Pb Hf Isotope Research Articles

Age and geochemistry of volcanic rocks from the Hikurangi and Manihiki oceanic Plateaus

Here we present the first radiometric age data and a comprehensive geochemical data set (including major and trace element and Sr–Nd–Pb–Hf isotope ratios) for samples from the Hikurangi Plateau basement and seamounts on and adjacent to the plateau obtained during the R/V Sonne 168 cruise, in addition to age and geochemical data from DSDP Site 317 on the Manihiki Plateau. The 40Ar/ 39Ar age and geochemical data show that the Hikurangi basement lavas (118–96 Ma) have surprisingly similar major and trace element and isotopic characteristics to the Ontong Java Plateau lavas (ca. 120 and 90 Ma), primarily the Kwaimbaita-type composition, whereas the Manihiki DSDP Site 317 lavas (117 Ma) have similar compositions to the Singgalo lavas on the Ontong Java Plateau. Alkalic, incompatible-element-enriched seamount lavas (99–87 Ma and 67 Ma) on the Hikurangi Plateau and adjacent to it (Kiore Seamount), however, were derived from a distinct high time-integrated U/Pb (HIMU)-type mantle source. The seamount lavas are similar in composition to similar-aged alkalic volcanism on New Zealand, indicating a second wide-spread event from a distinct source beginning ca. 20 Ma after the plateau-forming event. Tholeiitic lavas from two Osbourn seamounts on the abyssal plain adjacent to the northeast Hikurangi Plateau margin have extremely depleted incompatible element compositions, but incompatible element characteristics similar to the Hikurangi and Ontong Java Plateau lavas and enriched isotopic compositions intermediate between normal mid-ocean-ridge basalt (N-MORB) and the plateau basement. These younger (∼52 Ma) seamounts may have formed through remelting of mafic cumulate rocks associated with the plateau formation. The similarity in age and geochemistry of the Hikurangi, Ontong Java and Manihiki Plateaus suggest derivation from a common mantle source. We propose that the Greater Ontong Java Event, during which ∼1% of the Earth’s surface was covered with volcanism, resulted from a thermo-chemical superplume/dome that stalled at the transition zone, similar to but larger than the structure imaged presently beneath the South Pacific superswell. The later alkalic volcanism on the Hikurangi Plateau and the Zealandia micro-continent may have been part of a second large-scale volcanic event that may have also triggered the final breakup stage of Gondwana, which resulted in the separation of Zealandia fragments from West Antarctica.

Ridge subduction and crustal growth in the Central Asian Orogenic Belt: Evidence from Late Carboniferous adakites and high-Mg diorites in the western Junggar region, northern Xinjiang (west China)

The Central Asian Orogenic Belt (CAOB) is a natural laboratory for the study of accretionary tectonics and crustal growth owing to its massive generation of juvenile crust in the Paleozoic. There is a debate, however, on the mechanism of this growth. In the Baogutu area of the western Junggar region, northern Xinjiang (west China), diorite–granodiorite porphyry plutons and dikes are widely associated with Cu–Au mineralization. In this study, we present new results of zircon U–Pb geochronology, major and trace elements, and Sr–Nd–Pb–Hf isotope analyses for two diorite–granodiorite porphyry plutons and two dikes from this area. LA-ICP-MS zircon U–Pb analyses of four plutonic and dike samples yield Late Carboniferous ages of 315–310 Ma. The Baogutu diorite–granodiorite porphyries exhibit low-Fe and calc-alkaline compositions. They are also characterized by high Sr (346–841 ppm) contents, low Y (9.18–16.5 ppm) and Yb (0.95–1.60 ppm) contents, and relatively high Sr/Y (31–67) ratios, which are similar to those of typical adakites. In addition, some samples have relatively high MgO (2.35–8.32 wt.%) and Mg # (48–75), and Cr (22.7–291 ppm) and Ni (32.0–132 ppm) values, which are similar to those of high-Mg andesites. All rock samples exhibit mid-oceanic ridge basalt (MORB)-like Nd–Sr–Pb–Hf isotope features: high ε Nd(t) (+ 5.8–+8.3) and ε Hf(t) (+ 13.1–+15.7) values, and relatively low ( 87Sr/ 86Sr) i (0.7033 to 0.7054) and ( 206Pb/ 204Pb) i (17.842–18.055). The Baogutu adakitic rocks also contain reversely zoned clinopyroxene phenocrysts, which have low MgO cores and relatively high MgO rims. Geochemical modeling indicates that the Baogutu adakitic rocks could have been derived by mixing ~ 95% altered oceanic crust-derived melts with ~ 5% sediment-derived melts. Taking into account the regional geology, I- and A-type granitoids and Cu–Au mineralization, and the presence of Carboniferous ophiolite mélanges in northern Xinjiang, we suggest that the Baogutu adakitic rocks were most probably generated by partial melting of a slab edge close to a subducting spreading ridge in the Late Carboniferous. Ridge subduction and the resultant slab window probably caused strong extension in the overlying lithosphere, extensive melting of subducting oceanic crust, mantle and juvenile lower crust, and interaction between slab-derived melts and the mantle. Thus, events associated with ridge subduction are likely to have played an important role in crustal growth in the CAOB in addition to previously recognized accretion of subduction and arc complexes and post-collisional crustal melting.

Melt–Peridotite Reactions and Fluid Metasomatism in the Upper Mantle, Revealed from the Geochemistry of Peridotite and Gabbro from the Horoman Peridotite Massif, Japan

An investigation of the petrology and geochemistry of peridotites and gabbros in the Horoman massif, Hokkaido, Japan was undertaken to constrain geochemical processes in the upper mantle. Two types of sample were studied: one type comprises peridotites and gabbros forming thin layers varying from a few millimeters to centimeters in scale (thin-layer peridotites and gabbros); the other comprises thick layers (>1 m scale; massive peridotites and gabbros). There is no clear trace element evidence for metasomatism in the thin-layer peridotites. Instead, they have melt–rock reaction textures interpreted in terms of the formation of secondary pyroxene at the expense of primary porphyroclastic olivine and dissolution of primary porphyroclastic pyroxene to form secondary olivine. The thin-layer gabbros also exhibit no metasomatic effects; they have incompatible element depleted trace element characteristics and mid-ocean ridge basalt (MORB)-like isotopic signatures consistent with the presence of a new type of gabbro that previously has not been described from the Horoman Massif. The whole-rock chemistry of the thin-layer peridotites and thin-layer gabbros can be explained by melt–peridotite reactions between isotopically highly depleted MORB mantle (represented by the thin-layer peridotites) and melt with geochemical affinity to Pacific MORB (represented by the thin-layer gabbros). Sm–Nd and Lu–Hf isotope systematics suggest that these reactions might have occurred at ∼300 Ma. Some of the plagioclase lherzolites and all of the spinel lherzolites and harzburgites within the massive peridotites show enrichment in incompatible trace elements and more radiogenic Hf–Nd–Pb isotopic compositions than the incompatible-element depleted thin-layer peridotites. The analyzed massive gabbros are interpreted as subduction-related magmas formed in a MORB-source mantle wedge, which have subsequently interacted with a fluid or melt in the Hidaka subduction zone. Hf–Nd–Pb isotope systematics reveal that this interaction may have occurred at an age younger than ∼50 Ma. Melt- and fluid-related processes occurring in the upper mantle are systematically identified from the samples of the Horoman Massif based on petrography, major and trace element, and Sr–Nd–Pb–Hf isotope geochemistry. These processes occurred in different tectonic settings such as the convecting oceanic mantle and supra-subduction zone mantle wedge and have variably modified the original chemistry of residual mantle protolith, formed by partial melting of a depleted MORB source mantle at ∼1 Ga.

Slab and Mantle Controls on the Sr–Nd–Pb–Hf Isotope Evolution of the Post 42 Ma Izu–Bonin Volcanic Arc

The arc outflux is a hybrid of material fluxes from slab, mantle and crustal sources, whose proportions and composition may vary through time. Here we investigate the causes of temporal variability in the post 42Ma eruptive products of the Izu^Bonin arc in the NW Pacific Ocean basin using Sr^Nd^Pb^Hf isotopes in combination with major and trace element systematics in mafic to evolved arc magmas. Following arc initiation at 50Ma, Izu^Bonin arc formation was continuous except for a short period of quiescence ( 20^23Ma) during back-arc basin formation in the early Miocene.The Sr^Nd^Pb^Hf isotope trends do not co-vary through time. An increasing trend of Pb isotope ratios with time reflects a change in the trench sediment input as it becomes richer in radiogenic Pb. Sr isotopes show no unidirectional trend with time owing to the dominant contribution of unradiogenic Sr from the subducting igneous crust ( 66% of arc Sr) and mantle ( 31%) that buffer the minor amount of radiogenic Sr contributed from subducting sediment or altered oceanic crust. Nd and Hf isotope compositions are both controlled by the depleted sub-arc mantle. Nd isotope ratios increase significantly with time, whereas any temporal increase in Hf isotope ratios remains negligible relative to the inherent data variability. This temporal divergence is due to the much higher parent/daughter ratios of the sub-arc mantle source (Sm/Nd1⁄4 0·24; Lu/Hf1⁄4 0·04), which result in significant ingrowth in Nd but a negligible ingrowth of Hf. Overall, the isotopic evolution is unaffected by along-strike arc rifting and back-arc basin formation, or by the evolution of the siliceous Izu^Bonin arc crust. Compared with the greater isotopic variability of the broader Izu^Bonin^Mariana arc system, the post 42Ma Izu^Bonin arc exemplifies the long-term evolution of a simple arc system with comparatively uniform input from slab and mantle sources. Its isotopic evolution is hence not representative of that of other arcs, and this may reflect the different subduction geometry and components involved.

On the difficulty of assigning crustal residence, magmatic protolith and metamorphic ages to Lewisian granulites: constraints from combined in situ U–Pb and Lu–Hf isotopes

Abstract Zircons from two granulite facies gneisses from the central region of the Lewisian Complex have been investigated by high spatial resolution ion-microprobe U–Pb dating and laser ablation combined Pb–Hf isotope methods. The ion-microprobe data reveal a complex pattern of zircon ages distributed along the concordia curve between the time of granulite facies metamorphism at c . 2.5 Ga and the oldest zircon in each sample (respectively 2.89 Ga and 3.04 Ga). This Pb-loss pattern complicates assignment of an unambiguous magmatic protolith age to the zircon although cathodoluminescence (CL) imaging is used to suggest a preferred age of c . 2.85 Ga for both samples, with older grains being inherited. In situ Hf isotopes show a larger spread in the sample containing older grains which is also consistent with inheritance and further suggests that several crust extraction events are represented in the inherited population. Comparison of Hf isotope compositions with plausible model evolution curves suggests that crustal precursors to the Lewisian granulites were derived from their mantle source at c . 3.05–3.2 Ga. Supplementary material: Analytical methods and data tables are available at: http://www.geolsoc.org.uk/SUP18395

40Ar/ 39Ar eruption ages and geochemical characteristics of Late Tertiary to Quaternary intraplate and arc-related lavas in interior Alaska

Late Tertiary to Quaternary basaltic rocks from three locations in interior Alaska — Fort Hamlin Hills, Prindle Volcano, and Buzzard Creek — have been characterized geochemically using major and trace element concentrations and radiogenic isotopic ratios. The rocks from Fort Hamlin Hills and Prindle Volcano have well-constrained 40Ar/ 39Ar eruption ages ranging from 32.83 ± 0.16 Ma for the Fort Hamlin Hills basalts, to 0.20 ± 0.06 Ma for the Prindle Volcano basanites. Ages for the Buzzard Creek basalts have large errors because of the low K 2O concentration and young ages of the lavas, which we estimate to be < 0.01 Ma. On the basis of Sr–Nd–Pb–Hf isotope and rare earth element data, we have determined that the primary melts for the Fort Hamlin Hills and Buzzard Creek basalts were produced in the lithospheric mantle at spinel facies depths. The Prindle Volcano basanites were produced in asthenospheric mantle in garnet facies conditions. Although involvement in the source components of small volumes of enriched materials related to Late Jurassic to Early Cretaceous flat-slab subduction is not ruled out for the Fort Hamlin Hills basalts and Prindle Volcano basanites. These lavas do not exhibit the arc signature typified by depletion in high-field-strength elements. We conclude that the volcanism at the Fort Hamlin Hills and Prindle Volcano localities in interior Alaska represents intraplate activity due to lithospheric extension accompanied by decompression melting and additional heating. Buzzard Creek basalts, on the other hand, exhibit an arc signature characterized by troughs at Ta–Nb, and peaks at Pb and Sr on element distribution diagrams. These basalts occur in a 400 km volcano gap between the Aleutian arc system and Wrangell Volcanic Province.

Boninites as windows into trace element mobility in subduction zones

Boninites are subduction-related rocks originating from re-melting of highly depleted mantle sources left after extraction of tholeiitic melts. Due to their depleted nature, the incompatible trace element inventory of boninites is virtually entirely inherited from slab components without a significant contribution from the refractory mantle wedge. Thus, boninites constitute an excellent window into processes controlling trace element mobilization at the slab–mantle wedge interface. In order to constrain the behaviour of trace elements in subduction zones with a special emphasis on high field strength elements, we analyzed low-Ca boninites and associated tholeiitic basalts from Cape Vogel, Papua New Guinea (PNG) and compare them with compositions of high-Ca boninites and associated tholeiitic basalts from Cyprus. High-precision HFSE (Nb, Ta, Zr, Hf, W) concentration data of the boninites and associated tholeiitic basalts were obtained by isotope dilution. Major, trace element, and Sr–Nd–Hf–Pb isotope compositions clearly document a significant contribution of slab-derived melts involved in the petrogenesis of the PNG boninites, whereas only fluid-like subduction components were involved in the petrogenesis of the PNG basalts and the Cyprus suite. Low-Ca boninites from PNG are derived from a more refractory mantle source (∼21% depletion) than the high-Ca boninites from Cyprus (∼11% depletion) and their respective tholeiitic precursors (<10% depletion). In agreement with the more depleted nature of their mantle source, boninites exhibit a significantly stronger overprint by slab components. High-precision HFSE data indicate that, in comparison to LILE, a somewhat lower but measurable mobilization of all investigated HFSE in both slab-derived fluids and melts is evident. Modelling calculations demonstrate that the subduction components dominate the LILE budget and also largely control LREE and HFSE abundances in the boninite sources. Notably, the increasing influence of slab-derived fluids results in a decrease of the negative Nb–Ta anomaly, most likely reflecting a similar mobility of Nb–Ta and LREE at higher pressures near the critical point of fluid-melt miscibility. Ratios of Zr/Hf and Nb/Ta in the melt-like slab components dominating in the sources of the PNG boninites were probably fractionated in equilibrium with garnet–amphibolitic mafic oceanic crust. HFSE ratios in the Cyprus boninites are best explained by dehydration of subducted pelagic sediments in the absence of Ti-rich phases such as rutile. Our results also confirm previous assertions that the mobility of HFSE decreases in the order Sb > W–Mo > Nb–Ta > Zr–Hf. Furthermore, Mo–W systematics may provide a potential novel tracer for the amount, composition and redox state of subducted pelagic sediments that contribute to the geochemical budget of intra-oceanic arc systems.

Geochemistry and Sr–Nd–Pb–Hf isotopes of Early Cretaceous basalts from the Great Xinggan Range, NE China: Implications for their origin and mantle source characteristics

Late Mesozoic volcanism is widespread throughout northeastern China, mainly clustering in the Great Xinggan Range, where the major period of volcanic eruption was during the Early Cretaceous. The spatial distribution of volcanic rocks shows a NNE-extending pattern. The lithology of Mesozoic basaltic rocks in the Great Xinggan Range ranges from trachytic basalts, basalts to basaltic andesites. On the basis of lithological associations and spatial relationships, we divide the basalts into four groups, i.e. Tahe, Genhe, Zalute and Xiwu. The Early Cretaceous volcanism in the Great Xinggan Range is characterized by significant large ion lithophile elements (LILE) and light rare earth elements (LREE) enrichment and high field strength elements (HFSE) depletion, with a slightly enriched to depleted Nd–Hf and weakly enriched Sr isotopic compositions (initial 87Sr/ 86Sr = 0.7046–0.7079, ε Nd( t) = + 3.6 to − 6.9 and ε Hf( t) = + 8.9 to − 4.6). The Pb isotopic compositions are also variable with 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb values of 17.791–18.530, 15.477–15.597 and 37.761–38.497, respectively. The Sr–Nd–Pb–Hf isotopic data indicate that the magmatic sources are heterogeneous, showing an enriched to depleted signature of continental lithospheric mantle. The magmatic sources of the Genhe and Zalute groups are characterized by mixing among PM, EM II and DM, whereas the Tahe and Xiwu groups derive from enriched lithospheric mantle that had experienced metasomatism by fluids or melts from subducted Paleo-Asian and Mongol–Okhotsk ocean slabs and possible influence of Precambrian blocks. The Late Mesozoic volcanic rocks in the Great Xinggan Range resemble those of the Cenozoic calc-alkaline magmatism in the Basin and Range Province of the United States. We therefore suggest that the widespread Mesozoic volcanic rocks in the Great Xinggan Range were generated in an extensional tectonic setting.

Mobility of tungsten in subduction zones

Tungsten is highly incompatible and belongs to the high field strength element group (HFSE) that is traditionally regarded as immobile in subduction zone fluids. Due to the low concentrations of W in silicate rocks, a closer assessment of the geochemical behaviour of W in subduction systems was long hampered by analytical limitations. Here we present high-precision concentration measurements of W and other HFSE (Nb, Ta, Zr, Hf) by isotope dilution in combination with incompatible trace element (including Mo and Sb) and Sr–Nd–Hf–Pb isotope measurements. Two well-characterized representatives of subduction-related rock suites from the Solomon Islands and Cyprus were examined. The sources of the Cyprus lavas were mostly enriched by fluids originating from subducted pelagic sediment, whereas Solomon arc lavas originate from mantle sources that have only been overprinted by fluids and melts derived from subducted oceanic crust. Measured Ta/W in both suites (0.3 to 2.9) are always lower than values for MORB (ca. 5), indicating that W is more mobile in subduction components than other HFSE. Ratios of W–Th (up to 1) also extend towards higher values in both suites than MORB values (ca. 0.19). The Cyprus rocks display the highest W/Th, possibly reflecting an initially higher W/Th in subducted sediments. However, the elevated W/Th in Solomon Islands lavas argue for selective W mobilization by slab fluids. Correlations between Ce/Pb, Sb/Ce and W/Th support this model. In the Cyprus lavas, the W enrichment is coupled to Sb, but decoupled from Mo, possibly reflecting different redox conditions. Collectively, our data therefore suggest a selective enrichment of W relative to Th, U, and Ta in the sub-arc mantle via fluids derived from subducted oceanic crust. As subduction zone volcanism significantly contributes to the growth of continental crust, continued recycling of continental material into the sources of subduction zone magmas may have amplified the extent of W enrichment, possibly leading to a higher W/Th and W/U in the Earth's crust compared to the Earth's mantle.

Concurrent Pb–Hf isotope analysis of zircon by laser ablation multi-collector ICP-MS, with implications for the crustal evolution of Greenland and the Himalayas

The in situ Lu–Hf isotope analysis of zircon by laser ablation has emerged as a high-calibre tool for tackling magmatic and crustal evolution. The strength of the approach lies with the ability to target specific zircon growth domains identified by imaging, and thus to unravel polyphase crystallisation histories. However, due to the volume of material being sampled during analysis there remains the possibility of ablation-induced mixing between Hf from domains of different age. Inaccurate Hf isotope ratios and spurious geological interpretations could result. One approach to this problem involves dating the same volume of material analysed for Hf isotopes by concurrently measuring 207Pb/ 206Pb ratios during ablation [Woodhead, J.D., Hergt, J.M., Shelley, M., Eggins, S., Kemp, R. 2004. Zircon Hf-isotope analysis with an excimer laser, depth profiling, ablation of complex geometries, and concomitant age estimation. Chemical Geology 209, 121–135.]. This paper explores the viability of this dual analysis by investigating complex zircons from three different geological contexts, detrital zircons in sedimentary rocks, inherited zircons in granites, and zircons in metamorphosed Eo-Archaean TTG gneisses from Greenland. The implications of the Greenland data for Archaean crustal evolution are discussed in the light of published solution zircon Hf isotope datasets from these gneisses. A case study of detrital zircons from modern river sands in the Himalayas highlights the potential of the technique for providing a rapid, cost-effective picture of crustal evolution that should complement regional bulk rock studies.

Between carbonatite and lamproite—Diamondiferous Torngat ultramafic lamprophyres formed by carbonate-fluxed melting of cratonic MARID-type metasomes

New U–Pb perovskite ages reveal that diamondiferous ultramafic lamprophyre magmas erupted through the Archean crust of northern Labrador and Quebec (eastern Canada) between ca. 610 and 565 Ma, a period of strong rifting activity throughout contiguous Laurentia and Baltica. The observed Torngat carbonate-rich aillikite/carbonatite and carbonate-poor mela-aillikite dyke varieties show a large spread in Sr–Nd–Hf–Pb isotope ratios with pronounced correlations between isotope systems. An isotopically depleted component is identified solely within aillikites ( 87Sr/ 86Sr i = 0.70323–0.70377; εNd i = +1.2–+1.8; εHf i = +1.4–+3.5; 206Pb/ 204Pb i = 18.2–18.5), whereas some aillikites and all mela-aillikites range to more enriched isotope signatures ( 87Sr/ 86Sr i = 0.70388–0.70523; εNd i = −0.5 to −3.9; εHf i = −0.6 to −6.0; 206Pb/ 204Pb i = 17.8–18.2). These contrasting isotopic characteristics of aillikites/carbonatites and mela-aillikites, along with subtle differences in their modal carbonate, SiO 2, Al 2O 3, Na 2O, Cs–Rb, and Zr–Hf contents, are consistent with two distinctive metasomatic assemblages of different age in the mantle magma source region. Integration of petrologic, geochemical, and isotopic information leads us to propose that the isotopically enriched component originated from a reduced phlogopite-richterite-Ti-oxide dominated source assemblage that is reminiscent of MARID suite xenoliths. In contrast, the isotopically depleted component was derived from a more oxidized phlogopite-carbonate dominated source assemblage. We argue that low-degree CO 2-rich potassic silicate melts from the convective upper mantle were preferentially channelled into an older, pre-existing MARID-type vein network at the base of the North Atlantic craton lithosphere, where they froze to form new phlogopite-carbonate dominated veins. Continued stretching and thinning of the cratonic lithosphere during the Late Neoproterozoic remobilized the carbonate-rich vein material and induced volatile-fluxed fusion of the MARID-type veins and the cold peridotite substrate. Isotopic modelling suggests that only 5–12% trace element contribution from such geochemically extreme MARID-type material is required to produce the observed compositional shift from the isotopically most depleted aillikites/carbonatites towards enriched mela-aillikites. We conclude that cold cratonic mantle lithosphere can host several generations of contrasting vein assemblages, and that each may have formed during past tectonic and magmatic events under distinctively different physicochemical conditions. Although cratonic MARID-type and carbonate-bearing veins in peridotite can be the respective sources for lamproite and carbonatite magmas when present as the sole metasome, their concomitant fusion in a complex source region may give rise to a whole new variety of deep volatile-rich magmas and we suggest that orangeites (formerly Group 2 kimberlites), kamafugites, and certain types of ultramafic lamprophyre are formed in this manner.

Petrogenesis of Cogenetic Silica-Oversaturated and -Undersaturated Syenites by Periodic Recharge in a Crustally Contaminated Magma Chamber: the Kangerlussuaq Intrusion, East Greenland

The Palaeogene Kangerlussuaq Intrusion (∼50 Ma) of East Greenland displays concentric zonation from quartz-rich nordmarkite (quartz syenite) at the margin, through pulaskite, to foyaite (nepheline syenite) in the centre; modal layering and igneous lamination are locally developed but there are no internal intrusive contacts. This is an apparent violation of the phase relations in Petrogeny's Residua System. We propose that this intrusion is layered, grading from quartz syenite at the bottom to nepheline syenite at the top. Mineral and whole-rock major and trace element data and Sr–Nd–Hf–Pb isotope data are presented that provide constraints on the petrogenesis of the intrusion. Radiogenic isotope data indicate a continuously decreasing crustal component from the quartz nordmarkites (87Sr/86Sr = 0·7061; ɛNdi = 2·3; ɛHfi = 5·2; 206Pb/204Pbmeas = 16·98) to the foyaites (87Sr/86Sr = 0·7043–0·7044; ɛNdi = 3·8–4·9; ɛHfi = 10·7–11·1; 206Pb/204Pbmeas = 17·78–17·88); the foyaites are dominated by a mantle isotopic signature. The average Mg-number of amphibole cores becomes increasingly primitive, varying from 26·4 in the nordmarkites to 57·4 in the pulaskites. Modal layering, feldspar lamination and the presence of huge basaltic xenoliths derived from the chamber roof, now resting on the transient chamber floor, demonstrate bottom-upwards crystallization. The intrusion cannot, therefore, have formed in a system closed to magmatic recharge. The lack of gneissic xenoliths in the nordmarkites suggests that most contamination took place deeper in the crust. In the proposed model, the nordmarkitic magma formed during crustal assimilation in the roof zone of a large, silica-undersaturated alkali basaltic/basanitic, stratified magma chamber, prior to emplacement in the uppermost crust. The more primitive syenites, terminating with foyaite at the top of the intrusion, formed as a consequence of repeated recharge of the Kangerlussuaq Intrusion magma chamber by tapping less contaminated, more primitive phonolitic melt from deeper parts of the underlying chamber during progressive armouring of the plumbing system.

Simultaneous determinations of U–Pb age, Hf isotopes and trace element compositions of zircon by excimer laser-ablation quadrupole and multiple-collector ICP-MS

We describe an in situ method for simultaneous measurement of U–Pb–Hf isotopes and trace element compositions of zircons using a quadrupole and multiple-collector inductively-coupled-plasma mass spectrometer (Q-ICP-MS and MC-ICP-MS, respectively) connected to a single excimer laser-ablation system. A laser-generated zircon aerosol was split behind the ablation cell into two transport tubes via a Y-shaped connector and simultaneously introduced into the two mass spectrometers. Hafnium isotopes were measured on the MC-ICP-MS instrument, while U–Pb ages and trace element compositions were determined using the Q-ICP-MS. The precision and accuracy of this method was evaluated using six well-known and widely used zircon standards (91500, Temora-2, GJ-1, Mud Tank, BR266 and Monastery). Analyses were carried out using spot sizes of 32, 44 and 60 μm. For the 44 and 60 μm spot, the resulting U–Pb ages, Hf isotopic and rare earth element (REE) compositions of these six zircons agree with recommended/reported values within 2 σ error. The difference in relative standard deviations (RSD) of 206Pb/ 238U ages between split-flow measurements and those obtained separately on the Q-ICP-MS is within ∼ 20% for 91500, Temora-2 and GJ-1, and ∼ 60% for Mud Tank (due to its lower U and Pb concentrations). Our method provides a precise approach for determining the U–Pb age and the Hf isotopic and trace element compositions of zircon within a single ablation event. This is in particular important for analysis of zircons that are small or contain complicated zoning patterns. Finally, the REE composition of zircon BR266 is more homogeneous than other zircons and could be a suitable standard by which to benchmark new standards for microprobe analyses of zircons.

Geochemistry of a new enriched mantle type locality in the northern hemisphere: Implications for the origin of the EM-I source

Late Cretaceous (66.2 ± 0.5 Ma amphibole and 66.7 ± 0.2 Ma phlogopite 40Ar/ 39Ar ages) nephelinitic volcanic rocks from Godzilla Seamount in the eastern North Atlantic (34°N latitude) have trace element and Sr–Nd–Pb–Hf-isotope compositions similar to the Enriched Mantle I (EM-I) endmember, except for their low 207Pb/ 204Pb relative to 206Pb/ 204Pb ratios ( 206Pb/ 204Pb in = 17.7, 207Pb/ 204Pb in = 15.34) plotting below the Northern Hemisphere Reference Line on the uranogenic Pb isotope diagram. O isotope data on amphibole separates are mantle-like ( δ 18O = 5.6–5.8‰). Age and location of the isolated Godzilla Seamount, however, preclude it from being derived from the Madeira or Canary hotspots, making a lower-mantle origin unlikely. Therefore we propose derivation from a shallow (lithospheric/asthenospheric) melting anomaly. As observed in mid-ocean-ridge and ocean-island basalts, there is a systematic decrease of 207Pb/ 204Pb ratios (and Δ7/4) in the individual EM-I endmember type localities towards northern latitudes with Godzilla lying on the extension of this trend. This trend is mirrored in ultra-potassic volcanic rocks such as lamproites and kimberlites, which reflect the composition of enriched subcontinental lithospheric mantle. Therefore, a global pattern in 207Pb/ 204Pb ratios and Δ7/4 is suggested. The geochemical composition of EM-I endmember type localities, including Godzilla lavas, and the enriched (DUPAL) anomaly in the southern hemisphere could reflect derivation from ancient, metasomatized subcontinental lithospheric mantle. We propose a two-stage model to explain the trace element and isotopic composition of the EM-I mantle endmember localities worldwide: 1) during the early history of the Earth, subcontinental lithosphere was metasomatized by melts from subducted slabs along convergent margins generating high μ ( 238U/ 204Pb) sources, and 2) as the Earth cooled, hydrous fluids replaced hydrous melts as the main slab component metasomatizing the subcontinental lithospheric mantle (generating EM-I sources with lower μ). In accordance with this model, the global variations in 207Pb/ 204Pb ratios and Δ7/4 could reflect geographic differences in μ and/or the age at which the transition from stages 1 to 2 took place in the Archaean lithosphere. The model would require a re-definition of the EM-I endmember to low 206Pb/ 204Pb, high 208Pb/ 204Pb (positive Δ8/4) but variable 207Pb/ 204Pb (positive and negative Δ7/4).

The Geochemistry of the Arabian Lithospheric Mantle--a Source for Intraplate Volcanism?

We present trace element and Sr–Nd–Hf–Pb isotope compositions for clinopyroxenes from anhydrous spinel peridotite and garnet ± spinel pyroxenite xenoliths of Pan-African lithospheric mantle from Jordan, including the first high-precision double-spike Pb isotope measurements of mantle clinopyroxene. Clinopyroxenes from the peridotites are variably Th–U–LILE–LREE enriched and display prominent negative Nb, Zr and Ti anomalies. MREE–HREE abundances can generally be modelled as partial melting residues of spinel lherzolite with primitive-mantle-like composition after extraction of 5–10% melt, whereas the enrichments in Th–U–LILE–LREE require a Pan-African or later metasomatic event. The large range of Nd, Sr, Pb and Hf isotope ratios in both peridotites and pyroxenites (e.g. εNd ∼ 1·4–17·5; 206Pb/204Pb ∼ 17·2–20·4; εHf ∼ 0·6–164·6) encompasses compositions more radiogenic than mid-ocean ridge basalt (MORB), and Pb isotopes cover almost the entire range of oceanic basalt values. εHf values are some of the highest ever recorded in mantle samples and are decoupled from εNd in the same samples. Marked correlations between Sr–Nd–Pb isotopes, LILE–LREE enrichments and HFSE depletion suggest that the metasomatizing agent was a carbonatitic-rich melt and isotopic data suggest that metasomatism may have been related to Pan-African subduction. The metasomatic melt permeated depleted upper mantle (<16 kbar) during Pan-African subduction at ∼ 600–900 Ma, and the variably metasomatized material was then incorporated into the Arabian lithospheric mantle. There is no evidence for recent metasomatism (<30 Ma) related to the Afar plume like that postulated to have affected southern Arabian lithospheric mantle. Hf isotopes in the mantle clinopyroxenes are unaffected by metasomatism, and even some strongly overprinted lithologies record ancient (>1·2 Ga) pre-metasomatic Lu–Hf signatures of the depleted upper mantle that was the protolith of the Arabian lithospheric mantle. The ‘resistance’ of the Lu–Hf isotopic system to later metasomatic events resulted in the development of extremely heterogeneous Hf isotopic signatures over time that are decoupled from other isotopic systems. No mantle sample in this study exactly matches the chemical and isotopic signature of the source of Jordanian intraplate basalts. However, the xenolith compositions are broadly similar to those of the source of Arabian intraplate basalts, suggesting that the numerous Cenozoic intraplate volcanic fields throughout Arabia may be the product of melting upper mantle wedge material fertilized during Pan-African subduction and incorporated into the Arabian lithospheric mantle. We propose a model whereby the proto-Arabian lithospheric mantle underwent a major melting event in early Proterozoic–late Archean times (at the earliest at ∼1·2 Ga). Island-arc volcanism and major crust formation occurred during the Pan-African orogeny, which liberated fluids and possibly small-degree melts that migrated through the mantle creating zones of enrichment for certain elements depending upon their compatibility. Immobile elements, such as Nb, were concentrated near the base of the mantle wedge providing the source of the Nb-rich Jordanian volcanic rocks. More mobile elements, such as LILE and LREE, were transported up through the mantle and fertilized the shallow mantle source of the Jordanian xenoliths. Following subduction, the mantle wedge became fossilized and preserved distinct enriched and depleted zones. Lithospheric rifting in the Miocene triggered partial melting of spinel-facies mantle in the lower lithosphere, which mixed with deeper asthenospheric garnet-facies melts as rifting evolved. These melts entrained segments of variably carbonatite-metasomatized shallow lithospheric mantle en route to the surface.

The peculiar geochemical signatures of São Miguel (Azores) lavas: Metasomatised or recycled mantle sources?

The island of São Miguel, Azores consists of four large volcanic systems that exhibit a large systematic intra-island Sr–Nd–Pb–Hf isotope and trace element variability. The westernmost Sete Cidades volcano has moderately enriched Sr–Nd–Pb–Hf isotope ratios. In contrast, lavas from the easternmost Nordeste volcano have unusually high Sr and Pb and low Nd and Hf isotope ratios suggesting a long-term evolution with high Rb/Sr, U/Pb, Th/Pb, Th/U and low Sm/Nd and Lu/Hf parent–daughter ratios. They have trace element concentrations similar to those of the HIMU islands, with the exception of notably higher alkali element (Cs, Rb, K, Ba) and Th concentrations. The time-integrated parent–daughter element evolution of both the Sete Cidades and Nordeste source matches the incompatibility sequence commonly observed during mantle melting and consequently suggests that the mantle source enrichment is caused by a basaltic melt, either as a metasomatic agent or as recycled oceanic crust. Our calculations show that a metasomatic model involving a small degree basaltic melt is able to explain the isotopic enrichment but, invariably, produces far too enriched trace element signatures. We therefore favour a simple recycling model. The trace element and isotopic signatures of the Sete Cidades lavas are consistent with the presence of ancient recycled oceanic crust that has experienced some Pb loss during sub-arc alteration. The coherent correlation of the parent–daughter ratios (e.g. Rb/Sr, Th/U, U/Pb) and incompatible element ratios (e.g. Nb/Zr, Ba/Rb, La/Nb) with the isotope ratios in lavas from the entire island suggest that the Sete Cidades and Nordeste source share a similar genetic origin. The more enriched trace element and isotopic variations of Nordeste can be reproduced by recycled oceanic crust in the Nordeste source that contains small amounts of evolved lavas (∼ 1–2%), possibly from a subducted seamount. The rare occurrence of enriched source signatures comparable to Nordeste may be taken as circumstantial evidence that stirring processes in the Earth's mantle are not able to homogenise material within the size of seamounts over timescales of mantle recycling.

Craton reactivation on the Labrador Sea margins: 40Ar/ 39Ar age and Sr–Nd–Hf–Pb isotope constraints from alkaline and carbonatite intrusives

The once-contiguous North Atlantic craton (NAC) is crosscut by the Labrador Sea that opened during the Early Cenozoic after extensive Mesozoic continental rifting and removal of cratonic mantle. This large-scale structural change within the cratonic lithosphere was followed at about 150 Ma by the cessation of ultrapotassic and potassic-to-carbonatitic magma production, which had prevailed throughout much of the NAC history. At Aillik Bay, a sequence of olivine lamproites (1374.2 ± 4.2 Ma, 2 σ), aillikites/carbonatites (590–555 Ma), and nephelinites (141.6 ± 1.0 Ma, 2 σ) erupted through the southern NAC edge on the present-day Labrador Sea margin. Links between these alkaline magma types with diverse petrogeneses as a consequence of large-scale processes in the lithospheric mantle over a period of 1200 Myr are demonstrated utilizing their Sr–Nd–Hf–Pb isotope compositions. The Mesoproterozoic olivine lamproites are characterized by unradiogenic Nd ( ε Nd( i) = − 8.4 to − 5.4), Hf ( ε Hf( i) = − 11 to − 7.8), and Pb ( 206Pb/ 204Pb ( i) = 14.2–14.8) but moderately radiogenic Sr isotope compositions ( 87Sr/ 86Sr ( i) = 0.7047–0.7062) fingerprinting long-term enriched cratonic mantle, which must have reached to depths of more than 150 km at this time. In contrast, Neoproterozoic carbonate-rich aillikites and carbonatites have fairly radiogenic Nd ( ε Nd( i) = 0.1–1.8), Hf ( ε Hf( i) = − 0.9 to + 2.6), and Pb ( 206Pb/ 204Pb ( i) = 17.5–18.8) but unradiogenic Sr isotope compositions ( 87Sr/ 86Sr ( i) = 0.7033–0.7046) that point to the involvement of convective upper mantle material during melting. Simple binary mixing calculations coupled with the observation that carbonate-rich magmatism prevailed for over 30 Myr in the area imply a complex pattern of lithosphere–asthenosphere interaction at depths between ∼180 and 140 km. The Cretaceous nephelinites have slightly unradiogenic Nd ( ε Nd( i) = − 4 to − 1.4), moderately radiogenic initial 87Sr/ 86Sr (0.7044–0.7062), but initial ε Hf (− 3.3 to + 1.4) similar to the aillikites and highly radiogenic Pb ( 206Pb/ 204Pb ( i) = 19.1–20.2) isotope compositions. Their sodic mafic alkaline nature reflects partial melting at a higher level of the cratonic mantle tapping metasomatic components that had been introduced during the > 30 Myr of Neoproterozoic aillikite/carbonatite magmatism. The new 40Ar/ 39Ar age and Sr–Nd–Hf–Pb isotope data, along with petrological arguments, suggest that at least 30 km of the cratonic mantle beneath the southern NAC edge had been replaced by the hotter upwelling asthenosphere between ca. 550 Ma, when a thick diamond-bearing lithosphere was present, and 150 Ma. This lithospheric thinning presumably occurred shortly prior to Cretaceous continental rifting in response to enhanced plate-tectonic stresses focused at this zone of persistent lithospheric weakness. It appears, however, that the recurrent volatile-rich alkaline magmatism and associated mantle metasomatism played an important role in destroying the structural integrity of the cratonic mantle thereby aiding the subsequent lithosphere thinning.

The role of slab melting in the petrogenesis of high-Mg andesites: evidence from Simbo Volcano, Solomon Islands

The petrogenesis of high-Mg andesites (HMA) in subduction zones involves shallow melting of refractory mantle sources or, alternatively, the interaction of ascending slab-derived melts with mantle peridotite. To unravel the petrogenesis of HMA, we report major, trace element and Sr–Nd–Hf–Pb isotope data for a newly found occurrence of HMA in the New Georgia group, Solomon Islands, SW-Pacific. Volcanism in the Solomon Islands was initiated by subduction of the Pacific plate beneath the Indian–Australian plate until a reversal of subduction polarity occurred ca. 10 Ma ago. Currently, the Indian–Australian plate is subducted northeastwards along the San Cristobal trench, forming the younger and still active southwestern Solomon island arc. However, a fossil slab of Pacific crust is still present beneath the arc. The edifice of the active volcano Simbo is located directly in the San Cristobal trench on top of the subducting Indian–Australian plate. Simbo Island lies on top of a strike-slip fault of the adjacent Woodlark spreading centre that is subducted beneath the Pacific plate. Geochemical and petrological compositions of volcanic rocks from Simbo are in marked contrast to those of volcanic rocks from islands north of the trench (mostly arc basalts). Simbo-type rocks are opx-bearing HMA, displaying 60–62 wt% SiO2 but rather primitive Mg–Ni–Cr characteristics with 4–6 wt% MgO, up to 65 ppm Ni, up to 264 ppm Cr and Mg# from 67 to 75. The compositions of the Simbo andesites are explained by a binary mixture of silicic and basaltic melts. Relict olivine phenocrysts with Fo88–90 and reaction-rims of opx also support a mixing model. The basaltic endmember is similar to back-arc basalts from the Woodlark Ridge. A slab melt affinity of the silicic mixing component is indicated by Gd(N)/Yb(N) of up to 2.2 that is higher if compared to MORB and other arc basalts from the Solomon Islands. 87Sr/86Sr, ɛNd and ɛHf values in the analysed rocks range from 0.7035 to 0.7040, +6.4 to +7.9 and +12 to +14.4, respectively. These values reveal the presence of the Indian–Australian mantle domain beneath Simbo (i.e. the Indian–Australian plate) and also beneath all other volcanic islands of the New Georgia group, which are located north of the San Cristobal trench. 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values (18.43–18.52, 15.49–15.55 and 18.13–18.34, respectively) confirm the presence of slab melts from the subducted Pacific plate beneath southern Simbo where the highest Gd(N)/Yb(N) ratios are reported. A spatial shift towards an Indian–Australian slab signature is observed when approaching the active San Cristobal trench on northern Simbo, reflecting the decreasing influence of slab melts from the old subducted Pacific plate.

Zircon U–Pb age and Hf isotope in granulite-facies rocks from the Dabie orogen: Evidence for archean crustal relict and paleoproterozoic reworking

Zircon U–Pb age and Hf isotope in granulite-facies rocks from the Dabie orogen: Evidence for archean crustal relict and paleoproterozoic reworking

Closed- to open-system differentiation at Arenal volcano (1968–2003)

Arenal volcano, located in northern Costa Rica, has been continuously erupting since 1968. Magmas during the first half of the eruption by volume (Stage 1: 1968–1971) were related by largely closed-system crystal fractionation that had produced a compositionally zoned magma chamber prior to 1968. It erupted downward from the most differentiated magma in 1968 to the most mafic by early 1971. In contrast, the second half of the eruption has been dominated by recharge and compositions have become more evolved with time (Stage 2: 1971–current). We base these conclusions on new major and trace element plus Sr–Nd–Hf–Pb isotope analyses of 56 whole rocks from throughout the eruption. Differentiates are enriched in incompatible elements in both stages, but compatible element concentrations drop much more during Stage 1 than 2. Changes during Stage 1 were successfully modeled using least squares and MELTS models despite the mineral complexity of the rocks. About 19% fractional crystallization of phenocryst phases (plagioclase > orthopyroxene > clinopyroxene > magnetite) is required, consistent with crystallization at 4 kb and from 1145 to 1088 °C of a melt initially containing 2.5 wt.% H 2O at quite oxidizing conditions (QFM + 2). An implication is that most phenocrysts formed during decompression and degassing. Changes during the second stage were successfully modeled using EC-E′RAχFC with the ratio of recharge to crystallization decreasing from 17 to 5 over ∼ 30 years. Crystallization rates (d F S / d t) increase from 0.05 to 0.4%/a from closed- to open-system behavior and are even faster than inferred from U-series disequilibria. The recharging magma results from a smaller degree of flux melting of a mostly similar source than for the resident magma prior to the eruption, with the two events separated by ∼ 450 years. The most recent compositions have no precedent at Arenal.