HIMU Lavas Research Articles

A peridotite source for strongly alkalic ultrabasic HIMU lavas of the Oslo Rift, Norway

High alkali (Na2O + K2O) low SiO2 lavas generally have highly fractionated trace element compositions and their petrogenesis is debated. Both pyroxene-rich (pyroxenite) and olivine-rich (peridotite) mantle sources have been proposed for such lavas. HIMU (high 238U/204Pb) lavas usually have such compositions and are often interpreted as being derived from garnet-pyroxenite (eclogite) residue of subducted recycled oceanic crust, possibly including a carbonate component. In some cases, an origin from a garnet peridotite source has also been considered.We present new Mg, Pb, Nd, and Sr isotopic data and elemental data for alkalic ultrabasic lavas (melilitites, nephelinites) from the Oslo Rift, Norway. The magmatic province of the Permo-Carboniferous Oslo Rift is interpreted as part of the much larger Skagerrak-Centered Large Igneous Province (SCLIP) which may be related to a mantle plume source originating from the African Large Low Shear Velocity Province (LLSVP) at the core-mantle boundary. The range of δ26Mg is narrow (−0.32 to −0.28) and is similar to typical mantle values, and therefore is inconsistent with a carbonated source as has been suggested for other similar lavas. The 206Pb/204Pb values of all the lavas are high, which identifies them as HIMU lavas. The εNd of +1 to +2 are substantially lower than for typical HIMU lavas, and in the εNd-εSr diagram they plot on the LoNd array between HIMU and EMI. The lavas are more like archetypal kimberlites (formerly, Group I kimberlites), as these also have εNd slightly higher than the chondritic value and Pb isotopes consistent with a HIMU source. The REE patterns of the lavas are also almost as fractionated as in Group I kimberlites.We used major and trace elements to infer the primary magma composition of these lavas. This primary magma composition is compared with experimental melts of both carbonated peridotite and carbonated eclogite source compositions and shows that their major element compositions are only consistent with either a very high degree of melting of eclogite or a very low degree melting of garnet peridotite. The strongly fractionated REE patterns of the lavas can only be made by low-degree melting of either eclogite or garnet peridotite, which therefore effectively rules out eclogite as a major component of the mantle source of these lavas. Further trace element modeling points to a mantle source composition with >97% garnet peridotite that was lightly carbonated (<0.5 wt% CO2) and melted to a very low degree (∼0.4%) to produce these lavas. The modeled source trace element pattern was constrained by the Nd, Sr, and Pb isotopic compositions of the lavas. The measured high Ti contents of these lavas (3.8–6 wt%) are quantitatively shown to be the result of enrichment by both partial melting and fractional crystallization, using experimentally determined DTi for melting of this type of source. We also show that a low degree of melting of a slightly carbonated source is sufficient to result in the likely very high CO2 content (10–15 wt%) of the primary magma.The rather uniform Pb isotope composition of HIMU magmas suggests that the HIMU source is rather uniform in composition, but the Pb isotope composition requires this source to form late in Earth's history. The similarity of these lavas and kimberlites suggests a very deep source, possibly in the LLSVP that gave origin to the SCLIP. This means that LLSVPs do not represent some primordial layer around the core that escaped homogenization of the mantle by the Moon-forming giant impact. Instead, this layer must have formed late in Earth's history. The constraint that this source contains <3% of garnet-pyroxenite (eclogite) residue of recycled basaltic oceanic crust shows that this layer is primarily not a subducted slab graveyard. The process by which the LLSVPs formed, however, remains obscure.

A HIMU volcanic belt along the SW African coast (∼83–49 Ma): New geochemical clues to deep mantle dynamics from carbonatite and silica-undersaturated complexes in Namibia

The origin of carbonatitic and highly silica-undersaturated volcanism, common along the SW coast of Africa extending from Angola through Namibia to the tip of South Africa, is still poorly understood. Here we present new geochemical data (major and trace element and Sr-Nd-Pb-Hf-O-C isotopes) from the Agate Mountain calcio- to magnesio‑carbonatites (∼83 Ma), Dicker Willem calcio‑carbonatites (49 Ma) and Swakopmund basanitic plugs (76–72 Ma) along the coast of Namibia that were emplaced after the EMI (enriched mantle one) type Etendeka flood basalts. The trace element and isotopic composition of Agate Mountain carbonatites and Swakopmund basanites indicate that they were derived from a HIMU-type (high time-integrated 238 U/ 204 Pb with radiogenic Pb isotope ratios) magma source, similar to the St. Helena global HIMU endmember in the South Atlantic. The Agate Mountain carbonatites form part of the late-stage Walvis Ridge HIMU hotspot track overlying the EM1-type Walvis Ridge basement forming part of the Tristan-Gough hotspot track. The Dicker Willem carbonatites, however, extend to higher 206 Pb/ 204 Pb than St. Helena, but have similar 206 Pb/ 204 Pb to Mangaia HIMU lavas in the Pacific. Compared to Mangaia HIMU, the Dicker Willem carbonatites with mantle-type O and C isotopes have higher 207 Pb/ 204 Pb and 87 Sr/ 86 Sr but lower 143 Nd/ 144 Nd, suggesting it may represent a new HIMU endmember flavor. The HIMU carbonatitic and silica-undersaturated rocks form a belt of age-progressive volcanic tracks, including: 1) from the Walvis Ridge, through NW Namibia to central Angola, 2) from the Vema Seamount via Dicker Willem carbonatite to Gibeon kimberlites and carbonatites, 3) from the Namaqualand to Bushmanland and to Warmbad volcanic centers in northwestern South Africa, and 4) along the older end of the Shona EMI-type volcanic track extending into South Africa. Geochemical and seismic tomographic data suggest that the EMI and HIMU mantle plumes are generated from different geochemical domains at the base of the lower mantle. The Tristan-Gough, Discovery and Shona EM1 volcanic tracks are derived from a common low-velocity anomaly (superplume-like structure with three branching arms) ascending from the outer margin, possibly lower primoridal layer, of the African large low-shear-velocity province (LLSVP). Seismic low-velocity anomalies can be traced from beneath the belt of HIMU volcanism to an internal and shallower part of the LLSVP, located ∼900–1200 km east of the outer LLSVP margin and suggest that HIMU-type (possibly subducted oceanic lithospheric) material overlies EMI-type (possibly primordial) material in the internal part of the LLSVP. • A HIMU-like volcanism belt along the southwest Africa. • The HIMU-like volcanic complexes form age-progressive volcanic tracks. • EMI and HIMU mantle plumes are from different domains in the lower mantle.

Zinc isotopic evidence for recycled carbonate in the deep mantle

Carbonate, the major carbon reservoir on Earth’s surface, can enter into the mantle by subduction. However, evidence for recycled surficial carbonates in the deep mantle is still scarce. Ocean island basalts from Cook-Austral islands and St. Helena Island, widely called HIMU basalts because of their high μ = 238U/204Pb sources, are thought to be fed by mantle plumes originating in the lower mantle. Here we report exceptionally high δ66Zn values (δ66Zn = 0.38 ± 0.03‰) of these HIMU lavas relative to most published data for oceanic basalts (δ66Zn = 0.31 ± 0.10‰), which requires a source contributed by isotopically heavy recycled surficial carbonates. During subduction of the oceanic lithosphere, melting of mixed surficial carbonates and basaltic crust in the deep mantle generates carbonatite melts, which metasomatizes the nearby mantle and the resultant carbonated mantle ultimately evolves into a high-δ66Zn HIMU source. High-δ66Zn signatures of HIMU basalts, therefore, demonstrate that carbonates can be transported into Earth’s deep mantle.

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.

The nature and evolution of mantle upwelling at Ross Island, Antarctica, with implications for the source of HIMU lavas

The multiple, proximal, young and/or active volcanic centers of Ross Island, Antarctica, provide a unique opportunity to investigate both deep and shallow processes of alkaline magma genesis and the length scales of mantle heterogeneity. Ross Island, Antarctica is an assembly of four silica-undersaturated alkaline volcanic centers, including the active phonolitic Erebus volcano (1.2 to 0 Ma). Mt. Terror, Mt. Bird, and Hut Point Peninsula surround Erebus on the periphery of Ross Island and are mostly older (∼0.3 to 4 Ma) and mainly basanitic in composition. While the geochemical compositions and HIMU isotopic signature of Erebus lavas are well characterized, the geochemistry of the peripheral volcanic centers was, until this study, poorly known. To further investigate the nature of mantle sources and magmatic processes beneath Ross Island, we therefore measured major and trace element and Sr, Nd, Hf, and Pb isotope compositions on fifty-seven samples from the three volcanoes peripheral to Erebus volcano and compared the results to previous geochemical and isotopic studies of the latter. Mt. Terror, Mt. Bird, and Hut Point Peninsula have 87Sr/86Sr ranging from 0.702907 to 0.703147, εNd ranging from +4.3 to +6.3, εHf ranging from +5.6 to +8.6, and 206Pb/204Pb ranging from 19.282 to 20.241. The Sr, Nd, Hf, and Pb isotope compositions of all four volcanoes (Erebus, Terror, Bird, and Hut Point Peninsula) fall on a mixing line between the HIMU and DMM mantle end-members, with a minor contribution from an EM component. Small, but distinct differences in the isotopic compositions of the four volcanoes, most notably Mt. Bird, imply mantle source heterogeneity beneath Ross Island on a length scale comparable to or smaller than inter-volcano distances (i.e., less than 50 km). The major and trace element systematics and isotopic signatures of the Ross Island volcanic samples are distinct from those for samples from Zealandia and older Antarctic samples that typify magmas from the Cenozoic alkaline magmatic province. The chemical and isotopic compositions of the Ross Island lavas and tephras are best explained by small-degree melting of an ancient asthenospheric source containing residual garnet. Additionally, recent global tomographic modeling indicates low shear wave velocity anomalies with depths down to ∼1200 km beneath Ross Island. This suggests a deep-seated upwelling plume impinging on shallower metasomatized mantle which has been gradually eroded away and is no longer a potential source of Ross Island magmatism. Rather, the source of Ross Island lavas is old (Archean to early Proterozoic) material upwelling from the deep mantle as opposed to melting of subcontinental lithospheric mantle metasomatized by Paleozoic subduction. While our findings have local significance in terms of understanding Ross Island volcanism and providing a new perspective on the evolution of volcanism in the surrounding region during the Phanerozoic, they also have important global implications for gauging the nature, age, and evolution of the HIMU mantle source.

Slab-derived components in the subcontinental lithospheric mantle beneath Chilean Patagonia: Geochemistry and Sr–Nd–Pb isotopes of mantle xenoliths and host basalt

In subduction zones, ultramafic xenoliths hosted in alkaline basalts can yield significant information about the role of potential slab-derived components in the subcontinental lithospheric mantle (SCLM). Chemical and isotopic heterogeneities in such xenoliths are usually interpreted to reflect melt extraction followed by metasomatic re-enrichment. Here we report new whole-rock major, trace element and isotopic (Sr–Nd–Pb) data for a Proterozoic suite of 17 anhydrous spinel-lherzolites and Eocene (new K–Ar data) host alkaline basalt found near Coyhaique (~46°S), Aysén Region, Chile. These Patagonian nodules are located in a current back-arc position, ~100km east of the present day volcanic arc and ~320km from the Chile Trench.The mantle xenoliths consist of coarse- to medium-grained spinel-lherzolites with trace element compositions characteristic of a subduction zone setting, such as pronounced negative Nb, Ta and Ti anomalies coupled with significant enrichment of LILEs (e.g., U) and chalcophile elements (W, Pb and Sn). Most of them are characterized by flat to depleted light-rare earth element (LREE) patterns (Ce/YbN=0.6–1.1) coupled with less radiogenic Sr–Pb (87Sr/86Sr=0.702422–0.703479; 206Pb/204Pb=18.212–18.539) and more radiogenic Nd isotopic compositions (143Nd/144Nd=0.512994–0.513242), similar to the depleted mantle component (DMM or PREMA). In contrast, samples with slight LREE enrichment (Ce/YbN=1.3–1.8) show more radiogenic Sr–Pb (87Sr/86Sr=0.703791–0.704239; 206Pb/204Pb=18.572–18.703) and less radiogenic Nd isotopic compositions (143Nd/144Nd=0.512859–0.512934), similar to the EM-2 reservoir. These new geochemical and isotope data suggest that the Coyhaique spinel-lherzolites are derived from a heterogeneous SCLM resulting from mixing between a depleted mantle component and up to 10% of slab-derived components. The enriched component added to the SCLM represents variable extents of melts of both subducted Chile Trench sediments and modified oceanic crust throughout the initial stages of the Farallón–Aluk ridge collision during Paleocene to Eocene time. However, based on the tectonic evolution of southern South America, we cannot exclude the influence of long-lived subduction events beneath south Patagonia. Although we believe that the studied samples were brought to the surface in this geodynamic context, there is no evidence that ocean island basalt (OIB)–like melts related to the Farallón–Aluk asthenospheric slab window affected the peridotite composition.The host alkaline basalt is a single unit with a HIMU-like OIB signature characterized by marked positive Nb–Ta anomalies coupled with negative anomalies in highly incompatible and fluid-mobile elements (Rb, K, Pb, and Sr). The compositional similarity between the HIMU-like OIB mantle source and the host basalt is also evident from trace element ratios [(Ba–Th–K–La–Zr)/Nb] as well as by the low 87Sr/86Sri (0.703039–0.703058) and relatively high 143Nd/144Ndi (0.512880–0.512874) and 206Pb/204Pb (19.333–19.389) isotopic ratios. The low 206Pb/204Pb ratios compared to end-member HIMU lavas (e.g., Sta. Helena and the Cook-Austral Islands) suggest that this region was modified by processes associated with a prolonged period of subduction related to the Andean orogenesis and the recycling of several oceanic plates beneath the continent, following the Mesozoic breakup of Gondwana or an even older subduction-related event with young recycling ages (<2Ga).

Key new pieces of the HIMU puzzle from olivines and diamond inclusions.

Mantle melting, which leads to the formation of oceanic and continental crust, together with crust recycling through plate tectonics, are the primary processes that drive the chemical differentiation of the silicate Earth. The present-day mantle, as sampled by oceanic basalts, shows large chemical and isotopic variability bounded by a few end-member compositions. Among these, the HIMU end-member (having a high U/Pb ratio, μ) has been generally considered to represent subducted/recycled basaltic oceanic crust. However, this concept has been challenged by recent studies of the mantle source of HIMU magmas. For example, analyses of olivine phenocrysts in HIMU lavas indicate derivation from the partial melting of peridotite, rather than from the pyroxenitic remnants of recycled oceanic basalt. Here we report data that elucidate the source of these lavas: high-precision trace-element analyses of olivine phenocrysts point to peridotite that has been metasomatized by carbonatite fluids. Moreover, similarities in the trace-element patterns of carbonatitic melt inclusions in diamonds and HIMU lavas indicate that the metasomatism occurred in the subcontinental lithospheric mantle, fused to the base of the continental crust and isolated from mantle convection. Taking into account evidence from sulfur isotope data for Archean to early Proterozoic surface material in the deep HIMU mantle source, a multi-stage evolution is revealed for the HIMU end-member, spanning more than half of Earth's history. Before entrainment in the convecting mantle, storage in a boundary layer, upwelling as a mantle plume and partial melting to become ocean island basalt, the HIMU source formed as Archean-early Proterozoic subduction-related carbonatite-metasomatized subcontinental lithospheric mantle.

Combined petrological, geochemical and isotopic modeling of a plume source: Example of Gambier Island, Pitcairn chain

Mantle plumes have been increasingly studied in the past years, yet they remain not well understood. While geophysical approaches attempt to locate the depth of sources, petrological approaches characterize the nature of the materials that melt and geochemical approaches trace their history and origin. However, combined studies remain rare while they could provide strong constraints on the processes that give rise to mantle plumes. Here we present a new model that combines petrological and geochemical approaches to constrain the nature of source materials, their temperature of melting, their composition and their age. We focus on Gambier Island, Pitcairn chain in Polynesia because it has the advantage of combining two important features, (a) most lavas are tholeiites, a necessary feature to perform an accurate petrological modeling that can be compared to other plumes and (b) it belongs to the Pitcairn Chain, whose composition varies through time and samples today the extreme EM1-type source.We present geochemical analyses as well as Sr–Nd–Pb–Hf isotopes of lavas and high-precision olivine analyses. Using in situ olivine analyses and REE modeling, we constrain the composition of the mantle source (5% recycled oceanic crust – 95% peridotite), how both components melt (25–30% versus 0.5–1.5%) and we estimate the temperature of Gambier source at about 1400 °C, a rather low temperature compared to Hawaii for example. We also constrain both the age and the nature of the recycled material using the isotopic data. They require the presence of about 3% sediment associated to recycled basalt in the pyroxenitic component and its age is about 1.5 Gyr. We also attribute the marked Nb–Ta positive anomalies and the elevated Ce/Pb ratios to the presence of recycled basalt in the mantle source. These features resemble typical HIMU lavas but the younger age of the recycled material together with the presence of sediment in the plume source explains the lower Pb isotopic ratios. The clear isotopic change between Gambier and Pitcairn Islands suggests that the plume source changed radically within 5 Ma. More generally, our new data and model suggest that the FOZO composition as sampled by Gambier lavas does not necessarily represent a large and rather primitive mantle source.

The helium flux from the continents and ubiquity of low-3He/4He recycled crust and lithosphere

New helium isotope and trace-element abundance data are reported for pyroxenites and eclogites from South Africa, Siberia, and the Beni Bousera Massif, Morocco that are widely interpreted to form from recycled oceanic crustal protoliths. The first He isotope data are also presented for Archaean peridotites from the Kaapvaal (South Africa), Slave (Canada), and Siberian cratons, along with recently emplaced off-craton peridotite xenoliths from Kilbourne Hole, San Carlos (USA) and Vitim (Siberia), to complement existing 3He/4He values obtained for continental and oceanic peridotites.Helium isotope compositions of peridotite xenoliths vary from 7.3 to 9.6RA in recently (<10kyr) emplaced xenoliths, to 0.05RA in olivine from cratonic peridotite xenoliths of the 1179Ma Premier kimberlite, South Africa. The helium isotope compositions of the peridotites can be explained through progressive sampling of 4He produced from radiogenic decay of U and Th in the mineral lattice in the older emplaced peridotite xenoliths. Ingrowth of 4He is consistent with generally higher 4He concentrations measured in olivine from older emplaced peridotite xenoliths relative to those from younger peridotite xenoliths. Collectively, the new data are consistent with pervasive open-system behaviour of He in peridotite xenoliths from cratons, mobile belts and tectonically-active regions. However, there is probable bias in the estimate of the helium isotope composition of the continental lithospheric mantle (6.1±2.1RA), since previously published databases were largely derived from peridotite xenoliths from non-cratonic lithosphere, or phenocrysts/xenocrysts obtained within continental intraplate alkaline volcanics that contain a contribution from asthenospheric sources. Using the new He isotope data for cratonic peridotites and assuming that significant portions (>50%) of the Archaean and Proterozoic continental lithospheric mantle are stable and unaffected by melt or fluid infiltration on geological timescales (>0.1Ga), and that U and Th contents vary between cratonic lithosphere and non-cratonic lithosphere, calculations yield a 3He flux of 0.25–2.2atoms/s/cm2 for the continental lithospheric mantle. These estimates differ by a factor of ten from non-cratonic lithospheric mantle and are closer to the observed 3He flux from the continents (<1atoms/s/cm2).Pyroxenites and eclogites from the continental regions are all characterized by 3He/4He (0.03–5.6RA) less than the depleted upper mantle, and relatively high U and Th contents. Together with oceanic and continental lithospheric peridotites, these materials represent reservoirs with low time-integrated 3He/(U+Th) in the mantle. Pyroxenites and eclogites are also characterized by higher Fe/Mg, more radiogenic Os–Pb isotope compositions, and more variable δ18O values (∼3‰ to 7‰), compared with peridotitic mantle. These xenoliths are widely interpreted to be the metamorphic/metasomatic equivalents of recycled oceanic crustal protoliths. The low-3He/4He values of these reservoirs and their distinctive compositions make them probable end-members to explain the compositions of some low-3He/4He OIB, and provide an explanation for the low-3He/4He measured in most HIMU lavas. Continental lithospheric mantle and recycled oceanic crust protoliths are not reservoirs for high-3He/4He and so alternative, volumetrically significant, He-rich reservoirs, such as less-degassed (lower?) mantle, are required to explain high-3He/4He signatures measured in some intraplate lavas. Recycling of oceanic crust represents a fundamental process for the generation of radiogenic noble gases in the mantle, and can therefore be used effectively as tracers for volatile recycling.

Isotope evolution in the HIMU reservoir beneath St. Helena: Implications for the mantle recycling of U and Th

HIMU (high-μ; 238U/204Pb) is a mantle reservoir that has been thought to form by subduction and subsequent storage of ancient oceanic crust and lithosphere in the mantle. In order to constrain the processes that acted on subducted materials over several billion years, we present precise Pb–Sr–Nd–Hf–He isotopic data together with 40Ar/39Ar and K/Ar ages of HIMU lavas from St. Helena in the Atlantic. Clinopyroxene separates were analyzed together with whole-rock samples to better describe the geochemical characteristics of the HIMU component. Although isotopic variations are small in the St. Helena lavas (20.6–21.0 for 206Pb/204Pb) between 12 and 8Ma, the younger lavas have more HIMU-like isotopic compositions than the older lavas. The mixing arrays defined by these lavas are remarkably similar to those observed in HIMU lavas from Austral Islands in the Pacific, suggesting that the two HIMU reservoirs located in different mantle domains are characterized by similar isotopic compositions with radiogenic 206Pb/204Pb and 208Pb/204Pb, enriched Nd and Hf isotopes, depleted Sr isotopes, and radiogenic 3He/4He. However, there is a significant difference between the St. Helena and Austral Islands lavas in 207Pb/204Pb. The St. Helena lavas show systematically higher 207Pb/204Pb for a given 206Pb/204Pb. Lead isotope evolution models suggest that both HIMU reservoirs formed around 2Ga; however, the HIMU reservoir for St. Helena is about 0.3Ga older than that for Austral Islands. The relation between 206Pb/204Pb and 208Pb/204Pb could reflect the time-integrated κ (232Th/238U) in the components. The HIMU components for St. Helena and Austral Islands have κ values between 3.3 and 3.7, which are intermediate between the present-day fresh mid-ocean ridge basalts (MORB; 2.6–3.2) and the chondritic silicate Earth (∼4). This is consistent with the model that the HIMU precursor is subducted oceanic crust created around 2Ga from depleted upper mantle, in which κ monotonously decreased from the chondritic to the present-day values since late Archean or early Proterozoic, because of enhanced U recycling from the Earth’s surface back to the mantle in response to the increasing oxygen levels in the hydrosphere. Moreover, the fact that the HIMU components have much higher κ than the present-day hydrothermally altered MORB (0.2–2) suggests that either the HIMU precursor was an unaltered ancient oceanic crust, or more likely, an altered oceanic crust with minimal U enrichment by hydrothermal fluids in the less oxic marine environment of the late Archean or early Proterozoic. The unradiogenic 87Sr/86Sr of the HIMU components also suggests formation of ancient oceanic crust altered with hydrothermal fluids having much lower 87Sr/86Sr in that eon than at present, followed by removal of Rb from it by subduction dehydration.

Constraints on the origin of the HIMU reservoir from He–Ne–Ar isotope systematics

We present a new set of He–Ne–Ar isotopic compositions for HIMU and EM1 lavas from the Cook–Austral Islands in the south Pacific. 3He/ 4He of the HIMU lavas are lower than MORB values, as previously demonstrated, but a coherent variation in 3He/ 4He with Pb isotopes indicates two-component mixing to form the lavas. One component is the HIMU reservoir with 3He/ 4He of 6 Ra or lower, and the other is the local lithosphere, which is commonly involved in EM1 lavas. Relative abundances of radiogenic and nucleogenic 4He, 21Ne and 40Ar show systematic variations. In 4He/ 40Ar*– 4He/ 21Ne* space, the HIMU lavas define a trend that is parallel to, but offset from the trend previously observed for other OIBs. Using 4He/ 21Ne* as a monitor of elemental fractionation of noble gasses, fractionation-corrected 4He/ 40Ar* is higher than the 4He/ 40Ar* production ratio in the mantle, reflecting a HIMU reservoir with a lower K/U (approximately 3000) than canonical mantle value (13000). Radiogenic 3He/ 4He and the low K/U are best explained by a model where the HIMU reservoir was formed by direct accumulation of, or deep mantle metasomatism with, ancient subducted oceanic crust modified by hydrothermal alteration and dehydration during its subduction. If the subducted oceanic crust with fractionated K/U forms a large isolated reservoir in the mantle, previous estimates of K/U and K concentrations for the bulk silicate Earth, that did not take this reservoir into consideration, will be too high. Moreover, the mass balance calculation indicates that the subducted oceanic crust may make a significant contribution to the U mantle budget, requiring there to be less in the primitive mantle. However, a contribution from the primitive mantle is not ruled out entirely, unless the subducted oceanic crust had a relatively high U concentration and low K/U over geologic time and was totally preserved in a convecting mantle.

Major element chemistry of ocean island basalts — Conditions of mantle melting and heterogeneity of mantle source

We estimate average compositions of near-primary, ‘reference’ ocean island basalts (OIBs) for 120 volcanic centers from 31 major island groups and constrain the depth of lithosphere–asthenosphere boundary (LAB) at the time of volcanism and the possible depth of melt–mantle equilibration based on recently calibrated melt silica activity barometer. The LAB depth versus fractionation corrected OIB compositions (lava compositions, X, corrected to Mg# 73, X OIB #73, i.e., magmas in equilibrium with Fo 90, if olivine is present in the mantle source) show an increased major element compositional variability with increasing LAB depths. OIBs erupted on lithospheres < 40 km thick approach the compositions (e.g. SiO 2 #73, TiO 2 #73, [CaO/Al 2O 3] #73) of primitive ridge basalts and are influenced strongly by depth and extent of shallow melting. However, X OIB #73 on thicker lithospheres cannot be explained by melt–mantle equilibration as shallow as LAB. Melt generation from a somewhat deeper (up to 50 km deeper than the LAB) peridotite source can explain the OIB major element chemistry on lithospheres ≤ 70 km. However, deeper melting of volatile-free, fertile peridotite is not sufficient to explain the end member primary OIBs on ≥ 70 km thick lithospheres. Comparison between X OIB #73 and experimental partial melts of fertile peridotite indicates that at least two additional melt components need to be derived from OIB source regions. The first component, similar to that identified in HIMU lavas, is characterized by low SiO 2 #73, Al 2O 3 #73, [Na 2O/TiO 2] #73, and high FeO⁎ #73, CaO #73, [CaO/Al 2O 3] #73. The second component, similar to that found in Hawaiian Koolau lavas, is characterized by high SiO 2 #73, moderately high FeO⁎ #73, and low CaO #73 and Al 2O 3 #73. These two components are not evenly sampled by all the islands, suggesting a heterogeneous distribution of mantle components that generate them. We suggest that carbonated eclogite and volatile-free, silica-excess eclogite are the two most likely candidates, which in conjunction with fertile mantle peridotite, give rise to the two primitive OIB end members.

Survival of lithium isotopic heterogeneities in the mantle supported by HIMU-lavas from Rurutu Island, Austral Chain

Thirty years ago, Hofmann and Hart [Hofmann, A.W., Hart, S.R., 1978. An assessment of local and regional isotopic equilibrium in the mantle, Earth Planet Sci. Lett. 38, 44–62] showed that local disequilibria of slowly diffusing radiogenic tracers (e.g. Sr) are preserved in the mantle over 1–2 Ga time scales. Recently, it was suggested that this is not the case for fast diffusing elements such as lithium [Halama, R., McDonough, W.F., Rudnick, R.L., Bell, K., 2008. Tracking the lithium isotopic evolution of the mantle using carbonatites, Earth Planet Sci. Lett. 265, 726–742], thus questioning the ability of lithium isotopes to constrain long-term effects of recycling of material with crustal signatures. A key issue in this debate is the identification in hotspot volcanism of Li isotopic fingerprint consistent with recycling. Previous studies proposed that HIMU type volcanism, which is thought to sample mantle domains that include subducted altered oceanic crust, has 7Li/ 6Li distinctively higher than the fresh mid-ocean ridge basalts. This work focuses on Rurutu island, where both HIMU (20.88 < 206Pb/ 204Pb < 21.42) and non-HIMU (19.11 < 206Pb/ 204Pb < 20.45) lavas occur. When considering only the freshest and most primitive lavas, the lithium isotopic signatures of HIMU (+ 5.4 < δ 7Li < + 7.9‰) and non-HIMU (+ 2.9 < δ 7Li < + 4.8‰) lavas do not overlap, thus supporting the idea that HIMU mantle has distinctly elevated δ 7Li. This result suggests that Li isotopic heterogeneities could survive diffusion over 1–2 billion years, the amount of time required to develop the highly radiogenic Pb signature of HIMU-lavas. Modeling lithium diffusion out of a lithium-rich, isotopically heavy altered oceanic crust reveals that isotopic disequilibrium persists over long time periods in comparison with the rapid decrease of chemical disequilibrium. For instance, a kilometer-thick altered oceanic crust loses most of its Li excess in a matter of a few tens of millions of years but could preserve 7Li/ 6Li distinctively higher than the ambient mantle over a time period in excess of 1.5 Ga. After 1.5 Ga, a kilometer-thick altered oceanic crust has heavy but uniform 7Li/ 6Li, while large isotopic variations persist in the nearby mantle (< 20 km). Thus, decoupling between Li and slowly diffusing radiogenic isotopes can be predicted in the mantle nearby subducted crust.

Lithium isotope systematics of lavas from the Cook–Austral Islands: Constraints on the origin of HIMU mantle

Radiogenic isotope variations in lavas from the Cook–Austral volcanic chain have delineated three distinct mantle sources: a HIMU component, a depleted component (DM), and an enriched component (EM). To better constrain the mantle sources for South Pacific hot spot volcanism, we determined lithium isotopic compositions of lavas from Raivavae, Rapa, Mangaia and Tubuai of the volcanic chain. The study includes whole rock and mineral analyses. In general, δ 7Li of most olivines resemble bulk rock composition whereas clinopyroxenes are variably lighter. This implies that clinopyroxene is more susceptible to diffusion-induced fractionation, in agreement with previous studies. Olivine δ 7Li values span a narrower range than whole rock values, and do not extend to the very heavy compositions previously reported in HIMU bulk lavas. This discrepancy likely reflects alteration of bulk lavas, and suggests that Li-isotope analyses of bulk lavas should be interpreted with caution. Olivines from HIMU lavas have heavy δ 7Li values (up to 6.2‰), and extend beyond the range reported for fresh MORB. Correlations between Li-isotopes and radiogenic isotopes suggest that the heavy δ 7Li values in HIMU olivines are a source characteristic and not the result of post-magmatic alteration. Although the Li-isotope composition of recycled, dehydrated oceanic crust is currently under debate, our results suggest that HIMU lavas derive from a source containing recycled dehydrated oceanic crust, and that the “heavy” Li-isotope signature of altered oceanic crust is partially preserved during passage through the subduction factory.

New constraints on the HIMU mantle from neon and helium isotopic compositions of basalts from the Cook–Austral Islands

High 4He/ 3He ratios of 100 000 to 160 000 found at HIMU ocean islands (“high μ,” where μ is the U/Pb ratio) are usually attributed to the presence of recycled oceanic crust in the HIMU mantle source. However, significantly higher 4He/ 3He ratios are expected in recycled crust after residence in the mantle for periods greater than 1 Ga. In order to better understand the helium isotopic signatures in HIMU basalts, we have measured helium and neon isotopic compositions in a suite of geochemically well-characterized basalts from the Cook–Austral Islands. We observe 4He/ 3He ratios ranging from 56 000 to 141 000, suggesting the involvement of mantle reservoirs both more and less radiogenic than the mantle source for mid-ocean ridge basalts (MORBs). In addition, we find that the neon isotopic compositions of HIMU lavas extend from the MORB range to compositions less nucleogenic than MORBs. The Cook-Austral HIMU He–Ne isotopic compositions, along with Sr, Nd, Pb, and Os isotopic compositions, indicate that in addition to recycled crust, a relatively undegassed mantle end-member (e.g., FOZO) is involved in the genesis of these basalts. The association of relatively undegassed mantle material with recycled crust provides an explanation for the close geographical association between HIMU lavas and EM (enriched mantle)-type lavas from this island chain: EM-type signatures represent a higher mixing proportion of relatively undegassed mantle material. Mixing between recycled material and relatively undegassed mantle material may be a natural result of entrainment processes and convective stirring in deep mantle.

Compositions of HIMU, EM1, and EM2 from global trends between radiogenic isotopes and major elements in ocean island basalts

Sr and Pb isotopes exhibit global trends with the concentrations of major elements (SiO 2, TiO 2, FeO, Al 2O 3 and K 2O) and major elements ratios (CaO/Al 2O 3 and K 2O/TiO 2) in the shield-stage lavas from 18 oceanic hotspots (including Hawaii, Iceland, Galapagos, Cook-Australs, St. Helena, Cape Verde, Cameroon, Canary, Madeira, Comoros, Azores, Samoa, Society, Marquesas, Mascarene, Kerguelen, Pitcairn, and Selvagen). Based on the relationships between major elements and isotopes in ocean island basalts (OIBs), we find that the lavas derived from the mantle end members, HIMU (or high ‘μ’ = 238U/ 204Pb), EM1 (enriched mantle 1), EM2 (enriched mantle 2), and DMM (depleted MORB [mid-ocean ridge basalt] mantle) exhibit distinct major element characteristics: When compared to oceanic hotspots globally, the hotspots with a HIMU (radiogenic Pb-isotopes and low 87Sr/ 86Sr) component, such as St. Helena and Cook-Australs, exhibit high CaO/Al 2O 3, FeO T, and TiO 2 and low SiO 2 and Al 2O 3. EM1 (enriched mantle 1; intermediate 87Sr/ 86Sr and low 206Pb/ 204Pb; sampled by hotspots like Pitcairn and Kerguelen) and EM2 (enriched mantle 2; high 87Sr/ 86Sr and intermediate 206Pb/ 204Pb; sampled by hotspots like Samoa and Societies) exhibit higher K 2O concentrations and K 2O/TiO 2 weight ratios than HIMU lavas. EM1 lavas exhibit the lowest CaO/Al 2O 3 in the OIB dataset, and this sets EM1 apart from EM2. A plot of CaO/Al 2O 3 vs K 2O/TiO 2 perfectly resolves the four mantle end member lavas. Melting processes (pressure, temperature and degree of melting) fail to provide an explanation for the full spectrum of major element concentrations in OIBs. Such processes also fail to explain the correlations between major elements and radiogenic isotopes. Instead, a long, time integrated history of various parent–daughter elements appears to be coupled to major element and/or volatile heterogeneity in the mantle source. End member lava compositions are compared with experimental partial melt compositions to place constraints on the lithological characteristics of the mantle end members.

Pb isotopic variability in melt inclusions from the EMI–EMII–HIMU mantle end-members and the role of the oceanic lithosphere

Melt inclusions from four individual lava samples representing the HIMU (Mangaia Island), EMI (Pitcairn Island) and EMII (Tahaa Island) end member components, have heterogeneous Pb isotopic composition larger than that defined by the erupted lavas in each island. The broad linear trend in 207Pb/206Pb–208Pb/206Pb space produced by the melt inclusions from Mangaia, Tahaa and fPitcairn samples reproduces the entire trend defined by the Austral chain, the Society islands and the Pitcairn island and seamount groups. The inclusions preserve a record of melt composition of far greater isotopic diversity than that sampled in whole rock basalts. These results can be explained by mixing of a common depleted component with the HIMU, EMI and EMII lavas, respectively. We favor a model that considers the oceanic lithosphere to be that common component. We suggest that the Pb isotopic compositions of the melt inclusions reflect wall rock reaction of HIMU, EMI and EMII melts during their percolation through the oceanic lithosphere. Under these conditions, the localized rapid crystallization of olivine from primitive basalt near the reaction zone would allow the entrapment of melt inclusions with different isotopic composition.

Isotope and trace element variations in lavas from Raivavae and Rapa, Cook–Austral islands: constraints on the nature of HIMU- and EM-mantle and the origin of mid-plate volcanism in French Polynesia

We have examined the major and trace element and Sr–Nd–Hf–Pb–Os isotope variations in two suites of lavas from the islands of Raivavae and Rapa from the Cook–Austral chain to better constrain the nature and origin of the source(s) of these lavas. Combined with data from other Cook–Austral islands and seamounts, the Raivavae and Rapa samples reveal the presence of three distinct components involved in the genesis of these lavas, a HIMU component, a depleted (or DM) component, and an “enriched” (EM) component. Northern Cook–Australs lavas (those north of the Austral Fracture Zone (AFZ)) falling along the “Macdonald” pseudo-age progression reflect mixing of the HIMU and DM components, whereas lavas south of the AFZ reflect mixing of the DM and EM components.Strong depletions in potassium and other fluid soluble elements in HIMU lavas suggest that the HIMU source contains ancient recycled (dehydrated) oceanic crust. The EM lavas have systematically less radiogenic Os-isotopes than the HIMU lavas. Therefore, the enriched or EM component cannot be generated through the addition of pelagic or terrigenous sediment to the HIMU source. Instead, the EM component in the southern Cook–Australs may represent recycled subduction-modified sub-arc mantle. High 208Pb/204Pb values in the DM component relative to East Pacific Rise mid-ocean ridge basalts (MORB) preclude derivation of this component from assimilated lithospheric mantle. The Sr–Nd–Pb isotope characteristics of the DM component resemble Pacific alkalic seamount lavas, and may derive from easily fusible components (e.g., pyroxenite veins) dispersed in the Pacific upper mantle.The Raivavae and Rapa lavas define a Hf–Nd isotope correlation that is considerably steeper than the mantle Hf–Nd isotope array. The unusually low εHf values (for a given εNd value) characteristic of the HIMU source are consistent with the presence of recycled oceanic crust. However, the presence of low εHf values in the Rapa EM lavas reveals that anomalously low εHf values are not restricted to the HIMU source. Regardless of the ultimate origin of the Raivavae and Rapa lavas, the comparatively low εHf values in these lavas indicate that multiple low-εHf components are present in the mantle. The addition of these components may help resolve the apparent terrestrial Hf–Nd mass-imbalance with respect to estimates of the bulk earth Nd- and Hf-isotope composition.Islands and seamounts from the Cook–Austral chain do not follow a clear age progression as predicted by the standard plume model for the origin of ocean island basalts (OIB). Previous models for the origin of the Cook–Austral chain have appealed to the existence of multiple small plumes. However, these models are implausible because they do not explain how several deep-seated plumes become aligned with each other in the direction of plate motion. We propose that volcanism along the Cook–Austral chain (and perhaps in much of French Polynesia) is generated through self-perpetuating melting anomalies that preferentially sample enriched components in a marble-cake mantle. These melting anomalies preferentially form or strengthen where lithospheric boundary conditions are favorable, such as at pre-existing transform faults or where the lithosphere has been thinned by a previous period of volcanism.

Noble Gases in the Cameroon Line

The Cameroon line is an intraplate volcanic province straddling both oceanic and continental lithosphere. Recent lavas of the Cameroon line have radiogenic Pb, ranging up to a maximum of 2~176 = 20.5 at the continent-ocean crust boundary, similar to other HIMU lavas and indicative of source regions with high U/Pb (Halliday et al., 1988). Plume impingement beneath the Gondwanan craton, immediately prior to Mesozoic rifting has been suggested as a means of generating symmetric spatial variability of Pb isotopic compositions along the volcanic chain. Another equally important aspect of the Cameroon line and other HIMU lava compositions is the distinct and uniform Sr, Nd, Hf and He isotopic compositions (Ballentine et al., 1996). Excepting the lack of 2~176 Cameroon line lavas are indistinguishable from other HIMU suites such as the Austral Islands and St. Helena, and are representative of HIMU source regions or processes leading to the development of HIMU compositions.

In situ Pb Isotope Ratios in Melt Inclusions from Oceanic Island Basalts: Mangaia, a Case Study

The study of oceanic basalts has provided unequivocal evidence for a chemically heterogeneous mantle 1. However, the nature, distribution and scale of these heterogeneities remain problematic. The aggregation of melts on their way to the surface, and magma chamber processes previous to eruption, obscure the characteristics of the different mantle sources. Although trace and major element studies of melt inclusions from high Mg# olivine phenocrysts have proved to be useful in defining the composition of unaggregated melts 2, the uncertainty remains in distinguishing if they represent different extents of melting from a single source or if they originate from different source compositions. To address these problems, we have measured Pb isotopes in single melt inclusion by 1270 ionmicroprobe; we chose Mangaia Island as a case study, because its lavas are isotopically homogeneous and they represent one of the end-member mantle components (HIMU) 3. Melt inclusions, in olivine and clinopyroxene phenocrysts (Mg# 86-89) from two primitive lavas from Mangaia are partially crystallized and ellipsoidal in shape (30 to 300 lam), indicative of their primary nature. Partially resorbed clinopyroxene inclusions in the olivine also occur. Some of the melt inclusions contain FeCuNi sulphide and MgFeCa carbonate globules. The (La/Yb)cN ratios in the inclusions range from -5 to ~40. In-run precision for the Pb isotopic measurements range from 0.4 to 1% (2or), depending on the Pb content of the melt inclusion. Loihi glass 158-4 previously analysed by TIMS 4 was used as a standard during the period of measurement of the melt inclusions. The Pb content of the inclusions are roughly the same as the standard. The ion probe analyses reproduced the TIMS value within <0.2% standard errors (2or n = 17). The fractionation per amu (<0.15%) was smaller than the inrun precision. No indication of interference or hidryde was found in the Pb measurement. Ion microprobe analyses of Pb isotope ratios (2~176 and 2~176 within single melt inclusion from one sample show large and systematic variations that overlap with the whole field for HIMU lavas (Mangaia, Tubuai and St. Helena islands). The 2~176 and 2~176 ratios range from 0.706 to 0.768 and from 1.840 to 1.940 respectively, which is much larger than the range observed in Mangaia lavas (0.726 to 0.735 and 1.869 to 1.883) 5. Sulphide and carbonate globules, also analysed for Pb isotopes, have isotopic compositions typical of HIMU, suggesting a primary origin. No clear correlation was found between (La/Yb)cN ratios and Pb isotopic composition. The rough linear trend produce by the melt inclusions from one sample in 2~176 2~176 space reproduce the trend defined by the HIMU islands, indicating that lavas from Mangaia do not derive from a single homogeneous mantle source and geochemical models should consider at least two mantle sources. The HIMU Pb isotopic composition of the carbonate globules in conjunction with their textures and composition provide direct evidence for a carbonated mantle source for the Austral Islands.