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Coupled oxygen-hafnium isotope and trace element geochemical data were obtained using thirty eight previously dated zircon grains extracted from five mafic to intermediate crustal xenoliths of the Wyoming Craton (Montana, USA). Xenoliths include mid to lower crustal (642–817 °C and 3.5–9.4 kbar) mafic granulites and amphibolites with dominantly Mesoproterozoic (1772–1874 Ma) and minor Paleoproterozoic to Late Archean (2004–2534 Ma) 207Pb/206Pb zircon ages. Zircon oxygen isotope data indicate derivation from melts in equilibrium with a mantle source that interacted with limited supracrustal material (δ18O = 4.4–5.7‰), as well as the incorporation of supracrustal fluids or melts into mantle source regions (δ18O = 6.0–8.1‰). The small within-sample isotopic variability suggests that primary zircon did not exchange with isotopically distinct fluids or melts after initial formation. Initial zircon Hf isotopic values are highly variable across all xenoliths (ε Hf = +3.7 to −17.6), consistent with protolith derivation from mantle sources that incorporated evolved, unradiogenic material or were modified by subduction-related fluids. Within a single granulite xenolith, two zircon types are recognized based on CL imagery, Hf isotopes and U-Pb ages (Type I and Type II). Type I magmatic zircons show dispersed ages (ca. 1700–2534 Ma) and unradiogenic initial Hf (ε Hf = −17.6 to −1.5, 176Hf/177Hf = 0.281074–0.281232). The spread in ages and initial ε Hf , but narrow range in initial 176Hf/177Hf, points to variable Pb loss in response to dissolution-recrystallization of pre-existing zircon. Type II metamorphic zircon yields a younger Proterozoic population (ca. 1700–2155 Ma) with more radiogenic initial Hf relative to Type I zircon (ε Hf = −7.9 to +1.4, 176Hf/177Hf = 0.281427–0.281578); this type represents newly grown metamorphic zircon that formed in the solid-state and incorporated Zr and Hf from pre-existing zircon and silicate matrix/metamorphic phases. REE patterns from all xenoliths are steep and positively sloping without discernible HREE depletion relative to LREE, implying zircon crystallization/recrystallization in the absence of garnet. Negative Eu anomalies signify simultaneous zircon and feldspar crystallization. Solid-state recrystallization may have lead to variations in LREE, Eu and Ce in certain xenoliths. Xenoliths containing magmatic zircon (1834 ± 19 Ma) with mantle-like δ18O (4.4–5.5‰) and radiogenic initial ε Hf (−2.3 to +3.7) likely formed through crystallization of melts derived from a mantle source that incorporated minor amounts of subducted sedimentary/supracrustal material. Proterozoic (1874 ± 8 Ma) xenoliths with elevated δ18O (6.0–7.0‰) and unradiogenic initial ε Hf (−8.2 to −9.6) within magmatic zircon represent melt products of subduction-induced melting and metasomatism of the overlying mantle wedge in the vicinity of the northern GFTZ. Older (ca. 2534 Ma) xenoliths containing zircons with elevated δ18O (6.4–7.2‰) and unradiogenic ε Hf (up to −17.6) represent crystallization of protolith magmas extracted from a mantle source metasomatized by subduction-derived fluids and melts in the Late Archean or earlier. Zircon geochronology and isotope systematics within Mesoproterozoic xenoliths support a model of ocean-closure and subsequent continental collision between the Medicine Hat Block and Wyoming Craton, resulting in the formation of subduction-related melts at ca. 1834–1874 Ma, followed by ca. 1770 Ma collision-related metamorphism thereafter. • Zircon from crustal xenoliths of the Wyoming Craton was investigated using Hf, O and REE isotopic analyses. • Mantle-like δ18O (4.4–5.7‰) and radiogenic εHf (−2.3 to +3.7) in zircon imply mantle-derived protolith magmas. • Elevated zircon δ18O (6–8‰) and unradiogenic εHf (−8.2 to −9.6) are products of subduction-related mantle metasomatism. • Data are consistent with ocean-closure between the Medicine Hat Block and Wyoming Craton at ca. 1.8 Ga. • Collisional metamorphism at ca. 1.7 Ga may recrystallize xenoliths to amphibolite/granulite facies. [ABSTRACT FROM AUTHOR]

The timing for the onset of plate tectonics, along with the secular changes in the tectonic settings of continental crust formation, continue to be debated. Recent interpretations based on the increase in zircon 176Hf/177Hf ratios at the time of crystallisation (expressed as εHf (t) with respect to chondritic evolution) have been used to ascertain changes in geodynamic settings in the early Earth, specifically in the 3.8–3.6 Ga interval. This increase is widely interpreted as a change in magma generation, from source(s) dominated by ancient crust to source(s) dominated by juvenile inputs from the mantle. At issue, Hf isotope variations remain limited in the early Earth due to the long decay of the Lu Hf system. This feature, along with the scarcity of rocks and minerals of Eo/Mesoarchaean and Hadean ages, generate large uncertainties over the nature and the timing of the interactions between mantle and crustal reservoirs. The distinction between mantle and crustal sources becomes much clearer in the Palaeoproterozoic, and the study of ancient terranes with several billion years of protracted crustal evolution may hold the key to unlock complex mantle–crust interactions. Here we investigate high-grade metamorphic rocks from the Anabar shield, which contain zircons that crystallised between the Eoarchaean (oldest core at 3814 ± 16 Ma) and the Palaeoproterozoic (youngest core at 2251 ± 15 Ma and youngest rim at 1910 ± 21 Ma). The combination of in situ U Pb and Hf isotope analyses in zircon indicates the formation of the continental crust in the Siberian Craton in the Eoarchaean, and a conspicuous metamorphic event at 2.0–1.9 Ga. We demonstrate that 2.0–1.9 Ga zircon ages reflect recrystallisation processes under subsolidus conditions, involving the breakdown of high-Lu/Hf minerals (i.e. garnet and pyroxene). The εHf (t) shift at 2.0–1.9 Ga towards more radiogenic values may not be related to a change in magmatic style and sources, but rather to resetting of the Lu Hf system in response to heating and metamorphic reactions on a mineral scale. Our findings challenge the widely-evoked mechanism of changes in tectonic style and magma sources to account for vertical arrays in the εHf (t) versus crystallisation age space. This calls for considering alternative options, including those based on petrographic data, when interpreting Hf isotope variations in the Hadean/Archaean detrital zircon record. [Display omitted] • Evidence for the oldest crust formation in the Siberian Craton at ∼3.8 Ga. • Hf isotope data show two main evolution stages of the Anabar shield: i) decreasing εHf (t) (0 to −15) from 3.5 to 2.0 Ga; and ii) a large εHf (t) variation (−25 to 10) at ∼1.9 Ga. • The breakdown of high-Lu/Hf minerals during high-grade metamorphism releases radiogenic Hf, which is incorporated into new or recrystalised zircon to produce a large εHf (t) range. • Metamorphic breakdown reactions are a complementary mechanism for 'vertical shifts' in εHf (t) , typically attributed to mixing of ancient crust and mantle-derived magmas. • Changes in geodynamic settings in the early Earth are not necessarily a direct cause of abrupt shifts in εHf (t) in detrital zircon datasets. [ABSTRACT FROM AUTHOR]

Eoarchean peridotite enclaves found in southern West Greenland's Itsaq Gneiss Complex (IGC) represent an important and valuable record of Earth's early geodynamic history. However, the origins of these rocks and the processes acting on them in the Eoarchean remain a subject of debate. Some researchers have proposed that these peridotites represent the oldest preserved pieces of Earth's mantle, while others have suggested that they represent ultramafic cumulates. Similarly, the geodynamic context in which they formed and were emplaced is subject to multiple interpretations. Some researchers argue that only vertical tectonic processes operated in the Eoarchean and others contend that these peridotites were embedded in the crust by horizontal tectonic processes. We present multiple sulfur isotope data from IGC peridotite enclaves, offering new insights into these rocks' origins and tectonic processes in the early Earth. Generally small but significant nonzero Δ33S values of 0.04 to 0.21‰ were detected in the studied peridotites. Peridotites with almost no petrographic signs of metasomatic alteration have on average slightly higher Δ33S values, whereas peridotites with clear petrographic and geochemical signatures of melt metasomatism were found to have slightly lower Δ33S values. These Δ33S values point to incorporation of surface-derived material of Archean age or older. Correlations between Δ33S values and previously published major and trace element data support the view that these peridotites were subject to hydrous melt depletion of incompatible elements, followed by variable melt re-enrichment. Notably, a distinct correlation also exists between previously published Hf isotope data and Δ33S values, indicating a depleted mantle source for melt percolating through the peridotites in the Eoarchean. The sulfur isotope data demonstrate that these processes included the introduction of surface-derived sulfur, pointing to an early onset of crustal recycling. These findings point to the existence of depleted mantle domains in the Hadean and suggest that crustal recycling processes operated during the Eoarchean or earlier. • 3.81 Ga peridotites from southern West Greenland contain a MIF-S signal • Sulfur isotope values indicate incorporation of surface-derived S in the Archean • Sulfide minerals crosscut by amphiboles show that sulfides predate metamorphism • Correlation between εHf and Δ33S values indicate mantle depletion and re-enrichment • This is evidence for Eoarchean recycling of surface material into the mantle [ABSTRACT FROM AUTHOR]

The Geysers Plutonic Complex (GPC) in the California Coast Ranges underlies one of the world's premier geothermal resources. The GPC consists of three major intrusive phases, orthopyroxene-biotite microgranite porphyry, orthopyroxene-biotite granite, and hornblende-biotite-orthopyroxene granodiorite (in sequence of emplacement). These are nearly coeval and compositionally equivalent to lavas of the overlying Cobb Mountain Volcanic Center which includes (from older to younger) rhyolite of Alder Creek, dacite of Cobb Mountain, and dacite of Cobb Valley. Zircon from GPC plutonic and associated volcanic rocks were analyzed at high spatial resolution for their trace element abundances along with oxygen and hafnium isotopic compositions. Pronounced negative Eu anomalies and high incompatible trace element (e.g., Y, Hf, and U) abundances in zircon along with low Ti-in-zircon temperatures reveal that the GPC microgranite porphyry magma was more evolved than GPC granite and granodiorite. Isotopically, GPC microgranite porphyry, granite, and granodiorite zircon crystals closely overlap (δ18O = +4.76 to +9.18; εHf = +1.4 to +10.7), but a subpopulation with elevated δ18O (∼8.05) and lower εHf (∼4.4) is only present in GPC granite and granodiorite. Zircon from coeval volcanic units share this dichotomy, with a dominant population at (δ18O = +4.92 to +9.38; εHf = +3.4 to +11.3) and a generally minor population with elevated δ18O and low εHf, which is particularly prominent in dacite of Cobb Valley. Anticorrelated δ18O and εHf values indicate progressive assimilation of Franciscan Complex basement rocks, but fractional crystallization was decoupled from crustal assimilation. Together with published U-Th-Pb geochronology, these data reveal distinct degrees of crust-mantle interaction and constrain the thermochemical conditions during different stages in the evolution of a composite intrusive body that fed the GPC and contemporaneous volcanoes. A thermal model for such a body emplaced at ∼7 km depth quantitatively matches the GPC zircon age distribution when magma accumulation started at low recharge rates (0.1 km3/ka), intermittently peaked during a brief flare-up (4 km3/ka for 50 ka), and then returned to a low recharge flux (0.1 km3/ka). This model also qualitatively explains the initial presence of small-volume, highly evolved melts forming the microgranite porphyry, followed by massive emplacement of less evolved magmas forming the GPC granite-granodiorite complex. During this second stage, crustal assimilation locally intensified due to prior thermal priming of the country rock. The total injected magma volume into the upper-crustal reservoir between c. 2.1 and 1.1 Ma amounts to ∼300 km3, which is about three to four times the known volume from geothermal well penetration into the GPC. The accumulation of large volumes of silicic magma along the western North American continental margin in the wake of the northward migrating Gorda slab edge thus appears limited to a brief (c. 50 ka) pulse of high magma influx from the mantle that was pre- and post-dated by protracted low-flux magmatism. [ABSTRACT FROM AUTHOR]

The eventual docking of an exotic continental mass over the course of a Wilson cycle is marked by intracontinental sites of prograde metamorphism, contractional deformation, and syn- or post-collisional magmatism. This study shows that inherited premagmatic zircon in subsequently emplaced granitoid rocks provides another line of evidence for the collisional orogeny. Following the continental collision between the North and South China Cratons, Jurassic granitoids intruded widely throughout the central Korean Peninsula in association with the subduction of the Paleo-Pacific plate. The latest Early to Middle Jurassic (177–167 Ma) granitoids occur throughout the cratonic Gyeonggi Massif, whereas Early Jurassic (194–184 Ma) granitoids occur locally along the Okcheon Metamorphic Belt to the southeast of the massif. These two spatiotemporal groups differ profoundly in their age patterns and O-Hf isotopic signatures of inherited zircon. The younger group yields a typically North China-like Archean-Paleoproterozoic (mostly 2.5–1.8 Ga) zircon inheritance. In contrast, inherited zircons from the older group exhibit exclusively Neoproterozoic (ca. 0.9–0.7 Ga) and Paleozoic (ca. 0.4 Ga) cluster along with Archean-Paleoproterozoic populations. The exotic feature of the protolith of the older group is further indicated by significantly lower δ 18 O (− 0.9 to + 4.9‰) and positive εHf (+ 3 to + 15) values of the Cryogenian zircon cores. The results of zircon analysis demonstrate that South and North China-like terranes were juxtaposed infracrustally along the southeastern margin of the Gyeonggi Massif during the pre-Jurassic collisional orogeny, and then selectively provided major source material to the older and younger Jurassic granitoids, respectively. [ABSTRACT FROM AUTHOR]

All other factors being equal, the calculation of zircon Hf two stage model ages (T DM Hf) depends on the particular Lu/Hf value assumed for the magmatic source, the effect being more pronounced as the age difference between zircon and magmatic source increases. It is generally considered that the Lu/Hf measured in the zircon-hosting rock does not represent the composition of the source because of potential garnet or zircon fractionation. Accordingly, most authors either assume a single fixed value for Lu/Hf source , often Lu/Hf ≈ 0.079 to 0.108, or use two alternative models, one for felsic sources, often Lu/Hf ≈ 0.09, and the other for mafic sources, often Lu/Hf ≈ 0.165. In contrast with these opinions, however, here we show that partial melting of peraluminous sources causes little decoupling of Lu from Hf because of similar solubilities of zircon and monazite. Furthermore, the effects of residual garnet are largely compensated by the numerous zircon inclusions that garnet and other residual minerals almost always contain. Partial melting of metaluminous sources may significantly decouple Lu from Hf if allanite and/or titanite are not present in the source, but the effect decreases as the melt fraction increases. Similarly, fractional crystallization of metaluminous magmas may decouple Lu from Hf if amphibole or clinopyroxene begin to crystallize before zircon saturation. The Lu/Hf distribution in 4784 rocks from different regions and ages is lognomal rather than normal, and the calculated medians, i.e. the maximum of the probability density function for the logarithmically transformed Lu/Hf, are Lu/Hf mafic rocks ≈ 0.08, Lu/Hf felsic rocks ≈ 0.05, i.e. notably lower than the above-mentioned felsic and mafic magmatic source averages. Magmatic sources may be remarkably heterogeneous with respect to Lu/Hf. Our calculations show that fixed Lu/Hf source values translate the Lu/Hf heterogeneity of the source to the T DM Hf thus producing an artificial distribution of model ages that may be erroneously interpreted as different episodes of crustal growth. Therefore, we propose that the best strategy to calculate two stage Hf model ages of zircon is to use the analytically determined whole-rock Lu/Hf ratio as a proxy of the source. In the case of detrital or inherited zircons, for which no whole-rock information is available, it is advisable first to determine whether they come from a mafic or felsic rock by interpreting cathodoluminescence images, Th/U ratios and other chemical parameters, and then venture an estimate of the Lu/Hf source from the SiO 2 average. [ABSTRACT FROM AUTHOR]

It is generally agreed that decoupling of the Hf and Nd isotope systems in modern aqueous systems is a result of incongruent release of Hf during terrestrial weathering of the continental crust, although the mechanism(s) behind this process are not yet fully understood. We here present Hf–Nd isotope data for the Neoarchean Krivoy Rog Banded Iron Formation (BIFs), Ukraine, and combine observations on modern aqueous environments with those of the early Earth to further evaluate the mechanism(s) behind Hf–Nd isotope decoupling in aqueous systems. The pure Late Archean Krivoy Rog chemical sediment endmember, representing the dissolved pool of ancient seawater, shows decoupled εNd 2.60Ga -εHf 2.60Ga values of −2.3 and +9.48, respectively, and suggests that decoupled Hf–Nd isotopes had been a global rather than a local phenomenon in Neoarchean seawater. This further reveals that incongruent Hf release via terrestrial weathering and erosion of emerged and evolved continental landmasses were widespread geological processes by Late Archean time. Impure Krivoy Rog BIF samples, composed of a mixture of seawater-derived and detrital Nd and Hf, show systematically more positive εNd 2.60Ga values, but still reveal decoupled εNd-εHf values relative to an associated schist that plots slightly below the “terrestrial array”. This suggests that mineral sorting between a zircon-bearing sand-sized fraction and fine-grained sediment particles occurred on/in Late Archean continents, rivers and oceans, and had significant impact on the chemical compositions of the suspended and dissolved element loads of Late Archean seawater. Less radiogenic Hf isotope compositions in the Krivoy Rog seawater relative to detritus-contaminated BIFs further suggest a pathway for high-temperature hydrothermal Hf into anoxic Archean seawater, that diluted the even more radiogenic Hf isotopic composition of continental run-off, created by the mineralogical composition of the continental hinterland and the “zircon effect”. Alternatively, the less pronounced decoupling of εHf-εNd in Late Archean seawater may be related to a shorter residence time of Hf relative to Nd. Furthermore, systematically more positive initial εNd values in detritus-contaminated Archean BIFs relative to respective dissolved seawater loads suggest that weathered and eroded material of (ultra)mafic rock suites had significant impact on the suspended and dissolved fractions in Archean seawater. [ABSTRACT FROM AUTHOR]

Five Hadean (> 4 Ga) aged zircon grains from the Jack Hills metasedimentary belt have been investigated by a secondary ion mass spectrometry scanning ion image technique. This technique has the ability to obtain accurate and precise full U-Pb systematics on a scale < 5 μm, as well as document the spatial distribution of U, Th and Pb. All five of the grains investigated here have complex cathodoluminescence patterns that correlate to different U, Th, and Pb concentration domains. The age determinations for these different chemical zones indicate multiple reworking events that are preserved in each grain and have affected the primary crystalized zircon on the scale of < 10 μm, smaller than conventional ion microprobe spot analyses. In comparison to the spot analyses performed on these grains, these new scanning ion images and age determinations indicate that almost half of the spot analyses have intersected several age and chemical domains in both fractured and unfractured parts of the individual crystals. Some of these unfractured, mixed domain spot analyses have concordant ages that are inaccurate. Thus, if the frequency of spot analyses intersecting mixed domains here is even close to representative of all other studies of the Jack Hills zircon population, it makes the interpretation of any trace element, Hf, or O isotopic data present in the literature tenuous. Lastly, all of the grains analysed here preserve at least two distinguishable 207 Pb/ 206 Pb ages. These ages are preserved in core-rim and/or complex internal textural relationships in unfractured domains. These secondary events took place at ca. 4.3, 4.2, 4.1, 4.0, 3.7, and 2.9 Ga, which are coincident with previously determined statistically robust age peaks present in this zircon population. [ABSTRACT FROM AUTHOR]

This study presents solution and laser ablation analysis methods suited to the determination of the Lu-Hf and U-Pb isotope signatures of small volumes of zircon corresponding to sub-nanogram amounts of Hf. A reduced-volume approach to laser ablation is taken with consecutive U-Pb and Lu-Hf isotopic analyses resulting in a total pit depth of 18 μm using a 25 μm laser ablation spot. This results in excavation of ~ 40 ng of zircon, equating to ~ 0.3 ng of Hf and uncertainties ~ 1 ε Hf (2 s ). The laser technique presented here is ideally suited to complex, finely-zoned zircons for which drilling to depth would intersect different zones. The possibility of reducing the total laser ablation pit depth to ~ 10 μm is demonstrated by decreasing the integration time for Hf isotope analysis without serious compromise of the uncertainty. Furthermore, the capability to determine the Hf isotope composition of the same amount of Hf using solution MC-ICP-MS is also demonstrated, as is the suitability of analyzing solutions not subject to Hf-HREE separation. For both solution and laser ablation methods, this study investigates possible methods of Yb interference correction, the potential for matrix effects, and the accurate determination of 176 Lu/ 177 Hf. Using the approaches described here, acceptable uncertainty levels are achieved to resolve complexity at the level of 25 × 18 μm (diameter × depth), and therefore this method has the potential to yield geologically meaningful results for rocks containing complexly-zoned zircons. [ABSTRACT FROM AUTHOR]

Early to mid Carboniferous I-type granites distributed in a broad meridional belt west of Sydney, southeastern Australia, represent the last phase of granite magmatism in the southern Lachlan Fold Belt. Rare inherited zircon in the granites, in combination with zircon precipitated from the melt phase of the magmas, provides direct evidence of the nature and age of the source rocks from which the granites were derived, and the isotopic compositions of those rocks. Most granites from the north and central parts of the belt, represented by the Wuuluman, Home Rule, Oberon and Lett plutons (334 ± 3, 328 ± 4, 339 ± 2 and 328 ± 2 Ma, respectively), are characterized by scarce inherited zircon dominantly of Siluro-Devonian age, consistent with all being derived from a common zircon-poor source rock of mid to late Devonian age. Based on the isotopic compositions of the igneous zircon rims, that source was relatively homogeneous and immature (δ 18 O zrn 6–7‰, εHf(t) 0–+4). Two samples of the Tarana pluton (331 ± 2 Ma), near the centre of the belt, show evidence for heterogeneity in that source and the presence of a more evolved component that is also more zircon rich. Together with the Lett pluton from the same geochemical suite, their zircon rim compositions define a mixing array between more and less evolved end members (δ 18 O zrn 5.5–9.5‰, εHf(t) -4–+4). Only in the southern Chapmans Creek pluton (327 ± 3 Ma) is there clear evidence that the source of the granite also contained a minor component with a similar older protolith to the LFB early Palaeozoic sediments that host most of the LFB pre-Carboniferous granites. Presence of that component, which overwhelmingly dominates the inheritance in the pre-Carboniferous granites, is also reflected in a slightly elevated whole rock initial 87 Sr/ 86 Sr (> 0.7050) and igneous δ 18 O zrn (ca. 7.5‰), but not in the igneous zircon εHf(t) (ca. +2). The amount of restitic zircon in granite is determined by not only the proportion of restite present but also the zircon contents of the various source rock components. The process of restite unmixing does not necessarily mean that more mafic, restite-rich granites will contain more inherited zircon. [ABSTRACT FROM AUTHOR]

Thermochemical convective instabilities in the mantle, often referred to as mantle plumes, cause mantle melting that give rise to ocean island basalts (OIB) in intraplate settings. The array of radiogenic isotope signatures in global intraplate OIB indicate that although plumes are individual entities, they share enriched components that resemble various parts of subducted crust and a hypothetical mantle matrix termed "focal zone" (FOZO). Each plume is expected to rise with an individual buoyancy flux, thereby producing variable volumes of melting with some OIB forming subaerial islands, whereas others produce submarine volcanoes. Here, we report the first radiogenic isotope data (Sr, Nd, Pb and Hf) for lavas of Lord Howe Island (LHI) in the Tasman Sea, the most prominent subaerial expression of the hypothesised Lord Howe mantle plume. Major element data are consistent with a moderate degree of partial melting, and heavy rare earth element depletion indicates melting occurred in the presence of garnet, consistent with other global plume lavas. Radiogenic Sr Nd isotopic data are similar to those defining the relatively primitive FOZO component with no clear enriched mantle affinity. The nearby Tasmantid Seamounts are also sourced from a mantle plume, and have similar Sr Nd character. However, combined 208Pb/204Pb and 207Pb/204Pb vs 206Pb/204Pb ratios of LHI lavas are inconsistent with a FOZO-type source. Instead, Pb isotopes overlap with typical enriched mantle 1 (EM1) lavas (e.g., Pitcairn, Tristan da Cunha), trending slightly higher than the Northern Hemisphere Reference Line (NHRL). Hafnium isotopes follow trends observed in lavas of the archetypal EM1 Pitcairn-Gambier islands, and are partially decoupled from 143Nd/144Nd; this decoupling is rare among OIB. The combined element-isotope data indicate that LHI forms the most recent expression of a mantle plume track in the Tasman Sea. Albeit unique in its 208Pb*/206Pb* isotope code among global mantle plumes, the resemblance of LHI lavas with the spatially related and contemporaneous Taupo seamount of the parallel Tasmantid plume track in the Tasman sea supports a genetic relation of both plumes. The proposed common origin of the two parallel plume tracks requires a larger plume underpinning the Tasman sea with individual plume fingers rising towards the surface, possibly in a triple plume network considering the on-shore Cosgrove track in eastern Australia. The subaerial expression of LHI contrasts the submarine Tasmantid Seamounts; this expression is only possible due to the continental ribbon that forms the island's foundation, suggesting secondary plume fingers display similar low buoyancy fluxes. Lord Howe Island is predominantly composed of basaltic rocks, which are created when mantle underneath the crust melts and the subsequent magma rises to the surface and erupts out of a volcano. The chemical composition of these volcanic rocks can provide us with valuable information about geologic processes. Moreover, we can compare the composition of islands like Lord Howe to better investigate and understand volcanic networks that exist deep within the Earth. The composition of Lord Howe Island basalts was investigated for the first time using specialised instruments that can separate individual elements into different isotopes. The ratios of these different isotopes reflects the composition of the mantle source which can be enriched by the presence of subducted crustal rocks, oceanic rocks and sediments. Our data indicate the distinctive reservoir that Lord Howe Island samples were derived from is the same reservoir as that of the nearby Taupo Seamount volcano. We infer from this that a piece of ancient crust is stuck in the mantle, and the two volcanoes are sampling it. This study supports a theory of a large, linked volcanic network under the Tasman Sea, and adds to the growing body of research investigating 'double-chain' volcanism in the Pacific and Atlantic oceans. [Display omitted] • Lord Howe Island is characterised in Sr-Nd-Hf-Pb radiogenic isotope space for the first time • Our results show isotopic similarities between both the Tasmantids and Lord Howe Seamount Chains in the Tasman Sea • We infer the two Tasman Sea plume-trails likely derive from a similar isotopic reservoir. • Our findings support the theory of plume-branching and double-chain volcanism in the Pacific Ocean. [ABSTRACT FROM AUTHOR]

Serpentinized abyssal peridotites are the product of the chemical interaction between the Earth's oceans and the peridotitic upper mantle. This reaction is dominated by the addition of water and fluid mobile elements to the peridotite, with implications on the cycling of water and fluid mobile elements in subduction zones. Rare earth elements (REE) were generally thought as immobile during serpentinization. However the identification of Nd (and Sr) isotopic disequilibrium between fresh clinopyroxene and their host bulk serpentinized peridotite suggest addition of Nd and other light REE to the peridotite during serpentinization ( Frisby et al. (2016) ). As with the REE, the high-field strength elements (HFSE) have also been considered immobile and non-reactive during serpentinization. However, this has yet to be explicitly tested. Here we present a modified dissolution and extraction method that improves on the yields of REE and HFSE from highly depleted ultramafic rocks (peridotites) through the use of B(OH) 3 during digestion dry-down steps, Fe-precipitation for REE-HFSE sequestration, and liquid–liquid exchange for Fe removal. Using this method we report on the Nd and Hf isotope compositions of bulk rock serpentinized abyssal peridotites from the South-West Indian Ridge (SWIR), Mid-Atlantic Ridge (MIR) and Mid-Caymen Rise (MCR) and on samples where their clinopyroxene Hf, Nd isotopes were previously reported. The bulk rock Hf isotopes are equal to clinopyroxene to within 1 ε Hf , while Nd isotopes in the bulk peridotite are up to 8 ε Nd units less radiogenic than their clinopyroxene. Melt-rock reaction between the depleted peridotite endmember and local enriched lavas cannot generate the observed decoupling between Hf and Nd isotopes. Our data is instead consistent with seawater — peridotite interaction as this results in significant shift in Nd isotopes with little to no change in Hf isotopes until extremely high water/rock mass ratios of > 100,000. Our data shows that Hf isotopes are not affected by serpentinization and that Hf isotopes in bulk peridotites better record the magmatic composition of peridotites than Nd or Sr. The largest Nd isotopic difference between clinopyroxene and bulk peridotite is observed in the most LREE-depleted samples, while refertilized samples show little change. Based on the bulk rock (serpentinized) and clinopyroxene (magmatic value) Nd isotope compositions we estimate that 1–34% of the bulk rock Nd budget is seawater derived, but only 1–6% of Sm and < 2% of Lu and Hf in these samples. Therefore serpentinization will lower the Sm/Nd relative to Lu/Hf ratio in a peridotite and this may lead to decoupling between Hf and Nd isotopes upon recycling and aging, assuming no additional fractionation of these elements during the subduction process. Alternatively, if Nd becomes fluid-mobile upon dehydration during subduction then the excess Nd in the serpentinized peridotites may become an additional reservoir for Nd in arc volcanism. [ABSTRACT FROM AUTHOR]

Protocols for solution nebulization (SN) multi collector (MC) ICP-MS analysis of the Hf isotopic composition of accessory minerals (zircon and baddeleyite) selected for high-precision ID-TIMS U Pb geochronology have improved significantly during the last two decades. Precise crystallization age and Hf isotopic composition from the exact same volume of mineral can be retrieved, provided that time-consuming sample preparation and purification is possible. The present work focuses on the analytical capabilities of SN-MC-ICP-MS for Hf isotope analysis on non-purified reference materials. Analytical issues arise for (i) igneous and metamorphic zircons, often enriched in trace elements and REEs, which increase the amount of isobaric interferences. (ii) Small minerals that challenge the limits of quantification of the apparatus. JMC475 Hf standard, Plešovice and Temora-2 zircons and Phalaborwa baddeleyite were used to characterize the sources of analytical bias and to set up an optimized protocol for Hf isotope analysis of small accessory minerals with accurate correction of the isobaric interferences and accounting for the matrix effects. Doping of JMC475 with major (Zr, Si), trace (U, Th, Y, Ti) and rare earth elements (Ce, Nd, Gd, Dy, Ho, Er) shows that oxide species produced in the ICP induce an offset of the measured 176 Hf/ 177 Hf ratios that is corrected by normalization to the reference value. The 176 Yb isobaric interference is efficiently removed for samples with Yb/Hf ratios < 0.6 using in-run determined 176 Yb/ 173 Yb reference ratios from Yb doped reference material. This isotopic ratio varies (i) with time and (ii) sample matrix. Plasma loading by Zr is not the source of these variations. We propose that Yb oxidation in the plasma torch is responsible for variations in the 176 Yb/ 173 Yb ratio. Our dataset shows that matrix matched standards should be preferred for data reduction, and that Plešovice is a good zircon standard because of its availability, quantifiable REEs composition and potential for wide range of Yb doping. Consequently, low Hf concentration solutions from Temora-2 zircons were accurately measured in a small sample volume (< 120 μl), using a short acquisition protocol (50 × 1 s cycles), and sample-standard bracketing for mass bias coefficient determination in a [Hf]~ 10 ng/g Plešovice solution measured through 80 × 4 s cycles. The repeatability of the measurement (2SD) is improved by factors of up to ~ 4.4 and the limit of quantification is brought down to < 2 ng/g Hf, corresponding to zircon minerals of < 24 ng. [ABSTRACT FROM AUTHOR]

Zircon Hf isotope compositions provide valuable constraints on granite petrogenesis and the evolution of continental crust. However, the causes of variations in zircon Hf isotope ratios during magma evolution are poorly understood. Here we compiled zircon U Pb ages and Lu Hf isotope compositions of the Permian-Triassic granites in South China to demonstrate that Hf isotope variability is common in the early stages of granitic magma evolution, but Hf isotope ratios are homogenized during the final stages of granite crystallization. Late-grown zircons have relatively high concentrations of U and have ε Hf (t) values approximating the median values of multiple analyses for the low-U zircon in the same samples. Hafnium isotope variations measured in low-U zircon could be related to incremental assembly of multiple isotopically distinct magma batches derived from partial melting of heterogeneous crustal rocks; these discrete magma batches may have remained isolated from each other during segregation and extraction in the source and during ascent through the crust. After emplacement, these magma batches may have experienced variable degrees of mixing resulting in homogenization of Hf isotopes in late crystallized zircon. Coupled with whole-rock geochemical data of the Permian-Triassic granites, we argue that the Hf isotope homogenization is related to crystallization of zircon from late-stage magmas or precipitation of hydrothermal zircon from exsolved hydrothermal fluids during the final stages of magma solidification. Hafnium isotope homogenization in late-stage melts/fluids may have been driven by an increased concentration of fluorine and the efficient diffusion of hafnium-fluoride complexes through silicic melt. These results provide a deeper understanding of the mechanisms that contribute to Hf isotope heterogeneity in zircon during granite crystallization. • 1206 zircon Hf isotopes from 73 granite samples from South China were compiled. • Hf isotopic ratios become less varied with increasing U concentrations. • Hf isotopic ratios were homogenized with magma evolution. • F-rich solutions probably play an important role in the homogenization process. [ABSTRACT FROM AUTHOR]

The composition of the mantle source from which continental crust is ultimately derived informs both the timing and rate of continental growth. This composition is, however, poorly constrained during the first billion or so years of Earth's evolution. Typically in zircon Hf isotope studies, a linearly evolving depleted mantle back-calculated from modern MORB-source mantle to chondrite at 4.57 Ga has been used to calculate model 'crust formation' ages, which are themselves used in crustal growth models. Yet, an increasing number of studies of Eo- to Paleoarchean continental crust suggest crust extraction from a relatively undifferentiated mantle. Zircon Hf isotope studies of ca. 2.9 Ga gneisses from the Neoarchean Lewisian Gneiss Complex of Scotland have previously revealed initial Hf isotope compositions that suggest derivation from such an undepleted mantle source. Here, we present combined zircon U-Pb and Lu-Hf isotope data from seventeen representative grey gneiss samples from across the Lewisian of the Scottish mainland and the Outer Hebridean archipelago, ranging in age from 3.1 to 2.7 Ga. Regardless of location and exact age, initial Hf isotope signatures are predominantly near- to slightly supra-chondritic. Several models are investigated to explain this observation, our favoured one involving episodic extraction of the grey gneiss precursors from a mildly depleted mantle that began to diverge from chondritic composition at ca. 3.5 Ga. Samples from the southern Outer Hebrides record slightly more radiogenic initial Hf signatures, consistent with a possible terrane boundary along the Paleoproterozoic South Harris shear zone. This study provides further confirmation that domains of Hadean to Paleoarchean, and possibly even locally Mesoarchean, mantle remained chondritic with respect to Hf isotopes. Initiation of a depleted mantle source at this later time and its limited divergence from chondritic mantle, even into the Neoarchean as suggested here, has implications for crustal growth models based on detrital zircon Hf model ages. On a regional basis, the signatures recorded in our data are consistent with contemporaneous gneisses across Fennoscandia and East Greenland and the mildly depleted mantle source of continental crust furthermore persists into the Mesoproterozoic. [ABSTRACT FROM AUTHOR]

The Turkish–Iranian Plateau grew after the Middle Miocene following the initial Paleogene Arabia–Eurasia collision. Authors attribute uplift to break-off of the southern Neo-Tethys slab beneath the Bitlis–Zagros Suture at ~ 15–10 Ma, coupled with continued plate convergence and regional crustal shortening. Since this time there has been an upsurge in mantle-derived collision magmatism over large parts of NW Iran, Eastern Anatolia and the Lesser Caucasus, potentially hundreds of kilometres from the site of southern Neo-Tethys slab break-off, > 10 Myr after the proposed break-off event. Whole rock elemental and Sr–Nd–Pb–Hf isotope data are presented for < 3 Ma trachy-basalt to trachy-basaltic andesite lavas erupted in Armenia in the South Caucasus. Samples formed by < 5% melting of fertile subduction-modified spinel-facies lithospheric mantle, and few display elemental or isotopic evidence for contamination by the 45-km thick Mesozoic–Paleogene arc crust or South Armenian Block continental crust. Recent magmatic activity in Armenia may not be a direct consequence of southern Neo-Tethys slab break-off 300–450 km away, beneath the Bitlis Suture. Late Miocene break-off of a second (northern Neo-Tethys) slab beneath the Pontide Arc may have allowed asthenospheric upwelling over a wider area than was affected by southern Neo-Tethyan break-off. However, whole-scale delamination of mantle lithosphere is ruled out due to the modest degrees of partial melting, a lack of asthenospheric components and limited crustal involvement in magmatism. Small-scale sub-lithospheric convection may be complementary to break-off, causing localised removal of lithospheric mantle and aiding the occurrence of melting for a significant time interval after the break-off event(s). Collision magmas such as those in Armenia represent mantle-derived additions to continental crust, enriched in incompatible elements but with Th/La ratios ≤ 0.2, much lower than those calculated for continental crust (0.25–0.3). Collision magmatism in Turkic-style orogens must be balanced by infra-crustal recycling and delamination to produce bulk continental crust. [ABSTRACT FROM AUTHOR]

Oceanic sediments deposited on continental margins consist mainly of erosion products of the nearby exposed continental areas. Detrital input usually dominates their geochemical budget, and the composition of these sediments should record potential changes in their continental sources. However, along margins, mineral sorting associated with transport and sedimentary processes induces significant chemical and isotopic fractionation over few tens of kilometres. The study of margin sediments should help to quantify the extent of modification of the continental terrigenous supply when it reaches deep oceans. Reported Sr, Nd, Hf and Pb isotopic compositions of fifty-seven sediments from the northernmost part of the Cascade forearc (Ocean Drilling Program, ODP, Sites 888 and 1027) suggest the involvement of two dominant end-members coming from the nearby Canadian Cordillera. Erosion products of the depleted, western part of the Cordillera dominate the detrital input, while the eastern enriched terranes of the Cordillera contribute only 10 to 28% of the input. There is no marked change of provenance of sediments during the last 3.5 Myr and they all appear unaffected by glacial-interglacial climate cycles. The average isotopic compositions of the two sites are slightly different, but are both dominated by continental signature; these values can be used in future studies to identify any subducted sediment contribution to the Cascades Arc. On a finer scale, there are differences in the isotopic signature between samples dominated by clay minerals and those with coarser lithologies. For a given Nd isotopic composition, fine sediments have more radiogenic Sr and Hf isotope ratios than sands, and we interpret the difference as resulting from mineral sorting during transport of the particles. Fine sediments concentrate minerals with radiogenic Sr and Hf such as clays and micas, while coarse-grained detritus carry the unradiogenic mineral component of a given source rock through plagioclase-epidote and zircon. ODP Site 1027 is located 100 km further away from continent than ODP Site 888 and contains more clay. As a consequence, it has significantly more radiogenic Sr and Hf bulk composition than ODP Site 888. Similar differences in isotopic signatures related to distance to continent certainly occur in other areas in the word, and will account for a large part of differences known between continental sources and deep-sea sediments. [ABSTRACT FROM AUTHOR]

The Camamu basin is related to the breakup of Gondwana and belongs to the group of sedimentary basins that are part of the eastern Brazilian passive margin. We have determined the provenance of the precursor sediments by investigating the petrography of fluvial and eolian units from the early-rift Brotas Group and syn-rift Almada Group, coupled to in situ laser ablation MC-ICPMS U–Pb and Lu–Hf isotopic analyses of the detrital zircon grains. Detrital zircon grains from the Sergi Formation, Brotas Group, have a main population of U–Pb ages of 480–600Ma with smaller populations of ages of 900–1000Ma, 1900–2100Ma, 2500–2800Ma and 3000–3200Ma. The Almada Group zircon grains are characterized by a distribution of ages predominantly in the Paleoproterozoic and Archean. The Hf TDM model ages of the Brotas and Almada Group zircon grains range from ca. 1.1 to 4.0Ga, and the εHf(t) values vary from −31.4 to +9.6, providing evidence that some detrital zircon grains in the Camamu Basin were derived from juvenile sources but that the majority have reworked crustal sources or formed from juvenile magmatic rocks that were contaminated by older crustal material. Whole rock Sm–Nd isotope data collected previously are largely consistent with the new U–Pb, Lu–Hf zircon data but do suggest that zircon-poor detritus from the Neoproterozoic to Eopaleozoic Brasiliano Araçuaí-West Congo Belt contributed to particularly the Brotas Group sediments more than would be indicated by the zircon data alone. Thus, integrated petrographic, U–Pb, Lu–Hf zircon and Sm–Nd whole rock studies provide very important clues to the provenance history of the initial sedimentation on Camamu basin passive margin, where the early-rift phase is marked by the input of detritus from the Araçuaí-West Congo belt situated south of the basin, whereas syn-rift sedimentation received major contributions from Paleoproterozoic (Transmazonian cycle) to Archean (Jequié cycle) São Francisco craton units, located to the west. [ABSTRACT FROM AUTHOR]

Invited Technical Paper: High quality analytical data are essential for the development of sound scientific interpretations. To ensure the quality of the data published in Chemical Geology, a new type of contribution has been introduced - the "Invited Technical Paper". When a particular technical issue needing discussion or clarification is identified, specialists in the technique will be invited to share their expertise. As is true for all articles published in the journal, these contributions will be subject to peer review before publication. The first topic to be treated concerns in situ Hf isotopic measurements in zircon by laser ablation inductively coupled plasma mass spectrometry. This is an extremely powerful and promising technique that has been adopted by many laboratories throughout the world. However, if insufficient care is taken during analysis and data processing, inaccurate results can be obtained, notably because of the existence of very large isobaric interferences on the isotope of interest. To address this issue, we have asked Christopher Fisher, Jeffery Vervoort and John Hanchar to provide a set of guidelines that can be adopted to assure that reliable Hf isotopic data are obtained by this technique. Abstract Over the past decade, the Hf isotope composition of zircon, as determined in situ by laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS), has been applied increasingly to a wide range of geological problems and has proven to be a valuable analytical tool. There has been no uniformity, however, in the reporting of in situ Hf isotopic data and, quite often, reviewers and readers of papers are not provided with sufficient information to assess data quality. The goal of this invited contribution is to provide the non-specialist with a brief outline and explanation of what is required for proper presentation and documentation of in situ Hf isotopic data from zircon, including details of how the challenging large isobaric interference corrections have been made. In addition, we discuss a number of potential pitfalls vis-à-vis the assignment of the incorrect age to the measured Hf isotope composition. Non-specialists should be aware of these important issues when doing their own analyses and evaluating the analyses done by others. [ABSTRACT FROM AUTHOR]

Abstract: Abundant volcanic ash layers, largely altered to clay, characterize the Permian–Triassic boundary beds of the Daxiakou section, South China. Volcanism may have had an important role in the Permian–Triassic mass extinction, so it is important to understand the origin of these ash beds. The lithology of ash bed 260, as constrained by modes of crystal fragments (66.7% plagioclase, 7.2% alkali feldspar, and 26.1% quartz), suggests that the ash represents dacitic volcanism. The U–Pb ages, trace elements and Hf-isotope compositions of zircons from ten ash beds have been analyzed using LA-ICPMS and LA-MC-ICPMS. The zircons can be divided into two groups: magmatic zircons (~97%, 227–279Ma) and inherited zircons (~3%, 678–2424Ma). Magmatic zircons have Y, Hf, Th and U contents and Nb/Ta ratios typical of zircons from silicic calc-alkaline volcanism. εHf(t) values of magmatic zircons vary from −10.9 to +5.3, implying magmatic mixing between juvenile crustal material (probably Neoproterozoic in age) and ancient crustal material (probably Archean). Beds 252–259-b have relatively depleted Hf-isotope compositions with εHf(t) values of −10.9–+5.3 and averages of −5.0–−2.5, while other beds have εHf(t) values of −10.8–+4.4 (mostly −10.8–−3.5) and averages of −8.9–−6.8, implying more input of juvenile crustal material in the volcanism of Beds 252–259-b. Integration of the Hf-isotope and trace-element compositions of magmatic zircons suggests that the volcanism producing the ashes took place along the convergent continent margins during the formation of the Pangea supercontinent. This intense silicic volcanism may have played an important role in causing the Permian–Triassic mass extinction in South China. [Copyright &y& Elsevier]

Abstract: Adakites, or adakitic rocks, in a broad sense, have been used to describe a large range of igneous rocks with the common feature of high Sr/Y and La/Yb ratios that can be achieved though different mechanisms. Among them, the continental, or C-type, adakitic rocks are particularly controversial in terms of their sources and genesis. In this study we revisit both Cu–Au ore-bearing and barren “C-type adakitic rocks” in the Lower Yangtze River Belt (LYRB) of central eastern China, including comprehensive analyses of their in-situ zircon Hf–O isotopes, whole-rock geochemistry and Sr–Nd isotopes. These “C-type adakitic rocks” consist of monzodiorite, granodiorite and quartz monzonite that are classified as shoshonitic to high-K calc-alkaline series according to their chemical compositions. They are characteristically high in potassium (K2O=2.4–4.5%, K2O/Na2O=0.6–1.3), with continental crust-like isotopic compositions, i.e., whole-rock εNd(T)=−3.9 to −7.7, initial 87Sr/86Sr=0.7054–0.7085, zircon εHf(T)=0 to −11, and δ18O=6‰ to 9‰. The ore-bearing and barren rocks are cogenetic. Fractional crystallization of hornblende, titanite, magnetite and apatite played a major role in their chemical variations, with the ore-bearing rocks being more felsic (SiO2 =63.3–69.6%) and higher in Sr/Y (41.2–75.6) than the barren rocks (SiO2 =57.3–65.0%, Sr/Y=30.4–51.8). All these geochemical and isotopic features, in combination with regional geological data, suggest that the LYRB “C-type adakitic rocks” were unlikely to have been formed by melting of either a thickened and/or delaminated lower continental crust, or an altered oceanic crust as previously thought. These rocks are in general akin in geochemistry and isotopes to the Archean sanukitoids and the Setouchi high-Mg andesites in Japan, and are thus interpreted as being formed by melting of an enriched mantle source metasomatized by dewatering from a delaminated flat-slab. The flat subduction of an oceanic plateau and its subsequent delamination and foundering since the early Mesozoic beneath southeastern China (Li and Li, 2007) thus not only explain the temporal and spatial propagation of widespread Yanshanian igneous rocks regionally since ca. 195Ma, but also the formation of a series of enigmatic “adakitic” rocks in the region, including the LYRB potassium-rich rocks that were inappropriately called the “C-type adakitic rock” by previous workers. [Copyright &y& Elsevier]

Abstract: The lifetimes and thermal histories of upper crustal plutons are increasingly determined using geochronology, but complex growth of datable minerals in magmas impedes simple age interpretations. Careful field observation helps constrain zircon U–Pb dates in terms of timing of magma injection, for example because relative ages of successive magma pulses must be honored. We use ID-TIMS U/Pb zircon geochronology and field geology to construct timescales of incremental pluton assembly in the St-Jean-du-Doigt (SJDD) bimodal layered intrusion (Brittany, France). Field evidence suggests that early pulses were injected into a cold environment with little supersolidus interaction among successive magma pulses. Later injections occurred in a progressively hotter environment with protracted mafic and felsic magma interaction. Zircon dates show that early activity ca. 347Ma predates the thermally mature episode by about 1Ma, which terminated at ca. 345Ma. Dates from samples displaying core-rim zircon overgrowths span about 5Ma (351–346Ma), which we interpret to represent two distinct crystallization events. Hf isotopic data from cores and rims are homogeneous, precluding zircon inheritance from basement rocks. These textures and dates could instead reflect zircon saturation fluctuations at the emplacement depth, or antecrystic zircon grains recording pre-emplacement magmatic growth. [Copyright &y& Elsevier]

Abstract: The Earth''s upper mantle is commonly depicted as a “marble cake” assemblage of a refractory component mingled with varying amounts of geochemically diverse enriched components. This depleted component controls the major element composition of Mid-Oceanic Ridge Basalts (MORBs). It has long been considered a fairly uniform reservoir, depleted by early melting and well homogenized by subsequent convective stirring. We present new Hf, Nd, Pb and Sr isotopes and trace element data for basalts from the center of the Lucky Strike segment of the Mid-Atlantic Ridge (MAR). Despite the limited size of our sampling area (1.6km along-axis by 5km across), these data show a large range of isotopic compositions (similar to that of the whole MAR). It has recently been shown that part of the reason for not observing more segments with correlated Hf–Nd isotope systematics may be the lack of fine-scale sampling. Our data confirm this idea and show strong correlations between all isotope ratios. The systematics of Hf–Nd isotopes in Lucky Strike basalts define an atypical correlation, distinct from the global mantle array, with an anomalously high εHf for a given εNd. We illustrate that this atypical trend is the signature of an ancient refractory mantle rather than the product of kinetic processes during the melting event. The existence of this mantle component at Lucky Strike allows us to discuss the structure of the upper mantle around the Azores. [Copyright &y& Elsevier]

Abstract: The Chilungshan andesite belongs to the Chilung Volcano Group in northern Taiwan and constitutes the western end of the island chains of the Ryukyu arc. Chemically it is a normal, medium-K calc-alkaline andesite. SHRIMP dating of magmatic zircons yielded an emplacement 206Pb/238U age of 1.04±0.06Ma with εHf(t) of 6.1 to 13.4 and δ18O of 5.02 to 6.28‰. The zircons contain inherited cores and felsic to andesitic silicate-melt (glass) inclusions besides apatite, quartz, K-feldspar, biotite, garnet, Fe-Mg minerals (pyroxene or hornblende) and plagioclase with variable An values. Plagioclase phenocrysts also contain quartz, K-feldspar and felsic melt inclusions. This study confirms the presence of micro-scale heterogeneity in the andesite as a result of mixing between mantle-derived and continental material. Contamination of continental crust occurs either within the magma chamber at a medium to deep crustal level and/or during dehydration and melting of a subducted oceanic slab and overlying sediments in an island-arc environment. [Copyright &y& Elsevier]

Abstract: Detrital zircon grains from Lower Paleozoic sedimentary rocks in the Yucatan Peninsula have an age distribution characterized by major probability peaks at ~1.0, ~1.2, and ~1.5Ga (Martens et al., 2010). Here, we present new Lu–Hf data (MC–ICPMS) paired with U–Pb ages (ID-TIMS) for additional zircon grains from the same rocks. This analytical approach yields precise information about the time and geochemical environment of zircon growth, which in turn helps to distinguish between different crustal source regions that just happen to host zircon populations of similar age. In addition, single zircon grains from granitoids that intruded the sedimentary rocks were dated by U–Pb laser ablation MC–ICPMS and ID-TIMS, and their Hf-isotope compositions were determined by solution MC–ICPMS. The zircon data are complemented by Sm–Nd analyses of the sedimentary and igneous whole rocks. The Yucatan Peninsula, which forms part of the Maya block of Central America, includes lower Paleozoic rocks in the Maya Mountains of Belize. The pre-Mesozoic geologic history of the Maya block is related to the evolution of the mid-Proterozoic basement in Mexico (Oaxaquia), other Paleozoic peri-Gondwanan terranes, and the Pan-African–Brasiliano type basement of Florida. The initial 176Hf/177Hf values of ~1.0Ga detrital zircon grains lie on a crustal evolution trajectory similar to that defined by older, ~1.2 to ~1.5Ga grains. This trajectory is consistent with those that would be produced by crustal reservoirs that separated from the depleted mantle between 1.70 and 2.05Ga. However, some grains have significantly less radiogenic 176Hf/177Hf(t), indicating influence from even older cratonic crust. Zircon grains from granitoids that intruded the Early Paleozoic sedimentary rocks of the Yucatan Peninsula yielded Late Silurian to Early Devonian (~415–400Ma) crystallization ages. More radiogenic Hf isotope ratios indicate anatexis of a crustal reservoir that is distinct from that of the Early Paleozoic sedimentary rocks. The Sm–Nd systematics of whole rock samples further support the results from the Hf isotopes in zircon grains. The data suggest a more continental provenance for the sedimentary rocks from Yucatan as compared to typical ca. 1.3 to 1.0Ga outcrops in southern and central Mexico. The results indicate that sediments were shed either from mid-Proterozoic complexes of NW Amazonia or from similar continental sequences that were thrust over Oaxaquia during the Grenville orogeny and subsequently eroded in the early Paleozoic. Integrating the data into a new model for early Paleozoic times, the southern Maya block is inferred to have formed during the opening of the Rheic Ocean along the western margin of Amazonia adjacent to Oaxaquia. The paucity of Ediacaran (Pan African–Brasiliano) signatures implies that before the Silurian, the southern Maya block evolved geographically separated from NW Yucatan and Florida, where Pan African–Brasiliano crystalline rocks have been reported. [Copyright &y& Elsevier]

Abstract: A series of flux-grown synthetic zircon crystals doped with Hf and selected rare earth elements (REE) were produced for use as a potential reference material for Lu–Hf isotopic analysis of zircon by in situ laser ablation multi-collector inductively-coupled plasma mass spectrometry (LA-MC-ICPMS). The synthetic zircon crystals were doped such as to produce a large range of REE/Hf, allowing for a robust means of monitoring and correcting the sometimes large 176Yb and 176Lu isobaric interferences on the low-abundance 176Hf in natural zircon, as well as potential oxide interferences from Gd, Tb, and Dy. The synthetic zircon crystals have a homogeneous Hf isotopic composition, both within and between grains, as documented by solution MC-ICPMS analyses of Hf chemically separated from multiple fragments of zircon from three different syntheses, and 321 laser ablation MC-ICPMS analyses done in two different laboratories. An additional 110 in situ analyses of natural zircon crystals reveal identical behavior of natural and synthetic zircon during Hf isotope analysis by laser ablation. Hence, the synthetic zircon crystals provide a means of monitoring a wide range of Yb and Lu interferences (up to 50% of the total signal intensity at mass 176) during routine isotopic analysis for geological studies. They also may be helpful in developing improved laboratory protocols for accurate in situ Lu–Hf isotopic measurement of natural zircon. The present data illustrate the importance of the 176Yb interference correction on 176Hf, which can result in large biases if not properly applied to natural samples with their variable and often high 176Yb/177Hf. This study definitively shows that use of high-REE/Hf synthetic zircon standards for calibrating the Yb mass bias correction is more accurate than use of Yb-doped natural Hf solutions. Finally, the results of the present work suggest that synthetic minerals may prove useful as both in situ method development tools and isotopic reference materials. [Copyright &y& Elsevier]

Abstract: The Dadian alkaline intrusive complex is located within the Sulu orogenic belt, and includes hornblende syenite, syenite, quartz syenite and syenite porphyry. All the rocks from the complex show high SiO2, K2O+Na2O and LREE and low CaO, FeOT, MgO and HFSE (Nb, Ta, P, Ti) concentrations. The Sr, Nd, and Hf isotopic compositions ((87Sr/86Sr)i =0.708505–0.70876, εNd(t)=−16.5 to −17.9, εHf(t)=−21.5 to −16.4) of the rocks fall within the compositional field of mafic dykes occurring nearby, suggesting that the Dadian alkaline complex was mainly derived from an enriched lithospheric mantle. The initial Pb compositions ((206Pb/204Pb)i =16.706–16.779, (207Pb/204Pb)i =15.427–15.450, (208Pb/204Pb)i =37.266–37.352) of the Dadian rocks are lower than those of the Yangtze lithospheric mantle, albeit similar to that reported from beneath the North China Craton, implying that the mantle source of the Dadian alkaline complex most likely belong to the North China Craton. The relatively high SiO2 and HREE contents and low Nb/Ta ratios as well as strongly negative Eu and Sr anomalies of some of the rocks from the Dadian complex suggest that crustal material was also involved in the formation of this complex and that the crustal component was probably derived through partial melting at low pressure. The high crystallization temperatures revealed by zircon saturation thermometry and low pressures suggested by Al-in-hornblende barometry indicate that the Dadian alkaline complex crystallized at high temperature and intruded into shallow crustal level. This intrusive complex does not show any relationship with post-orogenic process in the region, and its petrogenesis is inferred to be associated with the Paleo-Pacific subduction tectonics. [Copyright &y& Elsevier]

Abstract: There is hot debate on whether there is a Paleo-Tethyan suture within Qiangtang and on how the suture extends in central Tibet. In situ Hf and U–Pb isotopes, and rare earth element (REE) analyses of zircons, together with bulk chemistry, have helped us identify a significant, and previously unknown stage of subduction-related magmatism along the southern part of the eastern Qiangtang block, north of the Shuanghu suture, central Tibet. SHRIMP and LA-ICP-MS zircon U–Pb data demonstrate that the magmatism started as early as 275Ma, lasted until 248Ma, and took place at the same time as blueschist and eclogite were forming in the central Qiangtang high- to ultrahigh-pressure metamorphic belt (QMB). The positive εHf(t) values of the magmatic zircons range from 9.71 to 16.73, and indicate derivation of magma from a relatively uncontaminated, depleted mantle. Variations in εHf(t) values and the REE patterns of magmatic zircons, combined with substantial heterogeneities in the whole-rock chemistry, suggest that the arc-related magmatism occurred in pulses, and was long-lived. The present data can be interpreted in terms of a history of progressive subduction during the Early Permian and Early Triassic, during which time continental fragments of EQ affinity were tectonically eroded and subducted into the mantle, along with the subducted oceanic slab. The temporal and spatial correlations between the arc-like volcanic belt and the high-pressure metamorphic belt suggest a Late Paleozoic ocean (Paleo-Tethys) existed between the western and eastern Qiangtang blocks, and that the ocean was closed by northwards subduction. [Copyright &y& Elsevier]

Abstract: High-precision Hf isotopic data for selected historic (pre-1971 eruption) and recent (post-1971) Etnean lavas are discussed in light of their covariation with previously reported Sr–Nd–Pb isotope ratios. 176Hf/177Hf ranges between 0.282961 and 0.282989 in the historic lavas, and between 0.282943 and 0.282990 in the recent ones, displaying, especially among the most recently erupted products, an overall decrease. In Hf–Nd and, to a minor extent, Hf–Sr–Pb isotope spaces, Etnean lavas plot slightly below the linear array defined by MORB and OIB, between the enriched end of the FOZO and HIMU fields. This may suggest that, prior to the formation of Etnean melts, the garnet-bearing source experienced an episode of low-degree melting; this source then evolved for enough time to produce the 176Hf/177Hf and 143Nd/144Nd decoupling observed in the historic and recent lavas. In addition, the distinct εHf values of the historic and recent lavas may be accounted for by portions of the mantle with distinct Lu/Hf ratios, likely related to decreasing modal proportions of garnet in favour of metasomatic phases involved in partial melting. Integration of all the available data suggests that FOZO is the dominant component in the source of Etnean magmas, although this is not enough to solve the observed 176Hf/177Hf change coupled with the Sr–Nd–Pb variation through time. Binary mixing between FOZO and EM-type components demonstrates that addition of about 10% of an EM1-type component to FOZO can account for the isotopic variability of Etnean magmas, a feature more emphasized in the most recent lavas. Divergences from a dominant FOZO signature of erupted magmas at Mt. Etna may therefore be attributed to small-scale heterogeneity produced by access into the source of enriched material with an EM1-type signature. The coupled analysis of 176Hf/177Hf and trace element ratios, such as Sm/Hf and Th/Hf, provides evidence that this enriched infiltrating component could be metasomatizing silicate melts rather than aqueous, low-T fluids. [Copyright &y& Elsevier]

Abstract: This work presents a rigorous assessment of the accuracy and precision with which 176Hf/177Hf can be measured in rutile by laser ablation (LA) MC-ICPMS. For rutile with ≥40ppm Hf, we demonstrate that 176Hf/177Hf can be measured accurately and reproducibly with adequate precision for application to petrological problems. We present an analytical and data reduction protocol tailored to the specific challenges of measuring Hf isotope ratios in this low-Hf mineral. Precision and accuracy are optimised by interpolating between baselines measured every ~10 analyses. For many rutiles, the advantages of determining the Hf mass bias coefficient, β Hf, during analysis are negated by the low precision with which it can be measured at low Hf contents. Hf mass bias is therefore monitored by regular analysis of a synthetic rutile doped with ~5000ppm Hf (SR-2) to facilitate an external mass bias correction. Because rutile contains negligible Yb, the Yb mass bias coefficient β Yb must be inferred from β Yb/β Hf measured on zircon in the same session. To account for a systematic instrument bias of 0.5–1εHf units, rutile analyses are normalised to SR-2, for which 176Hf/177Hf has been determined by solution MC-ICPMS. In situ 176Hf/177Hf measurements for two natural rutile samples with 40–50ppm Hf are in excellent agreement with solution MC-ICPMS values. Ti/Hf exerts no influence on the accuracy of 176Hf/177Hf measurements on our LA–MC-ICPMS, as illustrated by indistinguishable 176Hf/177Hf measurements for a series of synthetic rutiles doped with varying amounts of Hf. Rutile and zircon from the Duria mantle peridotite (Central Alps), which formed at different parts of the P–T–time path, record contrasting Hf isotope signatures and emphasise the complementary nature of Hf isotope analysis of rutile and zircon. [Copyright &y& Elsevier]

Abstract: Although the calcium- and titanium-bearing zirconsilicates, zirconolite and calzirtite, are common minerals in carbonatites little is known regarding their trace element or isotopic characteristics. Available data indicate that they can have significant contents of U, Pb, Nb, Ta, Zr, Hf, rare earth elements (REEs), minor Sr and low Rb/Sr and Lu/Hf ratios. Their compositions indicate that they have potential for U–Pb age determination together with Nd–Hf and possibly Sr isotopic analyses. In this study, zirconolite and calzirtite from carbonatites occurring at Phalaborwa (South Africa), Prairie Lake (Canada), Afrikanda and Kovdor (Kola, Russia), Guli (Siberia, Russia) and Jacupiranga (Brazil) were selected for an isotopic analysis by in situ techniques. Ion microprobe analyses using a CAMECA 1280 ion microprobe show that reliable Pb–Pb ages can be obtained from these minerals as they have extremely high U concentrations, coupled with negligible common Pb. Although these minerals have low Rb/Sr ratios, their low Sr concentrations, in most cases, render in situ Sr isotopic analyses difficult. In contrast, their high Sm, Nd and Hf concentrations, with generally low Lu/Hf ratios, permit precise in situ isotopic analysis of Nd and Hf. [ABSTRACT FROM AUTHOR]

Abstract: In situ analyses of hafnium isotopes in zircon combined with U–Pb zircon ages, and trace element data, provide new information on the characterization and evolution of magnetite–apatite (“Kiruna-type”) deposits and the tectonic environments in which they occur. The Lyon Mountain granite in the Adirondack Mountains of New York is the host to numerous zircon-bearing magnetite–apatite deposits. Hafnium isotopic compositions and rare earth element contents in individual zircon crystals were measured in situ in both the host granites and the ore bodies by laser ablation inductively coupled plasma-mass spectrometry. Hafnium isotopic compositions in the ore zircon can be divided into two groups: those that have initial εHf (t) values that are indistinguishable from those of the host granites (e.g., εHf (t) less than +7) and are typical of relatively juvenile Proterozoic crust, and those that have extremely radiogenic εHf (t) values (as high +40). Two models are proposed to explain the observed εHf (t) values in the zircon crystals: 1) early-formed ore bodies containing magnetite, apatite, and clinopyroxene were remobilized by secondary fluid alteration, releasing Zr and Hf for the crystallization of new zircon; or alternatively, 2) fluids responsible for ore formation have interacted with garnet-bearing rocks during retrograde metamorphism, scavenging rare earth elements and radiogenic Hf. Previous work done to determine the U–Pb ages from the same zircon crystals, which were analyzed for the Hf isotopic composition in this study, revealed that ore bodies record a mineralizing event that is 20 to 60m.y. younger than the age of granite emplacement. This age discrepancy, plus the highly radiogenic εHf (t) values in the ore zircon crystals, suggests that the fluids responsible for this younger event could not have been derived from the granite hosts. These data argue that magnetite–apatite deposits in the LMG have multiple mineralizing events superimposed upon one another and that early-formed deposits may be reworked, modified and redeposited by fluids subsequent to magma crystallization. [Copyright &y& Elsevier]

Abstract: Eudialytes are a group of complex Na–Ca zirconosilicate minerals that generally occur in peralkaline agpaitic syenites. Given that eudialytes are easily altered and commonly contain inclusions of earlier-crystallized minerals, in situ laser ablation is considered as the most suitable method to determine their U–Pb ages, and Sr, Nd and Hf isotopic compositions. Electron microprobe and LA-ICP-MS analysis of eudialytes from nepheline syenites from Ilímaussaq (Greenland), Khibiny and Lovozero (Russia), Saima (China), Tamazeght (Morocco), Kipawa and Mont Saint Hilaire (Canada), Poços de Caldas (Brazil) and Langesund Fjord (Norway), indicate that this mineral typically has high contents of U, Pb, Nb, Ta, Zr, Hf and rare earth elements (REE). Analysis of an in-house standard eudialyte by both solution and laser ablation methods demonstrates that precise and accurate U–Pb ages can be obtained from eudialyte after correction for the common Pb content. The high Sr, Nd and Hf contents in eudialyte, coupled with the generally low Rb/Sr and Lu/Hf ratios, also permit the precise determination of in situ Sr, Nd and Hf isotopic ratios by LA-(MC)-ICP-MS methods. However, some eudialytes with Rb/Sr ratio of >0.02 and Yb/Sr>0.1, and Lu/Hf>0.08 (or Yb/Hf>0.4–0.5) cannot be used for in situ Sr and Hf isotopic measurement. Eudialyte is the only mineral investigated to date for which it is possible to determine simultaneously U–Pb ages and Sr, Nd and Hf isotopic compositions. [Copyright &y& Elsevier]

Abstract: 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 (>2.2m), the Sr isotopes of the saprolites show an upward increasing trend with ε Sr changing from ~50 at 2.2m to ~70 at 0.5m, accompanying a upward increasing of Sr concentrations, from ~10μg/g to ~25μg/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 139μg/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. [Copyright &y& Elsevier]

Abstract: Voluminous granitic intrusions are distributed in the West Junggar, NW China, and they can be classified as the dioritic rocks, charnockite and alkali-feldspar granite groups. The dioritic rocks (SiO2 =50.4–63.8wt.%) are calc-alkaline and Mg enriched (average MgO=4.54wt.%, Mg# =0.39–0.64), with high Sr/Y ratios (average=21.2), weak negative Eu (average Eu⁎/Eu=0.80) and pronounced negative Nb–Ta anomalies. Their Sr–Nd and zircon Hf isotopic compositions ((87Sr/86Sr)i =0.7035–0.7042, ε Nd(t)=4.5–7.9, ε Hf(t)=14.1–14.5) show a depleted mantle-like signature. These features are compatible with adakites derived from partial melting of subducted oceanic crust that interacted with mantle materials. The charnockites (SiO2 =60.0–65.3wt.%) show transitional geochemical characteristics from calc-alkaline to alkaline, with weak negative Eu (average Eu⁎/Eu=0.75) but pronounced negative Nb–Ta anomalies. Sr–Nd and zircon Hf isotopic compositions ((87Sr/86Sr)i =0.7037–0.7039, ε Nd(t)=5.2–8.0, ε Hf(t)=13.9–14.7) also indicate a depleted source, suggesting melts from a hot, juvenile lower crust. Alkali-feldspar granites (SiO2 =70.0–78.4wt.%) are alkali and Fe-enriched, and have distinct negative Eu and Nb–Ta anomalies (average Eu⁎/Eu=0.26), low Sr/Y ratios (average=2.11), and depleted Sr–Nd and zircon Hf isotopic compositions ((87Sr/86Sr)i =0.7024–0.7045, ε Nd(t)=5.1–8.9, ε Hf(t)=13.7–14.2). These characteristics are also comparable with those of rocks derived from juvenile lower crust. Despite of the differences in petrology, geochemistry and possibly different origins, zircon ages indicate that these three groups of rocks were coevally emplaced at ~305Ma. A ridge subduction model can account for the geochemical characteristics of these granitoids and coeval mafic rocks. As the “slab window” opened, upwelling asthenosphere provided enhanced heat flux and triggered voluminous magmatisms: partial melting of the subducting slab formed the dioritic rocks; partial melting of the hot juvenile lower crust produced charnockite and alkali-feldspar granite, and partial melting in the mantle wedge generated mafic rocks in the region. These results suggest that subduction was ongoing in the Late Carboniferous and, thus support that the accretion and collision in the Central Asian Orogenic Belt took place in North Xinjiang after 305Ma, and possibly in the Permian. [Copyright &y& Elsevier]

Abstract: A new method for the radiogenic isotope (U–Th–Pa–Ra, Sr, Nd, Hf) analysis of the soluble and insoluble components found within ice cores is presented. Melting experiments with rock standards in the presence of EDTA indicate that carbonates, as well as silicates, can be buffered sufficiently to preclude dissolution. The use of EDTA allows adsorbing species, such as Th and Hf, to remain in solution during melting thus fully separating the dust (insoluble) and sea salt (soluble) components of the ice after filtration. A new elemental separation scheme for low sample masses, less than 5 mg solid material, utilizes 4 primary ion exchange columns and two “clean-up” columns to fully isolate U, Th, Pa, Ra, Sr, Nd, and Hf while maintaining high yields. Elution schemes measured for USGS rock standards and a Chinese loess are presented to provide a comparison for variable matrix compositions. Mass spectrometer techniques were modified to measure small aliquots of the standards, equivalent to the amounts found in ice core samples, 10 ng and less. A MC-ICPMS was employed for the measurement of U, Th, Pa, Ra, and Hf; results of the experiments show that with ion yields up to 1%, rock standards have errors for 234U/238U of 1%, 230Th/232Th of 1.5%, [228Ra] of 9%, and 176Hf/177Hf of 100 ppm. MC-TIMS measurements of Sr and Nd show similar errors for small sample sizes: 87Sr/86Sr of 50 ppm and 143Nd/144Nd of 80 ppm. This new analytical method increases the number of possible tracers measured from a single sample, reducing separation times and sample consumption, as well as providing the addition of a radiometric clock, U-series, to the traditional suite of isotopic tracers, Sr, Nd, and Hf. [Copyright &y& Elsevier]

Abstract: O, Hf and Os isotope data are presented for lavas from the highly depleted Tonga–Kermadec arc. O isotope values overlap with those of MORB limiting the amount of interaction with the arc crust. δ 18O does not increase northwards as would be expected from the ~4 fold increase in subduction rate if slab-derived fluids had high 18O/16O ratios. Thus, the overall northward decrease in HFSE concentrations likely reflects depletion due to prior melt extraction, not increasing extents of melting. Hf isotopes are strongly negatively correlated with Be isotopes consistent with mixing of subducted pelagic sediment into the mantle wedge and do not require Hf to be fluid mobile. With the exception of a boninite from the north Tongan trench, the northern Tonga lavas do not overlap the Hf isotope composition of either the Samoan plume or the subducting Louisville volcaniclastic sediments. Thus, the Pb isotope signatures in these lavas must have been added by fluids and sediment melts derived from the Louisville volcaniclastics with minimal mobilisation of Hf. This suggests conservative behaviour for this element due to the formation of residual zircon during partial melting of the subducted sediments. 187Os/188Os ranges from 0.1275 to 0.4731 and the higher Os isotope ratios reflect the sensitivity of this system to even minor interaction with altered arc crust. Conversely, the lowest Os ratios are subchondritic and indicate that transfer of radiogenic Os from the slab is not all pervasive and provide an important constraint on the composition of the mantle wedge. Remarkably, the least radiogenic sample is a dacite demonstrating that evolved magmas can develop by fractionation from mantle-derived magmas with minimal interaction with the arc crust. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: We report the results of a comprehensive isotopic investigation of zircons from metamorphosed tonalites and amphibolites from Lushan in the far south of the Trans-North China Orogen of the North China Craton that reveal the oldest rocks yet dated from the orogen. A detailed investigation of the internal structures of zircons, combined with targeted SIMS U–Pb and oxygen isotope analyses and ICP-MS Hf zircon analyses, has resulted in the discovery of previously unknown geological events in the North China Craton. Magmatic zircon from the tonalite and amphibolite was found to be essentially coeval with a small spread of ages from 2829±18 Ma to 2832±11 Ma in the tonalites and from 2838±35 Ma to 2845±23 Ma in amphibolite. Hafnium isotope data suggest derivation of both tonalite and amphibolite from a depleted mantle source with minor crustal contamination. Oxygen isotope ratios in zircons range from 5.1 to 7.3‰ VSMOW, with high δ18O results recorded only from metamorphic domains. The earliest stage of metamorphic zircon growth, present in all four samples analyzed in this study, occurred between 2772±17/22 in the tonalites and 2776±20 and 2792±12 Ma in the amphibolites. A younger metamorphic event at 2638±61 in the tonalite and between 2671±13 and 2651±13 Ma in the amphibolites is recognized in zircons from three of the four samples investigated. This is the first time that metamorphic events at either of these times have been recorded in the North China Craton. Importantly, there is no evidence for the 1.8 Ga high-grade metamorphic event that is manifest in the northern and central parts of the Trans-North China Orogen. [Copyright &y& Elsevier]

Abstract: The Acasta Gneiss Complex of northwestern Canada comprises early Archean orthogneisses and includes the oldest known terrestrial rocks (3.94–4.03 Ga). Previous zircon geochronological studies revealed the presence of zircon xenocrysts with ages up to 4.2 Ga in the oldest rocks, indicating that the source of early Archean granitoids contained Hadean (>4.03 Ga) crust. In this study, we have determined the Lu–Hf isotopic compositions of zircon grains extracted from the early Archean Acasta gneisses to evaluate the extent of the Hadean crust contribution to the formation of these granitoids. Analyses of the Lu–Hf isotopes were carried out using in-situ laser ablation inductively coupled plasma mass spectrometry combined with cathodoluminescence images of the internal structure of the grains. Two ca. 3.59 Ga granitoids have ε Hf(T) of −2.4±2.2 and −3.2±2.5, suggesting that the source of the granitoids was extracted from the mantle as far back as 3.8 Ga. This is consistent with the presence of abundant zircon xenocrysts with ages up to 3.9 Ga. The Hf isotopic composition of a 3.72 Ga granitoid is significantly less radiogenic, with an ε Hf(T) of −6.1±2.5. This indicates that the granitoid formed by remelting of very old crust. The ε Hf(T) values for the two oldest rocks, the ca. 3.97 Ga tonalites, are −1.2±3.3 and −3.3±1.7, respectively, indicating that even the oldest known granitoids contain reworked older crustal materials. These results suggest that Hadean crust had significantly contributed to the genesis of some of the early Archean Acasta granitoids. [Copyright &y& Elsevier]

Abstract: Twenty four splits from two different GJ-1 reference material zircon crystals were analyzed for their Lu and Hf concentrations, Lu/Hf and Hf isotopic compositions by solution ID-MC-ICPMS. Lutetium and Hf concentrations are variable between single splits ranging from 9.34 ppm to 14.3 ppm and 4976 ppm to 7719 ppm, respectively, with an average 176Lu/177Hf=0.00025. Concentrations of both elements show a positive correlation, which argue for in-homogeneity of the zircon with respect to HREE and Hf, and however, do not affect the isotope composition. Solution data from two laboratories (Amsterdam and Münster) yield an average present day 176Hf/177Hf of 0.282000±0.000005 (2σ weighted mean), which we recommend as a reference and nomination value for laser-ablation Hf isotope analyses. Cl imaging shows no visible banding or zonation for CL-active elements. Laser-ablation Hf isotopic analyses yield homogeneous 176Hf/177Hf identical within analytical uncertainties with the value obtained by the solution work, which make these zircons suitable for Hf isotope determinations using LA-MC-ICPMS analyses. Notably, one laser-ablation analysis shows elevated Yb and Lu contents with a significantly lower 176Hf/177Hf, pointing to possible small scale in-homogeneity with elevated HREE contents. [Copyright &y& Elsevier]

Abstract: We have developed a new analytical technique for precise and accurate in situ Hf isotope ratio measurements for zircons by means of laser ablation-multiple collector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS). Newly designed sample cell and N2 mixing technique provided higher elemental sensitivity with smoother signal intensity profiles. In order to improve the accuracy of the Hf isotope ratio analysis, previously reported correction methods for isobaric interferences on 176Hf by 176Yb and 176Lu were rigorously evaluated, and we found that the use of independent mass bias factors for Hf and Yb in the isobaric interference correction resulted in more reliable Hf isotopic data. In order to test the reliability of the Hf isotopic data from ∼35 μm diameter ablation pits that are smaller than the typical size of most zircon grains, Lu–Hf isotopes for three zircons (Harvard 91500, Antarctic zircon PMA7 and Acasta zircon AY199) were measured. The present 176Hf/ 177Hf isotope ratio of 0.282321±46 (2 S.D.) and initial 176Hf/ 177Hf isotope ratio of 0.282315±47 (2 S.D.) were obtained for zircon standard 91500. The resulting present 176Hf/ 177Hf isotope ratio shows an excellent agreement with the previously reported data. Our in situ isotopic data revealed that the Lu/Hf ratio varied significantly within the 91500 grain (176Lu/ 177Hf=0.000130–0.000345), suggesting the existence of slight isotopic heterogeneity in present 176Hf/ 177Hf (approximately 15 ppm). For Antarctic zircon PMA7, despite the smaller size of the single grain (50–100 μm wide, 80–300 μm long) and the presence of various mineral inclusions, resulting analytical precision for the initial 176Hf/ 177Hf ratio was 128 ppm [0.281208±36 (2 S.D.)]. For Acasta zircon AY199, despite the high Yb/Hf (∼0.05) and Lu/Hf (∼0.01) isotope ratios with the early Archean crystallization age (3.74 Ga), resulting analytical precision for the initial 176Hf/ 177Hf ratio was 187 ppm [0.280160±52 (2 S.D.)]. These results demonstrate clearly that in situ Hf isotope ratio microanalysis developed here has the potential to become a significant tool for zircon Lu–Hf isotopic study. [Copyright &y& Elsevier]

This study examines the levels of precision and accuracy obtainable by laser ablation Hf-isotope analysis of zircons in real-world situations using a 193-nm ArF excimer laser coupled to a Nu Plasma MC-ICPMS. Three different reference materials (91500, BR266, and Temora-2) were studied with both solution MC-ICPMS (after Hf separation) and laser ablation. The long-term (9 months) levels of precision and accuracy obtained in these standards by laser ablation are ±90 and ca. 50 ppm (2σ), respectively, closely approaching theoretical limits. The theoretical limiting precision can be expressed in terms of dependence on beam size and integration time. The quality of these data is attributed to a combination of high system sensitivity and a more robust correction for Yb interference than considered in previous studies of this kind.Novel solutions to the analysis of complex zircon growth morphologies have been tested including the ablation of non-standard geometries and depth profiling techniques, the latter tested by use of a composite zircon standard. Depth profiling appears to be viable as long as ablation pit aspect ratios remain below 2:1 (depth/width). Finally, the Nu Plasma zoom lens is employed to explore the possibility of simultaneous 207Pb–206Pb age determination and Hf-isotope analysis. [Copyright &y& Elsevier]

Rifting between eastern India and western Australia in the Early Cretaceous (∼140–133 Ma) resulted in the formation of the eastern Indian Ocean. Tholeiitic flood basalts erupted on the southwestern margin of Australia (Bunbury basalts at ∼132 and ∼123 Ma) and the eastern margin of India (Rajmahal Traps at ∼118 Ma) shortly after this time, while the Kerguelen Plateau began to form in the nascent ocean basin (∼119 Ma). We studied the Hf and Os isotopic systematics of the continental basalts in order to better understand the relationship between these volcanic events and the Kerguelen plume. The objective of this work was to determine the mantle sources of the continental tholeiites and the nature and quantity of continental contamination. Both the Hf and Os isotopic compositions of these basalts vary with indicators of continental contamination (SiO2, (Th/Ta)PM, (La/Nb)PM). There is no obvious correlation of γOs(T) with Sr or Nd isotopic ratios, but rough correlations are present between Os and Hf isotopes and 208Pb/204Pb; in contrast, #x03B5;Hf(T) shows strong correlations with all isotopic systems and indicators of continental crust assimilation. We demonstrate that the mantle sources of these continental tholeiites cannot originate purely from the depleted asthenosphere. An enriched mantle source is required for both of these basalt suites and the geochemical characteristics of this source are broadly consistent with those inferred to be present in the Kerguelen plume during Early Cretaceous time. The Os isotopic results suggest that at least some continental contamination is present in all samples from both the Bunbury basalts and the Rajmahal Traps. In the Rajmahal Traps, this contaminant appears to be derived from the continental crust and there is little to no evidence for involvement of subcontinental lithospheric mantle (SCLM). The Bunbury basalts have consistently unradiogenic Os isotopic compositions and high #x03B5;Hf(T) for a given #x03B5;Nd(T), characteristics ascribed to the SCLM. Previous investigations of these basalt suites have concentrated on major and trace elements and Sr, Nd and Pb isotopes; none of these results suggested the involvement of SCLM in the Bunbury basalts. Here, we emphasize that the study of Hf and Os isotopes provide additional, useful tools for deciphering mantle sources and continental contaminants involved in the petrogenesis of continental tholeiites. [Copyright &y& Elsevier]

The accessory mineral zircon is the most widely used geological timekeeper and tracer of crustal growth processes. Specifically, U–Pb isotopes in zircon offer a means to accurately determine the timing of magmatic events and their Hf isotopic composition provides a means to constrain magma source composition and potentially approximate source age. The high spatial resolution provided by in situ techniques such as secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) have the advantage of being able to target different growth zones within individual zircon crystals, and unravel complex magmatic histories. However, utilising this power effectively requires calculation of accurate initial Hf compositions, which are founded on the assumption that the information obtained via the U–Pb and Lu–Hf systems are correctly integrated. A typical Hf isotope LA-ICP-MS analysis ablates a sample volume that is two orders of magnitude greater than a typical SIMS analysis. Thus, when age determination has been carried out by SIMS it is necessary to demonstrate that each subsequent Hf isotope analysis has sampled a similar isotopically homogeneous volume. Here, we use a combined SIMS and laser ablation split stream (LASS)-ICP-MS approach, whereby U–Pb isotopic measurement concurrently on the same sample volume as the Hf isotope measurement is compared to prior lower volume SIMS measurements. Using a suite of new drill core magmatic rock samples from the comparatively unexplored Coompana Province in South Australia, we demonstrate how such an approach can be used to filter Hf isotope datasets by identifying LASS-ICP-MS analyses that sampled mixtures of different zircon growth domains. The robust initial 176Hf/177Hf compositions obtained from the filtered Coompana data set indicate that the province represents part of a juvenile Paleoproterozoic-Mesoproterozoic arc system formed through hyper-extension of the margin of the Archean Gawler Craton. This arc system can be correlated to the Musgrave Province and Madura Province in central and Western Australia respectively. [ABSTRACT FROM AUTHOR]

Mafic microgranular enclaves (MMEs) are common in many granitoid plutons and their origin is crucial for understanding the petrogenesis and tectonic setting of their hosts. Most previous workers have studied MMEs by using in situ zircon U-Pb and Hf-O isotopes. However, whether the zircons in the MMEs recorded the primary isotopes of their host MMEs is not well constrained. Here, two MMEs from the Daocheng (DC-2) and Dongcuo (HZS-3) granitic plutons, as well as their host granites, in the Triassic Yidun arc belt, eastern Tibetan Plateau, were studied. The MMEs have zircon U-Pb ages of 219 ± 3 Ma and 225 ± 3 Ma respectively, similar to their host granites. They also have similar or slightly depleted zircon Lu-Hf isotopes to their host granites. Furthermore, zircons from the Dongcuo pluton have indistinguishable Ti-in-zircon temperatures (T zr) and Th/U ratios from those of their host granites, whereas zircons from the Daocheng pluton have higher T zr values and Th/U ratios than those of their host granites. Model calculations indicate that the whole-rock composition of the studied MMEs was different from the melt composition from which the zircon grains grew. Additionally, zircon from Daocheng sample DC-2 has larger variations of Zr stable isotopes than its host granite, whereas those of Dongcuo sample HZS-3 have lower δ94/90Zr IPGP-Zr values than its host granite. Previous study suggests that early crystallized zircons will have lighter Zr isotopes. Thus, based on our new data, we propose that zircons from Dongcuo were actually xenocrysts that formed at an early stage in the granitic magma chamber, whereas those from Daocheng crystallized during magma mixing. Thus, none of the zircons in this study have recorded the primary isotopes of the MMEs. In addition, there are also many An-rich plagioclase crystals in the MMEs from both localities and the initial 87Sr/86Sr ratios of these plagioclases are quite heterogeneous. Both of the samples (DC-2 and HZS-3) record three peaks of 0.7078, 0.7064 and 0.7030, and 0.7087, 0.7096 and 0.7105, respectively, which record the primary Sr isotopes of the MMEs, as well as details of the interaction between the mantle-derived and crust-derived magmas. The lowest initial 87Sr/86Sr ratio peak was lower than the arc volcanic rocks but similar to those of MORBs in the Jinshajiang suture zone of the area. Therefore, the primary magma of the MMEs was likely derived from a depleted mantle source. We propose that in situ Sr isotopes of An-rich plagioclase and Zr isotopes of zircon can be powerful tools in deciphering the petrogenesis of MMEs. To simply use the zircon Hf-O isotopes to constrain the primary composition of MMEs is not adequate for determining their origin. • In situ zircon Lu-Hf isotopes do not always record the primary composition of their host MMEs. • The Sr isotopes of An-rich plagioclase reveal complex magma processes during the formation of MMEs. • Zr stable isotopes provide additional information on the origin of zircons in MMEs. [ABSTRACT FROM AUTHOR]