Acasta Gneiss Research Articles

Zircon geochemistry from early evolved terranes records coeval stagnant- and mobile-lid tectonic regimes

Determining the mechanisms by which the earliest continental crust was generated and reworked is important for constraining the evolution of Earth's geodynamic, surface, and atmospheric conditions. However, the details of early plate tectonic settings often remain obscured by the intervening ~4 Ga of crustal recycling. Covariations of U, Nb, Sc, and Yb in zircon have been shown to faithfully reflect Phanerozoic whole-rock-based plate-tectonic discriminators and are therefore useful in distinguishing zircons crystallized in ridge, plume, and arc-like environments, both in the present and in deep time. However, application of these proxies to deciphering tectonic settings on the early Earth has thus far been limited to select portions of the detrital zircon record. Here, we present in situ trace-element and oxygen isotope compositions for magmatic zircons from crystalline crustal rocks of the Acasta Gneiss Complex and the Saglek-Hebron Complex, Canada. Integrated with information from whole-rock geochemistry and zircon U-Pb, Hf, and O isotopes, our zircon U-Nb-Sc-Yb results reveal that melting of hydrated basalt was not restricted to a single tectonomagmatic process during the Archean but was operative during the reworking of Hadean protocrust and the generation of juvenile crust within two cratons, as early as 3.9 Ga. We observe zircon trace-element compositions indicative of hydrous melting in settings that otherwise host seemingly differing whole-rock geochemistry, zircon Hf, and zircon O isotopes, suggesting contemporaneous operation of stagnant-lid (oceanic plateau) and mobile-lid (arc-like) regimes in the early Archean.

Regional-Scale Paleoproterozoic Heating Event on Archean Acasta Gneisses in Slave Province, Canada: Insights from K–Ar and 40Ar/39Ar Chronology

Slave Province in Canada is an Archean granite–supracrustal terrane at the northwestern corner of the Canadian Shield. It is bordered by the Thelon–Taltson orogen (2.0 to 1.9 Ga) to the southeast and the Wopmay orogen (1.9 to 1.8 Ga) to the west. Acasta gneisses, exposed in the westernmost Slave Province, and the Wopmay rocks, located close to the gneisses, were systematically collected for K–Ar and laser step-heating 40Ar/39Ar single-crystal analyses of the biotite and amphibole. The K–Ar biotite ages of the four Wopmay samples range from 1816 ± 18 Ma to 1854 ± 26 Ma. The 40Ar/39Ar biotite analyses of the three Wopmay samples yield plateau ages of 1826 ± 21 Ma, 1886 ± 13 Ma, and 1870 ± 18 Ma. These ages fall within the reported U–Pb zircon age range of the Wopmay orogen. The K–Ar biotite ages of the fifteen Acasta gneisses range from 1779 ± 25 Ma to 1877 ± 26 Ma, except for one younger sample (1711 ± 25 Ma). The 40Ar/39Ar analyses of the biotite crystals from three samples give the plateau ages of 1877 ± 8 Ma, 1935 ± 14 Ma, and 1951 ± 11 Ma. The K–Ar amphibole ages from twelve samples range from 1949 ± 19 Ma to 1685 ± 25 Ma. Two samples of them give ages older than the zircon U-Pb age of Hepburn plutons. The 40Ar/39Ar analyses of the amphibole crystals show varied age relations. The two samples give plateau ages of 1814 ± 22 Ma and 1964 ± 12 Ma. Some samples exhibit apparent old ages of ~2000 Ma in the middle temperature fractions. These old fractions result from the amphibole crystals, originally formed in the Archean, being affected by the thermal events during the Wopmay orogeny but not fully resetting. These observations suggest that the K–Ar system ages of the biotite and amphibole in the Archean Acasta gneiss were rejuvenated during the Paleoproterozoic ages. The Discussion explores the possibility that the heat source rejuvenating the K–Ar system ages may have arisen due to asthenospheric extrusion into the wedge mantle, a process likely triggered by subduction rollback.

Paleoarchean metamorphism in the Acasta Gneiss Complex: Constraints from phase equilibrium modelling and in situ garnet Lu–Hf geochronology

AbstractThe oldest known evolved (felsic) rocks on Earth (c. 4.03 Ga) are found in the Acasta Gneiss Complex (AGC) in north‐western Canada and represent a fundamental keystone in unravelling the geological processes governing crustal growth and differentiation during the Hadean and early Archean. Although the timing of multiple episodes of magmatism, metamorphism and deformation in these tonalitic gneisses has been investigated extensively, the metamorphic pressure–temperature (P–T) conditions recorded by the rocks are poorly constrained. Here, we use phase equilibrium modelling coupled with in situ garnet Lu–Hf geochronology and trace element analysis for two garnet‐bearing tonalitic gneisses to decipher the metamorphic history of the AGC. The observed mineral assemblages are consistent with peak metamorphic conditions of T = 725–780°C and P = 4.5–6.2 kbar and the generation of a small amount of melt (<7 vol.%). Garnet geochronology constrains the age of metamorphism to 3.3–3.2 Ga, consistent with previous evidence for a late Paleoarchean tectono‐metamorphic event in the AGC. Subsequent isotopic disturbance of garnet at c. 1.9 Ga is interpreted to correspond to a modification of the primary Lu–Hf systematics in response to garnet resorption/recrystallization during the Paleoproterozoic Wopmay orogeny, resulting in significant scatter between these two age components. Our study adds to the small number of published P–T data for metamorphic rocks older than 2.8 Ga and shows that tonalitic gneisses in the AGC record a high apparent thermal gradient of ~140°C/kbar in the late Paleoarchean. This thermal gradient is the highest among the limited dataset, but is broadly similar to data from other Paleoarchean‐Mesoarchean crustal rocks in recording high T/P ratios (>77.5°C/kbar).

Bimodality in zircon oxygen isotopes and implications for crustal melting on the early Earth

Zircons from the oldest dated felsic crust, the Acasta Gneiss Complex, Canada, provide key information that may help understand the generation of crust on our nascent planet. When screened to eliminate grains with secondary alteration by measuring relative hydration (Δ16O1H/16O), primary ≥ 3.99 Ga zircon cores show δ18O of 5.88 ± 0.15 ‰, at the extreme upper (heavy) range for mantle values. Another early (≥3.96 Ga) zircon component indicates distinctly different, primary light δ18O values (δ18O ≤ 4.5 ‰). This bimodality in ancient zircon oxygen isotopes implies partial melting of both deep (lower crustal) and shallower (near surface) source rocks, responsible for felsic crust production on the early Earth. A similar bimodality in zircon δ18O is recognised in data from other ancient cratons, albeit at different times. Although alternative (uniformitarian) interpretations may also satisfy the data, the tempo of this bimodality matches models of planetary high-energy impact flux, consistent with a fundamental role for bolide impacts in the formation of crustal nuclei on the early Earth.

No evidence of supracrustal recycling in Si-O isotopes of Earth's oldest rocks 4 Ga ago.

Identifying the oldest evidence for the recycling of hydrated crust into magma on Earth is important because it is most effectively achieved by subduction. However, given the sparse geological record of early Earth, the timing of first supracrustal recycling is controversial. Silicon and oxygen isotopes have been used as indicators of crustal evolution on Archean igneous rocks and minerals to trace supracrustal recycling but with variable results. We present Si-O isotopes of Earth's oldest rocks [4.0 billion years ago (Ga)] from the Acasta Gneiss Complex, northwest Canada, obtained using multiple techniques applied to zircon, quartz, and whole rock samples. Undisturbed zircon is considered the most reliable recorder of primary Si signatures. By combining reliable Si isotope data from the Acasta samples with filtered data from Archean rocks globally, we observe that widespread evidence for a heavy Si signature is recorded since 3.8 Ga, marking the earliest record of surface silicon recycling.

U‐Th‐Pb and Trace Element Evaluation of Existing Titanite and Apatite LA‐ICP‐MS Reference Materials and Determination of 208Pb/232Th‐206Pb/238U Date Discordance in Archaean Accessory Phases

The last decade has seen a marked increase in U‐Th‐Pb petrochronological studies focused on titanite and apatite. This has motivated the need for detailed, phase‐specific method development and well‐characterised reference materials with a wide variety of ages and chemical compositions. This study presents new U‐Th‐Pb isotope and major‐, minor‐ and trace‐element mass fraction data for ten titanite and five apatite reference materials based on integrated EPMA, LA‐ICP‐MS and isotope‐dilution (ID) multi‐collector (MC)‐ICP‐MS characterisation. Cross‐comparison of EPMA and LA‐ICP‐MS data for the same titanite reference material suite demonstrates that careful selection of EPMA primary reference materials is necessary to minimise inaccuracies. We further identify a significant X‐ray interference when measuring low Ce abundances in Ti‐bearing phases and outline a method for its correction. New ID‐MC‐ICP‐MS and LA‐ICP‐MS data suggest tri‐concordance (defined as concordant ages for the 238U‐206Pb, 235U‐207Pb and 232Th‐208Pb decay systems) of the MKED‐1 titanite reference material and demonstrate the reproducibility of 208Pb/232Th measurements in some secondary titanite reference materials. Integration of 208Pb/232Th results with U‐Pb geochronology provides a meaningful tool to help interpret complex U‐Th‐Pb isotopic age spectra. We provide example applications of 208Pb/232Th LA‐ICP‐MS measurements toward interpreting Archaean titanite ages from the Acasta Gneiss Complex, Northwest Territories, Canada.

Quantifying the effect of late bombardment on terrestrial zircons

The first 500 million years of Earth history is thought to be a period of intense planetary bombardment, but the timing and flux of this meteorite bombardment is poorly understood. In particular, on the basis of an inferred lunar impact history, some workers have hypothesized a ∼3.9 Ga terminal cataclysm (TC) in which there was marked increase in the impact flux affecting the Moon, the Earth and possibly other terrestrial planets. Minerals that survived this enigmatic period offer a way to test early planetary bombardment models as they may contain telltale micro- to nanoscale shock features. Here, we present results from a numerical modeling calculation that assesses the probability that a zircon residing in the crust would escape shock melting or shock deformation during a TC bombardment event. Even with conservative pressure estimates for zircon shock deformation and intermediate bombardment intensities, we find that only ∼6% of ≥4.0 Ga crust would be expected to survive a 3.9 Ga cataclysm without experiencing either complete melting or zircon shock metamorphism. We couple this modeling with a search for shock effects in the oldest zircons from the Acasta Gneiss Complex, which would have been present in the Earth's crust during a putative 3.9 Ga TC. Spatially correlated electron and NanoSIMS ion microscopy of 4.02 Ga igneous zircons from Acasta reveals no evidence of ancient shock. These data, together with similar results from other Hadean zircon suites, confirm that a post-Hadean TC is unlikely to have occurred. We suggest that the dearth of pre-3.9 Ga terrestrial crust and zircons is instead best explained by endogenic processes related to the mechanisms of early crust formation. Our modeling allows us to evaluate bombardment scenarios from the terrestrial zircon record by applying probabilistic interpretations to zircon shock deformation data. This approach will be valuable for other planetary bodies, allowing broader conclusions to be drawn from geographically limited datasets.

Evaluating the Age Distribution of Exposed Crust in the Acasta Gneiss Complex Using Detrital Zircons in Pleistocene Eskers

AbstractThe Acasta Gneiss Complex (AGC) is a ∼2,400 km2 Hadean‐Mesoarchean terrane that contains the oldest known zircon‐bearing rocks on Earth. Despite its importance for early Earth geology, only a small fraction (∼50 km2) of the AGC has been mapped in detail. We use detrital zircon grains from late Pleistocene eskers that transect the Complex to approximate the lateral extent and relative proportions of diverse‐aged ancient rock units within the vast, little‐studied parts of the AGC. The esker sediment was derived from glacially eroded bedrock and therefore zircon grains can serve as a proxy for the ages of exposed bedrock in the study area. U‐Pb dates on ∼2400 detrital zircons from coarse and fine grain‐size fractions along the esker transect yield age distributions that coincide with ages of regionally mapped AGC bedrock, the adjacent Wopmay Orogen, and granitoids of the Slave craton. Based on detrital zircon age distributions and new reconnaissance‐scale mapping, we infer that 3.37 Ga granitoids are a volumetrically significant component of the unmapped AGC. Esker zircons older than 3.7 Ga are present in most esker samples but at low abundance, which suggests that Eoarchean and Hadean rocks are a volumetrically subordinate component of the AGC. However, the data also suggest that unmapped rocks at least as old as 3.95 Ga are present toward the inferred eastern limit of the AGC, a location where Eoarchean rocks have not been recognized previously.

Titanium isotopes constrain a magmatic transition at the Hadean-Archean boundary in the Acasta Gneiss Complex.

Plate subduction greatly influences the physical and chemical characteristics of Earth's surface and deep interior, yet the timing of its initiation is debated because of the paucity of exposed rocks from Earth's early history. We show that the titanium isotopic composition of orthogneisses from the Acasta Gneiss Complex spanning the Hadean to Eoarchean transition falls on two distinct magmatic differentiation trends. Hadean tonalitic gneisses show titanium isotopic compositions comparable to modern evolved tholeiitic magmas, formed by differentiation of dry parental magmas in plume settings. Younger Eoarchean granitoid gneisses have titanium isotopic compositions comparable to modern calc-alkaline magmas produced in convergent arcs. Our data therefore document a shift from tholeiitic- to calc-alkaline-style magmatism between 4.02 and 3.75 billion years (Ga) in the Slave craton.

Understanding Preservation of Primary Signatures in Apatite by Comparing Matrix and Zircon‐Hosted Crystals From the Eoarchean Acasta Gneiss Complex (Canada)

AbstractA novel way to investigate the petrogenesis of ancient polymetamorphosed terranes is to study zircon‐hosted mineral inclusions, which are sensitive to melt evolution such as apatite. Recent contributions on such inclusions in unmetamorphosed granitoids can provide valuable petrogenetic information and, in turn, represent a way to circumvent effects of metamorphism. Yet the impact of metamorphism on apatite inclusion has never been studied in detail. To address the issue of chemical and isotopic preservation of primary signals in apatite crystals both in the matrix and armored within zircons, we have studied apatite crystals from four 3.6–4.0 Ga TTG granitoids from the Acasta Gneiss Complex (Canada). Our results demonstrate that U‐Th‐Pb isotope systematics in matrix apatite crystals were reset at 1.8–1.7 Ga (Wopmay orogen) whereas primary REE signatures were preserved in many crystals. In contrast, zircon‐hosted apatite inclusions all preserved primary REE signatures despite variable ages between 1.7 and 4.0 Ga. We interpret reset ages to be a consequence of metamorphism that managed to affect U‐Th‐Pb systematics because of advanced radiation damage accumulation in host‐zircon lattices. Only the most pristine zircon crystal has an apatite inclusion with a concordant age consistent with the magmatic age of the zircon (4.0 Ga). In addition, our results show that apatite crystals from TTG have distinct REE composition from post‐Archean granitoids apatites, that is preserved even in some apatites with reset ages. This capacity to retain primary information and discriminate granitoid types makes apatite a very valuable tool for reconstructing the nature and evolution of ancient crustal rocks through the use of detrital minerals.

Unraveling the complexity of zircons from the 4.0–2.9 Ga Acasta Gneiss Complex

The Acasta Gneiss Complex (AGC) of the Northwest Territories, Canada, contains some of the oldest evolved terrestrial crust and has a sustained record of more than a billion years of magmatism (>4.0–2.9 Ga). These rocks provide an opportunity to investigate the nature of continental crust formation on the early Earth. Because complexities in zircons and bulk rocks are characteristic of the early Earth record—including the AGC—strategies are needed to extract accurate and meaningful age and isotopic information. In order to evaluate the accuracy of the early Earth Hf isotope record, we examined AGC zircons from a range of lithologies using paired chemical abrasion isotope dilution U-Pb age and solution Lu-Hf isotope analysis and compared these with previous results obtained using laser ablation split-stream (LASS) analysis. We describe an approach whereby LASS is used to identify rocks with the least complex zircons, and, when appropriate, solution methods are then used to refine the age and Hf isotopic composition to the highest precision. This two-pronged analytical approach results in a more robust determination of the age and Hf isotopic record of complex rocks and zircons and allows identification of complexity in the Hf isotopic record that would not be apparent by solution analysis alone, thereby refining the record of magmatic evolution on the early Earth. Despite the better precision, solution techniques are unsuitable for rocks with complexly zoned zircons. In particular, zircons from the two oldest AGC rocks in this study have positive initial εHf values when determined by solution analysis (+2.8 and +4.9 at 4.0 and 3.9 Ga, respectively) but have negative εHf values when determined by LASS (−3.4 and −4.7, respectively). This is attributable to the presence of later radiogenic overgrowths on the zircon grains which are incorporated in solution analyses but can be avoided using LASS. This provides important clarity to the AGC Hf isotope record. In total, all but one of the AGC rocks we analyzed, including the oldest samples, have negative εHf values and indicate derivation from an enriched reservoir; none of these samples—in contrast to whole rock Nd isotope compositions—have sufficiently positive εHf values to indicate their derivation from a depleted mantle reservoir. Despite the presence of ancient AGC crust, there is no record of corresponding mantle depletion. This implies that extraction of Hadean crust in this region did not happen in sufficient volume to result in widespread mantle depletion in the AGC source by the Eoarchean.Our results underscore the importance of identifying different components in ancient zircons—and the rocks that contain them—and accurately determining the age and isotopic composition of those components. This is critically important for clarifying the record of the formation of enriched crust and development of the depleted mantle in Earth’s early history.

Detrital zircon evidence for change in geodynamic regime of continental crust formation 3.7–3.6 billion years ago

The nature of the early terrestrial crust and how it evolved through time remains highly controversial. Whether conventional plate tectonics operated in the Hadean and early Archean and when it came into existence remains unclear. Here, we describe U-Pb ages, Hf isotope composition and trace element chemistry of 3.95–3.10 Ga old detrital zircons from the Singhbhum Craton in eastern India. The >3.7 Ga old zircons of this suite have crust-like Hf isotope compositions with strongly negative εHfi and their granitoid sources formed by intra-crustal reworking of a Hadean protolith that was extracted from primitive mantle at 4.4–4.5 Ga. The trace element and Hf isotope compositions of the zircons record a transition from higher Nb/Th (0.070 ± 0.010), Nb/U (0.045 ± 0.005), crust-like Hf isotope compositions, and longer crustal residence times of the protoliths prior to 3.7–3.6 Ga, to lower Nb/Th (0.032 ± 0.012), Nb/U (0.024 ± 0.009; 1σ), mantle-like Hf isotope compositions, and shorter protolith residence times post 3.7–3.6 Ga. The Nb/Th and Nb/U fractionation at 3.7 Ga seen in the detrital zircon record can be explained by transition to granitoid production at greater depths in an oceanic plateau-like regime. However, had that been the case, the crustal residence times of the protoliths of the granitoids from which the detrital zircons were sourced should have progressively increased with time, given the >1.1 billion years protracted history of granitoid magmatism in the craton, which is contrary to what is observed. We suggest that the changes in the granitoid chemistry recorded by the detrital zircons document a significant change in the depth of melting of the protoliths as well as in the tectonic setting of continental crust formation, and argue that it marks the transition to granitoid production from oceanic plateaus to arc-like tectonic environments. Broadly similar transitions at ca. 3.6 Ga have been documented in gneisses from the Acasta Gneiss Complex, the Jack Hills zircons and in detrital zircons from the Wyoming Province, which suggest that the end of the Eoarchean may have been marked by widespread transition in planetary tectonic regime.

A comparison between zircons from the Acasta Gneiss Complex and the Jack Hills region

The composition and origin of the earliest continental crust is intensely debated. The understanding of its composition relies heavily on the oldest vestiges of continental crust, which includes detrital zircon grains that are up to 4.36 billion-years-old (Ga) and <4.03 Ga zircon-bearing rocks. However, the interpretation of these two sample suites has thus far remained largely separate. Here, we demonstrate that the trace-element compositions of magmatic zircons from the Acasta Gneiss Complex compare favorably with those of ancient detrital zircons from western Australia. We combine these new data with existing oxygen and hafnium isotope datasets to show that the petrological processes that formed the Acasta Gneiss Complex, namely partial melting of mafic crust at various depths, are an appropriate analogue for the formation of the Hadean crust parental to the Jack Hills zircon grains. We also suggest that a transition from shallow-, to deep-crustal melting occurred in the JH source region ca. 3.8 Ga.

Disturbances in the Sm–Nd isotope system of the Acasta Gneiss Complex—Implications for the Nd isotope record of the early Earth

The whole rock 143Nd isotope record of the Acasta Gneiss Complex (AGC) has been used to argue for the development of both depleted mantle and enriched crustal reservoirs during the Hadean. The 147Sm–143Nd isotope compositions of rocks from AGC, however, fall on an array with an errorchron date of ∼3.3 Ga, despite the majority of samples yielding substantially older zircon U–Pb ages. This has led to the suggestion of a major Sm–Nd “resetting” event at this time in the AGC. To better understand the Sm–Nd bulk-rock systematics of the AGC, we investigate the U–Pb age and Sm–Nd isotope compositions of apatite and titanite from the AGC, using the laser ablation split stream (LASS) method. Apatite from all samples yield broadly similar U–Pb dates of ∼1.89 to 1.83 Ga, in agreement with resetting during the 1.9 Ga Wopmay orogen. The Sm–Nd isotope compositions of the apatite, however, lie off the WR errorchron and show a high degree of initial Nd isotope heterogeneity. Conversely, titanite Sm–Nd systematics are consistent with the ∼3.3 Ga whole rock errorchron further supporting Sm–Nd modification at ∼3.3 Ga. Taken together, these show that the oldest rocks in the AGC underwent at least two post-crystallization events (at 3.3 and 1.9 Ga) that modified the Sm–Nd isotope system. These results also demonstrate the relative mobility of the Sm–Nd isotope system during orogenic events, which may compromise the ability to determine robust bulk-rock 143Nd isotope signatures.

Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust

The U–Pb isotopic system in zircon is the tool of choice to interrogate high-temperature geological processes, yet this system has potential to investigate lower temperature fluid–rock interaction as well. In some cases, removal of radiogenic Pb is incomplete, potentially allowing regression of discordant U–Pb data on a concordia diagram to determine both the age of crystallization and the timing of fluid-driven isotopic disturbance. However, in rocks preserving more complex histories, simple regression is not effective at resolving multiple Pb loss events. Here, we use a ‘concordant–discordant comparison’ (CDC) test to establish the times of U–Pb disturbance in the Acasta Gneiss Complex (AGC), Canada. AGC c. 4.03 to c. 3.40 Ga orthogneisses experienced a long and complex post-crystallization history, for which U–Pb zircon data reflects not only the heterogeneous nature of the rock, but also the varying degrees and duration of crustal reworking that inevitably involved open system processes. The CDC test calculates the similarity between the concordant age structure and a modelled age structure, the latter inferred from discordant analyses, over a wide range of potential disturbance times. Our analysis reveals concordant zircon components implying new growth and/or recrystallization at 3992 ± 5, 3501 ± 6, 3442 ± 5 and 3126 ± 6 Ma. In addition, we establish episodes of radiogenic-Pb loss driven by fluid–rock interaction at 3150 ± 50 Ma, and probably at 2875 ± 50 Ma and c. 2590 Ma. These Pb-loss episodes correlate with previously recognised events recording growth of zircon rims during metamorphism, granite emplacement, and unroofing. Pb-loss within the AGC shows an antithetic relationship in different samples that are in close geographic proximity. We suggest that zircon alteration and associated new growth effectively rendered those grains that underwent Pb-loss at a particular time less susceptible to alteration during the next episode of fluid interaction.

Tungsten isotopic constraints on homogenization of the Archean silicate Earth: Implications for the transition of tectonic regimes

The short-lived 182Hf-182W system has been used to constrain the early Earth’s accretion and differentiation. This paper reports high precision W isotopic compositions and trace element concentrations for Archean (4.0–3.0 Ga) intermediate to felsic rocks from the Slave, North China, and Kaapvaal cratons to discuss early Earth differentiation and mantle mixing processes. The 4.0–3.8 Ga diorite and trondhjemite samples from the Acasta Gneiss Complex (AGC) in the Slave Craton and Anshan Complex in the North China Craton have positive μ182W values ranging from 8.3 ± 3.6 to 14.5 ± 4.0, whereas the 3.36–2.95 Ga TTG rocks from the North China Craton have a uniform modern mantle-like μ182W value of 0.7 ± 3.9 (2 SD, n = 11); with one exception, the 3.32 Ga old sample F28-2, which has a positive μ182W anomaly (7.3 ± 3.9 and 13.0 ± 3.2 for two individual analyses). The Earth’s oldest potassic granites, occurring as conglomerates in the Moodies Group, an intrusive trondhjemite, and an amphibolite with ages of ~3.55 Ga in the Kaapvaal Craton exhibit W isotopic compositions indistinguishable from what is proposed for the modern mantle. The positive 182W anomalies in the 4.0–3.8 Ga rocks could possibly be the result of either early mantle differentiation that occurred within the lifetime of 182Hf or a partial lack of late accreted material. The signature of 182W excess in the 3.32 Ga sample (F28-2) is inherited from earlier crust, presumably similar to the 3.8 Ga TTGs, either by melting or crustal contamination. The transition in μ182W values from positive to near-zero would indicates that a significant event occurred at ∼3.6 Ga which caused efficient mixing of early differentiated mantle and late accreted material, possibly indicating the transition of tectonic regimes from plume to plate tectonics on the Earth.

An impact melt origin for Earth’s oldest known evolved rocks

Earth’s oldest evolved (felsic) rocks, the 4.02-billion-year-old Idiwhaa gneisses of the Acasta Gneiss Complex, northwest Canada, have compositions that are distinct from the felsic rocks that typify Earth’s ancient continental nuclei, implying that they formed through a different process. Using phase equilibria and trace element modelling, we show that the Idiwhaa gneisses were produced by partial melting of iron-rich hydrated basaltic rocks (amphibolites) at very low pressures, equating to the uppermost ~3 km of a Hadean crust that was dominantly mafic in composition. The heat required for partial melting at such shallow levels is most easily explained through meteorite impacts. Hydrodynamic impact modelling shows not only that this scenario is physically plausible, but also that the region of shallow partial melting appropriate to formation of the Idiwhaa gneisses would have been widespread. Given the predicted high flux of meteorites in the late Hadean, impact melting may have been the predominant mechanism that generated Hadean felsic rocks. Earth’s oldest known felsic rocks formed by partial melting at low pressures and high temperatures caused by impact melting of mafic Hadean crust, according to phase equilibria and trace element modelling.

Evidence for evolved Hadean crust from Sr isotopes in apatite within Eoarchean zircon from the Acasta Gneiss Complex

Current models for the properties of Hadean-Eoarchean crust encompass a full range of possibilities, involving crust that is anywhere from thick and differentiated to thin and mafic. New data are needed to test and refine these models, and, ultimately, to determine how continents were first formed. The Rb-Sr system provides a potentially powerful proxy for crustal evolution and composition. However, this system has thus far been underutilized in studies on early crustal evolution due to its susceptibility to re-equilibration. Overcoming this issue requires new analytical approaches to micro-sample ancient Sr-rich mineral relics that may retain primary Rb-Sr systematics, allowing for the precise and accurate determination of initial 87Sr/86Sr values. In this study, we used a novel application of laser-ablation multi-collector inductively coupled plasma mass spectrometry to determine the Sr isotope composition of apatite inclusions in >3.6 Ga zircon grains from Eoarchean granodiorite gneisses of the Acasta Gneiss Complex, Slave Province, Canada. The 87Rb-corrected 87Sr/86Sr values of these inclusions are largely identical and are distinctly different from values obtained from altered matrix apatite. The inclusion data provide the first direct estimate of initial 87Sr/86Sr for these ancient rocks. Combining this result with information on the protolith and source-extraction age yields estimates for the range of Rb/Sr values, and by extension composition, that the source of these rocks may have had. The data indicate that continental crust containing over 60 wt% of SiO2 was present in the ca. 4.2 Ga source of the Acasta Gneiss Complex. Thus vestiges of evolved crust must have existed within the primitive proto-continents that were present on the Hadean Earth.

Petrogenesis and tectonics of the Acasta Gneiss Complex derived from integrated petrology and 142Nd and 182W extinct nuclide-geochemistry

The timing and mechanisms of continental crust formation represent major outstanding questions in the Earth sciences. Extinct-nuclide radioactive systems offer the potential to evaluate the temporal relations of a variety of differentiation processes on the early Earth, including crust formation. Here, we investigate the whole-rock 182W/184W and 142Nd/144Nd ratios and zircon Δ17O values of a suite of well-studied and lithologically-homogeneous meta-igneous rocks from the Acasta Gneiss Complex, Northwest Territories, Canada, including the oldest-known zircon-bearing rocks on Earth. In the context of previously published geochemical data and petrogenetic models, the new 142Nd/144Nd data indicate that formation of the Hadean–Eoarchean Acasta crust was ultimately derived from variable sources, both in age and composition. Although 4.02 Ga crust was extracted from a nearly bulk-Earth source, heterogeneous μ142Nd signatures indicate that Eoarchean rocks of the Acasta Gneiss Complex were formed by partial melting of hydrated, Hadean-age mafic crust at depths shallower than the garnet stability field. By ∼3.6 Ga, granodioritic–granitic rocks were formed by partial melting of Archean hydrated mafic crust that was melted at greater depth, well into the garnet stability field. Our 182W results indicate that the sources to the Acasta Gneiss Complex had homogeneous, high-μ182W on the order of +10 ppm—a signature ubiquitous in other Eoarchean terranes. No significant deviation from the terrestrial mass fractionation line was found in the triple oxygen isotope (16O–17O–18O) compositions of Acasta zircons, confirming homogeneous oxygen isotope compositions in Earth's mantle by 4.02 Ga.

Using the magmatic record to constrain the growth of continental crust—The Eoarchean zircon Hf record of Greenland

Southern West Greenland contains some of the best-studied and best-preserved magmatic Eoarchean rocks on Earth, and these provide an excellent vantage point from which to view long-standing questions regarding the growth of the earliest continental crust. In order to address the questions surrounding early crustal growth and complementary mantle depletion, we present Laser Ablation Split Stream (LASS) analyses of the U–Pb and Hf isotope compositions of zircon from eleven samples of the least-altered meta-igneous rocks from the Itsaq (Amîtsoq) Gneisses of the Isukasia and Nuuk regions of southern West Greenland. This analytical technique allows a less ambiguous approach to determining the age and Hf isotope composition of complicated zircon. Results corroborate previous findings that Eoarchean zircon from the Itsaq Gneiss (∼3.85 Ga to ∼3.63 Ga) were derived from a broadly chondritic source. In contrast to the Sm–Nd whole rock isotope record for southern West Greenland, the zircon Lu–Hf isotope record provides no evidence for early mantle depletion, nor does it suggest the presence of crust older than ∼3.85 Ga in Greenland.Utilizing LASS U–Pb and Hf data from the Greenland zircons studied here, we demonstrate the importance of focusing on the magmatic (rather than detrital) zircon record to more confidently understand early crustal growth and mantle depletion. We compare the Greenland Hf isotope data with other Eoarchean magmatic complexes such as the Acasta Gneiss Complex, Nuvvuagittuq greenstone belt, and the gneissic complexes of southern Africa, and all lack zircons with suprachondritic Hf isotope compositions. In total, these data suggest only a very modest volume of crust was produced during (or survived from) the Hadean and earliest Eoarchean. There remains no record of planet-scale early Earth mantle depletion in the Hf isotope record prior to 3.8 Ga.