Crust-mantle System Research Articles

Scaling of earthquake waiting time distributions in northern Chile

SUMMARY In this study, we examine the scaling of earthquake waiting time distributions in Northern Chile. For this purpose, we analysed 7-yr high spatial resolution and low completeness magnitude IPOC seismic catalogue and 43-yr USGS catalogue. A unified earthquake moment–space–time scaling relation is empirically evaluated by calculating waiting times for different ranges of magnitude and epicentral area linear dimension and estimating the associated scaling coefficients, β analogue to b-value and γ, the correlation fractal dimension. We find a scaling function that can be characterized with three distinct regions, regions whose behaviour depends on whether seismicity is in the coastal area or at intermediate depth. Moreover, high resolution localizations from the IPOC catalogue allows us to further observe differences: the lower plane seismicity of the double seismic zone behaves just like that at intermediate depth, while the crustal upper plate and the upper plane from the double seismic zone seismicity behaves more like interface seismicity. Thus, the earthquake waiting time distribution primarily depends on whether seismicity is located in a crust-on-crust or in a crust–mantle system: having, respectively, high/low correlated behaviour in the short scale region, non-exponential/exponential decay in the transition middle region and, in all cases, long-term clustering with a slower than exponential decay in the long scale.

Magmatic Native Gold: Composition, Texture, Genesis, and Evolution in the Earth’s Crust

Abstract —Here we report results of microforms’ studies of native gold and its alloys in igneous rocks, modified to varying degrees by secondary processes. We discuss the composition and occurrence of both the deep-seated magmatic gold-bearing alloys and the products of their transformation under conditions of the upper Earth’s crust. Gold-bearing Kamchatka adakites and ankaramites, Ildeus massif mafic-ultramafic intrusions and adakites from the Stanovoy fold system as well as dacites from the Bolivian Andes were formed during melting of either the suprasubduction mantle wedge or the subducted oceanic crust. In depleted peridotites from the Avachinsky Volcano in Kamchatka as well as suprasubduction ophiolites from Polar Urals, Eastern Sayan and the Western Mediterranean Betic–Rifean belt, the gold-bearing mantle was hybridized by subduction-related melts and high-temperature fluids. Volcanic rocks associated with the Lesser Khingan Fe–Mn deposits and Zolotaya Gora Au deposit in Southern Urals as well as Taragai ultramafic rocks in the South Khingan Range display subduction-related geochemical characteristics. Gold-bearing trachytes in the Virginian Appalachians (USA) represent felsic differentiates of mafic intraplate magmas. We propose that one of the principal forms of gold transport into the upper crustal environments is represented by Cu–Ag–Au alloys, which precipitated from mantle-derived silicate melt enriched in chalcophile and siderophile elements. Such Cu–Ag–Au alloy-rich magmatic rocks can either constitute primary sources of precious metals in the mantle-crust system or serve as geochemical precursors to the formation of native gold assemblages in epithermal and mesothermal ore deposits. Presence of magmatic gold particles in subduction-related igneous rocks and mantle restites hybridized by subduction-derived melts and high-temperature fluids suggest the existence of gold-rich horizons in the Earth’s mantle at depths comparable to typical depths of generation of primary convergent zone and some within-plate magmas.

Open-system 182W[sbnd]142Nd isotope evolution of the Earth

We present a five-reservoir open-system model for the 182Hf182W and 146Sm142Nd isotope evolution of the Earth to constrain early mantle-crust exchange and late accretion processes. In the presented model, core formation is complete within 30–100 Myr after solar system formation. The complementary bulk silicate Earth (BSE) differentiates to form a continental crust (CC), an incompatible element depleted upper mantle (UM), an initially primitive but continuously evolving lower mantle (LM), and an isolated reservoir (IR) where the recycled crust is stored before mixing into the LM. In our default model, late accretion adds bulk Earth-like material to the mantle after core formation, concurrent to progressive silicate differentiation. The 182W and 142Nd isotope evolution in each reservoir is calculated with a series of differential equations that compute the changing abundance of each isotope from the start of solar system evolution (t = 4.56 Ga) to the present (t = 0 Ga). Core formation, by far, has the greatest influence on the 182Hf182W isotopic evolution of the bulk silicate Earth (BSE). It determines the ∼200 ppm offset between BSE's μ182W (μ182WBSE ∼ 0) and chondrites (μ182Wchondrite ∼ −200 ppm), whereas late accretion adds ∼10% of BSE's W which changes μ182WBSE by only ∼several 10s ppm. Most compatible with the modeled 182W evolution and W content of the silicate Earth are core formation processes lasting until ∼45–60 Myr after solar system initial and subsequent addition of about 1.0–0.5% late accreted material to Earth's mantle. However, both the pre-late accretion μ182W and W content of the BSE depend strongly on the rate and duration of core formation and ultimately determine the amount and type of late accreted material necessary to evolve to the present μ182WBSE ∼ 0, or any other value slightly higher or lower. Because tight constraints on the timing and rate of core formation during accretion are still lacking, the trade-off between core formation processes and the effects of late accretion on BSE's 182W evolution remains grossly under-constrained, making the μ182W evolution of Earth's mantle-crust system similarly under-determined. Nevertheless, robust constraints are that heterogeneous distribution of the late accreted material to Earth's mantle in combination with Hadean crust formation and recycling is required to form a mantle reservoir with a small range of 182W excesses between 10 and 15 ppm throughout the Archean, as observed in most Archean rocks. Reproducing the observed 142Nd and 182W signatures in Hadean-Eoarchean mantle-derived rocks with our model further requires that continental crust formation starts within the first ∼50 Myr after core formation. Continuous exchange between the crust and the different mantle reservoirs in our model leads to post-Archean homogenization of early-formed 182W and 142Nd heterogeneities and makes it difficult to preserve ancient μ182W and μ142Nd deviations from BSE values. Nevertheless, μ182W < 0 inherited from the combined effects of early crust-mantle differentiation and late accretion may under certain conditions be preserved in parts of Earth's mantle throughout the Archean, and perhaps even until today. The μ182W < 0 observed in recent oceanic basalts may therefore be inherited from early silicate differentiation or originate from potential core-mantle interaction at any time during Earth's history.

Re-Os systematics and chronology of graphite

Natural graphite forms in a range of metamorphic and hydrothermal environments across timelines spanning from the birth of the solar system, to the evolution of early Precambrian life, and the development of contemporary geotectonic cycles. A precise timeline of these and other graphite-forming events, however, has hitherto been obscured by a lack of radiometric ages and as such, chronologies are inferred from host-rock or hydrothermal mineral ages. Herein we examine the Re-Os systematics and chronology of graphite formed in a suite of terrestrial and extraterrestrial environments (n = 17) with the principal aim of establishing the viability of Re-Os geochronology of natural graphite.Graphite Re and Os contents and isotopic ratios exhibit a wide range of values that extend up to 1520 ppb Re, 19,577 ppt Os, and 4101 and 42.18 for 187Re/188Os and 187Os/188Os ratios, respectively. These values are broadly comparable to those reported for crustal sulfides, organic-rich sedimentary rocks, and hydrocarbons. X-ray diffraction crystallinity data reveals that graphite Re abundances show a broadly inverse correlation with graphite formation temperature and crystallinity (d002 and Lc(002)) with interplanar spacing (d002) having the strongest anti-correlation with graphite Re contents.Graphite Re-Os geochronology is demonstrated with two independent case studies (Wollaston-Mudjatik Transition shear zones, Saskatchewan, Canada and Merelani Hills, Tanzania) yielding precise (<1%) Re-Os isochron dates of 1731.52 ± 7.43 Ma (2σ; MSWD = 1.3) and 586.89 ± 2.39 Ma (2σ; MSWD = 1.2) that are consistent, within uncertainty, to their mineralization ages constrained by other radiometric methods. These data confirm that graphite mineralization was synchronous with Trans-Hudsonian exhumation and tsavorite-tanzanite gemstone mineralization, respectively. Method accuracy, however, appears contingent on the analytical protocols used to isolate graphite, e.g. handpicking vs. heavy liquids (SPT) and water, with the latter perturbing graphite Re-Os systematics by as much as 20%. We, therefore, recommend handpicking paired with magnetic separation and grain mount examination.Our Re-Os age results are then coupled with new SIMS carbon isotope data (Wollaston-Mudjatik Transition graphite: δ13C = −21.64 to −15.28‰; Merelani Hills graphite: δ13C = −25.90 to −24.36‰) and 187Os/188Osi isotope data (Wollaston-Mudjatik Transition graphite = 0.3119 ± 0.0037; Merelani Hills graphite = 1.680 ± 0.038) to constrain graphitic carbon to sedimentary carbonate/organic (Wollaston-Mudjatik Transition graphite) and organic (Merelani Hills graphite) carbon sources. This unique pairing of isotope systems in graphite provides the first detailed chronology of localized carbon mobility in the Earth’s crust.Re-Os graphite geochronology likely has wide applications in ore-deposit and metamorphic geology with the potential to reshape our understanding of carbon cycling in the crust-mantle system, and for graphite exploration initiatives that are critical for a global transition to a green economy.

The influence of oxygen fugacity and chlorine on amphibole–liquid trace element partitioning at upper-mantle conditions

Abstract. The recycling of volatile and incompatible elements through the entire Earth's history is one of the most important processes for the chemical evolution of the lithosphere. In this context, amphiboles are important hydrous minerals playing a key role in the chemical differentiation of the crust–mantle system. We carried out amphibole–liquid partitioning (Amph/LD) experiments starting from a hydrous alkali basalt with variable chlorine content and doped in 37 trace elements of geochemical interest. Experiments were conducted with an end-loaded piston cylinder apparatus at a fixed pressure of 1.4 GPa, temperature from 1015 to 1050 ∘C, and at two different oxygen fugacity conditions (fO2; ΔFMQ ≈-2.6 (log fO2 [experiment] − log fO2 [FMQ buffer]) and ΔFMQ ≈+ 1.7). These conditions approach those of the Earth's upper mantle chemically metasomatised by subduction-derived agents that may occur in several geological settings. All runs show Ti-pargasites and kaersutites, either as individual crystals or replacing former clinopyroxenes, in equilibrium with alkali-rich (4 wt %–6 wt %) and silica-intermediate (52 wt %–59 wt %) melts. A comprehensive trace element dataset (LLEs – light lithophile elements, LILEs – light ion lithophile elements, REEs – rare Earth elements, HFSEs – high field strength elements, actinides, transition metals, chalcophiles) has been determined in glass and amphibole with a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). Although the melt polymerization has a major influence on Amph/LDs, we provide evidence that changes in the fO2 of both the system and Cl content in amphibole (up to 0.88 wt %) also affect the Amph/LDs of several trace elements and their ratios. In particular, at higher fO2 conditions lower Amph/LD values for U and Mo are observed likely in relation to the low capability of amphibole to incorporate the more oxidized forms of these cations. The Amph/LDTh/Amph/LDU ratio is positively correlated with the Cl content in amphibole, while the opposite behavior is shown by Amph/LD ratios of other elements with the same charge and different ionic radius (e.g., Gd / Yb, Nb / Ta, and Pb / Ba). Dimensional modification of the amphibole structure in relation to the incorporation of Cl in the O(3) site are at the origin of the observed variations. General implications of our results are discussed.

Fe and O isotopes in coesite-bearing jadeite quartzite from the Western Alps record multistage fluid-rock interactions in a continental subduction zone

Fluid is a key agent for the mass transfer between the subducting slab and the mantle wedge, which greatly affects the evolution of the crust-mantle system at convergent plate boundaries. A geochemical study was carried out for coesite-bearing jadeite quartzite and its country rock granitic gneiss from the Dora Maira Massif in the Western Alps. The results provide new insights into the composition of metamorphic fluids and fluid-rock interaction processes in the continental subduction zone. Coesite inclusions are found for the first time in metamorphic zircons of the granitic gneiss. The ultrahigh-pressure (UHP) metamorphism was dated to occur at 34.7 ± 0.3 Ma, confirming that the country rock experienced the UHP metamorphism synchronously with the whiteschist and jadeite quartzite. The jadeite quartzite occurs as layers and boudins within the coesite-bearing whiteschist, and shows similar whole-rock REE distribution patterns to both granitic gneiss and whiteschist. In addition, relict domains of magmatic zircon in the three types of UHP metamorphic rocks exhibit similar U-Pb ages and δ18O values, indicating that they have the same protolith of granites. However, metamorphic zircons in the jadeite quartzite show significantly lower δ18O values of 6.1–7.3‰ than the relict magmatic domains of 9.4–10.6‰. Furthermore, the jadeite quartzite has whole-rock δ18O values of 7.9–8.7‰, significantly lower than those of 9.5–10.9‰ for the granitic gneiss but higher than 6.5–7.9‰ for the whiteschist. These differences suggest that the jadeite quartzite was probably formed through metasomatism of metagranite by external fluids with relatively low δ18O values. The jadeite quartzite shows high δ56Fe values of 0.69–0.87‰, considerably higher than those of 0.18–0.38‰ for the granitic gneiss, but falling within the range of 0.32–1.03‰ for the whiteschist from the same outcrop. The Fe isotope modeling results suggest that the metasomatic fluids responsible for the formation of jadeite quartzite were possibly derived from the host whiteschist under UHP metamorphic conditions. Such fluids are enriched in Si, Al and Mg, and have high δ56Fe but low δ18O values. Therefore, the jadeite quartzite records the action of metamorphic supercritical fluids at the subarc depth. If the metasomatized rocks in the continental subduction zone would dehydrate to generate fluids with variable geochemical compositions due to multistage fluid-rock interactions, the fluids could further infiltrate the surrounding rock and possibly the mantle wedge to result in further geochemical transfer from the subducting slab to the mantle wedge.

Crust-mantle geodynamic origin of ~2.7 Ga granitoid diversification in the Jiaobei terrane, North China Craton

The crust-mantle geodynamic origin of global late Archean granitoid diversification remains enigmatic. A series of early Neoarchean diversified granitoids were newly identified in the Jiaobei terrane of the North China Craton, including ~2749–2739 Ma Na-rich quartz dioritic, tonalitic, and trondhjemitic (DTTG) gneisses, and ~2718–2701 Ma K-rich quartz monzodioritic, granodioritic, and monzogranitic gneisses.The quartz dioritic and TTG gneisses commonly show transitional field relationships. They have low K2O/Na2O (0.19–0.59), and are characterized by decreasing MgO, FeOT, TiO2, P2O5, and CaO, changing Eu anomalies from negative to positive, and more fractionated REEs, with increasing SiO2. Both the quartz dioritic and TTG gneisses possess depleted zircon εHf(t2) values (+1.4 to + 7.0). Through quantitative petrogenetic modeling, a fractional crystallization model is invoked, i.e., the quartz dioritic gneisses were derived from partial melting of a depleted mantle source metasomatized by slab-derived melts, whereas TTG gneisses (a low silica and a high silica group) formed by different degrees of fractionation from the quartz dioritic magmas, with inferred liquidus minerals of hornblende, clinopyroxene, plagioclase, and apatite. In comparison, the K-rich granitoids show higher K2O/Na2O (0.69–2.87), with some containing euhedral muscovites. The quartz monzodioritic and granodioritic gneisses have higher MgO and Mg#, but lower Fe* (FeOT/(FeOT + MgO)) than the monzogranitic gneisses. The zircon εHf(t2) values (-0.5 to + 5.6) are lower than those of DTTG gneisses. It is suggested that these K-rich granitoids formed by partial melting of either a metasomatized mantle source (with recycled sediments) or metasedimentary rocks at diverse crustal levels.Early Neoarchean crust-mantle interactions of the Jiaobei terrane are reconstructed, including (1) earlier slab-mantle wedge interactions under a continental marginal arc, forming quartz dioritic and the derivative tonalitic and trondhjemitic rocks; (2) ongoing convergence led to high relief of the arc mountain, facilitating erosion and reworking/recycling of sediments; and (3) partial melting of coupled crust-mantle system following accretion between the above arc system and regional ~2.9 Ga or older continental nucleus generated different K-rich granitoids. It is emphasized that the construction and erosion of thickened arcs are prone to the formation of both high pressure TTGs and K-rich granitoids, leading to the late Archean granitoid diversification globally.

Compositional heterogeneity of Archean mantle estimated from Sr and Nd isotopic systematics of basaltic rocks from North Pole, Australia, and the Isua supracrustal belt, Greenland

Compositional variability found in modern mid-ocean ridge basalt (MORB) and ocean island basalt (OIB) reflects differentiation processes associated with material recycling in the mantle-crust system. To investigate the timing at which this recycling system was established and how it transformed into the present-day system, we present geochemical analyses of the Archean basalts from North Pole (NP) in the East Pilbara Craton, Western Australia, and the Isua supracrustal belt (ISB), southern West Greenland. These rocks represent Archean accretionary complexes with ages of ~3.5 Ga and 3.7–3.8 Ga, respectively. We analyzed the trace element contents including rare earth elements (REEs), and Sr and Nd isotopic compositions of the basalts, which may represent MORBs and OIBs, from NP and ISB.Their trace-element compositions are broadly similar, but show distinct geochemical characteristics particularly with respect to REEs that probably reflect differences in both the source mantle and degree of melting. Such differences are also evident in their initial Nd isotopic compositions, which were estimated based on equilibrium partitioning of REEs and well-defined isochron ages. In contrast, the Sr isotopic compositions of the NP and ISB basalts are highly variable and their isochron ages are inconsistent with previous studies. Furthermore, the partitioning of Rb and Sr in the NP basalts indicates disequilibrium, suggesting that the Rb-Sr system has been disturbed by post-igneous alteration and metamorphism.Based on these observations, we propose the following model to explain the temporal variations in the geochemical composition of the Archean mantle: (i) ~3800 Ma: recycling of plate material and melting occurred quite readily and, therefore, MORBs and OIBs were produced from differentiated mantle sources; (ii) 3460 Ma to ~3800 Ma: mantle-crust mixing occurred as the result of an extreme event, such as mantle overturning, reducing the compositional variation of the mantle; and (iii) after ~3460 Ma: mantle heterogeneity gradually developed in the material-recycling system, re-establishing the compositional differences between MORBs and OIBs. This model requires an extreme event to drive the homogenization during stage (ii), which may provide new insights into the evolution of the crust-mantle system.

Constraints on Archean crust formation from open system models of Earth evolution

Establishing the mode and rate of formation of the continental crust is crucial for quantifying mass exchange between Earth’s crust and mantle. The limited crustal rock record, particularly of early Archean rocks, has led to a variety of different models of continental growth. Here, we present an open-system model of silicate Earth evolution incorporating the Sm-Nd and Lu-Hf isotope systematics with the aim to constrain crustal growth during the Archean and its effect on the chemical and isotopic evolution of Earth’s crust-mantle system. Our model comprises four reservoirs: the bulk continental crust (CC), depleted upper mantle (UM), lower mantle (LM), and an isolated reservoir (IR) where recycled crust is stored transiently before being mixed with the LM. The changing abundance of isotope species in each reservoir is quantified using a series of first order linear differential equations that are solved numerically using the fourth order Runge–Kutta method at 1 Myr time steps for 4.56 Gyr (the age of the Earth). The model results show that only continuous and exponential crustal growth reproduces the present-day abundances and isotope ratios in the terrestrial reservoirs. Our preferred crustal growth model suggests that the mass of the CC by the end of Hadean (4.0 Ga) and end of Archean (2.5 Ga) was ∼30% and ∼75% of the present-day mass of the CC, respectively. Models proposing formation of most (∼90%) of the present-day CC during the initial 1 Gyr or nearly 50–60% during the last 1 Gyr are least favorable. Significant mass exchange between crust and mantle, that is, both the formation and recycling of crust, started in the Hadean with Sm-Nd and Lu-Hf isotope evolution typical for mafic rocks. Depletion of the UM (in incompatible elements) during the early Archean is mitigated by the input of recycled crust, so that the UM maintained a near-primitive Hf-Nd isotope composition. The LM also retained a near-primitive Hf-Nd isotope composition during the Archean, but for different reasons. In contrast to the UM, the crustal return flux into the LM is transiently stored (∼ 1 Gyr) in an isolated reservoir (IR), which limits the mass flux into and out of the LM. The IR in our model is distinct from other mantle reservoirs and possibly related to stable crustal blocks or, alternatively, to recycled crust in the mantle that remains temporarily isolated, perhaps at the core-mantle boundary (LLSVPs).

Testing the occurrence of Late Jurassic true polar wander using the La Negra volcanics of northern Chile

True polar wander (TPW) is the reorientation of the crust-mantle system driven by the redistribution of masses in the mantle and on the Earth's surface. In the ideal case, characterization of TPW requires paleomagnetic constraints on the motion of all major plates and independent reconstructions of relative plate positions. While such complete datasets are absent for pre-Mesozoic TPW inferences due to the absence of oceanic plates, they are available for the Late Jurassic (165-145 Ma) “monster shift”, a ∼30° amplitude proposed TPW event. Here we perform paleomagnetic sampling and Ar-Ar geochronology on the La Negra volcanics of Northern Chile, producing two new paleomagnetic poles with ages 165.8 ± 1.8 Ma (1σ; 84.3°N 0.9°E; α95=7.6°; N=28) and 152.8 ± 0.8 Ma (84.5°N 256.4°E; α95=10.8°; N=18). By combining these data with other recently published results, we compute a net lithospheric rotation of 25.3° ± 7.3° (1σ) at a mean rate of 1.21° ± 0.35° My−1 between 170 and 145 Ma with a peak rate of 1.46° ± 0.65° My−1 between 160 and 145 Ma. These rates are consistent with inferences from the Pacific Plate, implying true whole lithosphere rotation. Given coherent motion involving the entire lithosphere, we conclude that the Earth underwent rapid TPW between approximately 165 and 145 Ma, potentially driven by the cessation of subduction along the western North American margin.

Tracking Fe mobility and Fe speciation in subduction zone fluids at the slab-mantle interface in a subduction channel: A tale of whiteschist from the Western Alps

Fluids released by subducting slabs are important to the geochemical evolution of Earth’s crust-mantle system. However, it is still not well constrained for the mobility of redox sensitive elements (like Fe, C or S) in subduction zone fluids, and its effect on the redox property of the overlying mantle wedge and its derived arc magmas. Iron isotope fractionation is sensitive to the redox state and speciation of Fe in fluids, which can potentially provide clues on the oxygen fugacity (fO2) of slab-derived fluids. Whiteschists at the Dora-Maira Massif in the Western Alps have experienced ultrahigh-pressure metamorphism at subarc depths. They are rich in both SiO2 (mostly >65 wt.%) and MgO (4–10 wt.%), and mainly composed of pyrope, quartz/coesite, talc, phengite and kyanite. They were demonstrated to have a granite protolith, but metasomatized by serpentinite-derived Mg-rich fluids at the slab-mantle interface in a subduction channel. Thus these rocks provide an excellent target to explore the fluid-rock interaction by Fe isotopes. The whiteschists show extremely high δ56Fe values of +0.32 to +1.22‰, being much higher than those of +0.10 to +0.38‰ for the surrounding metagranites. The Fe isotope composition of whiteschists significantly deviates from the igneous differentiation trend defined by rocks with basaltic to rhyolitic compositions. The heavy Fe isotope compositions must be produced by a metasomatic process. The much lower Fe2O3 and FeOt contents and Fe3+/∑Fe ratios but higher δ56Fe values for the whiteschists relative to their protolith suggest the reduction of Fe3+ to Fe2+ and the loss of isotopically light Fe, probably in the form of Fe(II)-Cl and/or Fe(II)-(HS) complexes, through saline fluids and/or HS- bearing fluids. The reduction of Fe3+ to Fe2+ and the possible presence of HS- suggests the occurrence of relatively reduced fluids. Such reduced fluids were probably derived from serpentinite dehydration at the slab-mantle interface in the subduction channel. Therefore, the Fe isotope results demonstrate that the fO2 at the slab-mantle interface can be locally highly heterogeneous, leading to various oxidation states in the mantle wedge and arc magmas.

Hf isotopic fingerprinting of geodynamic settings: Integrating isotopes and numerical models

Hf isotopes have proven invaluable in understanding the evolution of Earth's crust-mantle system, but their use in reconstructing tectonic environments, in many cases, remains equivocal. In this study, we introduce a new approach to predict the Hf isotopic evolutionary pattern for rifting and collision based on the integration of numerical models and 176Hf/177Hf isotopes. The geodynamic numerical models allow us to estimate the proportion of juvenile material added to the crust through time. On the basis of this proportion, we calculate changing 176Hf/177Hf ratios using mixing models. Predicted Hf isotopic patterns generated through this numerical approach imply that juvenile signals are observed during back-arc extension, whereas evolved signatures dominate collisional settings. We use this novel modeling approach in the case study region of the Halls Creek Orogen to elucidate its tectonic setting through time. In addition, the geochemical features of magmatic rocks in the case study region imply partial melting of a sub-arc mantle wedge with magma-crust interaction on ascent in a convergent margin setting. The links between predicted Hf isotopic evolution, geodynamic numerical models, whole rock geochemistry and measured zircon Hf isotopic evolution trend resolve three discrete stages in the tectonomagmatic development of the Halls Creek Orogen: (1) oceanic crust subduction; (2) back-arc formation with addition of juvenile mantle input; and (3) docking of the North Australian and Kimberley cratons resulting in the development of mixed-source magmatism formed in a collisional setting. We provide a new method to validate geodynamic models with isotopic datasets, which should lead to more rigorous understanding of crustal evolution.

The role of subduction recycling on the selenium isotope signature of the mantle: Constraints from Mariana arc lavas

Investigating the isotope systematics and behavior of selenium in subduction zones provides valuable insights into mechanisms that contribute to chalcophile and moderately volatile element distribution between terrestrial reservoirs. In this study, we present high-precision Se isotope and Se–Te elemental data on subduction zone lavas from the Mariana arc system. Our results indicate that Se–Te concentrations are unaffected by submarine degassing but are affected by magmatic differentiation. In contrast, Se isotopes of submarine lavas are unaffected by both magmatic differentiation and degassing and thus may preserve their magmatic source signature. Compared to the average Se isotope composition previously obtained for mantle-derived samples (δ82/76Se = 0.23 ± 0.12‰, 2 s.d., n = 5), suitable to represent the Mariana pre-subduction wedge, Mariana lavas show a larger overall range (δ82/76Se from 0.03 to −0.33‰, n = 21) with a clear tendency towards lighter Se isotope compositions. The variable Se isotope signatures within the Mariana suite can further be attributed to different slab-derived fluid and melt-like subduction components. This provides evidence for a significant role of subduction recycling of hydrothermal sulfide-containing altered oceanic crust and pelagic sediments with possible implications for the Se isotope evolution of the crust-mantle system throughout geological time.

Anomalous Late Jurassic motion of the Pacific Plate with implications for true polar wander

True polar wander, or TPW, is the rotation of the entire mantle–crust system about an equatorial axis that results in a coherent velocity contribution for all lithospheric plates. One of the most recent candidate TPW events consists of a ∼30° rotation during Late Jurassic time (160–145 Ma). However, existing paleomagnetic documentation of this event derives exclusively from continents, which compose less than 50% of the Earth's surface area and may not reflect motion of the entire mantle–crust system. Additional paleopositional information from the Pacific Basin would significantly enhance coverage of the Earth's surface and allow more rigorous testing for the occurrence of TPW. We perform paleomagnetic analyses on core samples from Ocean Drilling Program (ODP) Site 801B, which were taken from the oldest available Pacific crust, to determine its paleolatitude during the Late Jurassic and Early Cretaceous (167–133 Ma). We find that the Pacific Plate underwent a steady southward drift of 0.49°–0.74° My−1 except for an interval between Kimmeridgian and Tithonian time (157–147 Ma), during which it underwent northward motion at 1.45° ± 0.76° My−1 (1σ). This trajectory indicates that the plates of the Pacific Basin participated in the same large-amplitude (∼30°) rotation as continental lithosphere in the 160–145 Ma interval. Such coherent motion of a large majority of the Earth's surface strongly supports the occurrence of TPW, suggesting that a combination of subducting slabs and rising mantle plumes was sufficient to significantly perturb the Earth's inertia tensor in the Late Jurassic.

Accurate stable tungsten isotope measurements of natural samples using a 180W-183W double-spike

Tungsten is a moderately siderophile element and, thus, enriched in the Earth's core. Moreover, W behaves incompatibly during partial melting, causing relative enrichment in the Earth's crust compared to the mantle. However, little is known about the geochemical cycle of the redox-sensitive element W in the crust-mantle system and in modern to ancient low-temperature environments. High resolution stable W isotope measurements of rock samples from different geochemical reservoirs might be a powerful tool to better constrain this cycle. So far, low relative mass differences between the different W isotopes and analytical challenges hampered such high-resolution measurements. Notably, some pioneering studies on the stable W isotope composition of geological reference material show inconsistent results, calling for further verification of the true compositions of these materials.This study presents an analytical protocol for stable W isotope measurements including the calibration of a 180W-183W double-spike as well as W isotope and W concentration data of several geological reference materials (BHVO-2, AGV-2, SDC-1, W-2a, ScO-2, NOD-A-1, NOD-P-1). The reproducibility of stable W isotope measurements (±0.018‰ in δ186/184W; 2s.d.) is significantly improved compared to previous studies, which allows resolving between the stable W isotope compositions of various rock reservoirs on Earth. Relative to the NIST SRM 3163 standard, the highest δ186/184W value was observed for the Pacific Mn crust NOD-P-1 (+0.154±0.013‰; 2s.d.; n=6), which is significantly different from the δ186/184W value of the Atlantic Mn crust NOD-A-1 (+0.029±0.014‰; 2s.d.; n=6). Considering equilibrium fractionation between seawater WO42− and slowly growing Mn oxides, this indicates an isotopically heterogeneous distribution of W in the modern oceans. Igneous rocks also show a resolvable range in δ186/184W values. Magmatic reference materials range in δ186/184W between +0.016±0.032‰ (andesite AGV-2; 2s.d.; n=5) and +0.082±0.010‰ (basalt BHVO-2; 2s.d.; n=5) showing relative enrichment of light isotopes in more evolved magmatic rocks. These isotopic differences might result from isotope fractionation during magmatic differentiation. Alternatively, the mobilization of W by hydrothermal and/or magmatic fluids might be accompanied by isotope fractionation.

The boron and lithium isotopic composition of mid-ocean ridge basalts and the mantle

A global selection of 56 mid-ocean ridge basalt (MORB) glasses were analysed for Li and B abundances and isotopic compositions. Analytical accuracy and precision of analyses constitute an improvement over previously published MORB data and allow a more detailed discussion of the Li and B systematics of the crust-mantle system. Refined estimates for primitive mantle abundances ([Li]=1.39±0.10μg/g and [B]=0.19±0.02μg/g) and depleted mantle abundances ([Li]=1.20±0.10μg/g and [B]=0.077±0.010μg/g) are presented based on mass balance and on partial melting models that utilise observed element ratios in MORB.Assimilation of seawater (or brine) or seawater-altered material beneath the ridge, identified by high Cl/K, causes significant elevation of MORB δ11B and variable elevation in δ7Li. The B isotope ratio is, hence, identified as a reliable indicator of assimilation in MORB and values higher than −6‰ are strongly indicative of shallow contamination of the magma.The global set of samples investigated here were produced at various degrees of partial melting and include depleted and enriched MORB from slow and fast-spreading ridge segments with a range of radiogenic isotope signatures and trace element compositions. Uncontaminated (low-Cl/K) MORB show no significant boron isotope variation at the current level of analytical precision, and hence a homogenous B isotopic composition of δ11B=-7.1±0.9‰ (mean of six ridge segments; 2SD). Boron isotope fractionation during mantle melting and basalt fractionation likely is small, and this δ11B value reflects the B isotopic composition of the depleted mantle and the bulk silicate Earth, probably within ±0.4‰.Our sample set shows a mean δ7Li=+3.5±1.0‰ (mean of five ridge segments; 2SD), excluding high-Cl/K samples. A significant variation of 1.0–1.5‰ exists among various ridge segments and among samples within individual ridge segments, but this variation is unrelated to differentiation, assimilation or mantle source indicators, such as radiogenic isotopes or trace elements. It, therefore, seems likely that kinetic fractionation of Li isotopes during magma extraction, transport and storage may generate δ7Li excursions in MORB. No mantle heterogeneities, such as those generated by deeply recycled subducted materials, are invoked in the interpretation of the Li and B isotope data presented here, in contrast to previous work on smaller data sets.Lithium and boron budgets for the silicate Earth are presented that are based on isotope and element mass balance. A refined estimate for the B isotopic composition of the bulk continental crust is given as δ11B=-9.1±2.4‰. Mass balance allows the existence of recycled B reservoirs in the deep mantle, but these are not required. However, mass balance among the crust, sediments and seawater shows enrichment of 6Li in the surface reservoirs, which requires the existence of 7Li-enriched material in the mantle. This may have formed by the subduction of altered oceanic crust since the Archaean.

Open system models of isotopic evolution in Earth’s silicate reservoirs: Implications for crustal growth and mantle heterogeneity

An open system evolutionary model of the Earth, comprising continental crust (CC), upper and lower mantle (UM, LM), and an additional isolated reservoir (IR) has been developed to study the isotopic evolution of the silicate Earth. The model is solved numerically at 1Myr time steps over 4.55Gyr of Earth history to reproduce both the present-day concentrations and isotope ratios of key radioactive decay systems (Rb–Sr, Sm–Nd, and U–Th–Pb) in these terrestrial reservoirs. Various crustal growth scenarios – continuous versus episodic and early versus late crustal growth – and their effect on the evolution of Sr–Nd–Pb isotope systematics in the silicate reservoirs have been evaluated. Modeling results where the present-day UM is ∼60% of the total mantle mass and a lower mantle that is non-primitive reproduce the estimated geochemical composition and isotope ratios in Earth’s silicate reservoirs. The isotopic evolution of the silicate Earth is strongly affected by the mode of crustal growth; only an exponential crustal growth pattern with crustal growth since the early Archean satisfactorily explains the chemical and isotopic evolution of the crust–mantle system and accounts for the so-called Pb paradoxes. Assuming that the OIB source is located in the deeper mantle, our model could, however, not reproduce its target ɛNd of +4.6 for the UM, which has been estimated from the average isotope ratios of 32 individual ocean island localities. Hence, either mantle plumes sample the LM in a non-representative way, or the simplified model set-up does not capture the full complexity of Earth’s lower mantle (Nd isotope) evolution. Compared to the results obtained for a 4.55Ga Earth, a model assuming a protracted U–Pb evolution of silicate Earth by ca. 100Myr reproduces a slightly better fit for the Pb isotope ratios in Earth’s silicate reservoirs. One notable feature of successful models is the early depletion of incompatible elements (as well as rapid decrease in Th/U) in the UM within the initial 500Myr, as a result of early formation of CC, which supports other evidence in favor of the presence of Hadean continental crust. Therefore, a chondritic Th/U ratio (4±0.2) in the UM until 2Gyr appears rather unlikely. We find that the κ conundrum – the observation that measured Th/U ratios and those deduced from 208Pb–206Pb isotope systematics differ – is a natural outcome of an open system evolution in which preferential recycling of U for the past 2Gyr has played a dominant role. Overall, our simulations strongly favor exponential crustal growth, starting in the early Hadean, the transient preservation of compositionally distinct mantle reservoirs over billion year time periods, and a generally less incompatible element depleted, but non-primitive composition of the lower mantle.

No evidence for Hadean continental crust within Earth’s oldest evolved rock unit

Little is known about the character of the Hadean crust. Geochemical analyses of the 4-billion-year-old Acasta Gneiss from Canada suggest Earth’s earliest crust formed from a mafic reservoir, similar to the formation of oceanic crust today. Due to the acute scarcity of very ancient rocks, the composition of Earth’s embryonic crust during the Hadean eon (>4.0 billion years ago) is a critical unknown in our search to understand how the earliest continents evolved. Whether the Hadean Earth was dominated by mafic-composition crust, similar to today’s oceanic crust1,2,3,4, or included significant amounts of continental crust5,6,7,8 remains an unsolved question that carries major implications for the earliest atmosphere, the origin of life, and the geochemical evolution of the crust–mantle system. Here we present new U–Pb and Hf isotope data on zircons from the only precisely dated Hadean rock unit on Earth—a 4,019.6 ± 1.8 Myr tonalitic gneiss unit in the Acasta Gneiss Complex, Canada. Combined zircon and whole-rock geochemical data from this ancient unit shows no indication of derivation from, or interaction with, older Hadean continental crust. Instead, the data provide the first direct evidence that the oldest known evolved crust on Earth was generated from an older ultramafic or mafic reservoir that probably surfaced the early Earth.

Last interglacial (MIS 5e) sea-level determined from a tectonically stable, far-field location, Eyre Peninsula, southern Australia

ABSTRACTThe last interglacial maximum (Marine Isotope Substage 5e [MIS 5e], 128–116 ka) is a distinctive event in recent Earth history. Shoreline successions of this age are important for calibrating climate models and defining the overall behaviour of the crust–mantle system to fluctuating ice and ocean-water volumes. In a global context, the recently intensified interest in last interglacial shoreline successions has revealed considerable variability in the magnitude of sea-level rise during this time interval and highlighted the need to examine paleosea-level evidence from tectonically stable, far-field settings. Situated in the far-field of continental ice sheets and on the tectonically stable Gawler Craton, the 300 km coastal sector of western Eyre Peninsula between Fowlers Bay and Lake Newland in southern Australia represents an important region for defining the glacio-eustatic (ice-equivalent) sea-level attained during the last interglacial maximum based on the relative sea-level observations from this region. Low-energy, shoaling upward, peritidal bioclastic carbonate successions of the last interglacial (locally termed Glanville Formation) formed within back-barrier, estuarine–lagoonal environments in the lee of eolianite barrier complexes (locally termed Bridgewater Formation) along this coastline. The well-preserved shelly successions (coquinas) contain diverse molluscan fossil assemblages including species no longer living in the coastal waters of South Australia (e.g. the Sydney cockle Anadara trapezia and the benthic foraminifer Marginopora vertebralis). The extent of amino acid racemisation (a measure of fossil age based on increasing d/l value) in a range of species, and in particular A. trapezia and Katelysia sp., confirms the time equivalence of the isolated embayment-fill successions, correlated with the informal type section of the Glanville Formation at Dry Creek, north of Adelaide. Preliminary U-series analyses on A. trapezia also suggest a correlation with the last interglacial maximum, but further highlight the complexity in dating fossil molluscs by the U-series method in view of their open-system behaviour. The shelly successions of the Glanville Formation occur at elevations higher than attained by sea-level in the current, Holocene interglacial. A higher sea-level of between 2.1 ± 0.5 and 4 ± 0.5 m above present sea-level is inferred for the last interglacial maximum (MIS 5e) along this coastline based on the elevation of sedimentary successions host to the shallow subtidal–intertidal fossil molluscs Katelysia sp., and Anadara trapezia. The paleosea-level observations place a lower limit on the sea-level attained during the last interglacial maximum and suggest that caution be exercised in the definition of the upper limit of sea-level during this interglacial.

Granitoides e séries magmáticas: o estudo contextualizado dos granitoides

A relação que muitas rochas graníticas possuem com magmas menos diferenciados provenientes do manto tem sido subestimada nas últimas décadas, perdendo-se assim uma poderosa ferramenta científica para investigação de ambientes geotectônicos antigos e da própria evolução do sistema manto-crosta. Propõem-se neste trabalho que os magmas graníticos passem a ser considerados como possíveis produtos de diferenciação de magmas provenientes do manto, com variável, e por vezes predominante, assimilação de fusões crustais. Com este objetivo, são revistos os critérios para enquadramento dos granitoides nas diferentes séries magmáticas, avaliadas as implicações geotectônicas deste enquadramento e também discutidas as características dos granitoides gerados por pura fusão crustal.