Evolution of the depleted mantle: Hf isotope evidence from juvenile rocks through time

The covariant behavior of Lu-Hf and Sm-Nd isotopes during most magmatic processes has long been recognized, but the details of this behavior in the depleted mantle reservoir have not been adequately examined. We report new whole-rock Hf and Nd isotope data for 1) juvenile, mantle-derived rocks, mid-Archean to Mesozoic in age, and 2) early Archean gneisses from West Greenland. Hf and Nd isotopic compositions of the juvenile rocks are well correlated, with the best fit corresponding to the equation ε_Hf = 1.40 ε_Nd + 2.1, and is similar to the collective Hf-Nd correlation for terrestrial samples of ε_Hf = 1.36 ε_Nd + 3.0. The early Archean Greenland gneisses, in contrast, have an extreme range in ε_Nd values (⁻4.4 to ⁺4.2; Bennett et al., 1993) that is not mirrored by the Hf isotopic system. The ε_Hf values for these rocks are consistently positive and have much less variation (0 to ⁺3.4) than their ε_Nd counterparts.The information from the Hf isotopic compositions of the West Greenland gneisses portrays an early Archean mantle that is relatively isotopically homogeneous at 3.8 to 3.6 Ga and moderately depleted in incompatible elements. There is no evidence that any of these gneisses have been derived from an enriched reservoir. The Hf isotopic data are in stark contrast to the Nd isotopic record and strongly imply that the picture of extreme initial isotopic heterogeneity indicated by Nd isotopes is not a real feature of the West Greenland gneisses but is rather an artifact produced by disturbances in the Sm-Nd isotope system of these rocks.Although Hf and Nd isotopic data do not uniquely constrain either the nature of the earliest crust or the timing of crustal growth, the most probable candidate for the enriched reservoir complementary to the depleted mantle in the pre-4.0 Ga Earth is a mafic, oceanic-type crust. In order to explain the predominantly positive ε_Hf and ε_Nd values for the early Archean rocks, this crust must have had a short residence time at the surface of the Earth before returning to the mantle where it was isolated from mixing with the depleted mantle for several hundred million years. The following period from 3.5 to 2.7 Ga may mark a transition during which this early formed mafic crust was mixed progressively back into the depleted mantle reservoir. While a present-day volume of continental crust at 4.0 Ga cannot be excluded on isotopic grounds, we find such a scenario unlikely based on the lack of direct isotopic and physical evidence for its existence. An important aspect of crustal growth and evolution, therefore, may be the transformation of the enriched reservoir from being predominantly mafic in the early Earth to becoming progressively more sialic through time.     

Mechanisms of Archean crust formation inferred from high-precision HFSE systematics in TTGs

It has been proposed that Archean tonalitic-trondhjemitic-granodioritic magmas (TTGs) formed by melting of mafic crust at high pressures. The residual mineralogy of the TTGs (either (garnet)-amphibolite or rutile-bearing eclogite) is believed to control the trace element budget of TTGs. In particular, ratios of high-field-strength elements (HFSE) can help to discriminate between the different residual lithologies. In order to place constraints on the source mineralogy of TTGs, we performed high-precision HFSE measurements by isotope dilution (Nb, Ta, Zr, Hf) together with Lu-Hf and Sm-Nd measurements on representative, ca. 3.85-2.8 Ga TTGs and related rock types from southern West Greenland, W-India and from the Superior Province. These measurements are complemented by major and trace element data for the TTGs. Texturally homogeneous early Archean (3.85-3.60 Ga old) and Mesoarchean (ca. 3.1-2.8 Ga old) TTGs have both low Ni (<11 ppm) and Cr contents (<20 ppm), indicating that there was little or no interaction with mantle peridotite during ascent. Ratios of Nb/Ta in juvenile Eoarchean TTGs range from ca. 7 to ca. 24, and in juvenile Mesoarchean TTGs from ca. 14 to ca. 27. Even higher Nb/Ta (14-42) were obtained for migmatitic TTGs and intra-crustal differentiates, most likely mirroring further fractionation of Nb from Ta as a consequence of partial melting, fluid infiltration and migmatisation. In the juvenile TTGs, positive correlations between Nb/Ta and Gd/Yb, La/Yb, Sr/Y, Zr/Sm and Zr/Nb are observed. These compositional arrays are best explained by melting of typical Isua tholeiites in both, the rutile-bearing eclogite stability field (>15 kbar, high Nb/Ta) and the garnet-amphibolite stability field (10-15 kbar, low Nb/Ta). With respect to the low end of Nb/Ta found for TTGs, there is currently some uncertainty between the available experimental datasets for amphibole. Independent of these uncertainties, the TTG compositions found here still require the presence of both endmember residues. A successful geological model for the TTGs therefore has to account for the co-occurrence of both low- and high-Nb/Ta TTGs within the same geologic terrane. An additional feature observed in the Eoarchean samples from Greenland is a systematic co-variation between Nb/Ta and initial εHf(t), which is best explained by a model where TTG-melting occured at progressively increasing pressures in a pile of tectonically thickened mafic crust. The elevated Nb/Ta in migmatitic TTGs and intra-crustal differentiates can shed further light on the role of intra-crustal differentiation processes in the global Nb/Ta cycle. Lower crustal melting processes at granulite facies conditions may generate high-Nb/Ta domains in the middle crust, whereas mid-crustal melting at amphibolite facies conditions may account for the low Nb/Ta generally observed in upper crustal rocks.     

Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland

Growing evidence from the accessible geological record reveals that crust-mantle differentiation on Earth started as early as 4.4 Ga. In order to assess the extent of early Archean mantle depletion, we obtained ¹⁷⁶Lu-¹⁷⁶Hf, ¹⁴⁷Sm-¹⁴³Nd, and high field strength element (HFSE) concentration data for the least altered, well characterized boninite-like metabasalts and associated metasedimentary rocks from the Isua supracrustal belt (southern West Greenland). The metasediments exhibit initial εHf₍₃₇₂₀₎ values from −0.7 to +1.5 and initial εNd₍₃₇₂₀₎ values from +1.6 to +2.1. Initial εHf₍₃₇₂₀₎ values of the least altered boninite-like metabasalts span a range from +3.5 to +12.9 and initial εNd₍₃₇₂₀₎ values from −0.3 to +3.2. These initial Hf-isotope ratios display coherent trends with SiO₂, Al₂O₃/TiO₂ and other relatively immobile elements, indicating contamination via assimilation of enriched components, most likely sediments derived from the earliest crust in the region. This model is also consistent with previously reported initial γOs₍₃₇₂₀₎ values for some of the samples. In addition to the positive εHf₍₃₇₂₀₎ values, the least disturbed samples exhibit positive εNd₍₃₇₂₀₎ values and a co-variation of εHf₍₃₇₂₀₎ and εΝd₍₃₇₂₀₎ values. Based on these observations, it is argued, that the most depleted samples with initial εHf₍₃₇₂₀₎ values of up to +12.9 and high ¹⁷⁶Lu/¹⁷⁷Hf of ∼0.05 to ∼0.09 tap a highly depleted mantle source with a long term depletion history in the garnet stability field. High precision high field strength element (HFSE) data obtained for the Isua samples confirm the contamination trend. Even the most primitive samples display negative Nb-Ta anomalies and elevated Nb/Ta, indicating a subduction zone setting and overprint of the depleted mantle sources by felsic melts generated by partial melting of eclogite. Collectively, the data for boninite-like metabasalts support the presence of strongly depleted mantle reservoirs as previously inferred from Hf isotope data for Hadean zircons and combined ¹⁴²Nd-¹⁴³Nd isotope data for early Archean rocks.     

Combined bulk-rock Hf- and Nd-isotope compositions of Mesoarchaean metavolcanic rocks from the Ivisaartoq Supracrustal Belt,SW Greenland: Deviations from the mantle array caused by crustal recycling

Bulk-rock Lu-Hf and Sm-Nd isotope compositions, as well as major and trace element data are presented for metavolcanic rocks from the Mesoarchaean (ca. 3075 Ma) Ivisaartoq Supracrustal Belt in the Nuuk region of southern West Greenland. The εHf_t calculated at 3075 Ma range from +0.8 to +3.1 and the corresponding εNd_t values range from +0.7 to +3.6, which forms an array that is displaced off the mantle array for these two isotopic systems. Primitive mantle normalized trace element plots of the metabasalts display negative Nb- and Ti-anomalies in combination with the elevated Th abundances, which is consistent with a subduction zone affinity as proposed by previous studies of this metavolcanic belt. No significant correlations are observed between the isotope compositions and proxies of shallow crustal contamination in the Ivisaartoq rocks, despite clear evidence for inherited Eoarchaean zircon [Polat et al. (2009a) Chemical Geology 268, 248-271], which would have dominated the bulk-rock Hf-isotope budget. Furthermore, the measured samples are less radiogenic than the estimate for the depleted mantle composition at 3075 Ma. The lack of isotope and trace element correlation suggests incomplete equilibration between the crustal contaminant and the parental Ivisaartoq melts. We prefer a petrogenetic model with some combination of slab-derived metasomatism of the mantle source region for the Ivisaartoq magmas, which homogenized their trace element contents, in combination with the incorporation of granitoid residue with unradiogenic Hf-isotope composition at higher degrees of partial melting and finally the eruption of mechanically entrained Eoarchaean crust without significant chemical equilibration. The geochemical arc-affinity and lower than depleted mantle (DM) isotope compositions of these metavolcanic rocks support the notion that crustal recycling and plate tectonics has been operating on Earth since at least the Mesoarchaean Eon.     

Hf-Nd-Sr isotope systematics of garnet pyroxenites from Salt Lake Crater, Oahu, Hawaii: Evidence for a depleted component in Hawaiian volcanism

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

Multi-stage crustal growth and Neoarchean geodynamics in the Eastern Dharwar Craton,southern India

The Dharwar Craton is a composite Archean cratonic collage that preserves important records of crustal evolution on the early Earth. Here we present results from a multidisciplinary study involving field investigations, petrology, zircon SHRIMP U–Pb geochronology with in-situ Hf isotope analyses, and whole-rock geochemistry, including Nd isotope data on migmatitic TTG (tonalite-trondhjemite-granodiorite) gneisses, dark grey banded gneisses, calc-alkaline and anatectic granitoids, together with synplutonic mafic dykes along a wide Northwest – Southeast corridor forming a wide time window in the Central and Eastern blocks of the Dharwar Craton. The dark grey banded gneisses are transitional between TTGs and calc-alkaline granitoids, and are referred to as ‘transitional TTGs’, whereas the calc-alkaline granitoids show sanukitoid affinity. Our zircon U–Pb data, together with published results, reveal four major periods of crustal growth (ca. 3360-3200 Ma, 3000-2960 Ma, 2700-2600 Ma and 2570-2520 Ma) in this region. The first two periods correspond to TTG generation and accretion that is confined to the western part of the corridor, whereas widespread 2670-2600 Ma transitional TTG, together with a major outburst of 2570–2520 Ma juvenile calc-alkaline magmatism of sanukitoid affinity contributed to peak continental growth. The transitional TTGs were preceded by greenstone volcanism between 2746 Ma and 2700 Ma, whereas the calc-alkaline magmatism was contemporaneous with 2570–2545 Ma felsic volcanism. The terminal stage of all four major accretion events was marked by thermal events reflected by amphibolite to granulite facies metamorphism at ca. 3200 Ma, 2960 Ma, 2620 Ma and 2520 Ma. Elemental ratios [(La/Yb)_N, Sr/Y, Nb/Ta, Hf/Sm)] and Hf-Nd isotope data suggest that the magmatic protoliths of the TTGs emplaced at different time periods formed by melting of thickened oceanic arc crust at different depths with plagioclase + amphibole ± garnet + titanite/ilmenite in the source residue, whereas the elemental (Ba–Sr, [(La/Yb)_N, Sr/Y, Nb/Ta, Hf/Sm)] and Hf-Nd isotope data [εHf_(T) = −0.67 to 5.61; εNd_(T) = 0.52 to 4.23; ] of the transitional TTGs suggest that their protoliths formed by melting of composite sources involving mantle and overlying arc crust with amphibole + garnet + clinopyroxene ± plagioclase + ilmenite in the residue. The highly incompatible and compatible element contents (REE, K–Ba–Sr, Mg, Ni, Cr), together with Hf and Nd isotope data [εHf_(T) = 4.5 to −3.2; εNd_(T) = 1.93 to −1.26; ], of the sanukitoids and synplutonic dykes suggest their derivation from enriched mantle reservoirs with minor crustal contamination. Field, elemental and isotope data [εHf_(T) = −4.3 to −15.0; εNd_(T) = −0.5 to −7.0] of the anatectic granites suggest their derivation through reworking of ancient as well as newly formed juvenile crust. Secular increase in incompatible as well as compatible element contents in the transitional TTGs to sanukitoids imply progressive enrichment of Neoarchean mantle reservoirs, possibly through melting of continent-derived detritus in a subduction zone setting, resulting in the establishment of a sizable continental mass by 2700 Ma, which in turn is linked to the evolving Earth. The Neoarchean geodynamic evolution is attributed to westward convergence of hot oceanic lithosphere, with continued convergence resulted in the assembly of micro-blocks, with eventual slab break-off leading to asthenosphere upwelling caused extensive mantle melting and hot juvenile magma additions to the crust. This led to lateral flow of hot ductile crust and 3D mass distribution and formation of an orogenic plateaux with subdued topography, as indicated by strain fabric data and strong seismic reflectivity along an E-W crustal profile in the Central and Eastern blocks of the Dharwar Craton.     

Hf isotope compositions and HREE variations in off-craton garnet and spinel peridotite xenoliths from central Asia

Garnet-facies continental mantle is poorly understood because the vast majority of mantle xenoliths in continental basalts are spinel peridotite. Peridotite xenoliths from Vitim (southern Siberia) and Mongolia provide some of the best samples of garnet and garnet-spinel facies off-craton lithospheric mantle. Garnets in those fertile to moderately depleted lherzolites show a surprisingly broad range of HREE abundances, which poorly correlate with modal and major oxide compositions. Some garnets are zoned and have Lu-rich cores. We argue that these features indicate HREE redistribution after the partial melting, possibly related to spinel-garnet phase transition on isobaric cooling. Most peridotites from Vitim have depleted to ultra-depleted Hf isotope compositions (calculated from mineral analyses: ε_Hf(0) = +17 to +45). HREE-rich garnets have the most radiogenic ε_Hf values and plot above the mantle Hf-Nd isotope array while xenoliths with normal HREE abundances usually fall within or near the depleted end of the MORB field. Model Hf isotope ages for the normal peridotites indicate an origin by ancient partial melt extraction from primitive mantle, most likely in the Proterozoic. By contrast, an HREE-rich peridotite yields a Phanerozoic model age, possibly reflecting overprinting of the ancient partial melting record with that related to a recent enrichment in Lu. Clinopyroxene-garnet Lu-Hf isochron ages (31-84 Ma) are higher than the likely eruption age of the host volcanic rocks (∼16 Ma). Garnet-controlled HREE migration during spinel-garnet and garnet-spinel phase transitions may be one explanation for extremely radiogenic ¹⁷⁶Hf/¹⁷⁷Hf reported for some mantle peridotites; it may also contribute to Hf isotope variations in sub-lithospheric source regions of mantle-derived magmas.     

The Hadean upper mantle conundrum: evidence for source depletion and enrichment from Sm-Nd, Re-Os, and Pb isotopic compositions in 3.71 Gy boninite-like metabasalts from the Isua Supracrustal Belt, Greenland

Here we present Sm-Nd, Re-Os, and Pb isotopic data of carefully screened, least altered samples of boninite-like metabasalts from the Isua Supracrustal Belt (ISB, W Greenland)that characterize their mantle source at the time of their formation. The principal observations of this study are that by 3.7-3.8 Ga melt source regions existed in the upper mantle with complicated enrichment/depletion histories. Sm-Nd isotopic data define a correlation line with a slope corresponding to an age of 3.69 ± 0.18 Gy and an initial εNd value of +2.0 ± 4.7. This Sm-Nd age is consistent with indirect (but more precise) U-Pb geochronological estimates for their formation between 3.69-3.71 Ga. Relying on the maximum formation age of 3.71 Gy defined by the external age constraints, we calculate an average εNd [T = 3.71 Ga] value of +2.2 ± 0.9 (n = 18, 1σ) for these samples, which is indicative of a strongly depleted mantle source. This is consistent with the high Os concentrations, falling in the range between 1.9-3.4 ppb, which is similar to the estimated Os concentration for the primitive upper mantle. Re-Os isotopic data (excluding three outliers) yield an isochron defining an age of 3.76 ± 0.09 Gy (with an initial γOs value of 3.9 ± 1.2), within error consistent with the Sm-Nd age and the indirect U-Pb age estimates. An average initial γOs [T = 3.71 Ga] value of + 4.4 ± 1.2 (n = 8; 2σ) is indicative of enrichment of their source region during, or prior to, its melting. Thus, this study provides the first observation of an early Archean upper mantle domain with a distinctly radiogenic Os isotopic signature. This requires a mixing component characterized by time-integrated suprachondritic Re/Os evolution and a Os concentration high enough to strongly affect the Os budget of the mantle source; modern sediments, recycled basaltic crust, or the outer core do not constitute suitable candidates. At this point, the nature of the mantle or crustal component responsible for the radiogenic Os isotopic signature is not known.Compared with the Sm-Nd and Re-Os isotope systems, the Pb isotope systematics show evidence for substantial perturbation by postformational hydrothermal-metasomatic alteration processes accompanying an early Archean metamorphic event at 3510 ± 65 Ma and indicate that the U-Th-Pb system was partially opened to Pb-loss on a whole rock scale. Single stage mantle evolution models fail to provide a solution to the Pb isotopic data, which requires that a high-μ component was mixed with the depleted mantle component before or during the extrusion of the basalts. Relatively high ²⁰⁷Pb/²⁰⁴Pb ratios (compared to contemporaneous mantle), support the hypothesis that erosion products of the ancient terrestrial protocrust existed for several hundred My before recycling into the mantle before ∼3.7 Ga.Our results are broadly consistent with models favoring a time-integrated Hadean history of mantle depletion and with the existence of an early Hadean protocrust, the complement to the Hadean depleted mantle, which after establishment of subduction-like processes was, at least locally, recycled into the upper mantle before 3.7 Ga. Thus, already in the Hadean, the upper mantle seems to be characterized by geochemical heterogeneity on a range of length scales; one property that is shared with the modern upper mantle. However, a simple two component mixing scenario between depleted mantle and an enriched-, crustal component with a modern analogue can not account for the complicated and contradictory geochemical properties of this particular Hadean upper mantle source.     

Non-chondritic Sm/Nd ratio in the terrestrial planets: Consequences for the geochemical evolution of the mantle-crust system

Super-chondritic ¹⁴²Nd signatures are ubiquitous in terrestrial, Martian and lunar samples, and indicate that the terrestrial planets may have accreted from material with Sm/Nd ratio higher than chondritic. This contradicts the long-held view that chondrites represent a reference composition for the ¹⁴⁷Sm-¹⁴³Nd system. Using coupled ¹⁴⁶Sm-¹⁴²Nd and ¹⁴⁷Sm-¹⁴³Nd systematics in planetary samples, we have proposed a new set of values for the ¹⁴⁷Sm/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd ratios of the bulk silicate Earth (Caro et al., 2008). Here, we revise the Bulk Silicate Earth estimates for the ⁸⁷Rb-⁸⁷Sr and ¹⁷⁶Lu-¹⁷⁶Hf systems using coupled Sr-Nd-Hf systematics in terrestrial rocks. These estimates are consistent with Hf-Nd systematics in lunar samples. The implications of a slightly non-chondritic silicate Earth with respect to the geochemical evolution of the mantle-crust system are then examined. We show that the Archean mantle has evolved with a composition indistinguishable from that of the primitive mantle until about 2 Gyr. Positive ε¹⁴³Nd and ε¹⁷⁶Hf values ubiquitous in the Archean mantle are thus accounted for by the non-chondritic Sm/Nd and Lu/Hf composition of the primitive mantle rather than by massive early crustal formation, which solves the paradox that early Archean domains only have a limited extension in the present-day continents. The Sm-Nd and Lu-Hf evolution of the depleted mantle for the past 3.5 Gyr can be entirely explained by continuous extraction of the continents from a well-mixed mantle. Thus, in contrast to the chondritic Earth model, Sm-Nd mass balance relationships can be satisfied without the need to call upon hidden reservoirs or layered mantle convection. This new Sm-Nd mass balance yields a scenario of mantle evolution consistent with trace element and noble gas systematics. The high ³He/⁴He mantle component is associated with ¹⁴³Nd/¹⁴⁴Nd compositions indistinguishable from the bulk silicate Earth, suggesting that the less degassed mantle sources did not experience significant fractionation for moderately incompatible elements.     

Isotopic and trace element constraints on the petrogenesis of lavas from the Mount Adams volcanic field,Washington

Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P) _n and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ ¹⁸O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust.     

The isotope composition of Hf in zircon from Paleoarchean plagiogneisses and plagiogranitoids of the Sharyzhalgai uplift (southern Siberian craton): Implications for the continental-crust growth

This paper presents results of U–Pb dating (SHRIMP-II) and Lu–Hf (LA–ICP MS) isotope study of zircon from Paleoarchean plagiogneisses and plagiogranitoids of the Onot and Bulun blocks of the Sharyzhalgai uplift. Magmatic zircons from the Onot plagiogneiss and Bulun gneissic trondhjemite are dated at 3388±11 and 3311±16 Ma, respectively. Magmatic zircons from plagiogneisses and plagiogranitoids of the studied tonalite–trondhjemite–granodiorite (TTG) complexes are characterized mainly by positive values of εHf indicating that felsic melts were generated mainly from juvenile (mafic) sources, which are derived from a depleted mantle reservoir. The variable Hf isotope composition in magmatic zircons and the lower average εHf values in comparison with the depleted mantle values suggest the contributions of both mafic and more ancient crustal sources to magma formation. Metamorphic zircons from the gneissic plagiogranite and migmatized plagiogneiss either inherited the Hf isotope composition from magmatic zircon or are enriched in radiogenic Hf. The more radiogenic Hf isotope composition of metamorphic zircons from the migmatized plagiogneisses is due to their interaction with melt during partial melting. Variations in the Lu–Hf isotope composition of zircon from the Bulun rocks in the period 3.33–3.20 Ga are due to the successive melting of mafic crust or the growing contribution of crustal material to their genesis. Correlation between the Lu–Hf isotope characteristics of zircon and the Sm–Nd parameters of the Onot plagiogneisses points to the contribution of ancient crustal material to their formation. The bimodal distribution of the model Hf ages of zircons reflects two stages of crustal growth in the Paleoarchean: 3.45–3.60 and ~ 3.35 Ga. The isotope characteristics of zircon and rocks of the TTG complexes, pointing to recycling of crustal material, argue for the formation of plagiogneisses and plagiogranitoids as a result of melting of heterogeneous (mafic and more ancient crustal) sources in the thickened crust.     

Mantle and crustal sources of Archean anorthosite: a combined in situ isotopic study of Pb–Pb in plagioclase and Lu–Hf in zircon

Isotopic analyses of ancient mantle-derived magmatic rocks are used to trace the geochemical evolution of the Earth’s mantle, but it is often difficult to determine their primary, initial isotope ratios due to the detrimental effects of metamorphism and secondary alteration. We present in situ analyses by LA-MC-ICPMS for the Pb isotopic compositions of igneous plagioclase (An_75–89) megacrysts and the Hf isotopic compositions of BSE-imaged domains of zircon grains from two mantle-derived anorthosite complexes from south West Greenland, Fiskenæsset and Nunataarsuk, which represent two of the best-preserved Archean anorthosites in the world. In situ LA-ICPMS U–Pb geochronology of the zircon grains suggests that the minimum crystallization age of the Fiskenæsset complex is 2,936 ± 13 Ma (2σ, MSWD = 1.5) and the Nunataarsuk complex is 2,914 ± 6.9 Ma (2σ, MSWD = 2.0). Initial Hf isotopic compositions of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 ε_Hf units. In terms of Pb isotopic compositions of plagioclase, both anorthosite complexes share a depleted mantle end member yet their Pb isotopic compositions diverge in opposite directions from this point: Fiskenæsset toward a high-μ, more radiogenic Pb, crustal composition and Nunataarsuk toward low-μ, less radiogenic Pb, crustal composition. By using Hf isotopes in zircon in conjunction with Pb isotopes in plagioclase, we are able to constrain both the timing of mantle extraction of the crustal end member and its composition. At Fiskenæsset, the depleted mantle melt interacted with an Eoarchean (~3,700 Ma) mafic crust with a maximum ¹⁷⁶Lu/¹⁷⁷Hf ~0.028. At Nunataarsuk, the depleted mantle melt interacted with a Hadean (~4,200 Ma) mafic crust with a maximum ¹⁷⁶Lu/¹⁷⁷Hf ~0.0315. Evidence from both anorthosite complexes provides support for the long-term survival of ancient mafic crusts that, although unidentified at the surface to date, could still be present within the Fiskenæsset and Nunataarsuk regions.     

Whole-rock Nd–Hf isotopic study of I-type and peraluminous granitic rocks from the Chinese Altai: Constraints on the nature of the lower crust and tectonic setting

The nature of the lower crust and tectonic setting of the Chinese Altai in the early to middle Paleozoic are still hotly debated. Decoupling between zircon Hf and whole-rock Nd isotopic systems for granites results in different interpretations for the above issues. In order to solve the problem, whole-rock Nd–Hf isotopic analyses were conducted on representative early to middle Paleozoic I-type granite and strongly peraluminous granites and rhyolites from the Chinese Altai. The I-type granites show metaluminous to weakly peraluminous feature and have ε_Nd(t) values ranging from − 2.2 to + 0.8 and ε_Hf(t) from + 3.9 to + 12.9, respectively. The strongly peraluminous granites and rhyolites have similar ε_Nd(t) and ε_Hf(t) values ranging from − 3.0 to + 1.7 and from + 2.1 to + 10.4, respectively. All samples plot above the Terrestrial Array on Nd–Hf isotopic diagram, indicating significant Nd–Hf isotopic decoupling in the magma sources. These samples show flatten HREE pattern and have Lu/Hf ratios similar to the average crust, suggesting that Nd–Hf isotopic decoupling was not originated from an ancient basement with elevated Lu/Hf ratios. The observed isotopic decoupling is similar to those modern island arcs, such as the Lesser Antilles and Sunda, where Nd selectively enriched over Hf due to metasomatism in the mantle wedge and consequently resulted in decoupling between the Sm–Nd and Lu–Hf isotopic systems. Our results, combined with the available data, show that prolonged subduction and crust–mantle interaction caused the Nd–Hf isotopic decoupling in the lithospheric mantle beneath the Chinese Altai. The crust of the Chinese Altai was extracted from the lithospheric mantle and inherited the Nd–Hf isotopic decoupling feature. Therefore, the Hf, rather than Nd, isotopic data more faithfully reflect the nature of the lower crust that was quite juvenile in the Paleozoic, and the Chinese Altai represents an early Paleozoic magmatic arc possibly built near western Mongolia.     

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

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

Episodic crustal growth in the Tanzania Craton: evidence from Nd isotope compositions

The analysis of available Nd isotope data from the Tanzania Craton places important constraints on the crust-mantle separation ages, and events marking juvenile crustal addition and crustal recycling. Nd model ages date the oldest crust extraction to 3.16 Ga in the Tanzania Craton, although a rock record of such antiquity is yet to be found there. The most significant period of juvenile crustal addition as well as crustal recycling is 2.7–2.6 Ga. The Nd isotopes of mafic samples show that chemical heterogeneity existed in the mantle beneath the Tanzania Craton, with some samples originating from significantly depleted mantle, and most samples originating from the mixture of primitive mantle and depleted mantle. The Nd isotope section reveals significant differences in Nd isotopes between the north craton and central craton; compared to the north craton, the central craton yields a Nd model age that is approximately 100 Ma older, and its ε_Nd(t) values are more negative, indicating that the two parts of the craton have different mantle source regions. Different types of granitoids are distributed in the Tanzania Craton, such as high-K and low-Al granite, calc-alkaline granite, peraluminous granite and transitional types of tonalite-trondhjemite-granodiorites (TTGs). Most of the granitoids formed later than the mafic rocks in syn-collision and post-collision events.     

Geochemistry,geochronology and Sr–Nd–Pb–Hf isotopic compositions of Middle to Late Jurassic syenite–granodiorites–dacite in South China: Petrogenesis and tectonic implications

In situ zircon U–Pb ages and Hf isotope data, major and trace elements and Sr–Nd–Pb isotopic compositions are reported for coeval syenite–granodiorites–dacite association in South China. The shoshonitic syenites are characterized by high K₂O contents (5.9–6.1 wt.%) and K₂O/Na₂O ratios (1.1–1.2), negative Eu anomalies (Eu/Eu* = 0.65 to 0.77), enrichments of Rb, K, Nb, Ta, Zr and Hf, but depletion of Sr, P and Ti. The adakitic granodiorite and granodiorite porphyry intrusions are characterized by high Al₂O₃ contents (15.0–16.8 wt.%), enrichment in light rare earth elements (LREEs), strongly fractionated LREEs (light rare earth elements) to HREEs (heavy rare earth elements), high Sr (438–629 ppm), Sr/Y (29.2–53.6), and low Y (11.7–16.8 ppm) and HREE contents (e.g., Yb = 1.29–1.64 ppm). The calc-alkaline dacites are characterized by LREE enrichment, absence of negative Eu anomalies, and enrichment of LILEs such as Rb, Ba, Th, U and Pb, and depletion of HFSEs such as Nb, Ta, P and Ti.Geochemical and Sr–Nd–Hf isotopic compositions of the syenites suggest that the shoshonitic magmas were differentiated from parental shoshonitic melts by fractional crystallization of olivine, clinopyroxene and feldspar. The parent magmas may have originated from partial melting of the lithospheric mantle with small amount contribution from crustal materials. The adakitic granodiorite and granodiorite porphyry have Sr–Nd–Pb isotopic compositions that are comparable to that of the mafic lower crust. They have low Mg# and MgO, Ni and Cr contents, abundant inherited zircons, low ε_Nd(t) and ε_Hf(t) values as well as old whole-rock Nd and zircon Hf model ages. These granodiorites were likely generated by partial melting of Triassic underplated mafic lower crust. The Hf isotopic compositions of the dacites are relatively more depleted than the Cathaysia enriched mantle, suggesting those magmas were derived from the partial melting of subduction-modified mantle sources. The coeval shoshonitic, high-K calc-alkaline and calc-alkaline rocks in Middle to Late Jurassic appear to be associated with an Andean-type subduction. This subduction could have resulted in the upwelling of the asthenosphere beneath the Cathaysia Block, which induced partial melting of the mantle as well as the mafic lower crust, and formed an arc regime in the coastal South China during Middle to Late Jurassic.     

Early Paleozoic lithospheric extension law,dynamic mechanism,origin of mafic dykes in Ziyang,South Qinling Block,ChinaSCIEI北大核心CSCD

南秦岭地块紫阳地区广泛出露早古生代基性岩墙群,其研究具有重要的地球动力学意义。虽然如此,已有相关研究目前主要集中于志留纪基性岩墙群方面,而对早古生代(如,寒武纪、奥陶纪)基性岩墙群的研究仍相对薄弱。从而制约了对南秦岭早古生代岩石圈伸展过程相关问题(如,时空分布规律、地幔性质、动力学机制和相关成矿作用)的总体把握。鉴于尚存的科学问题,本研究选取康家坪、梨树梁、大竹坪、清明寨、苟家山、庙梁上、曾家山和毛坝村的基性岩墙群为研究对象,开展了矿物学、岩石学、锆石U-Pb年代学、地球化学和Sr-Nd-Hf同位素方面的研究,对其精细时代格局、成因及成岩动力学背景进行了系统探讨。研究结果显示,紫阳早古生代基性岩墙以辉绿岩和辉绿玢岩为主,形成时代为奥陶纪(478.8-486Ma),属碱性系列岩石(K_(2)O+Na_(2)O变化范围为4.10%-5.14%)。另外,基性岩墙群具有富集LREE、Rb、Ba、Sr、Nb、Ta、Zr、Hf和Eu(Eu/Eu*=1.13-1.35),亏损HREE、U、Pb和Ti的典型地球化学特征。(87Sr/86Sr)i=0.7044-0.7050、εNd(t)=3.1-3.6、εHf(t)=4.4-12.1,176Hf/177Hf=0.282634-0.282846,暗示基性岩墙为地幔柱作用过程亏损岩石圈或软流圈地幔部分熔融作用的产物。另外,在辉绿岩成岩过程经历了橄榄石和单斜辉石的分离结晶,但地壳混染的影响不明显。     

Ancient depletion signals in lherzolites from forearc region: Constraints from Lu-Hf isotope compositions

The sub-arc mantle that experienced hydrous melting is commonly characterized by refractory geochemical compositions. Nevertheless, minor lherzolites with fertile compositions have also been reported for mantle peridotites from subduction zone. The petrogenesis and mantle source of the lherzolites are still controversial. The New Caledonia ophiolite(Peridotite Nappe) has been regarded as an allochthonous body of forearc lithosphere. This is supported by refractory compositions of its dominant mantle rocks.A few isolated lherzolitic massifs have also been observed in the northern part of New Caledonia.Those lherzolites are compositionally similar to abyssal peridotites, with negligible subduction-related modification. Here, we present new comprehensive geochemical compositions, in particular highprecision Sr-Nd-Hf isotope data, for the lherzolites. The initial¹⁷⁶ Hf/¹⁷⁷ Hf ratios display moderate correlations with sensitive indicators for the extent of melting(i.e., olivine Fo, whole-rock Mg# and Yb contents in clinopyroxene) and whole-rock initial¹⁸⁷ Os/¹⁸⁸ Os ratios. Some samples have ancient radiogenic Hf isotopes and unradiogenic Os isotope compositions, implying the preservation of ancient depletion signals in the lherzolites. The Nd isotope compositions, together with trace elements and mineral micro-textures, suggest that the lherzolites have been overprinted by a recent melt-rock interaction event. The high equilibrium temperatures of the studied samples have been estimated by the twopyroxene REE thermometer, yielding temperatures of 1066–1315 ℃. The lherzolites have more depleted Nd-Hf isotope compositions and higher equilibrium temperatures than the New Caledonia harzburgites.This indicates that the lherzolites may represent the residues of asthenosphere mantle trapped within the forearc region. Our studies on the New Caledonia lherzolites with ancient depletion signals suggest that ancient mantle domains in the convective mantle can be emplaced in forearc region by the upwelling of asthenosphere during the early stage of subduction initiation.     

Ca isotope fingerprints of early crust-mantle evolution

The utility of ⁴⁰Ca/⁴⁴Ca as a tracer of pre-existing crustal contributions in early Archaean cratons has been explored to identify traces of Hadean crust and to assess the style of continental growth. The relatively short half-life of ⁴⁰K (∼1.3 Gy) means that its decay to ⁴⁰Ca occurs dominantly during early Earth History. If Archaean crust had a significant component derived from a more ancient protolith, as anticipated by “steady state” crustal evolution models, this should be clearly reflected in radiogenic ⁴⁰Ca/⁴⁴Ca ratios (or positive initial ε_Ca) in different Archaean cratons. A high precision thermal ionisation technique has been used to analyse the ⁴⁰Ca/⁴⁴Ca ratios of plagioclase separates and associated whole rocks in ∼3.6 Ga (early Archaean) samples from Zimbabwe and West Greenland. Three out of four tonalite, trondhjemite, granodiorite (TTG) suite samples from Zimbabwe display initial ⁴⁰Ca/⁴⁴Ca ratios indistinguishable from our measured modern MORB value (i.e., ε_Ca(3.6) ∼ 0). Greenland samples, however, are very diverse ranging from ε_Ca(3.7) = 0.1 in mafic pillow lavas and felsic sheets from the Isua supracrustal belt, up to very radiogenic signatures (ε_Ca(3.7) = 2.9) in both mafic rocks of the Akilia association and felsic TTG from the coastal Amîtsoq gneisses.At face value, these results imply the Zimbabwe crust is juvenile whereas most Greenland samples include an earlier crustal component. Yet the west Greenland craton, as with many Archaean localities, has experienced a complex geological history and the interpretation of age-corrected initial isotope values requires great care. Both felsic and mafic samples from Greenland display ε_Ca(3.7) so radiogenic that they are not readily explained by crustal growth scenarios. The presence of such radiogenic ⁴⁰Ca/⁴⁴Ca found in low K/Ca plagioclases requires Ca isotope exchange between plagioclase and whole rock during later metamorphic event(s). In addition the unexpectedly radiogenic Ca isotope ratios in some mafic samples reflect anomalous K/Ca ratios as a result of intense K-metasomatism ∼3.6 Ga. Thus Ca isotope measurements are not a robust tracer of crustal growth in the presence of intense tectono-metamorphic processes. Coupled with other isotope data, however, the degree of overprint can be estimated and the ⁴⁰Ca/⁴⁴Ca ratio of a little disturbed sample hints at a small contribution of Hadean protocrust in the coastal part of the Godthåbsfjord area (Southwest Greenland). In the majority of Zimbabwe TTG samples, unradiogenic initial Ca isotope ratios point to very little prior crustal history and minor subsequent disturbance. We thus infer that the modest initial ε_Nd ∼0.8 of the Zimbabwean samples is representative of the depleted mantle at ∼3.6 Ga. Furthermore, Ca isotope systematics provide little support for a “steady state” model of crustal growth.     

A high field strength element perspective on early lunar differentiation

Lunar rocks are inferred to tap the different fossil cumulate layers formed during crystallisation of a lunar magma ocean (LMO). A coherent dataset, including Zr isotope data and high precision HFSE (W, Nb, Ta, Zr, Hf) and REE (Nd, Sm, Lu) data, all obtained by isotope dilution, can now provide new insights into the processes active during LMO crystallisation and during the petrogenesis of lunar magmas. Measured ⁹²Zr and ⁹¹Zr abundances agree with the terrestrial value within 0.2 ε-units. Incompatible-trace-element enriched rocks from the Procellarum KREEP Terrane (PKT) display Nb/Ta and Zr/Hf above the bulk lunar value (ca. 17), and mare basalts display lower ratios, generally confirming the presence of complementary enriched and depleted mantle reservoirs on the Moon. The full compositional spectrum of lunar basalts, however, also requires interaction with ilmenite-rich layers in the lunar mantle. Notably, the high-Ti mare basalts analysed display the lowest Nb/Ta and Zr/Hf of all lunar rocks, and also higher Sm/Nd at similar Lu/Hf than low-Ti basalts. The high-Ti basalts also exhibit higher and strongly correlated Ta/W (up to 25) and Hf/W (up to 140), at similar W contents, which is difficult to reconcile with ortho- and clinopyroxene-controlled melting. Altogether, these patterns can be explained via assimilation of up to ca. 20% of ilmenite- and clinopyroxene-rich LMO cumulates by more depleted melts from the lower lunar mantle. Direct melting of ilmenite-rich cumulates or the possible presence of residual metals in the lunar mantle both cannot easily account for the observed Ta/W and Hf/W patterns. Cumulate assimilation is also a viable mechanism that can partially buffer the Lu/Hf of mare basalts at relatively low values while generating variable Sm/Nd. Thus, the dichotomy between low Lu/Hf of lunar basalts and high time integrated source Lu/Hf as inferred from Hf isotope compositions can potentially be explained. The proposed assimilation model also has important implications for the short-lived nuclide chronology of the Earth-Moon system. The new Hf/W and Ta/W data, together with a compilation of existing W-Th-U data for lunar rocks, indicate that the terrestrial and lunar mantles are indistinguishable in their Hf/W. Virtually identical εW and Hf/W in the terrestrial and lunar mantle suggest a strong link between final core-mantle equilibration on Earth and the Moon forming giant impact. Previously, linear arrays of lunar samples in ¹⁸²W vs. Hf/W and ¹⁴²Nd vs. Sm/Nd spaces have been interpreted as isochrons, arguing for LMO crystallisation as late as 250 Myrs after solar system formation. Based on the proposed assimilation model, the ¹⁸²W and ¹⁴²Nd in many lunar magmas can be shown to be decoupled from their ambient Hf/W and Sm/Nd source compositions. As a consequence, the ¹⁸²W vs. Hf/W and ¹⁴²Nd vs. Sm/Nd arrays would constitute mixing lines rather than isochrons. Hence, the lunar ¹⁸²Hf-¹⁸²W and ¹⁴⁶Sm-¹⁴²Nd data would be fully consistent with an “early” crystallisation age of the LMO, even as early as 50 Myrs after solar system formation when the Moon was probably formed.