Controls on magmatism in an island arc environment: study of lavas and sub-arc xenoliths from the Tabar–Lihir–Tanga–Feni island chain, Papua New Guinea

The Tabar–Lihir–Tanga–Feni (TLTF) islands of Papua New Guinea mainly comprise high-K calc-alkaline and silica undersaturated alkaline rocks that have geochemical features typical for subduction-related magmatism. Numerous sedimentary, mafic, and ultramafic xenoliths recovered from Tubaf seamount, located on the flank of Lihir Island, provide a unique opportunity to study the elemental and isotopic composition of the crust and mantle wedge beneath the arc and to evaluate their relationships to the arc magmatism in the region. The sedimentary and mafic xenoliths show that the crust under the islands is composed of sedimentary sequences and oceanic crust with Pacific affinity. A majority of the ultramafic xenoliths contain features indicating wide spread metasomatism in the mantle wedge under the TLTF arc. Leaching experiments reveal that the metasomatized ultramafic xenoliths contain discrete labile phases that can account for up to 50% or more of elements such as Cu, Zn, Rb, U, Pb, and light REE (rare-earth elements), most likely introduced in the xenoliths via hydrous fluids released from a subducted slab. The leaching experiments demonstrated that the light REE enrichment pattern can be more or less removed from the metasomatized xenoliths and the residual phases exhibit REE patterns that range from flat to light REE depleted. Sr–Nd isotopic data for the ultramafic residues show a coupled behavior of increasing ⁸⁷Sr/⁸⁶Sr with decreasing ¹⁴³Nd/¹⁴⁴Nd ratios. The labile phases in the ultramafic xenoliths, represented by the leachates, show decoupling between Sr and Nd with distinctly more radiogenic ⁸⁷Sr/⁸⁶Sr than the residues. Both leachates and residues exhibit very wide range in their Pb isotopic compositions, indicating the involvement of three components in the mantle wedge under the TLTF islands. Two of the components can be identified as Pacific Oceanic mantle and Pacific sediments. Some of the ultramafic samples and clinopyroxene separates, however, exhibit relatively low ²⁰⁶Pb/²⁰⁴Pb at elevated ²⁰⁷Pb/²⁰⁴Pb suggesting that the third component is either Indian Ocean-type mantle or Australian subcontinental lithospheric mantle. Geochemical data from the ultramafic xenoliths indicate that although the mantle wedge in the area was extensively metasomatized, it did not significantly contribute to the isotopic and incompatible trace element compositions of TLTF lavas. Compared to the mantle samples, the TLTF lavas have very restricted Pb isotopic compositions that lie within the Pacific MORB range, indicating that magma compositions were dominated by melts released from a stalled subducted slab with Pacific MORB affinity. Interaction of slab melts with depleted peridotitic component in the mantle wedge, followed by crystal fractionation most likely generated the geochemical characteristics of the lavas in the area. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Petrology and Geochemistry of Eclogite Xenoliths from the Colorado Plateau: Implications for the Evolution of Subducted Oceanic Crust

Eclogite xenoliths from the Colorado Plateau, interpreted asfragments of the subducted Farallon plate, are used to constrainthe trace element and Sr–Nd–Pb isotopic compositionsof oceanic crust subducted into the upper mantle. The xenolithsconsist of almandine-rich garnet, Na-clinopyroxene, lawsoniteand zoisite with minor amounts of phengite, rutile, pyrite andzircon. They have essentially basaltic bulk-rock major elementcompositions; their Na₂O contents are significantly elevated,but K₂O contents are similar to those of unaltered mid-oceanridge basalt (MORB). These alkali element characteristics areexplained by spilitization or albitization processes on thesea floor and during subduction-zone metasomatism in the fore-arcregion. The whole-rock trace element abundances of the xenolithsare variable relative to sea-floor-altered MORB, except forthe restricted Zr/Hf ratios (36·9–37·6).Whole-rock mass balances for two Colorado Plateau eclogite xenolithsare examined for 22 trace elements, Rb, Cs, Sr, Ba, Y, rareearth elements, Pb, Th and U. Mass balance considerations andmineralogical observations indicate that the whole-rock chemistriesof the xenoliths were modified by near-surface processes afteremplacement and limited interaction with their host rock, aserpentinized ultramafic microbreccia. To avoid these secondaryeffects, the Sr, Nd and Pb isotopic compositions of mineralsseparated from the xenoliths were measured, yielding 0·70453–0·70590for ⁸⁷Sr/⁸⁶Sr, –3·1 to 0·5 for Nd and 18·928–19·063for ²⁰⁶Pb/²⁰⁴Pb. These isotopic compositions are distinctlymore radiogenic for Sr and Pb and less radiogenic for Nd thanthose of altered MORB. Our results suggest that the MORB-likeprotolith of the xenoliths was metasomatized by a fluid equilibratedwith sediment in the fore-arc region of a subduction zone andthat this metasomatic fluid produced continental crust-likeisotopic compositions of the xenoliths. KEY WORDS: Colorado Plateau; eclogite xenolith; geochemistry; subducted oceanic crust     

He, Sr and Nd isotopes of mantle-derived xenoliths in volcanic rocks of NE China

Abundances and isotopic ratios of He and isotopic ratios of Sr and Nd have been measured for 18 mantle-derived xenoliths from three Cenozoic volcanic provinces in NE China. The xenoliths are characterized by low He concentrations (2.3×10⁻⁹–2.5×10⁻⁷ cm³ STP g⁻¹) and large variations of ³He/⁴He ratios (1.6×10⁻⁷–15.8×10⁻⁶ or R/Ra=0.1–11). ³He/⁴He, ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios in xenoliths from Kuandian Holocene volcanic provinces are consistent with those observed in mid-oceanic ridge basalt (MORB), indicating that the source of He–Sr–Nd in the lithospheric mantle is chemically and isotopically identical to that of MORB reservoir. However, xenoliths in Huinan Pleisteocene–Holocene volcanic province are characterized by slightly low ³He/⁴He ratios (R/Ra=4.3–6.5) and more radiogenic Sr and Nd. The age-corrected ³He/⁴He ratios suggest that their time-integrated ratio of (U+Th)/³He is slightly elevated compared to the depleted upper mantle. The Hannuoba xenoliths in the Miocene volcanic province show much lower and wider variable ³He/⁴He ratios, which is most likely caused by in situ radiogenic ⁴He accumulation since it erupted on to the surface. The He–Sr–Nd results suggest chemical and isotopical heterogeneity in the subcontinental mantle beneath NE China.     

Tertiary basalts and peridotite xenoliths from the Hessian Depression (NW Germany), reflecting mantle compositions low in radiogenic Nd and Sr

A total of 17 alkali basalts (alkali olivine basalt, limburgite, olivine nephelinite) and quartz tholeiites, and of 10 peridotite xenoliths (or their clinopyroxenes) were analyzed for Nd and Sr isotopes. ¹⁴³Nd/¹⁴⁴Nd ratios and ⁸⁷Sr/⁸⁶Sr ratios of all basalts and of the majority of ultramafic xenoliths plot below the mantle array with a large variation in Nd isotopes and a smaller variation in Sr isotopes. The tholeiites were less radiogenic in Nd than the alkali basalts. Volcanics from the Eifel and Massif Central regions contain Nd and Sr, which is more radiogenic than that of the basalts from the Hessian Depression. Nd and Sr isotopic compositions of all rocks from the latter area, with the exception of one tholeiite and one peridotite plot in the same field of isotope ratios as the Ronda ultramafic tectonite (SW Spain), which ranges in composition from garnet to plagioclase peridotite. The alkali basaltic rocks are products of smaller degrees of partial melting of depleted peridotite, which has undergone a larger metasomatic alteration compared with the source rock of tholeiitic magmas. For the peridotite xenoliths such metasomatic alteration is indicated by the correlation of their K contents and isotopic compositions. We assume that the upper mantle locally can acquire isotopic signatures low in radiogenic Nd and Sr from the introduction of delaminated crust. Such granulites low in radiogenic Nd and Sr are products of early REE fractionation and granite (Rb) separation.     

A Study of REE and Pb,Sr and Nd Isotopes in Garnet—Lherzolite Xenoliths from Mingxi,Fujian Province

The REE and Pb, Sr, Nd isotopes in three xenoliths from limburgite and scoria-breccias, including spinel-lherzolite, spinel-garnet-lherzolite and phlogopite-gamet-lherzolite, were analysed. The REE contents of the xenoliths are 1.3 to 3.3 times those of the chondrites with their REE patterns characterized by weak LREE depletion. The¹⁴³Nd/¹⁴⁴Nd values of whole rocks and minerals range from 0.51306 to 0.51345 with εNd=+ 8.2− +15.8,²⁰⁶Pb/²⁰⁴ Pb < 18.673, and²⁰⁷Pb/²⁰⁴Pb < 15.574. All this goes to show that the upper mantle in Mingxi at the depth of 67–82 km is a depleted mantle of MORB type, with⁸⁷Sr/⁸⁶ Sr ratios 0.70237–0.70390. In Nd-Sr diagram the data points of whole rocks are all out of the mantle array, implying that the xenoliths from Mingxi have more radiogenic Sr isotopes than those of the mantle array.     

Sr-Nd-Pb isotope data for ultramafic xenoliths from Hierro,Canary Islands: Melt infiltration processes in the upper mantle

We present here Sr, Nd, and Pb-isotopic data from harzburgite (group I) and dunite-pyroxenite (group II) suite mantle xenoliths from the island of Hierro, one of the youngest and westernmost of the Canary Islands. A progressive leaching technique has been developed and applied to the whole-rock powder samples in order to identify and remove as far as possible any recent additions (host basalt and/or sea-water). Isotopic analyses of the leached residues show significant systematic differences between these two suites. Dunite-pyroxenite suite xenoliths (olivine pyroxenites, dunites and wehrlites) exhibit a relatively small range of isotopic compositions (⁸⁷Sr/⁸⁶Sr from 0.70292 to 0.70315; ¹⁴³Nd/¹⁴⁴Nd from 0.51295 to 0.51302; ²⁰⁶Pb/²⁰⁴Pb from 19.18 to 19.40) compared to the harzburgite suite (⁸⁷Sr/⁸⁶Sr from 0.70295 to 0.70320; ¹⁴³Nd/¹⁴⁴Nd from 0.51285 to 0.51296; ²⁰⁶Pb/²⁰⁴Pb from 18.85 to 19.41). In all isotope correlation diagrams the leached dunite-pyroxenite suite xenoliths plot between the Hierro basalt field and a hypothetical depleted mantle suggesting that these xenoliths may have been strongly infiltrated by Hierro-type basalt. Progressive leaching of this suite of samples showed removal of a component with more enriched Sr (higher ⁸⁷Sr/⁸⁶Sr relative to depleted mantle) and Nd (lower ¹⁴³Nd/¹⁴⁴Nd) isotopic compositions that is probably host basalt glass. The leached harzburgite suite xenoliths extend to more enriched Sr and Nd isotopic compositions than Hierro-type basalt but always have more depleted Pb. This relationship can best be explained if this suite has been subject to infiltration by earlier magmas of the Canary Island suite (in particular, those from Gran Canaria show appropriate compositional ranges), although additional infiltration by Hierro basalt cannot be ruled out. The leaching experiments for this suite mostly show removal of a radiogenic Sr component only (? seawater) which supports the interpretation of early infiltration and subsequent recrystallisation and equilibration prior to the Hierro event. Isotopic data presented in this study show that complex interaction with percolating basaltic melts of varying composition was occurring in the upper mantle beneath Hierro prior to and during the volcanic event and was probably related to the generation of earlier Canary Island magmas.     

Sr,Nd and Pb Isotopic Systematics of the Cenozoic Basalts of the Korean Peninsula and Their Implications for the Permo-Triassic Continental Collision Boundary

Sr, Nd and Pb isotopic compositions of the Cenozoic basalts were analyzed from Baengnyeongdo Island, Jeongok, Ganseong, and Jejudo Island of Korea. They reveal relatively enriched Sr and Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0.70330∼0.70555, ¹⁴³Nd/¹⁴⁴Nd = 0.51298∼0.51256) compared with MORB.²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values of all the analyzed Korean basalts lie above the Northern Hemisphere Reference Line (NHRL) defined by Hart (1984). Pb isotopic compositions of basalts from Jejudo Islands (²⁰⁶Pb/²⁰⁴Pb = 18.61∼19.12, ²⁰⁷Pb/²⁰⁴Pb = 15.54∼15.69, ²⁰⁸Pb/²⁰⁴Pb = 38.98∼39.72) are significantly more radiogenic than the rest (²⁰⁶Pb/²⁰⁴Pb = 17.72∼18.03, ²⁰⁷Pb/²⁰⁴Pb = 15.44∼15.58, ²⁰⁸Pb/²⁰⁴Pb = 37.77∼38.64). The Cenozoic Korean basalts thus can be divided into two groups based on their Sr, Nd and Pb isotopic compositions. The north group reveals mixing between DMM and EM1 while the south group displays DMM-EM2 mixing. Such a distribution is the same as Chinese Cenozoic basalts and it can be interpreted that the subcontinental lithospheric mantle under Korea represents simple lateral continuation of the South and North China Blocks. We suggest that Korean continental collision zone cross the Korean Peninsula through the region between the north and south basalt groups of Korea.     

Geochemical variations in the Cocos Plate subducting beneath Central America: implications for the composition of arc volcanism and the extent of the Galápagos Hotspot influence on the Cocos oceanic crust

New geochemical data from the Cocos Plate constrain the composition of the input into the Central American subduction zone and demonstrate the extent of influence of the Galápagos Hotspot on the Cocos Plate. Samples include sediments and basalts from Ocean Drilling Program (ODP) Site 1256 outboard of Nicaragua, gabbroic sills from ODP Sites 1039 and 1040, tholeiitic glasses from the Fisher Ridge off northwest Costa Rica, and basalts from the Galápagos Hotspot Track outboard of Central Costa Rica. Site 1256 basalts range from normal to enriched MORB in incompatible elements and have Pb and Nd isotopic compositions within the East Pacific Rise MORB field. The sediments have similar ²⁰⁶Pb/²⁰⁴Pb and only slightly more radiogenic ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb isotope ratios than the basalts. Altered samples from the subducting Galápagos Hotspot Track have similar Nd and Pb isotopic compositions to fresh Galápagos samples but have significantly higher Sr isotopic composition, indicating that the subduction input will have a distinct geochemical signature from Galápagos-type mantle material that may be present in the wedge beneath Costa Rica. Gabbroic sills from Sites 1039 and 1040 in East Pacific Rise (EPR) crust show evidence for influence of the Galápagos Hotspot ∼100 km beyond the morphological hotspot track.     

He,Pb, Sr and Nd isotope constraints on magma genesis and mantle heterogeneity beneath young Pacific seamounts

Pb, Sr and Nd isotope variations are correlated in diverse lavas erupted at small seamounts near the East Pacific Rise. Tholeiites are isotopically indistinguishable from MORB (²⁰⁶Pb/²⁰⁴Pb=18.1–18.5; ⁸⁷Sr/⁸⁶Sr=0.7023–0.7028; ¹⁴³Nd/¹⁴⁴Nd=0.51326-0.51308); associated alkali basalts always show more radiogenic Pb and Sr signatures (²⁰⁶Pb/²⁰⁴Pb=18.8–19.2; ⁸⁷Sr/⁸⁶Sr=0.7029–0.7031) and less radiogenic Nd (¹⁴³Nd/¹⁴⁴Nd=0.51289–0.51301). The isotopic variability covers 80% of the variability for Pacific MORB, due to the presence of small-scale heterogeneity in the underlying mantle. Isotope compositions also correlate with trace element ratios such as La/Sm. Tholeiites at these seamounts have ³He/⁴He between 7.8–8.7 R _A(R _A= atmospheric ratio), also indistinguishable from MORB. He trapped in vesicles of alkali basalts, released by crushing in vacuo, has low ³He/⁴He (1.2–2.6 R)_Ain conjunction with low helium concentrations ([He]<5×10^–8 ccSTP/g). In many cases post-eruptive radiogenic ingrowth has produced He isotope disequilibrium between vesicles and glass in the alkali basalts; subatmospheric ³He/⁴He ratios characterize the He dissolved in the glass which is released by melting the crushed powders. The narrow range of ³He/⁴He in the vesicles of the alkali basalts suggests that low ³He/⁴He is a source characteristic, but given their low [He] and high (U + Th), pre-eruptive radiogenic ingrowth cannot be excluded as a cause for low inherited ³He/⁴He ratios. Pb, Sr and Nd isotope compositions in lavas erupted at Shimada Seamount, an isolated volcano on 20 m.y. old seafloor at 17°N, are distinctly different from other seamounts in the East Pacific (²⁰⁶Pb/²⁰⁴Pb=18.8–19.0, ⁸⁷Sr/ ⁸⁶Sr0.7048 and ¹⁴³Nd/¹⁴⁴Nd0.51266). Relatively high ²⁰⁷Pb/²⁰⁴Pb (15.6–15.7) indicates ancient (>2 Ga) isolation of the source from the depleted upper mantle, similar to Dupal components which are more prevalent in the southern hemisphere mantle. ³He/⁴He at Shimada Seamount is between 3.9–4.8 R _A. Because the helium concentrations range up to 1.5×10^–6, the low ³He/⁴He can not be due to radiogenic accumulation of ⁴He in the magma for reasonable volcanic evolution times. The low ³He/⁴He may be due to the presence of enriched domains within the lithosphere with high (U + Th)/He ratios, possibly formed during its accretion near the ridge. Alternatively, the low ³He/⁴He may be an inherent characteristic of an enriched component in the mantle beneath the East Pacific. Collectively, the He-Pb-Sr-Nd isotope systematics at East Pacific seamounts suggest that the range of isotope compositions present in the mantle is more readily sampled by seamount and island volcanism than by axial volcanism. Beneath thicker lithosphere away from the ridge axis, smaller degrees of melting in the source regions are less efficient in averaging the chemical characteristics of small-scale heterogeneities.     

Nd,Sr and Pb isotopic and REE geochemical study of some Miocene submarine hydrothermal deposits (Kuroko deposits) in Japan

Neodymium, Sr and Pb isotopic compositions, along with rare earth element (REE) concentrations were determined for twelve black ores and one yellow ore from twelve localities of the Kuroko deposits, Japan. The ores were generated by submarine hydrothermal activity during the Miocene age. Neodymium isotopic compositions of the ores (_Nd: –4.9 to +6.5) mostly overlap with spatially associated igneous rocks. On a Nd versus Sr isotopic correlation diagram, however, ⁸⁷Sr/⁸⁶Sr ratios are shifted from the associated igneous rocks towards the higher contemporaneous seawater ratio. REE patterns are highly variable, ranging from light REE enriched to depleted, and show no Ce anomalies, as would be expected if they were derived from seawater. These results suggest that the REEs contained in ores were mainly derived from the associated igneous rocks, but that the ore Sr is a mixture derived from both seawater and the igneous rocks. Most Pb isotopic compositions fall within the range defined by the associated igneous rocks (²⁰⁶Pb/²⁰⁴Pb=18.35–18.84, ²⁰⁷Pb/²⁰⁴Pb=15.59–15.97 and ²⁰⁸Pb/²⁰⁴Pb=38.53–39.90), although several samples have very radiogenic compositions that were most likely derived from basement rocks. Our new Pb isotopic results display greater variation, and have a larger range of more radiogenic compositions than has been noted previously for these ores. In addition, the black ore with the most radiogenic Pb isotopic composition also has the least radiogenic Nd isotopic composition. This suggests that at least some of the Pb contained in the ores was derived mainly from older basement rocks. The large positive Eu anomalies for some black ores are consistent with a high-temperature origin for the parental fluids, irrespective of the source rock. The single yellow ore examined, however, has a small negative Eu anomaly, which may indicate derivation from a lower temperature fluid. Previous studies suggested that the Kuroko ores were formed in the presence of organic materials in an anoxic basin. Combined Nd, Sr, Pb and Os isotopic and REE abundance data indicate that multiple sources were involved in the genesis of Kuroko ores.     

Heterogeneous lithospheric mantle metasomatism in the eastern North China Craton: He–Ar isotopes in peridotite xenoliths from Cenozoic basalts

The abundances and isotopic compositions of Helium and Argon have been analyzed in a suite of fresh spinel peridotite xenoliths in Cenozoic basalts from the eastern North China Craton (NCC) by step-wise heating experiments, to investigate the nature of noble gas reservoirs in the subcontinental lithospheric mantle beneath this region. The xenoliths include one harzburgite collected from Hebi in the interior of the NCC, two lherzolites from Hannuoba at the northern margin of the craton, and three lherzolites from Shanwang and Nushan on the eastern margin. ³He/⁴He ratios in most of the xenoliths are similar to those of mid-ocean ridge basalts (MORB) or slightly lower (2–10.5 Ra, where Ra is the ³He/⁴He ratio of the atmosphere), suggesting mixing of MORB-like and radiogenic components. One olivine separate from Nushan has a helium value of 25.3 Ra, probably suggesting cosmogenic ³He addition. The ⁴⁰Ar/³⁶Ar ratios vary from atmospheric value (296) to 1625, significantly lower than the MORB value. Available data of the peridotite xenoliths indicate the He and Ar isotopic systematics of the mantle reservoirs beneath the NCC can be interpreted as mixtures of at least three end-members including MORB-like, radiogenic and atmospheric components. We suggest that the MORB-like noble gases were derived from the underlying asthenosphere during mantle upwelling, whereas the radiogenic and recycled components probably were incorporated into the lithospheric mantle during circum-craton subduction of oceanic crust. Available data suggest that the MORB-like fluids are better preserved in the interior of the NCC, whereas the radiogenic ones are more prevalent at the margins. The Paleo-Asian ocean subduction system probably was responsible for the enriched and recycled noble gas signatures on the northern margin of the craton, while the Pacific subduction system could account for the observed He–Ar isotopic signatures beneath the eastern part. Therefore, integration of helium and argon isotopes reflects heterogeneous metasomatism in the lithospheric mantle and demonstrates the critical importance of lithospheric mantle modification related to both circum-craton subduction of oceanic crust and asthenospheric upwelling beneath the eastern NCC.     

The role of mantle-hybridization and crustal contamination in the petrogenesis of lithospheric mantle-derived alkaline rocks: constraints from Os and Hf isotopes

The Rhön area as part of the Central European Volcanic Province (CEVP) hosts an unusual suite of Tertiary 24-Ma old hornblende-bearing alkaline basalts that provide insights into melting and fractionation processes within the lithospheric mantle. These chemically primitive to slightly evolved and isotopically (Sr, Nd, Pb) depleted basalts have slightly lower Hf isotopic compositions than respective other CEVP basalts and Os isotope compositions more radiogenic than commonly observed for continental intraplate alkaline basalts. These highly radiogenic initial ¹⁸⁷Os/¹⁸⁸Os ratios (0.268–0.892) together with their respective Sr–Nd–Pb isotopic compositions are unlikely to result from crustal contamination alone, although a lack of Os data for lower crustal rocks from the area and limited data for CEVP basalts or mantle xenoliths preclude a detailed evaluation. Similarly, melting of the same metasomatized subcontinental lithospheric mantle as inferred for other CEVP basalts alone is also unlikely, based on only moderately radiogenic Os isotope compositions obtained for upper mantle xenoliths from elsewhere in the province. Another explanation for the combined Nd, Sr and Os isotope data is that the lavas gained their highly radiogenic Os isotope composition through a mantle “hybridization”, metasomatism process. This model involves a mafic lithospheric component, such as an intrusion of a sublithospheric primary alkaline melt or a melt derived from subducted oceanic material, sometime in the past into the lithospheric mantle where it metasomatized the ambient mantle. Later at 24 Ma, thermal perturbations during rifting forced the isotopically evolved parts of the mantle together with the peridotitic ambient mantle to melt. This yielded a package of melts with highly correlated Re/Os ratios and radiogenic Os isotope compositions. Subsequent movement through the crust may have further altered the Os isotope composition although this effect is probably minor for the majority of the samples based on radiogenic Nd and unradiogenic Sr isotope composition of the lavas. If the radiogenic Os isotope composition can be explained by a mantle-hybridization and metasomatism model, the isotopic compositions of the hornblende basalts can be satisfied by ca. 5–25% addition of the mafic lithospheric component to an asthenospheric alkaline magma. Although a lack of isotope data for all required endmembers make this model somewhat speculative, the results show that the Re–Os isotope system in continental basalts is able to distinguish between crustal contamination and derivation of continental alkaline lavas from isotopically evolved peridotitic lithosphere that was contaminated by mafic material in the past and later remelted during rifting. The Hf isotopic compositions are slightly less radiogenic than in other alkaline basalts from the province and indicate the derivation of the lavas from low Lu–Hf parts of the lithospheric mantle. The new Os and Hf isotope data constrain a new light of the nature of such metasomatizing agents, at least for these particular rocks, which represent within the particular volcanic complex the first product of the volcanism.     

Lower crustal granulite xenoliths from the Pannonian Basin,Hungary, Part 2: Sr–Nd–Pb–Hf and O isotope evidence for formation of continental lower crust by tectonic emplacement of oceanic crust

Mafic granulite xenoliths from the lower crust of the Pannonian Basin are dominated by LREE-depleted bulk-rock compositions. Many of these have MORB-like ¹⁴³Nd/¹⁴⁴Nd but ⁸⁷Sr/⁸⁶Sr is elevated relative to most MORBs. Their '¹⁸O values cover a wide range from +3.8 to +9.5‰. A group of LREE-enriched mafic granulites have higher ⁸⁷Sr/⁸⁶Sr (0.704-0.708) and lower ¹⁴³Nd/¹⁴⁴Nd (0.5128-0.5124), with higher '¹⁸O values on average (+7.8 to +10.6‰) than the LREE-depleted granulites. The LREE-enriched granulites are, however, isotopically similar to newly discovered metasedimentary granulite xenoliths. A sublinear correlation in )Hf-)Nd isotope space has a shallower slope than the crust-mantle array, with the metasedimentary rocks forming the low )Hf end member; the radiogenic end is restricted to the LREE-depleted granulites and these overlap the field of MORB. Pb isotopes for the LREE-depleted samples are less radiogenic on average than those of the LREE-enriched and metasedimentary xenoliths, and metasedimentary granulites have consistently higher ²⁰⁸Pb/²⁰⁴Pb. The wide range in '¹⁸O over a restricted range in Nd and Sr isotope values, in combination with the predominance of LREE-depleted trace-element compositions, is consistent with an origin as a package of hydrothermally altered oceanic crust. The existence of '¹⁸O values lower than average MORB and/or mantle peridotite requires that at least some of these rocks were hydrothermally altered at high temperature, presumably in the oceanic lower crust. The low ¹⁴³Nd/¹⁴⁴Nd of the LREE-enriched mafic granulites cannot be explained by simple mixing between a LREE-depleted melt and an enriched component, represented by the recovered metasediments. Instead, we interpret these rocks as the metamorphic equivalent of the shallowest levels of the ocean crust where pillow basalts are intimately intercalated with oceanic sediments. A possible model is accretion of oceanic crustal slices during subduction and convergence, followed by high-grade metamorphism during the Alpine orogeny.     

Baseline determination of the atmospheric Pb,Sr and Nd isotopic compositions in the Rhine valley,Vosges mountains (France) and the Central Swiss Alps

The Pb, Sr and Nd isotopic compositions of biomonitors (lichen, moss, bark) and soil litter from different regions in the Rhine valley, as well as of <0.45 μm particles separated out of ice of the Rhône and Oberaar glaciers and lichens from the Swiss Central Alps, have been determined in order to deduce the natural baseline of the atmospheric isotopic compositions of these regions, which are suggested to be close to the isotopic compositions of the corresponding basement rocks or soils at the same sites. ²⁰⁶Pb/²⁰⁷Pb and ⁸⁷Sr/⁸⁶Sr isotope ratios are positively correlated. Most polluted samples from traffic-rich urban environments have the least radiogenic Pb and Sr isotopic compositions with ²⁰⁶Pb/²⁰⁷Pb and ⁸⁷Sr/⁸⁶Sr ratios of 1.11 and 0.7094, respectively. These ratios are very different from those of the atmospheric baseline for the Vosges mountains and the Rhine valley (²⁰⁶Pb/²⁰⁷Pb: 1.158–1.167; ⁸⁷Sr/⁸⁶Sr: 0.719–0.725; ε_Nd: −7.5 to −10.1). However, this study indicates that the baseline of the atmospheric natural Pb and Sr isotopic compositions is affected by anthropogenic (traffic, industrial and urban) emissions even in remote areas. Lichen samples from below the Rhône and Oberaar glaciers reflect the baseline composition close to the Grimsel pass in the Central Swiss Alps (⁸⁷Sr/⁸⁶Sr: 0.714 − 0.716; ε_Nd: −3.6 to −8.1). The ¹⁴³Nd/¹⁴⁴Nd isotope ratios are highly variable (8ε units) and it is suggested that the variation of the ¹⁴³Nd/¹⁴⁴Nd is controlled by wet deposition and aerosols originating from the regional natural and industrial urban environments and from more distant regions like the Sahara in North Africa. The least anthropogenetically affected samples collected in remote areas have isotopic compositions closest to those of the corresponding granitoid basement rocks.     

Geochemical diversity in submarine HIMU basalts from Austral Islands, French Polynesia

We present the first report of geochemical data for submarine basalts collected by a manned submersible from Rurutu, Tubuai, and Raivavae in the Austral Islands in the South Pacific, where subaerial basalts exhibit HIMU isotopic signatures with highly radiogenic Pb isotopic compositions. With the exception of one sample from Tubuai, the ⁴⁰Ar/³⁹Ar ages of the submarine basalts show no significant age gaps between the submarine and subaerial basalts, and the major element compositions are indistinguishable at each island. However, the variations in Pb, Sr, Nd, and Hf isotopic compositions in the submarine basalts are much larger than those previously reported in subaerial basalts. The submarine basalts with less-radiogenic Pb and radiogenic Nd and Hf isotopic compositions show systematically lower concentrations in highly incompatible elements than the typical HIMU basalts. These geochemical variations are best explained by a two-component mixing process in which the depleted asthenospheric mantle was entrained by the mantle plume from the HIMU reservoir during its upwelling, and the melts from the HIMU reservoir and depleted asthenospheric mantle were then mixed in various proportions. The present and compiled data demonstrate that the HIMU reservoir has a uniquely low ¹⁷⁶Hf/¹⁷⁷Hf decoupled from ¹⁴³Nd/¹⁴⁴Nd, suggesting that it was derived from an ancient subducted slab. Moreover, the Nd/Hf ratios of the HIMU basalts and curvilinear Nd–Hf isotopic mixing trend require higher Nd/Hf ratios for the melt from the HIMU reservoir than that from the depleted mantle component. Such elevated Nd/Hf ratios could reflect source enrichment by a subducted slab during reservoir formation.     

Pb-Sr-Nd isotopic compositions and trace element geochemistry of megacrysts and melilitites from the Tertiary Urach volcanic field: source composition of small volume melts under SW Germany

 The Urach volcanic field is unique within the Tertiary–Quaternary European volcanic province (EVP) due to more than 350 tuffaceous diatremes and only sixteen localities with extremely undersaturated olivine melilitite. We report representative Pb-Sr-Nd isotopic compositions and incompatible trace element data for twenty-two pristine augite, Cr-diopside, hornblende, and phlogopite megacryst samples from the diatremes, and seven melilitite whole rocks. The Pb isotopic compositions for melilitites and comagmatic megacrysts have very radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios of 19.4 to 19.9 and plot on the northern hemisphere mantle reference line (NHRL). The data indicate absence of an old crustal component as reflected in the high ²⁰⁷Pb/²⁰⁴Pb ratios of many basalts from the EVP. This inference is supported by ²⁰⁶Pb/²⁰⁴Pb ratios of ∼17.6 to 18.3 and ɛ_Nd of ∼−7.8 to +1.6 for five phlogopite xenocryst samples reflecting a distinct and variably rejuvenated lower Hercynian basement. The ⁸⁷Sr/⁸⁶Sr ratios of 0.7033 to 0.7035 in the comagmatic megacrysts are low relative to their moderately radiogenic Nd isotopic compositions (ɛ_Nd +2.2 to +5.1) and consistent with a long-term source evolution with a low Rb/Sr ratio and depletion in light rare-earth elements (LREE). The melilitite whole-rock data show a similar range in Nd isotopic ratios as determined for the megacrysts but their Sr isotopic compositions are often much more radiogenic due to surface alteration. The REE patterns and incompatible trace element ratios of the melilitites (e.g. Nb/Th, Nb/U, Sr/Nd, P/Nd, Ba/Th, Zr/Hf) are similar to those in ocean island basalts (OIB); negative anomalies for normalized K and Rb concentrations support a concept of melt evolution in the lithospheric mantle. Highly variable Ce/Pb ratios of 29 to 66 are positively correlated with La/Lu, La/K₂O, and Ba/Nd and interpreted to reflect melting in the presence of residual amphibole and phlogopite. The data suggest an origin of the melilitites from a chemical boundary layer very recently enriched by melts from old OIB sources. We suggest that the OIB-like mantle domains represent low-temperature melting heterogeneities in an upwelling asthenosphere under western Europe. Received: 9 March 1995/Accepted: 24 July 1995     

Elemental and isotopic variations in subduction related basalts: evidence for a three component model

Subduction related basalts display wide ranges in large ion lithophile element ratios (e.g., Rb/Ba and Rb/ Sr) which are unlikely to result from mixing, but suggest a role for small degree partial melting of a relatively Rb-poor mantle wedge source. However, these variations do not correlate with other trace element criteria, such as the depletions of high field strength elements (HFSE) and light rare earth elements (LREE) relative to the LILE, which characterise subduction related magmatism. Integration of radiogenic isotope and trace element data demonstrates that the elemental enrichment cannot be simply related to two component mixtures inferred from isotopic variations. Thus a minimum of three components is required to describe the geochemistry of subduction zone basalts. Two are subduction related: high Sr/Nd material is derived from the dehydration of subducted basaltic ocean crust, and a low Sr/Nd component is thought to be from subducted terrigenous sediment. The third component is in the mantle wedge, it is usually similar to the source of MORB, particularly in its isotopic composition. However, in some cases, notably continental areas, more enriched mantle wedge material with relatively high ⁸⁷Sr/⁸⁶Sr, low ¹⁴³Nd/¹⁴⁴Nd and elevated incompatible trace element contents may be involved Mixing of these three components is capable of producing both the entire range of Sr, Nd and Pb isotope signatures observed in destructive margin basalts, and their distinctive trace element compositions. The isotope differences between Atlantic and Pacific island arc basalts are attributed to the isotope compositions of sediments in the two oceans.     

Origin of the Dexing Cu-bearing porphyries,SE China: elemental and Sr–Nd–Pb–Hf isotopic constraints

The Dexing porphyry copper deposit, part of the circum-Pacific porphyry copper ore belt, is the largest porphyry copper deposit in China. We present new LA–ICP–MS zircon U–Pb and molybdenite Re–Os dating, bulk-rock elemental and Sr–Nd–Pb isotopic as well as in situ zircon Hf isotopic geochemistry for these ore-bearing porphyries, in an attempt to better constrain their petrogenesis. LA–ICP–MS zircon U–Pb dating shows that the Dexing porphyries were emplaced in the early Middle Jurassic (～171 Ma); molybdenite Re–Os dating indicates that the associated Cu–Mo mineralization was contemporaneous (～171 Ma) with the igneous intrusion. The rocks are mainly high-K calc-alkaline and show adakitic affinities, including high Sr and low Y and Yb contents, high Sr/Y and La/Yb ratios, and high Mg# (higher than pure crustal melts). These porphyries have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7044?0.7047, ?_Nd(T) values of –1.5 to?+0.6, and ?_Hf(T) (in situ zircon) values of?+2.6 to?+4.6. They show unusually radiogenic Pb isotopic compositions with initial ²⁰⁶Pb/²⁰⁴Pb ratios up to 18.41 and ²⁰⁷Pb/²⁰⁴Pb up to 15.61. These isotopic compositions are distinctly different from either Pacific MORB or Yangtze lower crust but are similar to the subducting sediments in the western Pacific trenches. Detailed elemental and isotopic data suggest that the Dexing porphyries were emplaced in a continental arc setting coupled with westward subduction of the palaeo-Pacific plate. Partial melting involved the subducted slab (mainly the overlying sediments), with generated melts interacting with the lithospheric mantle wedge, thereby forming the investigated high-K calc-alkaline porphyry magmas.     

Effects of basement composition and age on silicic magmas across an accreted terrane-Precambrian crust boundary,Sierra Madre Occidental,Mexico

Trace element and isotopic compositions of mid-Tertiary siliceous magma sequences from two localities of the Sierra Madre Occidental, northern Mexico, display differences that reflect the composition and age of the basement through which they erupted. The crust beneath the section at San Buenaventura is thicker and more evolved and forms part of the North American basement, while that under El Divisadero consists of allochthonous terranes of island arc/oceanic? crust accreted during the Mesozoic.The volcanics are highly differentiated and range in composition from basalt to rhyolite (SiO₂=50–76%). Those erupted through the accreted terranes display a small range of isotope ratios and have lowest initial (age-corrected) Sr isotope ratios (>0.7044) and the highest Nd (<0.5126) and Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb ∼18.9). Isotope ratios of the continental suite are more variable and form an array which trends away from that of the accreted terrane suite toward compositions more typical of old crust (to ⁸⁷Sr/⁸⁶Sr ∼0.710 and ¹⁴³Nd/¹⁴⁴Nd ∼0.5123). The volcanics in the continental zone are relatively more enriched in moderately incompatible elements compared with those within the accreted terranes (Ce/Yb=25–45 vs. 13–33, respectively), but are depleted in some highly incompatible elements such as U and Rb (e.g., Th/U=3.8–7.5 vs. 2.5–4.0, respectively). Those higher in the stratigraphic sections have higher ⁸⁷Sr/⁸⁶Sr, ²⁰⁸Pb/²⁰⁴Pb, and Th/U ratios, and lower ¹⁴³Nd/¹⁴⁴Nd ratios than those lower in the sections.The data have implications for the nature of the sources and the petrogenesis of these volcanics. The isotope ratios of both suites fall between those of mafic magma compositions from the Sierra Madre Occidental, and intermediate and felsic lower crustal xenoliths in northern Mexico and the southwestern USA. The relationship between the isotope ratios of the sequences and the age of the basement, combined with the fact that the overall data set forms well-defined isotopic arrays, demonstrates the strong effects of the crust on the chemistry of the silicic magmas. In the continental suite, isotope ratios covary with Th/Pb and U/Pb ratios, approaching the compositions found in the intermediate and felsic granulite facies xenoliths, strongly indicating that they are not anatectic melts of the lower crust but rather reflect interaction between mantle-derived basaltic parental magmas and the crust. Crustal contributions appear to be large, on the order of 20–70%. The small range of isotope ratios in the accreted terrane suite appears to reflect interaction of the basaltic parent with relatively juvenile crust whose isotopic composition is similar to the mantle-derived magmas. High Th/U and Th/Rb ratios indicate that the crustal contamination occurs in the lower crust. Moreover, the less radiogenic ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios in the continental suite indicate that the depletion in highly incompatible elements in the continental lower crust is an old feature. The secular changes in the isotope ratios within the stratigraphic sections indicate increasingly shallow crustal contributions with time, initially by predominantly mafic deep lower crust and later by more felsic middle crust. Using lavas from outside of the two heavily sampled stratigraphic sections, the differences in the isotopic compositions between volcanics erupted through the accreted terranes and the continental basement help to delineate the location of the boundary.     

Enriched mantle – Dupal signature in the genesis of the Jurassic Ferrar tholeiites from Prince Albert Mountains (Victoria Land, Antarctica)

The major, trace (including rare earth) element abundances, and Sr-Nd-Pb isotopic compositions, have been analysed for andesitic basalt and andesitic sills and lavas of the Jurassic Ferrar Magmatic Province, Prince Albert Mountains, Antarctica. The typical “crustal signature” of the Ferrar magmatism, characterized by relatively high SiO₂, LREE and LILE contents in these samples, is associated with high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd. Systematic correlations of major and trace elements indicate that fractional crystallization was important. However, increases in incompatible elements are positively correlated with initial ⁸⁷Sr/⁸⁶Sr, suggestive of crustal assimilation processes. The observed correlations between initial ⁸⁷Sr/⁸⁶Sr and LREE enrichments have been modelled by an AFC process, starting from the least evolved sample and assuming the compositions of the orthogranulites of Victoria Land as contaminants. The REE patterns of the least evolved Ferrar rocks approach those of E-type MORB, differing only by higher LREE/IREE. The enrichment in LREE, accompanying high initial ⁸⁷Sr/⁸⁶Sr, ²⁰⁷Pb/²⁰⁴Pb and low ¹⁴³Nd/¹⁴⁴Nd compared with E-type MORB, can be explained by interaction of “primary Ferrar basalt” with crystalline basement. We propose a petrological model whereby Ferrar magmas were generated through high degrees of melting of an E-type MORB mantle source, and subsequently these “primary” melts underwent AFC processes inheriting a crustal signature. The Sr-Nd-Pb isotopic compositions required by the AFC model for the primary Ferrar basalt are similar to those of the Dupal signature of the oceanic basalts of the Southern Hemisphere (Hart 1984). Transantarctic Mountains would have been located inside the Dupal anomaly in pre-Gondwana dispersion times. Received: 21 April 1998 / Accepted: 25 January 1999