Hafnium isotopes in Arctic Ocean water

The first isotopic compositions of dissolved hafnium in seawater from across the Arctic Ocean are reported. Most samples from the four sub-basins of the Arctic Ocean have values within error of an average of ε_Hf = +0.8. Combined Hf-Nd isotope compositions do not fall on the well-established positive correlation for mantle and crustal rocks. Instead, Arctic waters have Hf that is more radiogenic than that typically found in rocks with similar Nd isotope compositions, a feature previously found in ferromanganese crusts and waters from the Pacific Ocean. Arctic seawater samples generally fall on the lower part of the ferromanganese crust array, reflecting influences of inputs from Arctic rivers and interactions of shelf waters with underlying sediments. Arctic rivers have much higher Hf concentrations (7-30 pM) than Arctic seawater (0.36-4.2 pM). Water from the Mackenzie River has the least radiogenic Hf, with ε_Hf = −7.1 ± 1.7, and plots furthest away from the ferromanganese crust array, while waters from the Ob, Yenisey, and Lena Rivers have values that are indistinguishable from most Arctic waters. In the Amundsen, Makarov, and Canada basins, Hf concentrations are highest at the surface and lowest in the deeper waters, reflecting the influences of riverine inputs and of waters that have flowed over the extensive Siberian continental shelves and have Nd and Hf characteristics that reflect water-sediment interactions. This is in contrast to the relatively low near surface Hf concentrations reported for locations elsewhere. The Pacific water layer in the Canada Basin exhibits the highest value of ε_Hf = +6.8 ± 1.8, reflecting the Hf isotopic composition of waters entering the Arctic from the Pacific Ocean. Mixing relationships indicate that a substantial fraction of the Hf in the Mackenzie River is lost during estuarine mixing; the behaviour of Hf from other rivers is less constrained.     

Neoproterozoic subducted materials in the generation of Mesozoic Luzong volcanic rocks: Evidence from apatite geochemistry and Hf-Nd isotopic decoupling

A combined study including apatite geochemistry, zircon U-Pb, Lu-Hf isotopes and whole-rock geochemistry including Nd isotopes was carried out for the late Mesozoic volcanic rocks from the Luzong Basin, in the lower Yangtze River region, South China. Whole-rock geochemistry indicates the enrichments of large ion lithophile elements (LILE) and light rare earth elements (LREE) as well as depletions of Nb, Ta and Ti. The extremely low Cl contents in apatites strongly contrast with the rather high-K contents in whole rocks. Potential loss of Cl during syn- and post-magmatic processes having been ruled out, Cl-K decoupling is attributed to be a feature inherited from the primary magma, which indicates the involvement of highly dehydrated sediments and altered oceanic crust in the mantle source. A calculation based on apatite and whole-rock geochemistry further illustrates that the source was composed of four end-members in the perspective of Cl/K, Cl/Nb and F/K ratios. The Hf-Nd isotopes are decoupled for the basaltic trachytes from the lower volcanic sequences in the Luzong Basin, with rather low ε_Hf(t) values (mean = − 10.3) and inconsistent Hf-Nd model ages (Hf ~ 1.8 Ga, Nd ~ 1.3 Ga), which indicate the “zircon effect” that in turn requires the incorporation of continental detritus in the source via subduction. However, Hf and Nd isotopes are nearly coupled for the rocks from the upper volcanic sequences in the Luzong Basin. Late-Mesoproterozoic two-stage Hf and Nd model ages (ca. 1.2 Ga) of rocks from the upper volcanic sequences in the Luzong Basin are similar to those of the Neoproterozoic igneous rocks from the Jiangnan orogen, suggesting their relationship with the same subduction event. Based on the combined apatite geochemistry and Hf-Nd isotopes, this work suggests that the source of Luzong volcanic rocks might incorporate Neoproterozoic subducted slab fragments and detrital sediments that had been blocked in the deep lithospheric mantle below the Luzong area since the Neoproterozoic assembly between the Yangtze and Cathaysia blocks. The partial melting may be triggered by the back-arc lithospheric extension related to the subduction of Paleo-Pacific plate in the late Mesozoic.     

Comparative behavior of Sr, Nd and Hf isotopic systems during fluid-related deformation at middle crust levels

We have carried out a comparative Rb-Sr, Sm-Nd and Lu-Hf isotopic study of a progressively deformed hercynian leucogranite from the French Massif Central, belonging to the La Marche ductile shear zone, in order to investigate the respective perturbation of these geochronometers with fluid induced deformation. The one-meter wide outcrop presents a strongly deformed and mylonitized zone at the center, and an asymmetric deformation pattern with a higher deformation gradient on the northern side of the zone. Ten samples have been carefully collected every 10 cm North and South away from the strongest deformed mylonitic zone. They have been analyzed for a complete major, trace element data set, oxygen isotopes, Rb-Sr, Sm-Nd and Lu-Hf isotopic systematics.We show that most of major and trace elements except SiO₂, alkaline elements (K₂O, Rb), and some metal transition elements (Cu), are progressively depleted with increasing deformation. This depletion includes REE + Y, but also HFS elements (Ti, Hf, Zr, Nb) which are commonly considered as immobile elements during upper level processes. Variations in elemental ratios with deformation, e.g. decrease in LREE/MREE- HREE, Nd/Hf, Th/Sr, increase in Rb/Sr, U/Th and constant Sr/Nd, lead to propose the following order of element mobility: U ? Th > Sr = Nd ? Hf + HREE. We conclude in agreement with previous tectonic and metallogenic studies that trace element patterns across the shear zone result from circulation of oxidizing F-rich hydrothermal fluids associated with deformation. A temperature of the fluid of 470-480 °C can be deduced from the δ¹⁸O equilibrium between quartz-muscovite pairs.Elemental fractionation induces perturbation of the Rb-Sr geochronometer. The well-defined ⁸⁷Rb/⁸⁶Sr-⁸⁷Sr/⁸⁶Sr correlation gives an apparent age of 294 ± 19 Ma, slightly younger than the 323 ± 4 Ma age of leucogranites in this area. This apparent age is interpreted as dating event of intense deformation and fluid circulation associated with mass transfer, and exhumation of the ductile crust shortly after the leucogranite emplacement. Sm-Nd and Lu-Hf isochron-type diagrams do not define any correlation, because of the low fractionated Sm/Nd and Lu/Hf ratios. Isotopic data demonstrate that only the Lu-Hf geochronometer system is not affected by fluid circulation and gives reliable T_DM age (1.29 ± 0.03 Ga) and ε_Hf signatures. By contrast, the Sm-Nd geochronometer system gives erroneous old T_DM ages of 2.84-4 Ga. There is no positive ε_Nd-ε_Hf correlation, because of decreasing ε_Nd values with deformation at constant ε_Hf values. However, ε_Nd-ε_Hf values remain in the broad ε_Nd-ε_Hf terrestrial array, which strongly indicates that fluid-induced fractionation can contribute to the width of the terrestrial array. The strong ε_Hf negative values of the leucogranite are similar to metasedimentary granulitic xenoliths from the French Massif Central and confirm the generation of the leucogranite by several episodes of reworking of the lower crust.     

The origin of enriched mantle beneath São Miguel, Azores

Lavas from the island of São Miguel, Azores Archipelago have long been known to display large radiogenic isotopic variability, that ranges from “depleted” isotopic signatures (e.g. high ε_Nd ∼ +5) in the west, typical of many ocean island basalts, to more “enriched” compositions (e.g. low ε_Nd ∼ +1) in the east. Here, we further characterise the geochemistry of lavas from this remarkable locality, focussing on the nature and origin of the enriched source. Our new isotope data define a striking, linear array in Nd and Hf isotope space that points towards an unusual, enriched composition below the mantle array. This distinctive Hf-Nd isotope signature is associated with elevated values of all three radiogenic Pb isotope ratios. Although the enriched component has certain geochemical similarities to both terrigenous sediments and some samples of the continental mantle lithosphere, such comparisons do not stand closer examination. In the absence of a clear, modern analogue we explore the isotope evolution of some simple, model melt compositions to investigate plausible means of producing an appropriate enriched component. Nd-Hf isotope characteristics provide the tightest constraints and can be reproduced by an ancient (∼3 Ga), modest-degree melt (∼2%) from a garnet peridotite source. Currently, modest-degree melts from garnet-bearing sources are found forming some major oceanic islands. Subduction, isolation and later mixing of small amounts (<5%) of such basaltic material with more ubiquitous ambient mantle can account for the isotopic characteristics of the enriched São Miguel source. Yet the incompatible element ratios of the enriched São Miguel lavas do not show “recycled” signatures of near-surface alteration nor subduction zone dehydration. Thus, we infer that the enriched component was originally under-plated basalt, intruded into oceanic mantle lithosphere rather than forming the island edifice itself. Since the extreme isotope compositions of São Miguel reflect unextraordinary, albeit ancient, magmatic fractionation, the general rarity of such signatures indicates the efficiency of mantle processes in homogenising or hiding similar sources.     

The hafnium isotope composition of Pacific Ocean water

The first Hf isotope data for seawater are reported for a series of stations in the Northwestern Pacific and define a range from ε_Hf = 3.5 ± 1.4 to 8.6 ± 1.6. Most samples have values within error of the average of ε_Hf = 5.9, but significant variations are found in intermediate waters at a depth of 600 m, as well as in deep waters. The Nd and Hf isotope compositions of the deep waters fall within the range of values found for surfaces of hydrogenetic ferromanganese crusts in the region, confirming that Hf in the Fe-Mn crusts has been derived from the overlying water column, which thus provide an archive of past seawater compositions. Although the seawater samples are generally close to the global ε_Nd-ε_Hf correlation obtained from ferromanganese crusts, there are significant deviations from this correlation indicating that there is some additional decoupling between Nd and Hf isotope signals, most likely caused by local water mass mixing and differences in residence times. This is not resolved in the crust samples, which integrate seawater signals over 10⁴ years. The combined use of these two isotope systems in seawater therefore provides an additional dimension for tracing water masses in the oceans. Studies of the distribution of oceanic Hf isotope compositions that have been confined to deep water and boundary waters, as recorded in seafloor ferromanganese crusts, can now be extended and aimed at characterising the entire present-day water column. Average Hf concentrations measured in this study are somewhat lower than previously reported, suggesting a shorter residence time for Hf in the global oceans, although the uncertainty in the extent of Hf removal from the water column during estuarine mixing as well as a lack of data on hydrothermal and dust inputs remains a limit on how well the residence time can be defined.     

Early Archean crustal evolution of the Jack Hills Zircon source terrane inferred from Lu-Hf, Pb/Pb, and δO systematics of Jack Hills zircons

Several lines of isotopic evidence - the most direct of which is from Hadean Jack Hills zircons - suggest a very early history of crust formation on Earth that began by about 4.5 Ga. To constrain both the fate of the reservoir for this crust and the nature of crustal evolution in the sediment source region of the Jack Hills, Western Australia, during the early Archean, we report here initial ¹⁷⁶Hf/¹⁷⁷Hf ratios and δ¹⁸O systematics for <4 Ga Jack Hills zircons. In contrast to the significant number of Hadean zircons which contain highly unradiogenic ¹⁷⁶Hf/¹⁷⁷Hf requiring a near-zero Lu/Hf reservoir to have separated from the Earth’s mantle by 4.5 Ga, Jack Hills zircons younger than ca. 3.6 Ga are more radiogenic than -13ε (CHUR) at 3.4 Ga in contrast to projected values at 3.4 Ga of -20ε for the unradiogenic Hadean reservoir indicating that some later juvenile addition to the crust is required to explain the more radiogenic younger zircons. The shift in the Lu-Hf systematics together with a narrow range of mostly mantle-like δ¹⁸O values among the <3.6 Ga zircons (in contrast to the spread towards sedimentary δ¹⁸O among Hadean samples) suggests a period of transition between 3.6 and 4 Ga in which the magmatic setting of zircon formation changed and the highly unradiogenic low Lu/Hf Hadean crust ceased to be available for intracrustal reworking. Constraining the nature of this transition provides important insights into the processes of crustal reworking and recycling of the Earth’s Hadean crust as well as early Archean crustal evolution.     

Hafnium and neodymium isotopes in surface waters of the eastern Atlantic Ocean: Implications for sources and inputs of trace metals to the ocean

We present hafnium (Hf) and neodymium (Nd) isotopic compositions and concentrations in surface waters of the eastern Atlantic Ocean between the coast of Spain and South-Africa. These data are complemented by Hf and Nd isotopic and concentration data, as well as rare earth element (REE) concentrations, in Saharan dust.Hafnium concentrations range between a maximum of 0.52 pmol/kg in the area of the Canary Islands and a minimum value of 0.08 pmol/kg in the southern Angola Basin. Neodymium concentrations also show a local maximum in the area of the Canary Islands (26 pmol/kg) but are even higher between ∼20°N and ∼4°N reaching maximum concentrations of 35 pmol/kg. These elevated concentrations provide evidence of inputs from weathering of the Canary Islands and from the partial dissolution of dust from the Sahara/Sahel region. The inputs from ocean island weathering are also reflected in radiogenic Hf and Nd isotopes.The Hf isotopic compositions of dust samples themselves are highly variable, ranging between ε_Hf = −20 and −0.6. The combined Hf and Nd isotopic compositions of dust plot close to the “terrestrial array” during periods of appreciable dust load in the atmosphere. During low atmospheric dust loading combined Hf and Nd isotopic compositions similar to seawater are observed. Most of the variability can be explained in terms of variable degrees of zircon loss from the dust samples, which in turn is linked to sorting during atmospheric transport to the eastern Atlantic Ocean and possibly presorting by sedimentary redistribution on the continent. In addition, increasing relative proportions of radiogenic clay minerals with decreasing grain size may contribute to the radiogenic Hf isotopic compositions observed.While the Nd isotopic composition in the surface ocean reflects the Nd isotopic composition of the Saharan dust adjacent to the Sahara/Sahel region, the release of Hf from that dust appears to be incongruent and results in surface ocean Hf isotopic compositions which are ∼10 ε_Hf more radiogenic than the bulk dust. Radiogenic Hf appears to be released from clays and possibly from trace apatite. Rare earth element patterns of dust samples indicate the presence of apatite but provide no evidence for ferromanganese grain coatings, suggesting that such coatings are insignificant in the release of Hf and Nd from Saharan dust to the surface ocean.The Nd isotopic composition of the surface waters becomes less radiogenic south of the equator, most likely reflecting the release of Nd from Congo river sediments. The release of Hf from Saharan dust and the Congo river sediments, however, does not produce distinct Hf isotopic signatures in the surface ocean, implying that the mobile fraction of Hf integrated over large continental areas is isotopically uniform. The Hf isotopic uniformity in the surface ocean means that the limited variability in deep water isotopic compositions is consistent with a short deep water residence time and reflects homogenous continental inputs rather than efficient deep water homogenization.     

An alternative model for silica enrichment in the Kaapvaal subcontinental lithospheric mantle

The Mg- and Si-rich nature of the sub-cratonic lithospheric mantle (SCLM) beneath the Kaapvaal Craton indicates extensive melt depletion, followed by a Si-enrichment process. Six highly silica enriched peridotites from Kimberley containing high amounts of orthopyroxene (Opx) or garnet (Grt) that are locally concentrated in clots, were investigated to constrain the timing and nature of the Si-enrichment process. A clinopyroxene-bearing lherzolite containing an Opx-clot was studied to quantify the effects of recent metasomatism on the Si-enriched samples. Minerals from the lherzolite, together with Opx from harzburgites and Opx- and Grt-clots have Hf-Nd isotope ratios at the time of kimberlite eruption, 90 Ma, comparable to group I kimberlites and are close to trace element equilibrium with kimberlitic melts. This implies the xenoliths underwent major interaction with kimberlitic melts close to the time of kimberlite eruption.Harzburgites and mineral clots record equilibration pressures and temperatures of, respectively, between 3.5-4.3 GPa and 930-1060 °C. The garnets in Opx-clots have low Lu/Hf and ε_Hf(t) −15, whereas garnets from Grt-clots have high Lu/Hf and ε_Hf(t) +10. In contrast, Grt from both Grt- and Opx-clots have low Sm/Nd and ε_Nd −10. The whole rock platinum group element (PGE) concentrations are an order of magnitude higher in the Grt-clot than the Opx-clot. Measured ¹⁸⁷Os/¹⁸⁸Os range from 0.1085 to 0.1222. The Grt-clot bearing sample yields Nd-Hf-Os isotope model ages that suggest formation in the Neoproterozoic (∼650 Ma). In contrast, an Opx-clot yields T_RD ages of 2.8 Ga, which is interpreted as the time of formation of the host harzburgite. The Opx-clots and host harzburgites have comparable Lu-Hf isotope systematics that imply Opx growth at ∼1.3 Ga and hence their formation is not related to the Grt-clots.Garnets from Opx- and Grt-clots have elevated high-field strength element (HFSE) concentrations, and lack HFSE depletion relative to other trace elements with comparable degrees of incompatibility in the mantle (La/Nb < 0.5). In addition, calculated melts in equilibrium with Grt have strongly fractionated REE (Nd/Yb > 300) and HREE depletion (Yb_N < 0.1) suggesting equilibration with a hydrous melt that is more HREE depleted than a kimberlitic melt. Previous models that related Si-enrichment to subduction are inconsistent with the lack of HFSE depletion (La/Nb < 0.5). Therefore the favoured model for Opx- and Grt-clot formation is infiltration of a hydrous melt in a within plate geodynamical environment associated with volcanism in the Mid-proterozoic and Neoproterozoic, respectively. This implies that Si-enrichment of the Kaapvaal SCLM may be a consequence of numerous localised magmatic events rather than a single craton-wide process.     

Nd isotopic composition in the central North Pacific

Three vertical profiles of seawater concentration and isotopic composition of Nd were determined for the western to central North Pacific Ocean.In the subarctic oceanic region, at depths greater than 500 m, one vertical profile of Nd isotopic composition was indistinguishable from most previously reported profiles from here. The data indicate a rather homogeneous Nd isotopic composition in the subarctic oceanic region at middle to deep depths (>500 m). Two stations in the subtropical oceanic region exhibited similar Nd isotopic composition profiles to those previously reported. The maxima ε_Nd values at depths of 800-1000 m (ε_Nd = −3.4 to −2.7), which correspond to the North Pacific Intermediate Water (NPIW), are found at both subtropical stations. This implies a ubiquitous distribution of NPIW showing a radiogenic ε_Nd value in the North Pacific. The subsurface minimum at a depth of ∼200 m, which indicates the penetration of the North Pacific Tropical Water (NPTW) with an unradiogenic Nd isotopic signal, was observed at one station in the western Pacific. This station had much lower ε_Nd than the central station at depths around 5000 m, suggesting the greater prominence of Antarctic Bottom Water (AABW) in the western subtropical Pacific than in the central to eastern subtropical Pacific.Results of a model calculation assuming boundary exchange indicate that the Hawaiian Islands play an important role in supplying radiogenic Nd to the central Pacific, similar to some continental margins.We show that Nd isotopic composition is a versatile tracer for ocean circulation and the geochemical cycle of Nd in the North Pacific. Further studies on the distribution of Nd isotopic composition in the Pacific Ocean, including the Southern Pacific, will better elucidate the circulation and geochemical cycle of Nd in the Pacific.     

U-Pb, Hf and O isotope evidence for two episodes of fluid-assisted zircon growth in marble-hosted eclogites from the Dabie orogen

A combined study of internal structure, U-Pb age, and Hf and O isotopes was carried out for metamorphic zircons from ultrahigh-pressure eclogite boudins enclosed in marbles from the Dabie orogen in China. CL imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain, respectively. The metamorphic zircons have low Th and U contents with low Th/U ratios, yielding two groups of ²⁰⁶Pb/²³⁸U age at 245 ± 3 to 240 ± 2 Ma and 226 ± 4 to 223 ± 2 Ma, respectively. Anomalously high δ¹⁸O values were obtained for refractory minerals, with 9.9 to 21.4‰ for garnet and 16.9‰ for zircon. This indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction-zone metamorphism. Most of the zircons are characterized by very low ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.000001-0.000028, indicating their growth in association with garnet recrystallization. A few of them falling within the older age group have comparatively high ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.000192-0.000383, suggesting their growth prior to the formation of garnet in the late stage of subduction. The variations in the Lu/Hf ratios for zircons can thus be used to correlate with garnet growth during eclogite-facies metamorphism. In either case, the zircons have variable ε_Hf (t) values for individual samples, suggesting that their protolith is heterogeneous in Hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle. Nevertheless, a positive correlation exists between ²⁰⁶Pb/²³⁸U ages and Lu-Hf isotope ratios for the metamorphically recrystallized zircons, suggesting that eclogite-facies metamorphism in the presence of fluid has the identical effect on zircon Lu-Hf and U-Th-Pb isotopic systems. We conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh-pressure metamorphism, whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high-pressure eclogite-facies regime.     

Neodymium isotopes in seawater from the Barents Sea and Fram Strait Arctic-Atlantic gateways

The neodymium concentration, C_Nd, and isotopic composition, ε_Nd, in seawater have been determined in the water column at five sites in the Barents Sea-Fram Strait area where most of the water exchange between the Arctic Ocean and the North Atlantic takes place. In the main Arctic Ocean inflow branch across the Barents Sea the concentration and isotopic composition (C_Nd = 15.5 pmol/kg and ε_Nd = −10.8) are similar to those reported for the northeastern Nordic Seas, which is consistent with this region being a source area for the Arctic inflow. Due to the addition of Nd from Svalbard shelf sediments, the C_Nd in the surface waters above 150 m, in the Fram Strait inflow branch is higher by a factor of 2 and the ε_Nd is shifted to lower values (−11.8).In the stratified Nansen Basin, where cold low salinity water overlies warmer Atlantic water the C_Nd and ε_Nd do not vary with the vertical temperature-salinity structure but are essentially constant and similar to those of the Atlantic inflow throughout the entire water column, down to 3700 m depth, which indicates that the Nd is to a large extent of Atlantic origin.Compared to the Atlantic inflow water, the Nd in the major Arctic Ocean outflow, the Fram Strait, show higher C_Nd in the surface waters above 150 m, and a higher ε_Nd (−9.8) throughout the entire water column down to 1300 m depth. Sources for the more radiogenic Nd isotopic composition in deep water of the Fram Strait outflow most likely involve boundary exchange with sediments on the shelf and slope as the water passes along the Canadian archipelago. River water is a possible source in the surface water but it also seems likely that Pacific water Nd, modified by interactions on the shelf, is an important component in the Fram Strait surface outflow. Changes in the relative proportions of inflow of river water and flow of Pacific water through the Arctic Ocean could thus influence the isotopic composition of Nd in the North Atlantic.     

Zircon U-Pb and Hf isotope constraints on crustal melting associated with the Emeishan mantle plume

SHRIMP zircon U-Pb dates, combined with in-situ Hf isotopic data, provide new constraints on the petrogenesis and protolith of peralkaline, metaluminous and peraluminous intrusions and rhyolitic tuffs in the Emeishan large igneous province, with significant bearing on crustal melting associated with mantle plumes. Syenite and A-type granitic intrusions from Huili, Miyi and Taihe in the center of this large igneous province yield U-Pb dates at ∼260 Ma, consistent with the ages obtained for mafic layered intrusions in the same province. Zircon from these rocks exhibits a wide range of initial Hf isotope ratios (ε_Hf(t) = −1.4 to +13.4), with corresponding T_DM1 of 400-900 Ma. The highest ε_Hf(t) value is only marginally lower than that of depleted mantle reservoir at 260 Ma, suggesting that their source is primarily juvenile crust added during Emeishan volcanism, with incorporation of variable amounts of Neoproterozoic crust. The trigger of crustal melting is most likely related to advective heating associated with magmatic underplating. In contrast, the 255-251 Ma peraluminous granites from Ailanghe and 238 Ma rhyolitic tuff from Binchuan, have negative initial ε_Hf values of −1.3 to −4.4, and of −7.7 to −14, respectively. Hf isotopic model ages and presence of inherited zircons indicate their derivation from Mesoproterozoic and Paleoproterozoic crust, respectively. Given the time lag relative to the plume impact (∼260 Ma) and insignificant mantle contribution to 255-238 Ma magmatism, conductive heating is suggested as the trigger of crustal melting that resulted in formation of delayed felsic magmas. The involvement of older crust in younger felsic magmas is consistent with upward heat transfer to the lithosphere during plume impregnation, if the age of crust is inversely stratified, i.e., changes from Paleoproterozoic to Mesoproterozoic to Neoproterozoic to Permian with increasing depth. Such crust may have resulted from episodic, downward crustal growth during the evolution of the western Yangtze Craton.     

Effects of aridity and vegetation on plant-wax δD in modern lake sediments

We analyzed the deuterium composition of individual plant-waxes in lake sediments from 28 watersheds that span a range of precipitation D/H, vegetation types and climates. The apparent isotopic fractionation (ε_a) between plant-wax n-alkanes and precipitation differs with watershed ecosystem type and structure, and decreases with increasing regional aridity as measured by enrichment of ²H and ¹⁸O associated with evaporation of lake waters. The most negative ε_a values represent signatures least affected by aridity; these values were −125 ± 5‰ for tropical evergreen and dry forests, −130‰ for a temperate broadleaf forest, −120 ± 9‰ for the high-altitude tropical páramo (herbs, shrubs and grasses), and −98 ± 6‰ for North American montane gymnosperm forests. Minimum ε_a values reflect ecosystem-dependent differences in leaf water enrichment and soil evaporation. Slopes of lipid/lake water isotopic enrichments differ slightly with ecosystem structure (i.e. open shrublands versus forests) and overall are quite small (slopes = 0-2), indicating low sensitivity of lipid δD variations to aridity compared with coexisting lake waters. This finding provides an approach for reconstructing ancient precipitation signatures based on plant-wax δD measurements and independent proxies for lake water changes with regional aridity. To illustrate this approach, we employ paired plant-wax δD and carbonate-δ¹⁸O measurements on lake sediments to estimate the isotopic composition of Miocene precipitation on the Tibetan plateau.     

In situ U-Pb, O and Hf isotopic compositions of zircon and olivine from Eoarchaean rocks, West Greenland: New insights to making old crust

The sources and petrogenetic processes that generated some of the Earth’s oldest continental crust have been more tightly constrained via an integrated, in situ (U-Pb, O and Hf) isotopic approach. The minerals analysed were representative zircon from four Eoarchaean TTG tonalites and two felsic volcanic rocks, and olivine from one harzburgite/dunite of the Itsaq Gneiss Complex (IGC), southern West Greenland. The samples were carefully chosen from localities with least migmatisation, metasomatism and strain. Zircon was thoroughly characterized prior to analysis using cathodoluminescence, scanning electron, reflected and transmitted light imaging. The zircon from all but one sample showed only minor post-magmatic recrystallisation. ²⁰⁷Pb/²⁰⁶Pb dating of oscillatory-zoned zircon using SHRIMP RG (n = 142) indicates derivation of the felsic igneous rocks from different batches of magma at 3.88, 3.85, 3.81, 3.80 and 3.69 Ga.Analyses of ¹⁸O/¹⁶O compositions of olivine from a harzburgite/dunite (n = 8) using SHRIMP II in multi-collector mode, indicate that the oxygen isotopic composition of this sample of Eoarchaean mantle (δ¹⁸O_Ol = 6.0 ± 0.4‰) was slightly enriched in ¹⁸O, but not significantly different from that of the modern mantle. Zircon δ¹⁸O measurements from the six felsic rocks (n = 93) record mean or weighted mean compositions ranging from 4.9 ± 0.7‰ to 5.1 ± 0.4‰, with recrystallised domains showing no indication of oxygen isotopic exchange during younger tectonothermal events. δ¹⁸O_Zr compositions indicate that the primary magmas were largely in equilibrium with the mantle or mantle-derived melts generated at similar high temperatures, while calculated tonalite δ¹⁸O_WR compositions (6.7-6.9‰) resemble those of modern adakites.LA-MC-ICPMS zircon ¹⁷⁶Hf/¹⁷⁷Hf analyses were obtained from six samples (n = 122). Five samples record weighted mean initial ε_Hf compositions ranging from to 0.5 ± 0.6 to −0.1 ± 0.7 (calculated using λ¹⁷⁶Lu = 1.867 × 10⁻¹¹ yr⁻¹), while one sample records a composition of 1.3 ± 0.7, indicating the magmas were generated from a reservoir with a time averaged, near chondritic Lu/Hf. The derivation of TTG magmas from a chondritic Lu/Hf source implies either that there was not voluminous continental crustal growth nor major mantle differentiation leading to Lu/Hf fractionation during the Hadean or Eoarchaean, or alternatively that rapid recycling of an early formed crust allowed the early mantle to maintain a chondritic Lu/Hf.Previous studies have demonstrated that ancient TTG rocks were mostly produced by dehydration melting of mafic rocks within the stability field of garnet, probably in flatly-subducted or buried oceanic crust. The oxygen isotopic signatures measured here at high spatial resolution allow the source materials to be better defined. Melting of a mixed mafic source consisting of ∼80% unaltered gabbro (δ¹⁸O_WR = 5.5‰) with ∼20% hydrothermally altered gabbro/basalt (δ¹⁸O_WR = 4.0‰) would produce tonalite magmas within the average compositional range observed. ¹⁸O-enriched components such as altered shallow basaltic oceanic crust and pelagic or continental sediments were not present in the sources of these TTG melts. The absence of high ¹⁸O signatures may indicate either the rarity of low temperature altered sediments, or their effective removal from the down-going slab.     

Paleoenvironmental implications of taxonomic variation among δN values of chloropigments

Natural variations in the ratios of nitrogen isotopes in biomass reflect variations in nutrient sources utilized for growth. In order to use δ¹⁵N values of chloropigments of photosynthetic organisms to determine the corresponding δ¹⁵N values of biomass - and by extension, surface waters - the isotopic offset between chlorophyll and biomass must be constrained. Here we examine this offset in various geologically-relevant taxa, grown using nutrient sources that may approximate ocean conditions at different times in Earth’s history. Phytoplankton in this study include cyanobacteria (diazotrophic and non-diazotrophic), eukaryotic algae (red and green), and anoxygenic photosynthetic bacteria (Proteobacteria), as well as environmental samples from sulfidic lake water. Cultures were grown using N₂, NO₃⁻, and NH₄⁺ as nitrogen sources, and were examined under different light regimes and growth conditions. We find surprisingly high variability in the isotopic difference (δ¹⁵N_biomass − δ¹⁵N_{chloropigment}) for prokaryotes, with average values for species ranging from −12.2‰ to +11.7‰. We define this difference as ε_por, a term that encompasses diagenetic porphyrins and chlorins, as well as chlorophyll. Negative values of ε_por reflect chloropigments that are ¹⁵N-enriched relative to biomass. Notably, this enrichment appears to occur only in cyanobacteria. The average value of ε_por for freshwater cyanobacterial species is −9.8 ± 1.8‰, while for marine cyanobacteria it is −0.9 ± 1.3‰. These isotopic effects group environmentally but not phylogenetically, e.g., ε_por values for freshwater Chroococcales resemble those of freshwater Nostocales but differ from those of marine Chroococcales. Our measured values of ε_por for eukaryotic algae (range = 4.7-8.7‰) are similar to previous reports for pure cultures. For all taxa studied, values of ε_por do not depend on the type of nitrogen substrate used for growth. The observed environmental control of ε_por suggests that values of ε_por could be useful for determining the fractional burial of eukaryotic vs. cyanobacterial organic matter in the sedimentary record.     

Constraints on source-forming processes of West Greenland kimberlites inferred from Hf-Nd isotope systematics

Kimberlites from West Greenland have Hf-Nd isotope as well as major and trace element compositions that are similar to other Group I kimberlites, but that are distinctive in the spectrum of magmas sampled at Earth’s surface. The West Greenland kimberlites have ε_Ndi that ranges from +1.6 to +3.1 and ε_Hfi that ranges from −4.3 to +4.9. The samples exhibit ubiquitous negative Δε_Hfi (deviation from the ocean island basalt ε_Hf-ε_Nd reference line), ranging from −1.8 to −11.2. The kimberlites are characterized by steep heavy rare earth element patterns, positive Ta-Nb anomalies and negative Hf-Zr anomalies. These chemical signals are consistent with the presence of ancient, subducted oceanic crust in the kimberlite source region. In the model we present, dewatering and possibly partial melting of rutile-bearing oceanic crust during subduction results in characteristic trace element patterns in the residual crust. During aging, the Hf-Nd isotopic composition of this dewatered/partially melted EMORB-type crust evolves to negative Δε_Hfi values. Metasomatic fluids derived from this ancient subducted oceanic crust infiltrate and impart their trace element and isotopic signal on proximal peridotitic mantle. Melting of this metasomatized mantle peridotite results in kimberlite magmas.     

The origin of decoupled Hf-Nd isotope compositions in Eoarchean rocks from southern West Greenland

Radiogenic isotope compositions of Hf and Nd are typically coupled in Phanerozoic and Proterozoic mafic rocks due to a similar behaviour of Lu-Hf and Sm-Nd during mantle melting. Eoarchean rocks, for instance those from southern West Greenland, exhibit an apparent decoupling of Hf and Nd isotope compositions. This apparent decoupling may either indicate metamorphic disturbance or, alternatively, mirror early differentiation processes in the silicate Earth. To evaluate the issue, we performed combined measurements of Hf-Nd isotope compositions together with major and trace element concentrations for well preserved >3720 to >3800 Ma old tholeiitic metabasalts and gabbros from the ∼3700 Ma and ∼3800 Ma old terranes of the Isua Supracrustal Belt, southern West Greenland. In contrast to younger mafic rocks, calculated initial εHf-εNd values of the Isua tholeiites show similar spreads and are both near chondritic to strongly depleted (−0.7 to +6.3 and −0.8 to +4.4, respectively), also in contrast to previously reported more depleted signatures in nearby boninite-like metabasalts of the Garbenschiefer unit. An evaluation of alteration effects based on preserved major and trace element arrays reveals pristine magmatic trends and therefore the measured isotope compositions indeed in most cases characterize contrasting Eoarchean mantle sources. In accord with this view, compositions of the Isua metabasalts yield Eoarchean regression ages in Sm-Nd and Lu-Hf isochron spaces, overlapping with emplacement ages inferred from crosscutting relationships with tonalites. Lutetium-Hf systematics of the Isua metabasalts studied here, yield clear isochron relationships. For both terranes, there is some scatter in Sm-Nd space, indicating early disturbance of the Sm-Nd system close in time to the extrusion ages, possibly by seafloor alteration. Trace element compositions of the metabasalts indicate an arc setting and a strong source overprint by melt-like subduction components. It is likely, that the source overprint may have caused partial decoupling of the εHf-εNd values, due to selective addition of Nd as observed in modern subduction settings. In this case, the most radiogenic initial εNd and εHf isotope values characterize the most depleted mantle sources, and less radiogenic values would reflect increased contributions of isotopically more enriched subduction components. However, the most depleted samples still exhibit decoupled Hf-Nd compositions, making a case for the presence of even older mantle heterogeneities. A proposed superchondritic composition of the silicate Earth (SCHEM), however, cannot account for the most depleted sample compositions. Conversely, a depleted upper mantle formed by crystallization of perovskite-rich cumulates in the early Hadean may well explain these observed compositions. A literature survey reveals an overlap in initial Hf-Nd compositions between southern West Greenland TTGs and the metabasalts analyzed here. This overlap suggests a genetic relationship between these lithologies, where the TTGs may have inherited their unusual Hf-Nd compositions from mafic precursors isotopically similar in composition to the Isua tholeiites.     

Variations of Sr–Nd–Hf isotopic systematics in basalt during intensive weathering

The Sr–Nd–Hf isotopic compositions of both saprolites and parent rocks of a profile of intensively weathered Neogene basalt in Hainan, South China are reported in this paper to investigate changes of isotopic systematics with high masses. The results indicate that all these isotopic systematics show significant changes in saprolites compared to those in corresponding parent rocks. The ⁸⁷Sr/⁸⁶Sr system was more seriously affected by weathering processes than other isotope systems, with ε_Sr drifts 30 to 70 away from those of the parent rocks. In the upper profile (> 2.2 m), the Sr isotopes of the saprolites show an upward increasing trend with ε_Sr changing from ~ 50 at 2.2 m to ~ 70 at 0.5 m, accompanying a upward increasing of Sr concentrations, from ~ 10 μg/g to ~ 25 μg/g. As nearly all the Sr of the parent rock has been removed during intensive weathering in this profile, the upward increasing of Sr concentrations in the upper profile suggests import of extraneous Sr. Rainwater in this region, which enriches in Sr (up to 139 μg/L) from seawater, may be the important extraneous source. Thus, the Sr isotopes of the saprolites in the upper profile may be mainly influenced by import of extraneous materials, and the Sr isotopic characteristics may not be retained. In contrast, the ε_Nd and ε_Hf of the saprolites drift only 0–2.6 and 0–3.7 away from the parent rocks, respectively. The negative drifts of the ε_Nd and ε_Hf are coupled with Nd and Hf losses in the saprolites; i.e., larger proportions of Nd and Hf loss correspond to lower ε_Nd and ε_Hf. Compared with the relative high Nd and Hf concentrations of the saprolites, the contributions of extraneous Nd and Hf both from wet and dry deposits of aeolian input are negligible. Thus, the ε_Nd and ε_Hf changes in the profile are mainly resulted from consecutive removal of the Nd and Hf. Calculation indicates that the ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf ratios in saprolites are all significantly lower than their initial values in the parent rock. Simply removing part of the Nd and Hf by incongruent decomposing some of the minerals may not account for this. Fractionation should be happen, which ¹⁴³Nd and ¹⁷⁶Hf may be preferentially removed from the profile relative to ¹⁴⁴Nd and ¹⁷⁷Hf during intensive chemical weathering, resulting in lower ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf ratios in saprolites relative to the parent rock, even though details for this process is not known. A positive correlation is observed between the ε_Nd and ε_Hf of the saprolites. Interestingly, the saprolites with a net loss of Nd and Hf in the upper profile show good positive correlation, and the regression line parallels the terrestrial array. By contrast, saprolites with a net gain of Nd and Hf in the lower profile generally show higher ε_Hf values at a given ε_Nd value, and the regression line between these ε_Nd and ε_Hf appears to parallel the seawater array. This supports the hypothesis that the contribution of continental Hf from chemical weathering release is the key to the obliquity of the seawater array away from the terrestrial array of the global ε_Nd and ε_Hf correlation. Our results also indicate that caution is needed when using ε_Sr, ε_Nd, and ε_Hf to trace provenances for sediments and soils.     

High-precision high field strength element partitioning between garnet, amphibole and alkaline melt from Kakanui, New Zealand

The high field strength elements (HFSE: Zr, Hf, Nb, Ta, and W) are an important group of chemical tracers that are increasingly used to investigate magmatic differentiation processes. Successful modeling of these processes requires the availability of accurate mineral-melt partition coefficients (D). To date, these have largely been determined by ion microprobe or laser ablation-ICP-MS analyses of the run products of high-pressure, high-temperature experiments. Since HFSE are (highly) incompatible, relatively immobile, high-charge, and difficult to ionize, these experiments and their analysis are challenging. Here we explore whether high-precision analyses of natural mineral-melt systems can provide additional constraints on HFSE partitioning.The HFSE concentrations in natural garnet and amphibole and their alkaline host melt from Kakanui, New Zealand are determined with high precision isotope dilution on a multi-collector-ICP-MS. Major and trace element compositions combined with Lu-Hf isotopic systematics and detailed petrographic sample analysis are used to assess mineral-melt equilibrium and to provide context for the HFSE D measurements. The whole-rock nephelinite, ∼1 mm sized amphiboles in the nephelinite, and garnet megacrysts have similar initial Hf isotope ratios with a mean initial ¹⁷⁶Hf/¹⁷⁷Hf_{(34 Ma)} = 0.282900 ± 0.000026 (2σ). In contrast, the amphibole megacrysts are isotopically distinct (¹⁷⁶Hf/¹⁷⁷Hf_{(34 Ma)} = 0.282830 ± 0.000011). Rare earth element D values for garnet megacryst-nephelinite melt and ∼1 mm amphibole-nephelinite melt plotted as a function of ionic radii show classic near-parabolic trends that are in excellent agreement with crystal lattice-strain models. These observations are consistent with equilibrium between the whole-rock nephelinite, the ∼1 mm amphibole grains within the nephelinite and the garnet megacrysts.High-precision isotope dilution results for Zr and Hf in garnet (D_Zr = 0.220 ± 0.007 and D_Hf = 0.216 ± 0.005 [2σ]), and for all HFSE in amphibole are consistent with previous experimental findings. However, our measurements for Nb and Ta in garnet (D_Nb = 0.0007 ± 0.0001 and D_Ta = 0.0011 ± 0.0006 [2σ]) show that conventional methods may overestimate Nb and Ta concentrations, thereby overestimating both Nb and Ta absolute D values for garnet by up to 3 orders of magnitude and underestimating D_Nb/D_Ta by greater than a factor of 100. As a consequence, the role of residual garnet in imposing Nb/Ta fractionation may be less important than previously thought. Moreover, garnet D_Hf/D_W = 17 and D_Nb/D_Zr = 0.003 imply fractionation of Hf from W and Nb from Zr upon garnet crystallization, which may have influenced short-lived ¹⁸²Hf-¹⁸²W and ⁹²Nb-⁹²Zr isotopic systems in Hadean time.     

Late Neoarchean reworking of the Mesoarchean crustal remnant in northern Liaoning,North China Craton: A U–Pb–Hf–O–Nd perspective

How the earth's crust formed and evolved during the Precambrian times is one of the key questions to decipher the evolution of the early Earth. As one of the few cratons containing well-preserved Eoarchean to Neoarchean basement on Earth, the North China Craton is an ideal natural laboratory to unravel the early crustal evolution. It is controversial whether the Archean tectonothermal events in this area represents reworking or growth of the continental crust. To solve this issue, we have compelled field-based mapping, zircon U–Pb dating by SHRIMP RG and LA–ICP–MS U–Pb, zircon SHRIMP SI oxygen and LA–MC–ICP–MS Hf isotope, and whole-rock Nd–O isotope analyses from the Archean granitoids in northern Liaoning, North China Craton. On the basis of zircon U–Pb isotopic dating and measured geological section investigation, two distinct magmatic suites as enclaves in the Jurassic granites are recognized, viz. a newly discovered 3.0 Ga crustal remnant and a 2.5 Ga granitoid. The Mesoarchean zircons from the 3.0 Ga granodioritic gneisses exhibit heterogeneous Hf isotopic compositions, with the most radiogenic analysis (ε_Hf(t) = +3.8) following the depleted mantle evolution array and the most unradiogenic ε_Hf(t) extending down to −3.4. This implies that both ancient continental crust at least as old as 3.4 Ga and depleted mantle contributed to the magma source of the protoliths of the Mesoarchean gneisses. The ε_Hf(t) values of the Neoarchean zircons from these gneisses overlap the 3.4–3.0 Ga zircon evolution trend, indicating that the ancient crustal materials have been reworked during the late Neoarchean. The Neoarchean zircons from the 2.5 Ga granitoids have a relatively small variation in the Hf isotope and are mainly plotted in the 3.0–2.8 Ga zircon evolution field. However, taking all the ε_Hf(t) values of the Neoarchean zircons into the consideration, we find that the Hf model age of the Neoarchean zircon does not represent the time of crustal growth or reworking but are artifacts of magma mixing. The interaction between the magmas derived from the ancient crustal materials and the depleted mantle is also supported by zircon O isotopic data and Hf–O isotopic modeling of the Neoarchean granitoids. Both Mesoarchean and late Neoarchean tectonothermal events involved synchronous crustal growth and reworking, which may be applicable to other parts of the world.