Search

Mantle evolution is governed by continuous depletion by partial melting and replenishment by recycling oceanic and continental crust. Several important unknowns remain, however, such as the extent of compositional variability of the residual depleted mantle, the timescale, mass flux and composition of recycled oceanic and continental crust. Here, we investigate the Ce-Nd-Hf isotope systematics in a globally representative spectrum of mid ocean ridge and ocean island basalts. Using a Monte Carlo approach for reproducing the observed Ce-Nd-Hf isotope variation shows that the type and age of depleted mantle and recycled crust have the dominant influence on the slope, scatter, and extent of the modeled Ce-Nd-Hf isotope array. The model results suggest a relatively young (

There is debate to what degree mid-ocean ridge basalts (MORB) can be used to infer the composition of the sub-ridge mantle and whether the upper mantle contains components that are difficult to recognize by observations on MORB alone. Here we report trace element and isotope data on a fossil mantle section exposed in the ophiolitic Lanzo South massif, Italy. Our findings show the existence of clinopyroxenes in geochemically depleted melt-infiltrated harzburgites with an extreme radiogenic Hf-isotopic composition but MORB-like Nd isotopic compositions. The simplest way of explaining the depleted trace elements in combination with the decoupled isotopic compositions is that the harzburgites were infiltrated by a melt with isotopic compositions inherited from an anciently (>1 Ga) depleted source. The existence of melts from ultra-depleted sectors of the asthenosphere allows constraining of the configuration of the heterogeneities in the sub-ridge mantle. The parallel arrays in Hf–Nd isotope composition space of different Mid-Ocean ridge segments imply the ubiquitous presence of anciently depleted residual mantle as the matrix in which more enriched components occur in pockets.

The evolution of the mantle melting regime and the process of refertilization beneath cold lithosphere remain ambiguous at ultraslow spreading ridges. Few previous studies were concerned with the temporal variations of mantle melting indicated by peridotites on the flank of a single ridge segment. Here, we present in-situ major and trace element analyses of harzburgites from the Dragon Bone ridge segment both from near the spreading axis and from rift mountains up to tens of kilometers from the ridge axis. We propose that the Dragon Bone mantle had been depleted anciently at higher pressures, prior to the recent ascent under the ridge at lower pressures, and only a limited volume of melt was generated during recent melting beneath the ridge. This interpretation is consistent with the high degree of depletion of the Dragon Bone harzburgites with little trapped melt, and thin discontinuous igneous crust on the seafloor. Comparison of peridotites from near-ridge locations with those further out on the flanks indicates that the melting regime has progressively shrunk over time creating at present a nearly amagmatic segment. Peridotites from segment ends are seldom affected by late-stage refertilization, while those from the segment center are highly depleted mantle residues (resembling those from fast spreading ridges) that sustained syn-melting metasomatism.

Mantle source heterogeneity and magmatic processes have been widely studied beneath most parts of the Southwest Indian Ridge (SWIR). But less is known from the newly recovered mid-ocean ridge basalts from the Dragon Bone Amagmatic Segment (53°E, SWIR) and the adjacent magmatically robust Dragon Flag Segment. Fresh basalt glasses from the Dragon Bone Segment are clearly more enriched in isotopic composition than the adjacent Dragon Flag basalts, but the trace element ratios of the Dragon Flag basalts are more extreme compared with average mid-ocean ridge basalts (MORB) than the Dragon Bone basalts. Their geochemical differences can be explained only by source differences rather than by variations in degree of melting of a roughly similar source. The Dragon Flag basalts are influenced by an arc-like mantle component as evidenced by enrichment in fluid-mobile over fluid-immobile elements. However, the sub-ridge mantle at the Dragon Flag Segment is depleted in melt component compared with a normal MORB source owing to previous melting in the subarc. This fluid-metasomatized, subarc depleted mantle is entrained beneath the Dragon Flag Segment. In comparison, for the Dragon Bone axial basalts, their Pb isotopic compositions and their slight enrichment in Ba, Nb, Ta, K, La, Sr and Zr and depletion in Pb and Ti concentrations show resemblance to the Ejeda–Bekily dikes of Madagascar. Also, Dragon Bone Sr and Nd isotopic compositions together with the Ce/Pb, La/Nb and La/Th ratios can be modeled by mixing the most isotopically depleted Dragon Flag basalts with a composition within the range of the Ejeda–Bekily dikes. It is therefore proposed that the Dragon Bone axial basalts, similar to the Ejeda–Bekily dikes, are sourced from subcontinental lithospheric Archean mantle beneath Gondwana, pulled from beneath the Madagascar Plateau. The recycling of the residual subarc mantle and the subcontinental lithospheric mantle could be related to either the breakup of Gondwana or the formation and accretion of Neoproterozoic island arc terranes during the collapse of the Mozambique Ocean, and is now present beneath the ridge.

The upper mantle is a heterogeneous mixture of refractory and recycled crustal domains. The recycled portions, more fertile and thus preferentially melted, dominate the composition of the basalts erupted on the surface, whereas the imprint of melting of the refractory counterparts is more difficult to discern from the basalt chemistry. Contrasting radiogenic isotopic signatures of mid-ocean ridge basalts and oceanic mantle, however, show that Hf isotope ratios may provide hints for melting of refractory source materials despite ubiquitous magma mixing during ascent and stalling in the crust. This property may allow identifying contributions from depleted mantle materials unseen in other isotope systematics in basalts. Here, we show that basalts from Mohns and Knipovich ridges, two >500-km long oblique super-segments in the Arctic Atlantic, have distinctly high Hf isotope ratios, not mirrored by comparatively high Nd and low Sr and Pb isotope ratios. These compositions can be explained if a highly depleted asthenospheric mantle melts beneath this section of the Arctic Mid Atlantic Ridge. We argue that this depleted source consists of high proportions of ancient (>1 Ga), ultra-depleted mantle, previously drained of enriched components before being re-melted in its current location following a recent ridge-jump, allowing the identification of ultra-depleted mantle components in the arctic subridge mantle.

The amount of recycled oceanic crust in the mantle is constrained from elemental observations of melts., Mid-oceanic ridge basalts (MORBs) are depleted in incompatible elements, but ridge segments far from mantle plumes frequently erupt chemically enriched MORBs (E-MORBs). Two major explanations of E-MORBs are that these basalts are generated by the melting of entrained recycled crust (pyroxenite) beneath ridges or by the melting of refertilized peridotites. These two hypotheses can be discriminated with compatible element abundances from Sc to Ge, here termed the ScGe elements. Here, we demonstrate that E-MORBs have systematically lower Ge/Si and Sc contents and slightly higher Fe/Mn and Nb/Ta ratios than depleted MORBs (D-MORBs) due to the mixing of low-degree pyroxenite melts. The Ge/Si ratio is a new tracer that effectively discriminates between melts derived from peridotite sources and melts derived from mixed pyroxenite-peridotite sources. These new data are used to estimate the distribution of pyroxenite in the mantle sources of global MORB segments.

The concentration of carbon in primary mid-ocean ridge basalts (MORBs), and the associated fluxes of CO2 outgassed at ocean ridges, is examined through new data obtained by secondary ion mass spectrometry (SIMS) on 753 globally-distributed MORB glasses. MORB glasses are typically 80-90% degassed of CO2, thus we use the limited range in CO2/Ba (81.3±23) and CO2/Rb (991±129), derived from undegassed MORB and MORB melt inclusions, to estimate primary CO2 concentrations for ridges that have Ba and/or Rb data. When combined with quality-controlled volatile-element data from the literature (n=2,446), these data constrain a range of primary CO2 abundances that varies from 104 ppm to 1.90 wt%. Segment-scale data reveal a range in MORB magma flux varying by a factor of 440 (from 6.8x105 to 3.0x108 m3/yr) and an integrated global MORB magma flux of 16.5±1.6 km3/yr. When combined with CO2/Ba and CO2/Rb-derived primary magma CO2 abundances, the calculated segment-scale CO2 fluxes vary by more than three orders of magnitude (3.3x107 to 4.0x1010 mol/yr) and sum to an integrated global MORB CO2 flux of x1012 mol/yr. Variations in ridge CO2 fluxes have a muted effect on global climate, however, because the vast majority of CO2 degassed at ridges is dissolved into seawater and enters the marine bicarbonate cycle. MORB degassing would thus only contribute to long-term variations in climate via degassing directly into the atmosphere in shallow-water areas or where the ridge system is exposed above sea level.

To understand the dynamics of solid mantle upwelling and melting in the Hawaiian plume, we present new major and trace element data, Nd, Sr, Hf, and Pb isotopic compositions, and 238U–230Th–226Ra and 235U–231Pa–227Ac activities for 13 Haleakala Crater nepheline normative basanites with ages ranging from ∼900 to 4100 yr B.P. These basanites of the Hana Volcanics exhibit an enrichment in incompatible trace elements and a more depleted isotopic signature than similarly aged Hawaiian shield lavas from Kilauea and Mauna Loa. Here we posit that as the Pacific lithosphere beneath the active shield volcanoes moves away from the center of the Hawaiian plume, increased incorporation of an intrinsic depleted component with relatively low 206Pb/204Pb produces the source of the basanites of the Hana Volcanics. Haleakala Crater basanites have average (230Th/238U) of 1.23 (n = 13), average age-corrected (226Ra/230Th) of 1.25 (n = 13), and average (231Pa/235U) of 1.67 (n = 4), significantly higher than Kilauea and Mauna Loa tholeiites. U-series modeling shows that solid mantle upwelling velocity for Haleakala Crater basanites ranges from ∼0.7 to 1.0 cm/yr, compared to ∼10 to 20 cm/yr for tholeiites and ∼1 to 2 cm/yr for alkali basalts. These modeling results indicate that solid mantle upwelling rates and porosity of the melting zone are lower for Hana Volcanics basanites than for shield-stage tholeiites from Kilauea and Mauna Loa and alkali basalts from Hualalai. The melting rate, which is directly proportional to both the solid mantle upwelling rate and the degree of melting, is therefore greatest in the center of the Hawaiian plume and lower on its periphery. Our results indicate that solid mantle upwelling velocity is at least 10 times higher at the center of the plume than at its periphery under Haleakala.

Approximately coeval, relatively primitive (∼5–10% MgO with exception of a trachyandesite) alkaline mafic dikes and sills at or near Spanish Peaks, CO are divided into relatively sodic and potassic varieties on the basis of K 2 O/Na 2 O. Many of these dikes are true lamprophyres. In spite of variable alkali element ratios, the alkaline rocks share a number of geochemical similarities: high LIL element contents, high Ba and similar Sr, Nd and Hf isotope ratios near that of Bulk Earth. One important difference is that the potassic rocks are characterized by lower Al 2 O 3 contents, typically less than 12 wt.%, than the sodic dikes/sills which typically have more than 13 wt.% Al 2 O 3 , and this difference is independent of MgO content. We attribute the distinct Al 2 O 3 contents to varying pressure during melting: a mica-bearing, Al-poor vein assemblage for the potassic magmas melted at higher pressure than an aluminous amphibole-bearing vein assemblage for the sodic magmas. Remarkable isotopic and trace element similarities with approximately contemporaneous, nearby Rio Grande rift-related basalts in the San Luis Valley, indicate that the magmatism at Spanish Peaks was rift-related, and that lithosphere sources were shared between some rift magmas and those at Spanish Peaks. High Zn/Fe ratios in the Spanish Peaks mafic rocks point to a clinopyroxene- and garnet-rich source such as lithosphere veined by pyroxenite or eclogite. Lithospheric melting was possibly triggered by foundering of cool, dense lithosphere beneath the Rio Grande rift during the initiation of rifting with the potassic parent magmas generated by higher pressure melting of the foundered lithosphere than the sodic parent magmas. This process, caused by gravitational instability of the lithosphere (Elkins-Tanton, 2007) may be common beneath active continental rifts.

Geochemical data for abyssal peridotites are used to determine the relationship to mid-ocean ridge basalts from several locations at ridge segments on the SW Indian Ridge (SWIR), the Mid-Cayman-Rise (MCR), and the Mid-Atlantic Ridge (MAR). Based on chemical and petrological criteria peridotites are categorized as being either dominantly impregnated with melt or being residual after recent melting. Those that are considered impregnated with melt also have isotopic compositions similar to the basalts indicating impregnation by an aggregate MORB melt. A SWIR and MCR residual peridotite Nd-isotopic compositions partly overlap the Nd-isotopic compositions of the basalts but extend to more radiogenic compositions. The differences between peridotite and basalt Nd-isotopic compositions can be explained by incorporating a low-solidus component with enriched isotopic signature in the subridge mantle: a component that is preferentially sampled by the basalts. At the MAR, peridotites and associated basalts have overlapping Nd-isotopic compositions, suggesting a more homogeneous MORB mantle. The combined chemistry and petrography indicates a complex history with several depletion and enrichment events. The MCR data indicate that a low-solidus component can be a ubiquitous component of the asthenosphere. Residual abyssal peridotites from limited geographic areas also show significant chemical variations that could be associated with initial mantle heterogeneities related to events predating the ridge-melting event. Sm-Nd model ages for possible earlier depletion events suggest these could be as old as 2.4 Ga.

The forearc of the southern Mariana arc preserves igneous suites formed during the initiation of subduction between the Pacific and Philippine Sea plates about 50 Ma ago. We have studied rare suites of gabbroic to tonalitic plutonic rocks dredged from two locations in the Mariana forearc by cruise by University of Hawai'i cruise KK81-06-26. Comparison of the chemical and isotopic (Sr, Nd, Pb, and Hf) characteristics of these rocks with well-studied volcanics from the forearc reveals that the plutonics from dredge RD63 and RD64 are chemically related to boninites erupted at 48–43 Ma. This is the first report of boninite-like plutonics in the southern Mariana trench. These suites have trace element characteristics consistent with island arc settings (U/Th: 0.58–1.44, Nb/La: 0.18–0.79) and other features uniquely connected with boninites: TiO2 25. RD63 plutonics resemble nearby boninite volcanics and were likely derived from differentiated boninite magma with 58% SiO2, forming gabbro by crystal accumulation, diorite and quartz diorite by crystallization, and tonalite by crystallization and/or partial melting. The RD64 suite (gabbro through tonalite) may have had a more depleted magma source and formed by accumulation and crystallization only. Although the physical dimensions of the plutonic body are unknown, the relationship with boninites indicates that felsic intrusives can form during early stages of island arc development. Such rocks could form part of midcrustal low-velocity layers detected in arc crust by seismic studies. Tonalites similar to those studied here are also found in some ophiolites.

Quaternary Mount Rainier (Washington, USA) of the Cascades magmatic arc consists of porphyritic calc-alkaline andesites and subordinate dacites, with common evidence for mingling and mixing with less evolved magmas encompassing andesites, basaltic andesites, and rarely, basalts. Basaltic andesites and amphibole andesites (spessartites) that erupted from vents at the north foot of the volcano represent some of Mount Rainier’s immediate parents and overlap in composition with regional basalts and basaltic andesites. Geochemical (major and trace elements) and isotopic (Sr, Nd, Pb, O) compositions of Mount Rainier andesites and dacites are consistent with modest assimilation (typically ≤20 wt%) of evolved sediment or sediment partial melt. Sandstones and shales of the Eocene Puget Group, derived from the continental interior, are exposed in regional anticlines flanking the volcano, and probably underlie it in the middle to lower crust, accounting for their assimilation. Mesozoic and Cenozoic igneous basement rocks are unsuitable as assimilants due to their high 143 Nd/ 144 Nd, diverse 206 Pb/ 204 Pb, and generally high δ 18 O. The dominant cause of magmatic evolution at Mount Rainier, however, is inferred to be a version of in situ crystallization-differentiation and mixing (Langmuir, 1989) wherein small magma batches stall as crustal intrusions and solidify extensively, yielding silicic residual liquids with trace element concentrations influenced by accessory mineral saturation. Subsequent magmas ascending through the intrusive plexus entrain and mix with the residual liquids and low-degree re-melts of those antecedent intrusions, producing hybrid andesites and dacites. Mount St. Helens volcanic rocks have geochemical similarities to those at Mount Rainier, and may also result from in situ differentiation and mixing due to low and intermittent long-term magma supply, accompanied by modest crustal assimilation. Andesites and dacites of Mount Adams isotopically overlap the least contaminated Mount Rainier magmas and derive from similar parental magma types, but have trace element variations more consistent with progressive crystallization-differentiation, probably due to higher magma fluxes leading to slower crystallization of large magma batches, allowing time for progressive separation of minerals from melt. Mount Adams also sits atop the southern projection of a regional anticlinorium, so Eocene sediments are absent, or are at shallow crustal levels, and so are cold and difficult to assimilate. Differences between southwest Washington stratovolcanoes highlight some ways that crustal geology and magma flux are primary factors in andesite generation.

[1] Rejuvenated volcanism refers to the reemergence of volcanism after a hiatus of 0.5–2 Ma following the voluminous shield building stage of Hawaiian volcanoes. The composition of the rejuvenated source and its distribution relative to the center of the plume provide important constraints on the origin of rejuvenated volcanism. Near-contemporaneous lavas from the Kaula-Niihau-Kauai ridge and the North Arch volcanic field that are aligned approximately orthogonally to the plume track can constrain the lateral geochemical heterogeneity and distribution of the rejuvenated source across the volcanic chain. Nephelinites, phonolites and pyroxenite xenoliths from Kaula Island have radiogenic Hf, Nd and unradiogenic Sr isotope compositions consistent with a time-integrated depleted mantle source. The pyroxenites and nephelinites extend to the lowest 208Pb/204Pb reported in Hawaiian rocks. These data, along with new Pb isotope data from pyroxenites from the Salt Lake Crater (Oahu) redefine the composition of the depleted end-member of the Hawaiian rejuvenated source at 208Pb/204Pb=37.35±0.05, 206Pb/204Pb = 17.75±0.03, eNd = 9–10, eHf ∼16–17 and 87Sr/88Sr

Strata-bound cobalt-copper deposits occur in the metasedimentary rocks of the Apple Creek Formation (Mesoproterozoic) in a linear belt called the Idaho cobalt belt, near Salmon, Idaho. We report the first set of lead isotope measurements on silicate rocks and sulfide ores and new sulfur isotope data on sulfide ores. The Pb isotope compositions of cobalt-copper (Co-Cu) ores (206Pb/204Pb = 30.8–40.4; 207Pb/204Pb = 16.8–17.6; 208Pb/204Pb = 49.7–63.9) are very radiogenic, much more radiogenic than any known sedimentary exhalative (sedex) deposits. The data plot well beyond the crustal growth model curves, a feature shared by the Mississippi Valley-type (MVT) deposits which is characteristic of an upper crustal source. It is unlikely that the ore lead could have come from any mafic igneous source of mantle origin (Proterozoic Moyie sill and its equivalent, or the Tertiary Challis Volcanic Group). The Cretaceous felsic igneous rocks of the region (the Idaho batholith and related rocks) also have much lower Pb isotope ratios than the Co-Cu ores. The Pb isotope ratios of Proterozoic crystalline rocks partially overlap Pb isotope ratios of the Apple Creek Formation but would have been much less radiogenic than the ores at an assumed mineralization age of about 1400 Ma. Only the host Apple Creek Formation is known to have appropriate Pb isotope compositions (206Pb/204Pb = 26.8–86.7; 207Pb/204Pb = 16.3–21.1; 208Pb/204Pb = 47.9–64.8) to be the source of Co-Cu ores. Leaching of metals from the host sedimentary sequence by hydrothermal fluids and subsequent deposition of ores at chemically and structurally favorable sites could have resulted in the formation of Co-Cu deposits in the Idaho cobalt belt.

The proposed origins for the Enriched Mantle I component are many and various and some require an arbitrary addition of an exotic component, be it pure sediment or an enriched melt from the subcontinental lithosphere. With Pitcairn, Walvis Ridge is the ‘type-locality’ for the Enriched Mantle I (EMI) component. We analyzed basalts from DSDP Site 525A, Site 527 and Site 528 on the Walvis Ridge with the aim to constrain the history of its source. The isotopic compositions we measured for the three sites overlap with the values obtained by Richardson et al. (1982a) and extend towards less radiogenic Sr and more radiogenic Pb and Nd isotopic compositions. We used our new trace element and radiogenic isotope (Hf, Nd, Pb and Sr) characterization in combination with the literature data to produce the simplest possible model that satisfies the trace element and isotopic constraints. Although the elevated 207Pb/204Pb with respect to 206Pb/204Pb predicts an ancient origin for EMI, none of the proposed origins had modeled it as such. The data is consistent with the EMI composition being formed by the addition of a melt to a mantle with bulk Earth-like composition followed by melt extraction of a low degree melt. The timing of these two events is such that the metasomatism has to have taken place prior to 4 Ga and the subsequent melt removal before 3.5 Ga. This confirms the expectation of an ancient character for the EMI component. The Walvis Ridge data shows two distinct two component mixing trends: one formed by the less enriched Site 527 and Site 528 basalts and one formed by the Site 525A basalts. The two trends have the EMI endmember in common. The less depleted end of the Site 527–Site 528 basalts is FOZO-like and can be explained by the addition of a recycled component (basaltic oceanic crust plus sediment). This recycled component was altered during subduction. The sense and magnitude of the chemical fractionation resulting from the subduction alteration are in agreement with dehydration experiments on basalts and sediment. Compared to other EMI like basalts the Walvis Ridge basalts have flatter REE patterns and show less fractionation between large ion lithophile and heavy REE elements. Using the isotopic compositions as constrains for the parent–daughter ratios we were able to model the trace element patterns of the basalts as melting between 5 and 10% for Site 525A and between 10 and 15% for the depleted end of the Site 528–Site 527 array. In all cases a significant portion of melting takes place in the garnet stability field.

The Emperor Seamount-Hawaiian island chain is thought to originate from melting of a heterogeneous mantle plume, but the composition of the plume has always been inferred from the compositions of the erupted lavas. It has been suggested that recycled (i.e. previously subducted) basaltic oceanic crust (with or without sediments) is part of this plume and responsible for the enriched isotopic compositions of the Hawaiian lavas. Here we present the first combined Hf–Os isotopic study on peridotite mantle xenoliths from the island of Oʻahu, Hawaiʻi. The clinopyroxene and spinel major element compositions overlap the global range of compositions of abyssal peridotites, suggesting that the Oʻahu peridotites are samples from the oceanic mantle and residues of variable degrees of melting. The peridotite xenoliths from Salt Lake Crater have both highly unradiogenic Os (down to 187Os/188Os = 0.1138) and radiogenic Hf (up to eHf = 114) isotopic compositions and minimum rhenium-depletion ages up to ∼ 2 Ga. Such extreme Hf and Os isotopic compositions are not observed in Hawaiian lavas and are far removed from the composition of the depleted mantle, as it is sampled by mid oceanic ridge basalts and abyssal peridotites. Importantly, both Hf and Os isotopic compositions correlate with degree of depletion, suggesting that they are related to an ancient melting event. In contrast, peridotites from the Pali and Kaau vents have Os and Hf isotope ratios consistent with an origin from the ∼ 100 Ma Pacific lithosphere that lies beneath the Hawaiian islands. We suggest that the Salt Lake Crater peridotites are fragments of an ancient (> 2 Ga) depleted and recycled mantle lithosphere that is part of the upwelling Hawaiian plume. Such depleted peridotites have higher solidus temperature than other more fertile mantle components, so that their contribution to the erupted lavas compositions is minor, if any. The recognition of such isotopically and compositionally depleted materials within the Hawaiian plume suggests that depleted components are more common in mantle plumes than previously realized.

Sulfide ores containing principally argentiferous galena, sphalerite, and tetrahedrite occur both in Paleozoic sedimentary rocks and in Late Cretaceous granites along the eastern margin of the Idaho batholith in central Idaho. Two genetic models have been proposed for the sediment-hosted deposits, one involving vein and replacement styles of mineralization during the Cretaceous and Tertiary and the other representing initial Paleozoic sedimentary exhalative (sedex) style of mineralization and subsequent remobilization during the Cretaceous and Tertiary. There is also uncertainty about the sources of ore components; studies based on fluid inclusions and stable isotopes suggest a shallow crustal source, whereas the Pb isotope studies favor sources deeper than the exposed crustal rocks. We present new lead isotope data on 16 sulfide ore samples and 24 whole-rock and mineral separate samples, and compare them with data from the literature to assess the possible sources of base and precious metals in the deposits of central Idaho. Two potential ore metal sources are recognized: a sedimentary source and an igneous source. Deposits that derived their metals from a sedimentary source are characterized by radiogenic Pb and are subdivided into three types: Triumph ( 206 Pb/ 204 Pb = 19.4–20.1, 207 Pb/ 204 Pb = 15.7–15.9, and 208 Pb/ 204 Pb = 39.0–39.9), Minnimoore ( 206 Pb/ 204 Pb = 19.9–20.6, 207 Pb/ 204 Pb = 15.8–16.0, and 208 Pb/ 204 Pb = 40.1–41.3), and Pacific ( 206 Pb/ 204 Pb = 20.4–21.4, 207 Pb/ 204 Pb = 15.7–15.8, and 208 Pb/ 204 Pb = 40.8–41.1). The Pb isotope compositions of sediment-hosted deposits can be explained by the mixing of Pb derived from Paleozoic sedimentary rocks and Early Proterozoic crystalline rocks or by the mixing of Pb derived from the Cretaceous granite and Early Proterozoic crystalline rocks. Alternatively, the Pb in sediment-hosted deposits may represent a mixture of Pb derived from all three sources in varying proportions. None of these scenarios requires Pb from deep sources. The results of this study and prior fluid inclusion and stable isotope studies support a genetic model involving shallow crustal sources for metals and sulfur, mobilized by meteoric water-dominated hydrothermal systems. Deposits in which the metals were derived from an igneous source are subdivided into Carrietown and non-Carrietown types. Carrietown-type ores have relatively low uranogenic Pb isotope ratios ( 206 Pb/ 204 Pb = 17.3–19.1 and 207 Pb/ 204 Pb = 15.4–15.7) and variable thorogenic Pb isotope ratios ( 208 Pb/ 204 Pb = 38.9–41.0). The non-Carrietown–type ores are isotopically similar to their host granites and have higher uranogenic Pb isotope ratios ( 206 Pb/ 204 Pb = 19.3–20.6, 207 Pb/ 204 Pb = 15.7–15.8, and 208 Pb/ 204 Pb = 39.3–40.1) compared to the Carrietown-type ores. We propose that the two groups are dominated by Pb sources from different depths; the Carrietown-type ores from middle crustal sources (uranium-depleted) and the non-Carrietown–type ores from upper crustal sources (enriched in both U and Th). A magmatic hydrothermal origin for both types of ores is supported by their granitic host rocks, hydrothermal alteration of intrusive granite and surrounding rocks, high temperatures of ore formation, and similar radiometric ages for the granite and mineralized veins.

We present the first comprehensive major, trace element and Hf, Nd and Sr isotope investigation of clinopyroxene and garnet mineral separates from a set of garnet clinopyroxenite xenoliths from the Salt Lake Crater, Oahu, Hawaii. These xenoliths occur in the posterosional Honolulu Volcanics Series lavas and represent some of the deepest samples from the oceanic mantle lithosphere. Our study shows that the Salt Lake Crater pyroxenites represent high pressure (>20 kb) accumulates from melts similar (but not identical) to the erupted Honolulu Volcanics, and unlike MORB or E-MORB-type melts. All clinopyroxene-garnet mineral pairs in these xenoliths show, within error, zero-age Lu-Hf and Sm-Nd isotope systematics. These pyroxenites have relatively radiogenic Hf isotope compositions (for a given Nd) and define a distinct steep slope (3.3) in eHf-eNd isotope space, similar to the Honolulu Volcanics but unlike other ocean island basalts (OIB). These compositions require an end-member component that falls above the OIB array in Nd-Hf space. This component is different than present-day MORB-mantle and it is best explained by an old depleted oceanic lithosphere. We suggest that this depleted component most likely represents a recycled depleted lithosphere that is intrinsic to the Hawaiian plume. In this respect, the Hawaiian plume is sampling both the enriched portion of a subducted oceanic crust (basalt and sediments) as well as the depleted lithospheric portion of it. This suggests that, at least for Hawaii, the whole subducted oceanic slab package has retained its integrity during subduction and subsequent mixing and storage in the mantle, probably in the order of a billion years, and that the plume is sampling the full range of these compositions.

Eclogites from the Roberts Victor mine, Kaapvaal craton are classic examples of subducted Achaean oceanic crust brought up as xenoliths by kimberlite. New in situ trace element and oxygen isotope data (δ18O=3.09–6.99‰ SMOW) presented here reemphasise their origin from seawater-altered plagioclase-rich precursors. Their Hf–Nd isotopic compositions are not in agreement with compositions predicted by geochemical modelling of the isotopic composition of aged subducted oceanic crust. Instead, Hf isotopic compositions are very heterogeneous, varying between 0.281625 and 0.355077 (−37.8 and +2561 ɛHf) at the time of kimberlite emplacement (128 Ma) in keeping with equally variable Nd isotopic compositions (0.511124–0.545092; −26.3 to +636 ɛNd). However, most samples plot on the terrestrial array. The isotopic compositions of some samples are too extreme to play a major role in mixed peridotite-eclogite melting in basalt source regions, whereas the isotopic composition of other samples is reconcilable with a contribution of up to ca. 15% of eclogite partial melt to the MORB source. Most importantly, our results show that ancient subducted oceanic crust is not isotopically homogeneous and should not be treated as a “component” or “reservoir” during geochemical modelling. The heterogeneity reflects radiogenic in-growth starting from small compositional heterogeneities in gabbroic protoliths, followed by modification during sea-floor alteration, subduction and emplacement into the subcratonic lithosphere.

Sm–Nd and Lu–Hf isotopic data are presented for 19 chondritic meteorites: six carbonaceous chondrites, five L-chondrites, seven H-chondrites, and a single enstatite chondrite. The primary goal of the study is to better define the Bulk Silicate Earth (BSE) reference values for Hf isotopes. Except for one sample with lower Sm/Nd, the Sm–Nd data define a cluster around the accepted reference values for chondrites and terrestrial planets, giving a mean 147Sm/144Nd of 0.1960±0.0005, and a mean 143Nd/144Nd of 0.512631±0.000010 (uncertainties are two standard errors). It seems appropriate to retain the presently accepted Sm–Nd reference parameters, 147Sm/144Nd=0.1966 and 143Nd/144Nd=0.512638 (when fractionation-corrected to 146Nd/144Nd=0.7219).Lu–Hf isotopic data are not clustered, but spread along an approximate 4.5-Ga isochron trend, with a range of 176Lu/177Hf from 0.0301 to 0.0354. The data are similar to many of the samples of chondrites presented by Bizzarro et al. [Nature 421 (2003) 931], but lack the range to lower Lu/Hf shown by those authors. Our chondrite data define a regression line of 4.44±0.34 Ga when 1.867×10−11 year−1 is used for the decay constant of 176Lu [Science 293 (2001) 683; Earth Planet. Sci. Lett. 219 (2004) 311–324]. Combining our data with the main population of analyses from Bizzarro et al. [Nature 421 (2003) 931] yields 4.51±0.24 Ga. Unless samples of eucrite meteorites and deviating replicates of chondrites with 176Lu/177Hf less than 0.030 are employed, no combination of the main population of chondrite Lu–Hf data yields a regression with sufficiently low error to constrain the decay constant of 176Lu. Sample heterogeneity seems to hinder the acquisition of reproducible Lu–Hf analyses from small, manually ground pieces of chondrites, and we suggest that analysis of powders prepared from large volumes of meteorite will be needed to adequately characterize the Lu–Hf isotope systematics of chondritic reservoirs and of BSE. Our results for carbonaceous chondrites show higher average 176Lu/177Hf and 176Hf/177Hf than ordinary chondrites, and the mean of carbonaceous chondrites also coincides with replicate analyses of a powder representing a large volume of meteorite, the Allende powder from the Smithsonian Institution. Use of the carbonaceous chondrite mean for BSE Lu–Hf characteristics results in a BSE Hf–Nd point that lies well within the array of terrestrial compositions, and leads to plausible initial eHf values for Precambrian rocks. An improved objective resolution of meteorite data and of meteoritic models for the Earth needs to occur before BSE can be established for Lu–Hf.

We present a detailed geochemical investigation on the Hf, Nd and Sr isotope compositions and trace and major element contents of clinopyroxene mineral separates from spinel lherzolite xenoliths from the island of Oahu, Hawaii. These peridotites are believed to represent the depleted oceanic lithosphere beneath Oahu, which is a residue of a MORB-related melting event some 80–100 Ma ago at a mid-ocean ridge. Clinopyroxenes from peridotites from the Salt Lake Crater (SLC) show a large range of Hf isotopic compositions, from ϵHf=12.2 (similar to the Honolulu volcanics series) to extremely radiogenic, ϵHf=65, at nearly constant 143Nd/144Nd ratios (ϵNd=7–8). None of these samples show any isotopic evidence for interaction with Koolau-type melts. A single xenolith from the Pali vent is the only sample with Hf and Nd isotopic compositions that falls within the MORB field. The Hf isotopes correlate positively with the degree of depletion in the clinopyroxene (e.g. increasing Mg#, Cr#, decreasing Ti and heavy REE contents), but also with increasing Zr and Hf depletions relative to the adjacent REE in a compatibility diagram. The Lu/Hf isotope systematics of the SLC clinopyroxenes define apparent ages of 500 Ma or older and these compositions cannot be explained by mixing between any type of Hawaiian melts and the depleted Pacific lithosphere. Metasomatism of an ancient (e.g. 1 Ga or older) depleted peridotite protolith can, in principle, explain these apparent ages and the Nd–Hf isotope decoupling, but requires that the most depleted samples were subject to the least amount of metasomatism. Alternatively, the combined isotope, trace and major element compositions of these clinopyroxenes are best described by metasomatism of the 80–100 Ma depleted oceanic lithosphere by melts products of extensive mantle–melt interaction between Honolulu Volcanics-type melts and the depleted lithosphere.

Hf, Zr and Ti in carbonatites primarily reside in their non-carbonate fraction while the carbonate fraction dominates the Nd and Sr elemental budget of the whole rock. A detailed investigation of the Hf, Nd and Sr isotopic compositions shows frequent isotopic disequilibrium between the carbonate and non-carbonate fractions. We suggest that the trace element and isotopic composition of the carbonate fraction better represents that of the carbonatite magma, which in turn better reflects the composition of the carbonatitic source. Experimental partitioning data between carbonatite melt and peridotitic mineralogy suggest that the Lu/Hf ratio of the carbonatite source will be equal to or greater than the Lu/Hf ratio of the carbonatite. This, combined with the Hf isotope systematics of carbonatites, suggests that, if carbonatites are primary mantle melts, then their sources must be short-lived features in the mantle (maximum age of 10–30 Ma), otherwise they would develop extremely radiogenic Hf compositions. Alternatively, if carbonatites are products of extreme crystal fractionation or liquid immiscibility then the lack of radiogenic initial Hf isotope compositions also suggests that their sources do not have long-lived Hf depletions. We present a model in which the carbonatite source is created in the sublithospheric mantle by the crystallization of earlier carbonatitic melts from a mantle plume. This new source melts shortly after its formation by the excess heat provided by the approaching hotter center of the plume and/or the subsequent ascending silicate melts. This model explains the HIMU-EMI isotope characteristics of the East African carbonatites, their high LREE/HREE ratios as well as the rarity of carbonatites in the oceanic lithosphere.

We measured trace element and Rare Earth Element (REE) contents of clinopyroxenes (cpx) in peridotites from ophiolite complexes from the Hellenic Peninsula: Vourinos, Pindos, Othris (Greece), and Bulqiza (Albania). Compared to abyssal peridotites the Hellenic peridotites have a highly depleted mineralogy (

The similarity of the Lu–Hf and Sm–Nd isotope system during most mantle differentiation processes makes the combination of 176Hf/177Hf and 143Nd/144Nd a very sensitive indicator of a select number of processes. This paper present new Hf-isotope data for a large number of ocean islands and examines the Hf–Nd–Pb isotope relations of oceanic volcanics. Except for HIMU islands, St. Helena and Tubaii, the Hf and Nd isotope ratios in ocean island basalts (OIB) are extremely well correlated. It is argued that crustal recycling (by either continental or oceanic sediments) most likely did not cause the Hf–Nd variations. The correlated 176Hf/177Hf–143Nd/144Nd variations in OIB most likely represent the time integrated fractionations which are the result of melting in the presence of garnet. The Hf-isotope systematics of HIMU-type OIB are consistent with these basalts representing recycled oceanic crust and thus support the earlier hypothesis on the origins of HIMU basalts. Chondrites form an array that is at high angle with the OIB array. This allows a choice in the 143Nd/144Nd and 176Hf/177Hf values for chondritic bulk earth. With a choice of bulk earth at the extreme end of the OIB array the shift of OIB to higher 176Hf/177Hf can be explained by either isolation of a significant amount of basalts from the mantle for several billions of year or by fractionation and isolation of small amounts (

We present Hf and Pb isotopic data, and chemical compositions of the outermost layers of marine ferromanganese deposits of different types (hydrogenous and hydrothermal) with a worldwide distribution. The Hf isotopic compositions display a broad range and refine previously reported regional differences as follows: Atlantic Ocean ɛHf = −4to+2, Indian Ocean ɛHf = +2to+4, Pacific Ocean ɛHf = +3to+10. The most radiogenic Hf isotopic compositions in the Pacific samples are for hydrothermal manganese deposits that also have low 207Pb204Pb, demonstrating that this signature reflects a contribution from hydrothermal venting of Hf leached from oceanic volcanic rocks rather than from riverine inputs, volcanic ash, or eolian dust. Hafnium concentrations in the deposits increase from 20 ppb to 20 ppm with decreasing ɛHf, The Hf and Pb isotopic compositions for ferromanganese crusts define an apparent mixing trend between literature values of average continental crust and MORB. The range in ɛHf for ferromanganese crusts is narrower than it is for 206Pb204Pb compared to the differences in isotopic composition of the sources of Hf and Pb. This is consistent with Hf having a longer residence time than Pb. The concentration of Hf in ferromanganese crusts has been found to co-vary with growth rate, and inversely correlates with Hf isotopic compositions. Hf isotope ratios may be used to determine not only the source of Hf, but possibly the source of Fe and Mn. Measurements of ɛHf and Hf concentrations in nodule tops, bottoms and associated sediments show that the ɛHf of nodules is sensitive to sedimentary oxic and sub-oxic diagenetic processes and thus most nodules may not reliably reflect the isotopic composition of Hf in seawater.

This paper presents new Hf and Nd isotope and trace element data for mid-ocean ridge basalts. The intention of this study was to investigate whether melt parameters derived from the isotopic systematics of MORB ( δ (Lu) (Hf) and δ (Sm) (Nd) ) support the variations in depth and degree of melting as inferred from the major elements and ridge depth; that is, the ‘local’ and ‘global’ trend [1]. Variation on a ‘local’ scale is defined as variation within a ridge segment, while the ‘global’ scale variation is defined as variation between ridge segments. Both on a ‘local’ and a ‘global’ scale, δ (Lu) (Hf) and δ (Sm) (Nd) show consistent variations with the major element melt parameters, Na 8 and Fe 8 . In addition, the δ melt parameters vary consistently with ridge depth. Basalts from some of the deepest ridges, Australian-Antarctic Discordance and Mid-Cayman Rise, show evidence that a relatively large proportion of the basalt was generated in the presence of garnet. A melting model is derived using best available estimates for a large number of parameters that influence the chemistry of MORBs. With this melting model, the additional constraints of δ (Lu) (Hf) and δ (Sm) (Nd) further define the melting regime beneath the ocean ridges. The onset of melting for shallow ridges, like Kolbeinsey Ridge, starts deeper than for deep ridges, the aggregated melts from the Kolbeinsey Ridge sample a columnar shaped melting regime. Basalts from deep ridges represent a smaller degree of melting, which was generated in a triangular-shaped melting regime. The translation of δ (Lu) (Hf) and δ (Sm) (Nd) in absolute values for degree and depth of melting for individual ridge segments is hampered by uncertainties in the details of the heterogeneity of the MORB reservior.

Lavas from several major bathymetric highs in the eastern Indian Ocean that are likely to have formed as Early to Middle Cretaceous manifestations of the Kerguelen hotspot are predominantly tholeiitic; so too are glass shards from Eocene to Paleocene volcanic ash layers on Broken Ridge, which are believed to have come from eruptions on the Ninetyeast Ridge. The early dominance of tholeiitic compositions contrasts with the more recent intraplate, alkalic volcanism of the Kerguelen Archipelago. Isotopic and incompatible-element ratios of the plateau lavas are distinct from those of Indian mid-ocean ridge basalts; their Nd, Sr, 207Pb204Pb and 208Pb204Pb isotopic ratios overlap with but cover a much wider range than measured for more recent oceanic products of the Kerguelen hotspot (including the Ninetyeast Ridge) or, indeed, oceanic lavas from any other hotspot in the world. Samples from the Naturaliste Plateau and ODP Site 738 on the southern tip of the Kerguelen Plateau are particularly noteworthy, with ϵNd(T) = − 13 to −7, (87Sr86Sr)T=0.7090 to 0.7130 and high 207Pb204Pb relative to 206Pb204Pb. In addition, the low-ϵNd(T) Naturaliste Plateau samples are elevated in SiO2 (> 54 wt%). In contrast to “DUPAL” oceanic islands such as the Kerguelen Archipelago, Pitcairn and Tristan da Cunha, the plateau lavas with extreme isotopic characteristics also have relative depletions in Nb and Ta (e.g., ThTa, La Nb > primitive mantle values); the lowest ϵNd(T) and highest ThTa and La Nb values occur at sites located closest to rifted continental margins. Accepting a Kerguelen plume origin for the plateau lavas, these characteristics probably reflect the shallow-level incorporation of continental lithosphere in either the head of the early Kerguelen plume or in plume-derived magmas, and suggest that the influence of such material diminished after the period of plateau construction. Contamination of asthenosphere with the type of material affecting Naturaliste Plateau and Site 738 magmatism appears unlikely to be the cause of low-206Pb04Pb Indian mid-ocean ridge basalts. Finally, because isotopic data for the plateaus do not cluster or form converging arrays in isotope-ratio plots, they provide no evidence for either a quickly evolving, positive ϵNd, relatively high-206Pb204Pb plume composition, or a plume source dominated by mantle with ϵNd of −3 to ∼ 0.

Components: 80 words.Keywords: MORB; hafnium; isotopes; neodymium.Index Terms: 1025 Geochemistry: Composition of the mantle; 1038 Geochemistry: Mantle processes (3621); 1040Geochemistry: Radiogenic isotope geochemistry; 9900 Corrections.Received 13 September 2011; Published 25 October 2011.Salters, V. J. M., S. Mallick, S. R. Hart, C. E. Langmuir, and A. Stracke (2011), Correction to “Domains of depleted mantle:New evidence from hafnium and neodymium isotopes,” Geochem. Geophys. Geosyst., 12, Q10017,doi:10.1029/2011GC003874.

Isotope systematics of basalts provide information on the distribution of mantle components and the length scale of mantle heterogeneity. To obtain this information, high data and sampling density are crucial. We present hafnium and neodymium isotope data on more than 400 oceanic volcanics. Over length scales of several hundred to over one thousand kilometers hafnium and neodymium isotopes of mid-ocean ridge basalts are correlated and form an array of parallel trends on a global scale. On a larger scale these domains differ in the amount of highly depleted mantle material with radiogenic hafnium and neodymium isotope ratios. Compared to the Atlantic and Indian Ocean basins the asthenosphere of the Pacific basin seems to have a more uniform and a less radiogenic Hf isotopic composition for a given Nd isotopic composition. The parallel arrays of mid-ocean ridge basalts provide strong constraints on the makeup of the MORB mantle and are evidence for the presence of a highly depleted and highly radiogenic neodymium and hafnium component. This component, because of its highly depleted character, is unrecognized in the strontium-neodymium-lead isotope systems alone. Alternatively, the parallel arrays can have an ancient origin of systematic variations in the degree of depletion. Each array then represents the variations in this fossil melting regime. Individual ocean island basalt suites display different slopes in hafnium-neodymium isotope space, which are also best explained by varying amounts of highly residual mantle rather than isotopic differences in enriched mantle components as previously invoked. The ocean island basalt arrays diverge at the depleted end and project to radiogenic compositions that are similar to those of the asthenosphere through which they travel. This is strong evidence that the plume material interacts with its surrounding mantle as it ascends. The isotopic compositions of the ocean island and ridge basalts suggest that their systematics are influenced by a heretofore unrecognized depleted component.

This paper presents Nd- and Hf-isotope data for oceanic volcanics and island arc basalts in order to (1) further examine the correlation between the two isotopic systems, (2) determine 176Hf/177Hf of the mantle components as described by Zindler and Hart (1986) [1], and (3) assess the origin of HFSE depletions in calc-alkaline volcanics. Ocean island basalts (OIB) form an array on a 143Nd/144Nd versus 176Hf/177Hf diagram, whereby the mantle endmembers as defined in SrNdPb isotopic space can be extended to the Hf system. For MORB, however, the decoupling of Hf and Nd isotopes requires a variable MORB endmember. Melting models in which Lu/Hf and Sm/Nd are fractionated during the formation of the MORB reservoir can explain the complete range in Nd and Hf isotopes in MORB as long as variable amounts of both diopside and garnet are present during melting. A model in which a MORB-type mantle is contaminated with high 176Hf/177Hf, high 143Nd/144Nd material is also consistent with the isotope data. This high 176Hf/177Hf component may be generated by high Lu/Hf caused by metasomatism with a melt/fluid which was in equilibrium with phases with a high D for Hf like perovskite. The existence of high-field-strength-element (HFSE) depletions has often been thought to be a widespread characteristic of subduction-related volcanism (Gill, 1981) [2]. However, a literature survey shows that Ti, Zr and Hf depletions are not a ubiquitous a feature in calc-alkaline volcanics (see also White and Patchett, 1984 [3] and Woodhead, 1989 [4]), and that, except for Zr and Hf, the HFSE (Nb, Ta, Ti, Zr, and Hf) behave independently from each other. 176Hf/177Hf ratios in island arc lavas are not related to the degree of Hf/REE variation, strongly suggesting that the HfZr depletion is less than 250 m.y. old. The HFSE variations are not compatible with generation during melting nor are the HFSE variations solely due to the addition of a slab-derived component. However, the available data are compatible with the variations being a reflection of a HFSE-depleted mantle source of calc-alkaline volcanics. The HFSE depletions in this sub-arc mantle have to be less than 250 m.y. old.

We have measured the Hf and Nd isotopic compositions of 38 basalts from the Ko’olau drill hole, Hawai’i. The basalts show limited variations in both 176Hf/177Hf and 143Nd/144Nd (e Nd varies from +4.2 to +7.3 and e Hf from +8.0 to +12.3). Their correlated variation has an R 2 of 0.86. The data form an array with a slope of 1.2 on an e Hf–e Nd isotope correlation diagram, while the slope of all Hawai’ian basalt data is 0.98. Both slopes are significantly shallower than that of the mantle array of 1.4 defined by ocean island basalts. Previous studies have shown that a shallow slope in Hf–Nd isotope space can be related to ancient pelagic sediments in the mantle source (Blichert-Toft et al. 1999; Salters and White 1998). However, the combined variations in Ko’olau basalts of Hf–Nd–Pb–Os isotopic compositions and trace element ratios, such as La/Nb, Th/La and Sr/Nd, are not consistent with the simple addition of a sediment component to the mantle. We instead propose that the shallow slope on the Hf–Nd isotope correlation diagram for Ko’olau shield stage basalts can be better explained if the enriched endmember contains either an ancient oceanic lithosphere component or the high-176Hf/177Hf component observed in the Salt Lake Crater (SLC) peridotite xenoliths (which also have a depleted lithosphere origin). Since Ko’olau basalts have high 187Os/188Os (0.135–0.160) and the SLC xenoliths have 187Os/188Os up to 0.13 (Lassiter et al. 2000) Os-isotopes are consistent with the latter being a component in the enriched Ko’olau source.

[1] We present an estimate for the composition of the depleted mantle (DM), the source for mid-ocean ridge basalts (MORBs). A combination of approaches is required to estimate the major and trace element abundances in DM. Absolute concentrations of few elements can be estimated directly, and the bulk of the estimates is derived using elemental ratios. The isotopic composition of MORB allows calculation of parent-daughter ratios. These estimates form the “backbone” of the abundances of the trace elements that make up the Coryell-Masuda diagram (spider diagram). The remaining elements of the Coryell-Masuda diagram are estimated through the composition of MORB. A third group of estimates is derived from the elemental and isotopic composition of peridotites. The major element composition is obtained by subtraction of a low-degree melt from a bulk silicate Earth (BSE) composition. The continental crust (CC) is thought to be complementary to the DM, and ratios that are chondritic in the CC are expected to also be chondritic in the DM. Thus some of the remaining elements are estimated using the composition of CC and chondrites. Volatile element and noble gas concentrations are estimated using constraints from the composition of MORBs and ocean island basalts (OIBs). Mass balance with BSE, CC, and DM indicates that CC and this estimate of the DM are not complementary reservoirs.

[1] Recycled ancient oceanic crust with variable amounts of aging, or inclusion of sediments of differing types and origins has often been invoked as a source for present-day ocean island basalts (OIB), but the current evidence remains largely qualitative. Previous quantitative modeling has shown that much has to be learned in order to better understand the implications of crustal recycling on mantle heterogeneity. Here, we present new model calculations incorporating recent constraints on subduction-zone processes and the composition of subducted sediments. Modeled compositions of the recycled oceanic crust vary widely as a function of the recycling age and composition of the oceanic crust. HIMU-type sources can only be created by recycling igneous oceanic crust if it has undergone substantial modification during subduction. Although the required modifications are qualitatively consistent with dehydration processes in subduction zones, the many uncertainties prevent a precise estimate of the isotopic composition of ancient recycled igneous crust. Inclusion of sediments increases the isotopic variability and although the resulting Sr and Nd isotopic signatures can be similar to enriched mantle (EM) signatures, the Pb isotopic composition of EM-type OIB is difficult to reconcile with the presence of sediment in their sources. The large variability of modeled compositions of the subducted crust suggests that if mantle heterogeneity is largely formed by crustal recycling, each OIB is likely to have a unique isotopic composition resulting from specific combinations of composition, age and subduction modification of the subducted crust. Given the variability of the recycled components, a small number of relatively well-defined enriched compositions can only be explained if either the subduction processing of oceanic crust is a far better defined process than observation would seem to indicate, or, the intramantle disaggregation and mixing of compositionally diverse recycled materials is surprisingly efficient.

Inferring the melting process at mid-ocean ridges, and the physical conditions under which melting takes place, usually relies on the assumption of compositional similarity between all mid-ocean-ridge basalt sources. Models of mantle melting therefore tend to be restricted to those that consider the presence of only one lithology in the mantle, peridotite. Evidence from xenoliths and peridotite massifs show that after peridotite, pyroxenite and eclogite are the most abundant rock types in the mantle. But at mid-ocean ridges, where most of the melting takes place, and in ophiolites, pyroxenite is rarely found. Here we present neodymium isotopic compositions of abyssal peridotites to investigate whether peridotite can indeed be the sole source for mid-ocean-ridge basalts. By comparing the isotopic compositions of basalts and peridotites at two segments of the southwest Indian ridge, we show that a component other than peridotite is required to explain the low end of the (143)Nd/(144)Nd variations of the basalts. This component is likely to have a lower melting temperature than peridotite, such as pyroxenite or eclogite, which could explain why it is not observed at mid-ocean ridges.

[1] The existence of an enriched component in the mantle with a pyroxenitic or eclogitic composition and its importance for basalt genesis has been discussed for over two decades. Inferences about the depth of melting as well as the dynamics of melting based on the presence of garnet and the location of the spinel-garnet transition are different if garnet-pyroxenite is present in the peridotitic mantle. Trace element partition coefficients are dependent on composition, and the differences between garnet-pyroxenite and peridotite are large enough to produce significant differences in trace element fractionation between melts derived from these different lithologies. Melts derived from garnet-pyroxenite or eclogite-bearing sources will have small or no 230Th excesses, which are largely independent of melting and upwelling rate. Melts derived from garnet-peridotite will have significant 230Th excesses, which are dependent on melting and upwelling rate. We show that the combined hafnium-neodymium-thorium (Hf-Nd-Th) isotope and trace element data can distinguish between melts derived from peridotitic and pyroxenitic or eclogitic sources. We also present new Hf isotope data for Hawaiian basalts and use the combined Hf-Nd-Th isotope and trace element systematics to argue against the existence of garnet-pyroxenite or eclogite in the source of Hawaiian basalts. It is especially the large variation in degree of melting for relatively constant isotopic composition that allows us to rule out garnet-pyroxenite in the source of the Hawaiian basalts.

We report here the widespread occurrence of mantle peridotites in both suboceanic and subcontinental lithosphere that show trace element characteristics similar to those of island arc volcanics. These anhydrous peridotites, which reside at high levels in the mantle, cannot generate nor be in equilibrium with mid-ocean ridge or ocean island basalts, and have complex geochemical histories involving partial melting, melt extraction and metasomatism.

87 Sr/ 86 Sr, major and trace element data are presented for calc-alkaline lavas from Santorini and Milos, Aegean Sea, Greece. 87 Sr/ 86 Sr ratios of fresh lavas from Santorini range from 0.70472 to 0.70509 whereas those of fresh lavas from Milos lie in the range 0.70540–0.70620. Altered lavas from both islands have higher 87 Sr/ 86 Sr ratios (up to 0.70573 and 0.70662 respectively). The range of 87 Sr/ 86 Sr ratios of fresh lavas from Santorini is only just greater that the maximum analytical uncertainty and is thus considered to reflect that of the parental magmas from which the lava series was derived (by fractional crystallization at low pressures). For the analysed Milos lavas (dacite-rhyolite) there is no correlation between 87 Sr/ 86 Sr and any major or trace element concentration and this, together with the fact that the 87 Sr/ 86 Sr ratios are similar to those of basaltic andesites and andesites from Aegina and Methana in the northwestern part of the arc, likewise indicates that the 87 Sr/ 86 Sr ratios are identical to those of the parental magmas from which the lava series was derived (also by fractional crystallization). The relatively high 87 Sr/ 86 Sr ratios of the Santorini and Milos parental magmas indicate that terrigenous material has played a role in their genesis. This could be either via the subduction process or by assimilation of lower crustal material during ascent of mantle-derived magmas. The weight of the available evidence favours the latter possibility, but, even so, there are some indications that unusual mantle source material was involved in the genesis of the magmas.

Volcanic activity on Santorini started in Upper Pliocene times and is still in progress (the last eruption occurred in 1950). At least seven eruptive centers were active at various times, and some centers were active contemporaneously. The overall chemical variations displayed by lavas and pyroclastics from the volcanic complex are largely explicable in terms of fractional crystallization involving removal of the observed phenocryst phases, together with variable amounts of magma mixing and assimilation. Comparison of the chemistry of volcanic products from different eruption centers indicates long-term variations in magma chemistry (progressive depletion in LIL elements and enrichment in compatible elements with time) that cannot be explained in terms of fractionation, mixing and assimilation. Such variations are attributed to processes occurring within the upper mantle. Trace element data require that the primitive lavas erupted on Santorini were derived from an upper mantle source which was relatively depleted in LIL elements (compared with other calc-alkaline volcanics) and it is proposed that the source region contained a (dominant) refractory component which had experienced earlier events and a component derived from the subducted slab. As magmatism progressed, the contribution to the source region from the slab decreased, due to either (1) cessation of subduction and hence a decrease in the proportion of slab-derived material in the source region; (2) a rise in geothermal gradient during melting so that the earliest melts contained a higher proportion of (low-melting) slab derived material; or (3) a combination of these processes. We conclude that it is possible to “see through” intracrustal processes and to evaluate the processes which occur in the upper mantle beneath island arcs and we also conclude that the time-integrated history of the mantle wedge above the subducted slab beneath island arcs must be taken into account when constructing viable models for are magmatism.

MID-OCEAN-RIDGE basalts (MORBs) are thought to result from melting in the mantle at depths of less than 60 km, in the spinel stability field1–3. MORBs have 176Hf/177Hf ratios indicating derivation from a mantle reservoir with a long-term Lu/Hf ratio greater than that of Cl chondrite meteorites, yet the measured Lu/Hf ratios in MORB are lower than in Cl chondrites: this is the 'hafnium paradox'4. Here we show that the Lu–Hf and Sm–Nd systematics of MORBs require garnet to be a residual phase in MORB melt genesis. This places the onset of melting beneath a mid-ocean ridge at depths greater than 80 km. A sequential melting model, in which melting starts in the garnet stability field and then continues at shallower levels, best explains the combined Nd and Hf isotope systematics, and is compatible with our present geophysical and geochemical knowledge of mid-ocean-ridge magmatism.

We report here the widespread occurrence of mantle peridotites in both suboceanic and subcontinental lithosphere that show HFSE depletions similar to those of island arc volcanics. The generation of these depletions is not related to the subduction process. These anhydrous peridotites, which reside at high levels in the mantle, cannot generate, nor be in equilibrium with, mid-ocean ridge or ocean island basalts, and have a complex geochemical history involving partial melting, melt extraction and metasomatism.