Tungsten Isotope Constraints on Archean Geodynamics

Geodynamics on Earth are active since more than 4 billion years and have continuously lead to differentiation and homogenization of mantle and crust. However, evidence from short-lived nuclide decay systems (e.g. 129I – 129Xe and 146Sm – 142Nd) suggest that primordial heterogeneities, which formed in the early Hadean Eon (i.e. during the first ca. 100 million years after formation of the Earth), survived for a long period of time – in some cases for Eons and until present day. Studying rocks that display variations in the decay products of short-lived nuclide decay systems offer two intriguing perspectives: (1) The cause for their formation refers to processes that must have had operated at a time when no other witnesses preserved in the geological rock record. (2) Understanding the mechanisms that allowed for their long-term preservation provides important insights into the temporal evolution of the bulk silicate Earth (BSE) and into the processes that lead to the present-day state of the mantle, including the onset of plate tectonic processes on Earth. One of the short-lived decay series that has increasingly been applied in geochemistry is the 182Hf-182W decay system with a half-life of 8.9 million years. Over the past decade, several studies investigated W isotope systematics in terrestrial rocks and found differences in the relative abundance of 182W. While Archean rocks were found to exhibit predominantly elevated 182W isotope compositions compared to the modern depleted mantle, modern oceanic island basalts (OIBs) and one Archean komatiites system (the ca. 3.55 Ga Schapenburg komatiite suite from the Kaapvaal Craton, southern Africa) were shown to display negative anomalies. These findings were interpreted as evidence for the preservation of early-formed heterogeneities in the sources of Archean rocks and taken as evidence for the presence of primordial mantle components that participate to the provenance of modern igneous reservoirs. However, the processes that lead to the formation of these reservoirs remained ambiguous. These processes include (1) incomplete equilibration of the mantle source with late accretionary material (late accretion hypothesis), (2) early fractionation of Hf from W by silicate crystal-liquid fractionation, e.g., in an early magma ocean, or (3) core-mantle interaction. Matters are further complicated because secondary processes (fluid-mediated alteration) often obscured primary W budgets of metamorphosed Archean rock assemblages and the analytical standards to obtain high-precision 182W isotope measurements turned out to be challenging. In this study, an analytical protocol is presented to obtain high-precision 182W isotope measurements on samples with low bulk-rock W concentrations (several ng/g). In three chapters we report high-precision 182W isotope data for Archean rocks from the ca. 3.9-3.6 Ga Itsaq Gneiss Complex of southern West Greenland (chapter 1), the ca. 3.6-3.2 Ga Pilbara Craton, NW Australia (chapter 2), and the ca. 3.6-3.2 Ga Kaapvaal Craton, southern Africa (chapter 3). In all studies, we combine 182W isotope analysis with high-precision isotope dilution measurements for high field strength element (HFSE), U, and Th abundances, to assess the elemental W systematics in our samples. This allows us to obtain a precise understanding of the primary and secondary processes that modified the W abundances and isotope compositions. As we demonstrate, the elemental W budgets of many mantle-derived rocks are dominated by metasomatic agents that mix reservoirs of variable 182W isotope compositions and obscure primary signatures (chapters 1 and 2). If not taken into consideration, this can lead to ambiguous interpretations of 182W isotope compositions observed in Archean lithostratigraphic successions. Our studies on rocks from different Archean cratons reveal that several processes are responsible for the origin of 182W isotope anomalies. Excesses of 182W in rocks from the Pilbara Craton (chapter 2) are best explained by missing late accreted additions in their mantle sources. Anomalies in rocks from the Itsaq Gneiss Complex (chapter 1) and the Kaapvaal Craton (chapter 3) instead were inherited from mantle sources that underwent early silicate differentiation during the lifetime of 182Hf (i.e. in the first ca. 60 million years after Solar System formation). Our results demonstrate that these Hadean signatures remained isolated in the mantle for several hundred million years. Understanding the evolution of 182W isotope systematics in the BSE through time requires comprehensive studies of lithostratigraphic successions that cover relatively long time frames of Archean geodynamic evolution, as shown by our study on the geological rock record of the Pilbara Craton (chapter 2). We further demonstrate that information about the temporal evolution of 182W isotope systematics of individual cratons is archived in Archean shales, which provide an average of the 182W isotope composition of the upper crust (chapter 2). These findings allow for observational constraints, which have important implications for understanding timescales of geodynamic processes on the early Earth (e.g. mantle stirring rates). As reported in chapter 3, rocks from the Kaapvaal Craton display correlations between 182W isotope compositions and initial εNd(t) and εHf(t) values. To our knowledge, this is the very first co-variation observed between 182W isotope systematics and long-lived radiogenic nuclides (147Sm-143Nd and 176Lu-176Hf systematics). The only plausible model to explain these patterns is the presence of recycled mafic restites from Hadean protocrust in the ancient mantle beneath the Kaapvaal Craton. As further demonstrated by our model, the striking isotopic similarity between recycled restites from Hadean protocrust and the low 182W endmember of modern OIBs might also be the missing link bridging 182W isotope systematics in Archean and young mantle-derived rocks. This finding offers important constraints on the geodynamic evolution of Earth’s mantle through time, indicating inefficient homogenization of Hadean silicate reservoirs

Temporal evolution of 142Nd signatures in SW Greenland from high precision MC-ICP-MS measurements

Measurements of 142Nd isotope signatures in Archean rocks are a powerful tool to investigate the earliest silicate differentiation events on Earth. Here, we introduce a new analytical protocol that allows high precision radiogenic and mass-independent Nd isotope measurements by MC-ICP-MS. To validate our method, we have measured well-characterized ∼3.72 to ∼3.8 Ga samples from the Eoarchean Itsaq Gneiss Complex and associated supracrustal belts, as well as Mesoarchean greenstones and a Proterozoic dike in SW Greenland, including lithostratigraphic units that were previously analyzed for 142-143Nd isotope systematics, by both TIMS and MC-ICP-MS. Our μ142Nd values for ∼3.72 to ∼3.8 Ga rocks from the Isua region range from +9.2 ± 2.6 to +13.2 ± 1.1 ppm and are in good agreement with previous studies. Using coupled 142,143Nd/144Nd isotope systematics from our data for ∼3.8 Ga mafic-ultramafic successions from the Isua region, we can confirm previous age constraints on the earliest silicate differentiation events with differentiation age of 4.390−0.060+0.045 Ga. Moreover, we can resolve a statistically significant decrease of 142Nd/144Nd isotope compositions in the ambient mantle of SW Greenland that already started to commence by Eoarchean time, between ∼3.8 Ga (μ142Nd = +13.0 ± 1.1) and ∼ 3.72 Ga (μ142Nd = +9.8 ± 1.0). Even lower but homogeneous μ142Nd values of +3.8 ± 1.1 are found in ∼3.4 Ga mantle-derived rocks from the Ameralik dike swarms. Our study reveals that ε143Nd(i) and εHf(i) values of Isua rocks scatter more than it would be expected from a single stage differentiation event as implied from nearly uniform μ142Nd values, suggesting that the previously described decoupling of Hf and Nd isotopes is not a primordial magma ocean signature. Instead, we conclude that some of second stage processes like younger mantle depletion events or recycling of subducted material affected the 147Smsingle bond143Nd isotope systematics. The preservation of pristine whole-rock isochrons largely rules out a significant disturbance by younger alteration events. Based on isotope and trace element modelling, we argue that the temporal evolution of coupled 142,143Nd/144Nd isotope compositions in the ambient mantle beneath the Isua rocks is best explained by the progressive admixture of material to the Isua mantle source that must have had present-day-like μ142Nd compositions. In contrast, Mesoarchean mafic rocks from the ∼3.08 Ga Ivisaartoq greenstone belt and the 2.97 Ga inner Ameralik Fjord region as well as a 2.0 Ga Proterozoic dike within that region all have higher μ142Nd values as would be expected from our simple replenishment model. This argues for reworking of older Isua crustal material that carried elevated μ142Nd compositions

Geodynamic implications of synchronous Norite and TTG formation in the 3 Ga Maniitsoq Norite Belt, West Greenland

This study was supported by Villum Fonden through grant VKR18978 to K.S. Funding for article fees was supplied by the Ministry of Mineral Resources, Government of Greenland.We present new data for the ∼3.0 Ga Maniitsoq Norite Belt of the Akia Terrane, West Greenland, with the aim of understanding its petrogenesis. The Maniitsoq Norite Belt is hosted in regional tonalite-trondhjemite-granodiorite (TTG) and dioritic orthogneisses, intruded by later sheets of TTG and granite pegmatites, and comprises two main rock types: plagioclase-rich “norites” and pyroxene-rich “melanorites”. Both norites and melanorites have high SiO2 contents (52–60 wt% SiO2), high bulk rock Mg# (0.57–0.83), and low TiO2 contents (0.1–0.7 wt%). Their trace element patterns are defined by depleted heavy Rare-Earth elements, highly enriched light Rare-Earth elements, negative anomalies in Nb, Ta, and Ti, and variable anomalies in Zr, Hf, and Eu. New zircon U-Pb geochronology data and previously published ages establish an emplacement age of 3,013 ± 1 Ma for the majority of the Maniitsoq Norite Belt, with magmatism continuing until 3,001 ± 3 Ma. This ∼12 Myr period of norite magmatism is coeval with an ongoing period of TTG production in the Akia Terrane. Norite Belt emplacement was closely followed by high temperature, low pressure granulite-facies metamorphism at ∼800°C and 900°C/GPa) and that the norite magmas were emplaced into thin crust and lithosphere. Compositions of the norites and melanorites can be explained by derivation from a single mafic parental melt (∼13 wt% MgO), with the norites predominantly accumulating plagioclase and the melanorites predominantly accumulating pyroxene. Evidence from field relationships, the presence of xenocrystic zircon, major element compositions and combined trace element and Hf-isotope modelling suggests the norites were contaminated by assimilation of ∼20–30% continental TTG crust. Geochemical and Hf-Nd isotopic constraints indicate that the norite mantle source was depleted, and that this depletion occurred significantly before the emplacement of the norite magmas. Contemporaneous production of both TTGs and norite, their emplacement in thin crust, and the rapid transition to high temperature, low pressure granulite-facies metamorphism is best explained by their formation in an ultra-hot orogeny. Formation of norites in this setting may be restricted to >2.7 Ga, when geothermal gradients were higher on Earth.Publisher PDFPeer reviewe

Upper mantle control on the W isotope record of shallow level plume and intraplate volcanic settings

Several studies have revealed small heterogeneities in the relative abundance of 182W, the radiogenic nuclide of short-lived 182Hf (t1/2 = ∼9 Ma), in terrestrial rocks. Whereas the majority of Archean rocks display 182W excesses relative to bulk silicate Earth, many young ocean island basalts show small 182W deficits, in particular if they are sourced from deep-rooted mantle plumes. The origin of this anomaly is still ambiguous, proposed models focus on core-mantle interaction or the presence of reservoirs in the lower mantle that have been isolated since the Hadean. In order to evaluate the role of upper mantle reservoirs, we report the first 182W data for intraplate basalts where a deep plume origin is still debated (Ascension Island, Massif Central, Siebengebirge and Eifel) and intraplate volcanic rocks associated with either plume or subduction zone environments (Italian Magmatic Provinces) and compare them to new data for basalts that have a deep mantle plume origin (La Réunion and Baffin Island). The proto-Iceland plume basalts from Baffin Island have uniform and modern mantle-like W of around 0 despite extremely high (3He/4He). In contrast, basalts from both volcanic edifices from La Réunion span a range from modern upper mantle values to deficits as low as W = −8.8 ppm, indicating a heterogeneous source reservoir. The W in all other intraplate volcanic provinces overlap the composition of modern upper mantle to within 3 ppm. The absence of resolvable 182W anomalies in these intraplate basalts, which partially tap the lithospheric mantle, suggests that primordial components are neither present in the central and southern European lithosphere nor in the European asthenospheric reservoir (EAR). The general absence of 182W anomalies in European plume-related basalts can either be explained by a shallow mantle source or by the absence of isotopically anomalous and isolated domains in the deep mantle beneath the northern hemisphere, as also suggested by geophysical evidence

Accurate stable tungsten isotope measurements of natural samples using a W-180-W-183 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 W-180-W-183 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 delta(186)/W-184; 2 s.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 delta W-186/184 value was observed for the Pacific Mn crust NOD-P-1 (+ 0.154 +/- 0.013%; 2 s.d.; n = 6), which is significantly different from the delta(186)/W-184 value of the Atlantic Mn crust NOD-A-1 (+ 0.029 +/- 0.014%; 2 s.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 delta(186)/W-184 values. Magmatic reference materials range in d(186)/W-184 between + 0.016 +/- 0.032% (andesite AGV-2; 2 s.d.; n = 5) and + 0.082 +/- 0.010% (basalt BHVO-2; 2 s.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

Two metamorphic gold mineralization events confirmed by Lu-Hf isotope dating of garnet in the Late Archean StorO Au deposit, Nuuk region of SW Greenland

We report Lu-Hf isotope ages for garnet from the Late Archean StorO Supracrustal Belt (SSB) ranging from 2697 +/- 8 Ma to 2646 +/- 13 Ma. These ages essentially overlap with two previously reported Re-Os isotope ages for arsenopyrite associated with Au mineralization within the SSB, which implies that both Au-forming events took place while garnet was stable and thus occurred during amphibolite-facies metamorphic conditions. Furthermore, we report bulk-rock Lu-Hf isotope data for amphibolite and aluminous gneiss from the SSB. The latter rock type has previously been suggested to represent the pre-metamorphic hydrothermal alteration product associated with low temperature epithermal Au formation. However, the initial Hf isotope compositions of these two rock types are significantly different, and therefore demonstrate that they do not share a common origin, because the Lu-Hf isotope system is essentially fluid-immobile. Instead, the aluminous gneiss represents a pelitic rock with mixed mafic and felsic sources, which is in agreement with the formation of both Au mineralization events during orogenic amphibolite-facies metamorphic processes, which were therefore structurally controlled. A too young bulk-rock errorchron age of 2553 +/- 72 Ma for the amphibolite unit (>2720 Ma) is interpreted to reflect resetting of the system during the intrusion and melt infiltration of the ca. 2560 Ma Qorqut Granite Complex

Late Eocene Impact ejecta in Italy: Attempt to constrain the impactor composition from isotopic analyses of spinel rich samples.

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Reworking of aged mafic crust in a Palaeoarchaean layered complex inferred from coupled Sm-Nd and Lu-Hf isotope systematics, Stolzburg Complex, Barberton Greenstone Belt, South Africa

The Stolzburg Complex is a prominent and well-preserved Palaeo- to Mesoarchaean layered ultramafic-mafic complex in the vicinity of the Barberton Greenstone Belt. Whole rock Sm- Nd, Lu-Hf and 142Nd isotopic data obtained from a variety of lithologies, and augmented by major and trace element geochemistry, are presented to examine petrogenesis, magma source composition, and mantle differentiation as well as the geodynamic setting of emplacement. Geochemically, all samples are characterized by unfractionated normalized trace element patterns (La/Luchond. = 0.7-2.3, La/Gdchond. = 0.6-1.7, Sm/Luchond. = 0.9-1.4) associated with weak HFSE anomalies (Nb/[0.5Th+0.5La]PRIMA=0.5-2.3). Trace element concentrations vary as a function of mineralogy, but differences in trace element compositions are unsystematic across lithologies. The coherence of trace element characteristics is consistent with a common mantle source that underwent moderately large degrees of melting at mantle pressures within the spinel stability field. Whole rock Sm-Nd and Lu-Hf data yield well-defined apparent isochrons corresponding to ages of 3367±62 (n=12, MSWD=1.5) and 3396±36 (n=11, MSWD=2.1), respectively. Initial Hf and Nd isotopic compositions vary from +3.3 to +5.7 and +0.9 to +1.7, respectively, indicative of derivation from slightly to moderately depleted mantle source(s). Isochron ages and a regressed initial emplacement age (zircon, titanite) of 3.25 Ga and Hf values for zircon of -2.5 to +3.0. Hydrothermal alteration or weathering, incorporation of evolved pre-existing (continental) crust and fractionation by high-pressure phases prior to melt extraction can all be ruled out to have significantly affected isotope systematics. Protracted magma chamber activity and non-synchronous emplacement of unrelated gabbroic magmas also fail to fully account for the discrepancy between whole rock and mineral age and isotopic data. Instead, the coherence in apparent isochron ages and variability of initial Hf and Nd isotopic composition is adequately explained by reworking of aged mafic-ultramafic (likely oceanic) crust in staging chambers with compositions indistinguishable from newly formed magmas. Thus, apparent isochrons represent mixing lines and do not record timing of crystallization. A subset of samples was also analyzed for 142Nd isotopic compositions to trace preserved mantle heterogeneities caused by early Archaean crust-mantle differentiation. The results (142Nd = -2.3±2.2 to +1.7±2.1) are not resolvable from the modern mantle value, indicating that 142Ndenriched or depleted mantle reservoirs had not remained isolated from convective homogenization in the asthenosphere, or the volume of mantle underlying the layered complex was too small to sample mantle heterogeneities. The Stolzburg Complex was most possibly emplaced in ancient oceanic lithosphere, followed by pervasive but inhomogeneous Ca-metasomatism as documented by exposed rodingites

Long-term preservation of Hadean protocrust in Earth's mantle

With plate tectonics operating on Earth, the preservation potential for mantle reservoirs from the Hadean Eon (>4.0 Ga) has been regarded as very small. The quest for such early remnants has been spurred by the observation that many Archean rocks exhibit excesses of 182W, the decay product of short-lived 182Hf. However, it remains speculative whether Archean 182W anomalies and also 182W deficits found in many young ocean island basalts (OIBs) mirror primordial Hadean mantle differentiation or merely variable contributions from older meteorite building blocks delivered to the growing Earth. Here, we present a high-precision 182W isotope dataset for 3.22-to 3.55-Ga-old rocks from the Kaapvaal Craton, southern Africa. In expanding previous work, our study reveals widespread 182W deficits in different rock units from the Kaapvaal Craton and also the discovery of a negative covariation between short-lived 182W and longlived 176Hf 143Nd 138Ce patterns, a trend of global significance. Among different models, these distinct patterns can be best explained by the presence of recycled mafic restites from Hadean protocrust in the ancient mantle beneath the Kaapvaal Craton. Further, the data provide unambiguous evidence for the operation of silicate differentiation processes on Earth during the lifetime of 182Hf, that is, the first 60 million y after solar system formation. The striking isotopic similarity between recycled protocrust and the low -182W endmember of modern OIBs might also constitute the missing link bridging 182W isotope systematics in Archean and young mantle-derived rocks