Source Of Ocean Island Basalts Research Articles

Magmatic water content in HIMU basalts from the Cook-Austral Islands: constraints on degassing processes and source composition from clinopyroxene phenocrysts

The amount of water recycled during subduction is unclear. Water/Ce estimates in HIMU (high-238U/204Pb) basalts are variable, ranging from < 100 to > 250 in glasses and melt inclusions. Because clinopyroxene (cpx) is a common early liquidus phase and the cpx/melt partitioning of water is well constrained, cpx phenocrysts provide an additional constraint on magmatic water contents. We present water and trace element concentrations in HIMU-basalt-hosted cpx phenocrysts from the Austral Islands. Calculated melt [H2O] (up to 4.3 wt.%) and H2O/Ce ratios (22–825) are higher than in olivine-hosted inclusions, as are estimated equilibration pressures. Correlation between estimated melt [H2O] and crystallization or entrapment pressure suggests significant water loss during magma ascent. Most ocean island basalts (OIBs) span a limited range in H2O (~ 1–1.5 wt.%), and low (< 100) H2O/Ce ratios are primarily observed in melts with unusually high [Ce] (up to 350 ppm). Additionally, [H2O] and H2O/Ce in some suites correlate with entrapment pressure despite having quench pressures high enough to prevent significant water loss from open- or closed-system degassing (< 100 MPa). Polybaric “sparging”, whereby low-P melts re-equilibrate with CO2-rich fluids exsolved at higher pressure, may result in water loss at pressures less than CO2 saturation. This may more accurately describe OIB degassing processes than open or closed system degassing. After correcting for degassing, primary Australs melts likely have H2O/Ce of ~ 300–600. If applicable to OIB sources in general, this limits the total water budget of the mantle, including the mantle transition zone, to < 2.4 ocean masses.

The role of calcium phosphates and silicates in the storage and transport of phosphorus at the upper-to-lower mantle transition: An experimental study to 25 GPa in a model peridotitic bulk composition

Calcium (Ca) phosphates are major hosts for phosphorus, halogens and incompatible trace elements in the Earth’s crust and mantle and, by supplying almost all phosphorus to the biosphere, are indispensable for sustaining life on Earth. To better understand the role of Ca-phosphates in the deep silicate Earth portion of the global phosphorus cycle at depths of the transition zone, we performed experiments in the P-T range of 15–25 GPa and 1600–2000 °C using a moderately fertile peridotite doped with 3 % synthetic β-Ca3(PO4)2 and 1 % of a trace element mix containing a range of HFSE, LILE, REEs, Cl and Br. Tuite is stable to at least 25 GPa at which pressure its upper T-stability limit is between 1700 and 1750 °C. At 15 GPa, by comparison, the upper T-stability limit is located between 1750 and 1800 °C, indicating a negative slope of the tuite-out reaction(s). Thus, tuite is stable to at least 700-km depth with a maximum T-stability close to or slightly above the average current mantle adiabat (ACMA) at 20–25 GPa and significantly above the ACMA at 15 GPa. Beyond the P-T stability limit of majoritic garnet, tuite is the principal subsolidus phosphorus carrier in the uppermost lower mantle. Beyond the P-T stability limit of tuite, phosphorus is likely to either (i) enter a Ca3(PO4)2 polymorph with an even higher density, (ii) form an entirely new phosphate phase or (iii) be accommodated in lower mantle silicates in six-fold coordination. Within the P-T range of our experiments, the phases encountered show the following trend of phosphorus contents:Pmelt>Pmajoritic garnet > Polivine ≅ Pringwoodite > Pdavemaoite ≅ Pwadsleyite > Pclinoenstatite ≅ Pbridgmanite ≅ Pferropericlase. Even when coexisting with tuite, bridgmanite and ferropericlase show phosphorus contents < 100 µg/g, and davemaoite has phosphorus contents that do not exceed 390 µg/g, testifying to the very limited phosphorus storage capacity of the lower mantle. Model calculations based on the phosphorus contents of ocean island basalts (OIBs) compared with those of mid ocean ridge basalts (MORBs) indicate that the OIB source is significantly richer in phosphorus than the MORB source and, for a wide range of OIB phosphorus contents, is likely to contain modal apatite. The relative importance of Ca-phosphates and silicates as phosphorus carriers in the peridotitic upper and uppermost lower mantle is strongly dependent upon pressure and the bulk phosphorus content. With increasing P, phosphorus is progressively transferred from Ca-phosphates to silicates with an ensuing decrease in the modal amount of the Ca-phosphates. Dependent upon the bulk phosphorus content, this may cause the Ca-phosphates to disappear before the upper-to-lower mantle boundary is reached, leaving majoritic garnet as the major phosphorus host. Breakdown of garnet will subsequently cause tuite to re-appear in the uppermost lower mantle and to resume its role of the principal subsolidus phosphorus carrier. For sufficiently high bulk phosphorus contents, tuite will remain stable throughout the transition zone, thereby maintaining phosphorus saturation in the coexisting silicates, and will eventually be joined by the tuite newly generated by garnet breakdown.

A carbon, nitrogen, and multi-isotope study of basalt glasses near 14°N on the Mid-Atlantic Ridge. Part B: Mantle source heterogeneities

Geochemical variations along mid-ocean ridges reveal the heterogeneous nature of the convecting upper mantle and geodynamic evolution of our planet (e.g., Hofmann, 2007; Parai et al., 2012). Although the occurrence of incompatible element and isotopically enriched mid-ocean ridge basalts (E-MORB) commonly arises from interaction with nearby mantle plumes, the source of E-MORBs far from known hotspots is debated. A well-known example of an enigmatic geochemical enrichment is found at 14°N on the Mid-Atlantic ridge (MAR). This is also one of the few locations worldwide where volatile-saturated E-MORBs, often referred to as “popping rocks” (PR), have been recovered. Although the mechanism(s) involved in popping rock generation remain elusive, compressional regimes associated with the exhumation and formation of oceanic core complexes (OCC) may be required to produce popping rocks via protracted volatile accumulation. However, the geochemical signature of OCC samples and their potential relationship to local geochemical enrichments associated with popping rock-affiliated MORBs remain unknown. Here, we present a comprehensive volatile characterization of popping rocks and associated MORBs (n = 19) sampled at 14°N on the MAR, including (n = 2) normal MORBs (N-MORB) from an oceanic core complex (OCC) and (n = 17) E-MORBs (Bekaert et al., Part A). We use isotopic and abundance data for volatile (carbon, nitrogen, noble gases) and radiogenic (Pb, Sr, Nd) elements, as well as the abundances of major and trace elements, to elucidate on the source(s) of E-MORBs at 14°N and discuss the potential origin(s) of geochemical heterogeneities within the upper mantle. We observe co-variations of helium and radiogenic element isotopes, suggesting potential contributions from a young HIMU-type (high μ = time-integrated 238U/204Pb) component in the OCC mantle source. The mantle source of the OCC samples is clearly distinct from that of PR-affiliated samples, suggesting no genetic relationship between OCC samples and E-MORBs. In addition, elevated Dy/Yb in OCC samples likely point to the incorporation of a subducted crustal component that is not observed in the mantle source of other MORB samples analyzed in this study. We report mantle source 40Ar/36Ar variations at 14°N, which, in line with previous studies, are interpreted as primarily reflecting variations in the amounts of recycled atmospheric Ar. Despite extensive evidence for drastic geochemical heterogeneities at 14°N on the MAR, we observe no significant δ15N variation (average δ15N = -4.49 ± 1.40 ‰) across N-MORB and E-MORB samples. This is a fundamental constraint, as the absence of significant N isotope variations across the upper mantle may imply that sedimentary N (with a typical δ15N ∼+6‰) is not extensively introduced within this reservoir during subduction. We investigate several scenarios that could explain this observation, including a significant contribution of altered oceanic crust to the overall budget of subducting slabs, quantitative return of subducting sedimentary N to the Earth's surface by arc volcanism, and/or preferential transport/preservation of sedimentary N in the lower mantle source of oceanic island basalts.

Concordance of V-in-olivine and Fe-XANES oxybarometry methods in mid-ocean ridge basalts

Oxygen fugacity (fO2) is a fundamental parameter controlling the behavior of redox-sensitive elements, as well as the speciation of degassing volatiles in igneous systems. Therefore, the average fO2 of typical mid-ocean ridge basalts (MORB) is important to constrain for models of atmospheric and mantle chemical evolution. Although there are several independent oxybarometry methods, in recent years, the Fe X-ray absorption near edge spectroscopy (Fe-XANES) oxybarometry of volcanic glasses has become the gold standard for evaluating the fO2 of modern lavas. Other oxybarometry methods, such as the V-in-olivine method, are useful for samples in which fresh glass is not preserved, but it is not clear whether the two methods give comparable results when applied to the same samples due to the lack of inter-calibration studies. Presented here are V-in-olivine oxybarometry data for eleven MORB lavas, eight of which have been previously characterized by Fe-XANES oxybarometry. The new data show that the V-in-olivine method is not applicable to MORB with glass containing <8.3 wt. % MgO, and that MORB with glass having MgO >8.3 wt. % have an fO2 averaging -0.28 ± 0.28 ΔFMQ, overlapping with the global Fe-XANES MORB average of -0.18 ± 0.16 ΔFMQ. It is therefore recommended that this oxybarometry technique only be applied to MORB with primitive (>8.3 wt. % MgO) glass. The lack of any systematic offset between the data from the two oxybarometers supports our previous conclusion that ocean island basalt sources (OIB) are more oxidized than average MORB, and that the majority of Archean komatiite sources record significantly lower fO2 than modern MORB.

Heavy Mo isotope enrichment in the Pitcairn plume: Implications for the subduction cycle of anoxic sediments

Subduction redistributes elements between Earth's principal geochemical reservoirs, modifying the chemical composition of Earth's mantle, crust, atmosphere, and hydrosphere, and consequently having an impact on the evolution of life itself. Subduction of surface material that has been geochemically modified by low-temperature processes leads to mineralogical and chemical heterogeneities in mantle reservoirs over time and is recorded in modern ocean island basalts. One of the principal geochemical end members of the heterogeneous deep mantle, the enriched mantle 1 (EM-1) source of Pitcairn Island, has been attributed to the contribution of crustal material with vastly different chemical compositions and ages. The Mo isotope composition of lavas from Pitcairn Island constrains the nature of this recycled crustal component. Pitcairn lavas have elevated δ98/95Mo relative to the depleted mantle. The high δ98/95Mo is associated with high time-integrated 232Th/238U and 87Rb/86Sr, and low time-integrated 147Sm/144Nd and 238U/204Pb. These characteristics can be attributed to the recycling of nearly pristine pelagic sediments that were deposited in a Proterozoic anoxic deep-ocean into the sources of the Pitcairn Island lavas. The isotope composition of these lavas is similar to that of EM-1 hotspots from the South Atlantic, indicating the addition of reduced sediments in both of Earth's large low shear wave velocity provinces (LLSVPs). Consistent data from both locations imply that the subduction cycling of sedimentary redox-sensitive elements such as Mo, S, Se, and U into arc magmas was in these cases inefficient in the Precambrian and the chemical and isotopic signature of reduced sediments is preserved in the source of ocean island basalts bearing the EM-1 characteristics.

Molybdenum isotope evidence for subduction-modified, recycled mafic oceanic crust in the mantle sources of ocean island basalts from La Palma and Hawaii

The recycling of mafic oceanic crust in subduction zones represents an important geological process to explain the geochemical diversity of the Earth's mantle. Based on radiogenic isotope and trace element data it has been proposed that recycled crust may form part of the deep-rooted mantle sources of ocean island basalts. Here we provide Mo isotope data for basalts from the ocean islands of La Palma and Hawaii to further address this issue. Samples have Mo isotope ratios (δ98/95Mo from −0.51 to −0.26 ± 0.06‰; 2σ; for La Palma and −0.50 to −0.11 ± 0.06‰; 2σ; for Hawaii) that extend to values significantly lower, and Ce/Mo ratios (112 to 32 for La Palma and 86 to 9 for Hawaii) that are significantly higher than those for the depleted mantle and bulk silicate Earth. Experimental data and model calculations show that neither magmatic processes (partial melting, residual sulphide) nor assimilation of marine sediments readily explain these observations as both processes result in δ98/95Mo values that are either too high, or Ce/Mo that are too low compared to the measured data for basalts from La Palma and Hawaii. Rather, their Mo isotope and Ce/Mo characteristics are uniformly matched by eclogite-facies, MORB-type meta-basalts that once formed part of oceanic lithosphere subducted to mantle depths. Our observations therefore provide strong evidence that subduction-modified, mafic oceanic crust is present in the mantle source of La Palma and Hawaii. As such, combined Mo isotopes and Ce/Mo ratios in ocean island basalts may provide an additional distinctive and robust tracer of recycled lithosphere in the mantle.

The origin of Ni and Mn variations in Hawaiian and MORB olivines and associated basalts

The origins of differences in the Mn and Ni contents of olivine phenocrysts from ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) are disputed. One prevalent hypothesis is mixing between peridotite melts and melts of an olivine-free pyroxenite that forms by reaction between large extents of melting of recycled ocean crust and peridotite. A second is peridotite melting at various pressures without contributions from recycled materials, and a third is mixing of recycled crust into peridotite sources. All models purport to account for olivine data, but an additional requirement is that the models account for corresponding liquid compositions. Here we demonstrate that none of the proposed models alone are able to account for both olivine and liquid data. The pyroxenite model predicts variations in liquid Mn to account for high Fe/Mn ratios, in contrast to the data where the high ratios result primarily from Fe variations. Using an updated model of Ni partitioning, combined with quantitative modeling of polybaric melting, confirms the important role of pressure on olivine compositions, and in addition shows that extent of melting and mantle fertility are also important. Most MORB are consistent with ∼10% -15% melting of peridotite at lower pressures, but low Ni olivines found on ridges adjacent to hot spots require a depleted mantle source. High Ni olivines can be produced by low degree melting of peridotite at high pressure, but such models do not produce the high Ni liquids that are also observed, nor the trace element abundances such as high TiO2. Mixing basalt and peridotite sources is not successful because it fails to account for the low Na/Ti ratios of OIB. A successful model for OIB involves adding ∼3–5% of a low degree eclogite melt with high trace element abundances and low Na/Ti to mantle peridotite, and then melting this source to small extents at high pressure. The need for a low degree melt of eclogite also rules out models that call upon high degrees of melting of eclogite that react with peridotite to produce OIB sources, and shows that this melting does not occur during upwelling beneath hot spots, where high degree melts of eclogite would occur. A firm conclusion of this work is that olivine compositions cannot be used to calculate the amount of recycled crust in mantle sources. Further work to explore variations within OIB and MORB will benefit from the realization that olivine compositions alone cannot be interpreted reliably without quantitative polybaric melting models and attention to co-existing liquids and trace element abundances.

A multi-siderophile element connection between volcanic hotspots and Earth's core

The existence of resolvable 182W/184W deficits in modern ocean island basalts (OIB) relative to the bulk silicate Earth has raised questions about the relationship of these rocks to Earth's core. However, because the core is expected to host high abundances of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re), it would be expected that such heterogeneity is accompanied by correlating variability in HSE abundances among OIB, but this has not been observed. We report instead a relationship between the W isotopic compositions and Ru/Ir ratios of Hawai‘i and Iceland OIB, which represent two of Earth's primary mantle plumes. Previous studies have highlighted the unique behavior of Ru relative to Os and Ir during metal-silicate fractionation, particularly when sulfide phases are segregated with metal. Using the information from these studies, we construct models predicting the consequences for HSE fractionation of various scenarios in which 182W/184W deficits can be created. These models show that the observed trends are likely inconsistent with modern, active core-mantle interaction at the CMB, and instead the observed low-Ru/Ir, low-182W/184W OIB are best explained by metal-silicate interaction that happened at significantly lower pressures. Such conditions may reflect what is expected for metal-silicate equilibration during the process of core formation itself, meaning that the deep mantle sources of OIB, such as ultra-low velocity zones, may instead reflect preserved relics of core formation. An ancient origin for distinct domains now residing at the core-mantle boundary is consistent with geophysical and petrological observations, for example that the Mg/Fe ratio of ferropericlase in the D″ layer is in significant disequilibrium with the modern core. Additional work is required to constrain the behavior of HSE during silicate differentiation processes that may also generate low 182W/184W ratios. However, if modern OIB represent a direct link to the ancient processes of core formation, future geochemical studies may be able to unlock new information about the formation and evolution of the Earth using OIB.

The role of recycled oceanic crust in the alkaline basalts: Evidence from Zn isotopic compositions of alkaline basalts from the Madeira hotspot (eastern North Atlantic)

Plate tectonic processes introduce oceanic crust (as eclogite) into the sources of oceanic island basalts (OIB). The fate of this recycled material in the deep mantle is still poorly understood. Here we present a systematic study of Zn isotopes on well-characterized alkaline basalts (<5 Ma) from the Madeira Islands in the eastern North Atlantic. Our analyses show that the δ66Zn values of alkaline basalts range from 0.25‰ to 0.34‰, with an average of 0.30 ± 0.05‰ (2SD, N = 15), which is similar to the average of mid-ocean ridge basalts (MORB) (0.28 ± 0.03‰, 2SD), and ∼ 0.14‰ higher than that of the asthenospheric mantle (0.16 ± 0.06‰, 2SD). However, these alkaline basalts have higher Zn/Fe ratios (up to 14.20) than MORB (generally less than 12). Model calculations show that the partial melting of mantle peridotite cannot simultaneously produce the observed MORB-like δ66Zn values and trace element ratios. After excluding the effects of post-eruption alteration, crystal fractionation, and the assimilation of crustal materials during magma upwelling on Zn isotopic compositions of the studied samples, we suggest that the MORB-like δ66Zn values reflect the mixing of peridotite melt and eclogite/pyroxenite-derived melt in the magma source of the Madeira hotspot. This is supported by trace element ratios and radiogenic isotopes, such as high Zn/Fe and Dy/Yb ratios and 206Pb/204Pb values. Our new data provide independent evidence in support of the important role of recycled oceanic crust in the source and generation of alkaline OIBs, and also highlight that such sources are not ubiquitously carbonated.

Carbon budget of Earth's deep mantle constrained by petrogenesis of silica-poor ocean island basalts

The convecting mantle is the Earth's largest carbon reservoir that controls the carbon outflux from the interior to surface, but its carbon content remains largely uncertain due to limited constraints on carbon inventory of the deep mantle. This knowledge gap can be filled by a global assessment of CO2 in ocean island basalts (OIBs), which are associated with upwelling mantle plumes from the deep mantle. However, this approach is hindered because of limited samples of undegassed, primary melts from intraplate magmatic settings and the debate on the origin of volatile-rich alkaline OIBs from plume versus lithospheric mantle. Here we examine the origin of silica-poor, alkaline OIBs and constrain CO2 in the primary melts of global OIBs by applying the newly developed liquid-based thermobarometer and primary melt correction scheme for silica-poor mantle-derived magmas. The averaged primary melt compositions of alkaline OIBs from individual island groups are more CO2-rich (∼3–11 wt% CO2) than those of subalkaline OIBs (∼0–7 wt% CO2), but both series from the same island groups yield final equilibration with the mantle at ∼3–5 GPa and mantle potential temperatures of ∼1430–1530°C. Our findings indicate that alkaline and subalkaline OIBs are generated from carbon-rich and carbon-poor domains, respectively, in the mantle plume sources, rather than deriving from two different sources in the plume and lithosphere mantle or variable extents of melting of a source with identical carbon content. Despite the source heterogeneity between alkaline and subalkaline OIBs, CO2 in the averaged primary melts of individual island groups displays strong positive correlations with Nb and Ba, which define robust CO2/Nb (1850 ± 196) and CO2/Ba ratios (226 ± 22) for the less-degassed deep, OIB source mantle. Combining these ratios with primitive mantle Nb and Ba contents, we estimate that the deep mantle on average has ∼330–400 ppm carbon. With an averaged CO2 content of ∼4 wt% in their primary melts, global OIBs are likely produced from the deep mantle source by ∼3% melting. The mean CO2 content in the primary melts of global OIBs, together with the previous estimate of plume magma production rate, yields a flux of ∼5.5 × 1012 mol/yr CO2 or ∼66 Mt/yr carbon through ocean island volcanism. If the OIB source mantle carbon budget is mostly primordial, our estimation provides a useful reference point for undifferentiated bulk silicate Earth to further examine carbon distribution and/or loss through Earth's history.

Barium isotope evidence of a fluid-metasomatized mantle component in the source of Azores OIB

This study presents Ba isotope compositions of ocean island basalts (OIB) from the Azores Archipelago to investigate the origin of the enriched components within their mantle sources. These samples have large variations in radiogenic Sr-Nd-Pb-Hf isotope compositions, indicating that they tap highly heterogeneous mantle sources. They display two types of Ba isotope characteristics. Samples from the Faial, Pico, São Jorge, and São Miguel Islands exhibit homogeneous and MORB-like Ba isotope compositions, whereas samples from Terceira Island have variable Ba isotopes. The δ138/134Ba values of the Terceira samples range from −0.03 to +0.24‰, extending to values significantly higher than those of the other islands.The MORB-like Ba isotope composition of the São Jorge samples indicates that the enriched HIMU/FOZO material in the Azores mantle plume source has a homogeneous and MORB-like Ba isotope composition. In contrast, the Terceira samples are characterized by heavy Ba isotope compositions, which are likely inherited from subducted altered oceanic crust (AOC) and may reflect the presence of a shallower enriched source beneath Terceira. As the Azores archipelago is located at a triple junction far from any active subduction zone, the signature of subducted AOC must be an ancient feature, potentially derived from a recycled fluid-metasomatized mantle component now present beneath the Azores archipelago. The negative correlation between δ138/134Ba and 206Pb/204Pb and the positive correlation between δ138/134Ba and Ba/Th among selected Terceira samples likely reflect mixing between the mantle plume and shallower, recycled, fluid-metasomatized mantle components enriched in heavy Ba isotopes.

Barium isotopes in ocean island basalts as tracers of mantle processes

The compositions of ocean island basalts (OIB) can record signatures of ancient subducted crust that has been recycled through the mantle. Based on their radiogenic isotope, major and trace element compositions, several geochemically-extreme mantle compositions are suggested to host components of recycled crust. Barium (Ba) stable isotopes can serve as a tracer of crustal recycling into the mantle sources of OIB because 1) Ba isotopes are fractionated in surface crustal reservoirs and 2) Ba is highly enriched in crust relative to the mantle. Therefore, the Ba isotopic compositions of the subducted crust could be detected in OIB. Here, we characterized Ba isotopic compositions of fresh OIB from hotspots sampling a range of mantle compositions. We first explore δ138/134Ba (permil deviation of the 138Ba/134Ba ratio from the standard SRM3104a) in a suite of Hekla samples and demonstrate that magmatic differentiation does not impact the δ138/134Ba values of OIB over MgO concentrations from 2.3 to 8.0 wt%, and higher MgO lavas are unlikely to experience δ138/134Ba fractionation. After excluding OIB samples that experienced low-temperature alteration—which may shift the δ138/134Ba values of samples away from their mantle sources—the δ138/134Ba values of Samoan (including EM2 and high 3He/4He) lavas range from -0.07 to 0.07‰, while for Cook-Austral (HIMU) lavas the δ138/134Ba values range from 0.00 to 0.11‰, and for St. Helena (HIMU) samples the δ138/134Ba values range from 0.02 to 0.05‰. High 3He/4He (> 20 Ra, ratio to atmosphere) lavas from Vestfirðir of Iceland (-0.09 to 0.14‰) and Samoa (-0.02 to 0.01‰) also exhibit Ba isotopic variability. An extreme EM2 lava examined here (143Nd/144Nd = 0.512582) has lighter Ba isotopic composition (-0.07 ± 0.02‰, 2SE) than the depleted mantle sampled by MORB (0.05 ± 0.05‰, 2SD). The Samoan lavas show a possible positive trend between δ138/134Ba values and 143Nd/144Nd ratios, and can be modeled as a binary mixing between the high 3He/4He (FOZO) mantle and recycled heterogeneous continental crust. In contrast, the extreme HIMU composition (206Pb/204Pb > 21.7) shows heavier Ba isotope signatures (0.11 ± 0.02‰, 2SE); δ138/134Ba values positively correlate strongly with 206Pb/204Pb ratios in HIMU lavas, consistent with a contribution from recycled oceanic crust.

Nitrogen isotope evidence for Earth’s heterogeneous accretion of volatiles

The origin of major volatiles nitrogen, carbon, hydrogen, and sulfur in planets is critical for understanding planetary accretion, differentiation, and habitability. However, the detailed process for the origin of Earth’s major volatiles remains unresolved. Nitrogen shows large isotopic fractionations among geochemical and cosmochemical reservoirs, which could be used to place tight constraints on Earth’s volatile accretion process. Here we experimentally determine N-partitioning and -isotopic fractionation between planetary cores and silicate mantles. We show that the core/mantle N-isotopic fractionation factors, ranging from −4‰ to +10‰, are strongly controlled by oxygen fugacity, and the core/mantle N-partitioning is a multi-function of oxygen fugacity, temperature, pressure, and compositions of the core and mantle. After applying N-partitioning and -isotopic fractionation in a planetary accretion and core–mantle differentiation model, we find that the N-budget and -isotopic composition of Earth’s crust plus atmosphere, silicate mantle, and the mantle source of oceanic island basalts are best explained by Earth’s early accretion of enstatite chondrite-like impactors, followed by accretion of increasingly oxidized impactors and minimal CI chondrite-like materials before and during the Moon-forming giant impact. Such a heterogeneous accretion process can also explain the carbon–hydrogen–sulfur budget in the bulk silicate Earth. The Earth may thus have acquired its major volatile inventory heterogeneously during the main accretion phase.

Size and Composition of the MORB+OIB Mantle Reservoir

AbstractMost efforts to characterize the size and composition of the mantle that complements the continental crust have assumed that the mid‐ocean ridge basalt (MORB) source is the incompatible‐element depleted residue of continental crust extraction. The use of Nd isotopes to model this process led to the conclusion that the “depleted MORB reservoir” is confined to the upper ∼30% of the mantle, leaving the lower mantle in a more “primitive” state. Here, we use Nb/U and Ta/U to evaluate mass and composition of the mantle reservoir residual to continent extraction and find that it exceeds 60% of the total mantle. Thus, the (Nb, Ta)/U‐based mass balance conflicts with the ε(Nd)‐based mass balance, and this invalidates the classical 3‐reservoir silicate Earth model (continental crust, depleted mantle, and primitive mantle). Including the combined MORB + ocean island basalt (OIB) sources in the ε(Nd)‐based mass balance does not reconcile the conflict as it would require their average ε(Nd) to be ≤3.0, much lower than observed MORB + OIB ε(Nd) averages. We resolve this conflict by invoking an additional, “early enriched reservoir” (EER), formed prior to extraction of significant continental crust, but now hidden or lost. This EER differs from EERs previously invoked by having no Nb‐Ta anomaly. We suggest that it originated as an early mafic crust, which had unfractionated (Nb, Ta)/U but fractionated Sm/Nd ratios. The corresponding “early depleted” reservoir generated the present‐day continental crust and the “residual mantle” MORB‐OIB reservoir, which occupies at least 63% of the present‐day mantle and is only moderately depleted in incompatible trace elements.

Noble gas isotope systematics in the Canary Islands and implications for refractory mantle components

Noble gas isotope systematics of ocean island basalts (OIB) provide evidence for relatively undegassed and primitive mantle sources. These OIB sources partly derive from the deep mantle by virtue of their distinctiveness from mid-ocean ridge basalts (MORB), which dominantly sample upper mantle. New helium, neon and argon isotope data are presented for Canary Islands lavas, carbonatites and cumulate and mantle xenoliths confirming 3He/4He ratios that are the same or lower than MORB, but that are heterogeneous (∼3–9.5RA) within and between islands in the archipelago. Neon and Ar isotope systematics for lavas are mostly within the range of air compositions. Harzburgite xenoliths from Lanzarote, which are interpreted to represent ancient refractory mantle residues, have distinct He-Ne-Ar isotope systematics from lavas or cumulate xenoliths. The harzburgites are characterized by forsteritic olivine (>Fo91) with uniformly low-3He/4He (6.6 ± 0.2RA), high 40Ar/36Ar (630–4900), and have Ne isotope compositions that range between air values or that are similar or more nucleogenic than depleted MORB mantle (DMM). Similar refractory mantle peridotites have been discovered as xenoliths at other OIB localities and are likely to be distinct from continental lithospheric mantle. Refractory mantle (RM), which by virtue of its high melting temperature is difficult to partially melt, may be a significant component in the convecting mantle and has potential to impart a cryptic noble gas signature to partial melts in intraplate, divergent and convergent margin settings. In this sense, RM may represent a ‘sixth mantle component’ after DMM, high-µ (high 238U/204Pb; HIMU), enriched mantle endmembers (EMI, EMII) and the ‘focus zone’ (FOZO). An RM-type component may be presented in the older eastern Canary Islands and possibly in recent rejuvenated volcanism from Teide (Tenerife). Canary Island intraplate volcanism samples multiple mantle components, confirming that the most gas-rich mantle sources involved in magmatism dominate OIB noble gas compositions.

Mantle noble gas abundance ratios inferred from oceanic basalts and model estimates

We investigated mantle noble gas abundance ratios using a global compilation of data from mid-ocean ridge basalts (MORB) and high 3He/4He ocean island basalts (OIB), coupled with calculated abundances based on closed-system evolution models. Assuming that the mantle sources of MORB and high 3He/4He OIB are derived from the same early Earth parental material, variations in their noble gas abundance ratios are useful to ascertain which source is more depleted or degassed, or if the mantle noble gases might be a hybrid of different parental materials. Therefore, noble gas compositions of mantle sources can be a key to elucidating mantle evolution and structure. We calculated the noble gas abundance patterns using a Monte Carlo approach with validated end-members, which provides new estimates for the abundance ratios and their uncertainties. The adequacy of the calculated values was confirmed by existing data. We compiled all published noble gas data extracted by total heating of glassy oceanic basalts, which enables estimation of the pristine abundance patterns of noble gases including Kr for both sources of MORB and OIB. The pristine abundance ratios of the mantle sources for oceanic basalts are estimated by correcting measured values for atmospheric influences and elemental fractionation. The corrected abundance ratios resemble mantle values calculated using the proposed end-members for MORB and high 3He/4He OIB. The estimated relative abundance ratios in the high 3He/4He OIB source are similar to those of C1 chondrites in the range of He to Kr, supporting the hypothesis that a less degassed primitive OIB source can exist in the Earth's mantle. By contrast, the MORB source shows depletion of lighter noble gases, but with high relative helium content compared to the OIB source. The contrast in He/Ne abundance ratios between MORB and OIB, is a key constraint on mantle evolution models, andis expected to reflect several stages of diffusive fractionation during Earth history, or different initial mantle sources, which would require heterogeneous Earth accretion.

A quantitative framework for global variations in arc geochemistry

Arc magmas are generated by partial melting of a mantle wedge modified by inputs from subducting sediment and igneous ocean crust. Despite this uniform tectonic process, global arc-front stratovolcano compositions exhibit trace element abundances that can vary by more than an order of magnitude. All incompatible elements correlate well, including those that are thought to be wedge derived, “fluid-mobile”, and sediment derived. It has been suggested that global variations in arc trace element abundances occur due to the addition of variable slab components to a depleted mantle wedge, though quantitative global models that include both trace elements and isotopes have been lacking. Here we present an alternative framework in which a relatively constant proportion of slab material, consisting of melts both of altered ocean crust and sediment, is added to ambient mantle wedge that varies from more depleted than MORB to as enriched as “EM1” ocean island basalt sources. The modified mantle melts to varying extents, with melting controlled primarily by the thickness of the overlying lithosphere. Thick lithosphere leads to lower extents of melting in the garnet stability field in most continental arcs. Continental arcs also have enriched ambient mantle, likely because of contributions from enriched sub-continental lithospheric mantle. High degrees of melting of more depleted sources cause oceanic arcs to have low incompatible element abundances. Quantitative modeling of these processes shows that this framework can account for the global arc systematics exhibited by arc-front stratovolcanoes and provides a tool to examine regional variations and more unusual magma compositions. Variations in slab temperature and amount of slab input remain important for second-order variations and certain element ratios.

An eclogitic component in the Pitcairn mantle plume: Evidence from olivine compositions and Fe isotopes of basalts

Subducted oceanic crust can transform into eclogite in the upper mantle, and generate chemical heterogeneity of mantle plumes, as recorded by elemental and radiogenic isotopic variations in oceanic island basalts (OIBs). The secondary pyroxenite produced by the reaction between eclogite-derived melt and peridotite is increasingly regarded as a major source component of OIBs, as well as peridotite. However, it remains unclear whether eclogite can be a direct source component of OIBs. To test this possibility, we present high-precision whole-rock Fe isotopes and the chemical compositions of olivine phenocrysts from well-characterized EM1-type basalts from Pitcairn Island. The Pitcairn basalts are characterized by moderate 87Sr/86Sr, low 143Nd/144Nd and 206Pb/204Pb isotopic ratios, and lowest δ26Mg values among OIBs, suggesting a contribution from recycled ancient crustal components (oceanic crust plus sediment). For comparison, we also report the Fe isotope compositions of FOZO-type basalts (with low 87Sr/86Sr, moderately high 143Nd/144Nd and moderate 206Pb/204Pb ratios) from the Louisville hot spot track, which were suggested to be a typical peridotite-derived OIB. The Louisville basalts have MORB-like δ57Fe values (0.06‰–0.15‰), whereas the Pitcairn lavas have substantially heavier Fe isotopic compositions (δ57Fe = 0.17‰–0.31‰) than MORBs. Quantitative evaluations suggest that magmatic differentiation, partial melting, and elevated oxygen fugacity in the source are insufficient to generate the heavy Fe isotopic compositions of the Pitcairn basalts.A good correlation between δ57Fe and εNd(i) (or δ26Mg) values in the Pitcairn basalts indicates binary mixing, and the melts derived from the EM1 endmember have unusually heavy Fe and light Mg isotopic compositions. In order to explain the origin of the Fe and Mg isotopic compositions, we calculated the Fe, Mg, and Nd isotopic compositions of partial melts of eclogite, secondary pyroxenite and peridotite, respectively. The results indicate that eclogite is the only suitable candidate to generate melts with both heavy Fe and light Mg isotopes. This inference is strengthened by the major and minor elemental compositions of olivine phenocrysts from the Pitcairn basalts, which show low Fo (73.2–82.5) and Ni contents (620–1949 ppm), and low Mn/Fe (0.011–0.015) and Ni/(Mg/Fe) (524–1041) ratios. These characteristics are markedly different from those of olivines from MORBs and the Koolau lavas from Hawaii, which represent phenocrysts that equilibrated with peridotite-derived and secondary pyroxenite-derived melts, respectively. We therefore argue that eclogite is the source lithology of the EM1 endmember of the Pitcairn basalts. Binary mixing between our modelled eclogite- and peridotite-derived melts produced magmas with relatively low Mg# value, Mg/Fe ratios and moderate Ni contents characteristics, which are preserved in the low-Fo Pitcairn olivines. Our results highlight that, in addition to peridotite and secondary pyroxenite, eclogite may survive in mantle plumes at shallow depths and make a substantial contribution to the source of OIBs.

Source and magmatic evolution of ocean island basalts from the Pohnpei Island, Northwest Pacific Ocean: Insights from olivine geochemistry

Source and magmatic evolution of ocean island basalts from the Pohnpei Island, Northwest Pacific Ocean: Insights from olivine geochemistry

Combined Lithophile‐Siderophile Isotopic Constraints on Hadean Processes Preserved in Ocean Island Basalt Sources

AbstractDetection of Hadean isotopic signatures within modern ocean island basalts (OIB) has greatly influenced understanding of Earth's earliest history and long‐term dynamics. However, a relationship between two isotopic tools for studying early Earth processes, the short‐lived 146Sm‐142Nd and 182Hf‐182W systems, has not been established in this context. The differing chemical behavior of these two isotopic systems means that they are complementary tracers of a range of proposed early Earth events, including core formation, magma ocean processes, and late accretion. There is a negative trend between 142Nd/144Nd and 182W/184W ratios among Réunion OIB that is extended by Deccan continental flood basalts. This finding is contrary to expectations if both systems were affected by silicate differentiation during the lifetime of 182Hf. The observed isotopic compositions are attributed to interaction between magma ocean remnants and Earth's core, coupled with later assimilation of recycled Hadean mafic crust. The effects of this scenario on the long‐lived 143Nd‐176Hf isotopic systematics mirror classical models invoking mixing of recycled trace‐element enriched (sedimentary) and depleted (igneous) domains in OIB mantle sources.If the core provides a detectible contribution to the tungsten element budget of the silicate Earth, this represents a critical component to planetary‐scale tungsten mass balance. A basic model is explored that reconciles the W abundance and isotopic composition of the bulk silicate Earth resulting from both late accretion and core‐mantle interaction. The veracity of core‐mantle interaction as proposed here would have many implications for long‐term thermochemical cycling.