4He of 3.0–9.5 times atmosphere (Ra). The neon isotope compositions overlap with the data of previous studies for lava flow samples of the Hawaiian Islands, MORB, and air. The 40Ar/36Ar range from atmospheric to 3100. They show very low 4He/40Ar* and 4He/21Ne*, where asterisks denote correction for atmospheric contributions. The low 4He/40Ar* and 4He/21Ne* are characteristic of small fluid inclusions, based on step crushing experiments, indicating that they record kinetic fractionation that occurred in the SOLM most likely due to melt infiltration. Using 4He/40Ar* and 20Ne/22Ne respectively as indices of the kinetic fractionation and atmospheric contribution, we corrected for these effects. The corrected noble gas isotopic compositions are best explained by interaction between plume-derived melts and SOLM. The kinetic fractionation yields a decrease in 3He/238U in SOLM, which should result in a rapid decrease in 3He/4He with time. The slightly elevated 3He/4He (above 8 times atmospheric), as found in the present xenoliths cannot be maintained for more than one million years in the fractionated SOLM with 3He/U molar ratio of ∼1 × 10−8, which is calculated assuming that the 3He/U has the same degree of kinetic fractionation as 4He/40Ar* and 4He/21Ne*. Therefore, the fractionation event is likely to be recent, and may be related to recent magmatism in the SOLM beneath Oahu Island. Such igneous activity is limited to Honolulu series volcanism (0.35–0.80 Ma). Therefore, the xenoliths contain a record of magmas with plume-derived noble gases, most likely associated with the Honolulu series volcanism.
•We analyzed noble gas isotopes of mantle xenoliths from Oahu Island, Hawaii.•They show plume-like neon isotopic signatures, combined with low He/Ne and He/Ar.•The noble gas isotope data were combined with petrographic and Raman spectroscopic studies.•The combination is useful to explore the complex record of noble gases in xenoliths.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Establishing when, and from where, carbon, nitrogen and water were delivered to Earth is a fundamental objective in understanding the origin of habitable planets such as Earth. Yet, volatile delivery ...to Earth remains controversial
. Krypton isotopes provide insights on volatile delivery owing to their substantial isotopic variations among sources
, although pervasive atmospheric contamination has hampered analytical efforts. Here we present the full suite of krypton isotopes from the deep mantle of the Galápagos and Iceland plumes, which have the most primitive helium, neon and tungsten isotopic compositions
. Except for
Kr, the krypton isotopic compositions are similar to a mixture of chondritic and atmospheric krypton. These results suggest early accretion of carbonaceous material by proto-Earth and rule out any combination of hydrodynamic loss with outgassing of the deep or shallow mantle to explain atmospheric noble gases. Unexpectedly, the deep-mantle sources have a deficit in the neutron-rich
Kr relative to the average composition of carbonaceous meteorites, which suggests a nucleosynthetic anomaly. Although the relative depletion of neutron-rich isotopes on Earth compared with carbonaceous meteorites has been documented for a range of refractory elements
, our observations suggest such a depletion for a volatile element. This finding indicates that accretion of volatile and refractory elements occurred simultaneously, with krypton recording concomitant accretion of non-solar volatiles from more than one type of material, possibly including outer Solar System planetesimals.
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GEOZS, IJS, IMTLJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK, ZAGLJ
We report new noble gas isotopic compositions of submarine basaltic glasses sampled from two seamounts discovered offshore of Chile, and inferred to have erupted as petit-spot volcanoes near the Juan ...Fernández hotspot. The samples have 3He/4He of 1–15 times atmosphere (Ra). Their neon isotope compositions are similar to those of the Hawaiian Islands and Réunion Island. Their 40Ar/36Ar range from atmospheric to 2300. Although the lavas are likely to be influenced by a hotspot-related component, the cause of the 3He/4He variation must be clarified to ascertain the mantle source. Variations in 3He/4He are not attributable to processes occurring at the Earth's surface such as degassing fractionation, mixing with atmosphere dissolved in seawater, or in-situ post-eruptive addition of 4He. A combination of the Ne-Ar isotope ratios corrected for atmospheric influence and He isotope ratios indicates that the noble gas isotopes of the lavas are a mixture of a hotspot magma, MORB-source, and radiogenic components. The lower 3He/4He are attributed to assimilation with the oceanic lithosphere, suggesting that the pristine 3He/4He of the lavas is hotspot-like. These features can be interpreted as indicating that part of the Juan Fernández plume infiltrated the lithosphere–asthenosphere boundary (LAB), and that the ponding magma has erupted as petit-spot volcanoes because of plate bending. The noble gas data indicate that LAB can be a reservoir for exotic melts, which might have lubricated plate tectonics.
•We analyzed noble gas isotopes of basaltic glasses from central Chile trench.•They show hotspot-like noble gas isotopic signatures.•It demonstrates the possibility of exotic melt ponding in the LAB.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Helium is a powerful tracer of primitive material in Earth's mantle. Extremely high (3)He/(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 (3)He/(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. Here we show that Cenozoic-era Baffin Island and West Greenland lavas, previously found to host the highest terrestrial-mantle (3)He/(4)He ratios, 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 (143)Nd/(144)Nd ratios similar to values recently proposed for an early-formed (roughly 4.5 Gyr ago) terrestrial mantle reservoir. 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.
Rare high-³He/⁴He signatures in ocean island basalts (OIB) erupted at volcanic hotspots derive from deep-seated domains preserved in Earth’s interior. Only high-³He/⁴He OIB exhibit anomalous 182W—an ...isotopic signature inherited during the earliest history of Earth—supporting an ancient origin of high ³He/⁴He. However, it is not understood why some OIB host anomalous 182W while others do not. We provide geochemical data for the highest-³He/⁴He lavas from Iceland (up to 42.9 times atmospheric) with anomalous 182W and examine how Sr-Nd-Hf-Pb isotopic variations—useful for tracing subducted, recycled crust—relate to high ³He/⁴He and anomalous 182W. These data, together with data on global OIB, show that the highest-³He/⁴He and the largest-magnitude 182W anomalies are found only in geochemically depleted mantle domains—with high 143Nd/144Nd and low 206Pb/204Pb—lacking strong signatures of recycled materials. In contrast, OIB with the strongest signatures associated with recycled materials have low ³He/⁴He and lack anomalous 182W. These observations provide important clues regarding the survival of the ancient He and W signatures in Earth’s mantle. We show that high-³He/⁴He mantle domains with anomalous 182W have low W and ⁴He concentrations compared to recycled materials and are therefore highly susceptible to being overprinted with low ³He/⁴He and normal (not anomalous) 182W characteristic of subducted crust. Thus, high ³He/⁴He and anomalous 182Ware preserved exclusively in mantle domains least modified by recycled crust. This model places the long-term preservation of ancient high ³He/⁴He and anomalous 182W in the geodynamic context of crustal subduction and recycling and informs on survival of other early-formed heterogeneities in Earth’s interior.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
The origin of volatiles on Earth is still a matter of debate. Noble gases are an efficient geochemical tool to constrain Earth formation processes due to their inertness. Several studies have focused ...on the neon isotopic composition of the lower mantle because the 20Ne/22Ne ratio is thought to reflect that of Earth's primordial components. Two models to explain the origin of light noble gases on Earth have been proposed: either solar wind implantation onto the Earth's solid precursors or dissolution into the mantle of a primordial atmosphere captured from solar nebula gas. In order to test these two models, we analyzed the noble gas compositions (helium, neon and argon) of two submarine oceanic island basalt glasses from Fernandina volcano (Galápagos archipelago), which have among the most primitive/unradiogenic terrestrial helium and neon isotopic compositions. Several sample pieces are studied both by step-crushing and by laser ablation analyses of single vesicles. Results of step-crushing are consistent with those of laser ablation analyses, but the latter results provide new insights into the origin of atmospheric contamination. The single-vesicle laser-ablation measurements overlap with the step crushing results, but have systematically higher 40Ar/36Ar, and 3He/36Ar, suggesting less atmospheric contamination using this method. The single vesicle data therefore suggest that atmospheric contamination is introduced by exposure to the modern atmosphere, after sample collection. 3He/4He values are about 23 times the atmospheric ratio (R/Ra) for the two Fernandina (Galápagos) samples, in agreement with previous studies. We obtain 20Ne/22Ne and 40Ar/36Ar isotopic ratios as high as 12.91 and 9400, respectively, for the mantle source of the Galápagos hotspot. The new data show that step-crushing and laser ablation analyses are complementary methods that should be used together to derive the noble gas ratios in uncontaminated samples. The results of neon compositions are consistent with previous hotspot studies and support the model of solar wind implantation associated with sputtering to explain helium and neon origins on Earth.
•Step-crushing and laser ablation are complementary methods to extract noble gases.•Laser ablation noble gas analyses are almost free of atmospheric contamination.•The 20Ne/22Ne ratio is close to the Neon B composition for the Galápagos source.•The model of solar wind irradiation explains the neon origin on Earth.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
We present new helium and neon measurements in dredged basaltic glasses from the Western Galápagos, with an emphasis on the submarine flanks of Fernandina volcano, but including the adjacent flanks ...of Darwin, Ecuador, Wolf, and Roca Redonda. The samples from the submarine flanks of Fernandina volcano have the least radiogenic helium and neon;
3He/
4He ratios vary from 18 to 29 times atmospheric (Ra), which spans the range previously observed at subaerial Fernandina. Samples from north of Fernandina have
3He/
4He ratios closer to Mid-ocean ridge basalt (MORB) values (7.6 to 11.8 Ra), and are similar to subaerial lava flows from Darwin, Ecuador, Wolf, and Roca Redonda volcanoes. On a three-isotope neon diagram, the new submarine Fernandina data define a line that is closer to “solar” than data from Hawaii and Iceland, and therefore are among the least radiogenic, most primitive, neon isotopic compositions found on Earth. In contrast, the northern dredges have neon isotopic compositions similar to MORB. This sharp isotopic contrast between Fernandina and the adjacent volcanoes, whose summit calderas are only 35 to 40 km apart, is consistent with the hypothesis that Fernandina lies over the hotspot center. Measured
3He/
22Ne ratios, coupled with helium and neon isotopic systematics show that
3He/
22Ne values are extremely low in the Fernandina samples compared to the northern dredges. Coupled crushing and melting experiments show that vesicles often have lower
20Ne/
22Ne and higher
3He/
22Ne, suggesting that vesicle formation is not necessarily in solubility equilibrium, and that vesicles are preferentially affected by atmospheric contamination (as compared to host glass). There is a crude correlation between Mg# and total helium content in the glasses, suggesting that fractionation/degassing in a shallow magma reservoir is a primary control on noble gas contents. Due to similarities in volcanic plumbing between the Galápagos volcanoes, and in major and trace elements, the large
3He/
22Ne variations (extrapolated to solar neon) between Fernandina (1.5) and the northern dredges (15) may be related to mantle source characteristics rather than the effects of recent degassing or melting. Simple closed-system evolution models show that the unradiogenic neon isotopic compositions require preservation since the first few hundred million years of Earth history, which is consistent with undegassed material in the lower mantle.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The upper mantle, as sampled by mid-ocean ridge basalts (MORBs), exhibits significant chemical variability unrelated to mechanisms of melt extraction at ridges. We show that barium isotope variations ...in global MORBs vary systematically with radiogenic isotopes and trace element ratios, which reflects mixing between depleted and enriched MORB melts. In addition, modern sediments and enriched MORBs share similar Ba isotope signatures. Using modeling, we show that addition of ~0.1% by weight of sediment components into the depleted mantle in subduction zones must impart a sedimentary Ba signature to the overlying mantle and induce low-degree melting that produces the enriched MORB reservoir. Subsequently, these enriched domains convect toward mid-ocean ridges and produce radiogenic isotope variation typical of enriched MORBs. This mechanism can explain the chemical and isotopic features of enriched MORBs and provide strong evidence for pervasive sediment recycling in the upper mantle.
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