Kristoffer Szilas | University of Copenhagen (original) (raw)
Papers by Kristoffer Szilas
Mineralium Deposita, 2024
northwards along the southwest coast of Greenland. The intrusions have previously been grouped in... more northwards along the southwest coast of Greenland. The intrusions have previously been grouped into the so-called "Greenland Norite Belt" (Nielsen 1976; Secher 1983; Garde et al. 2013), extending for ∼ 75 km in N-S, and 15 km in E-W direction. However, we will show in the following that most rocks in the belt contain significant clinopyroxene, i.e. are gabbronorites. True norites make up a smaller proportion, and pyroxenite and diorite are relatively rare (see also Waterton et al. 2020). It is possible if not likely that the pre-metamorphic mineral assemblage contained relatively less clinopyroxene, with relatively more calcic plagioclase. However, in order to reduce terminological ambiguity and to highlight the economic potential of the area, we will use the term "Maniitsoq Nickel Belt" for the studied intrusions. The Maniitsoq project area is underlain by the North Atlantic Craton. In western Greenland, the craton comprises six shear zone-bounded blocks, exposing granulite facies rocks in the north and prograde amphibolite facies rocks in the south (Windley and Garde 2009). From south to north, the blocks are: Ivittuut, Kvanefjord, Bjørnesund, Sermilik, Fiskefjord, and Maniitsoq. The Ni-bearing intrusions are hosted within the Fiskefjord block (Fig. 1A).
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Geochemical Perspectives Letters, 2024
The Ujaragssuit intrusion is thought to host the oldest chromitites (>3.8 Ga) on Earth, showing e... more The Ujaragssuit intrusion is thought to host the oldest chromitites (>3.8 Ga) on Earth, showing evidence of both Hadean mantle depletion events and nucleosynthetic isotopic heterogeneities. We set out to verify the age of the intrusion and identify the Os isotope composition of the Ujaragssuit mantle source. Here, we show that the only minimum age constraint is 2970 ± 8 Ma, provided by cross-cutting leucogranites. Concordant Re-Pt-Os isotope ages are consistent with formation of the intrusion from a chondritic primitive mantle source at 3246 ± 120 Ma; mean Pt-depletion ages of 3437 ± 587 Ma offer no direct evidence for Hadean mantle depletion. No nucleosynthetic Os isotopic anomalies could be identified, consistent with large scale Os homogeneity in the presolar nebular. The new 'young' age for Ujaragssuit means that nucleosynthetic anomalies occur repeatedly between ∼3.8 and >3.0 Ga in the NAC, suggesting its unique mantle source was repeatedly tapped over ∼600 Myr without significant mixing with the rest of Earth's mantle.
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JPet, 2022
Numerous ultramafic rocks occur as lens-shaped bodies in the Archaean continental crust exposed in... more Numerous ultramafic rocks occur as lens-shaped bodies in the Archaean continental crust exposed in southern West Greenland. As some of the oldest exposed ultramafic bodies, determining their origin, as mantle segments or magmatic cumulates, is an important yet controversial issue. The origin of these Archaean ultramafic rocks remains unclear, in-part because these rocks have undergone metasomatic modification since their formation, yet the effects of this metasomatism have so far not been assessed in detail, despite being crucial for understanding their geochemical evolution. Here, we examined the petrology, mineral and whole-rock chemistry of the largest ultramafic body located within the Mesoarchaean Akia terrane, known as the Ulamertoq ultramafic body, to elucidate the poly-metamorphic and metasomatic events that overprinted the protolith. Pronounced lithologic zoning from hydrous mineral-rich layers to orthopyroxene-rich ultramafic rocks at the boundaries between ultramafic rocks and the granitoid country rocks was formed locally by metasomatic reactions related to the granitoids. The main body of ultramafic rocks, far from the contacts, can be classified into four types based on mineral assemblage and chemistry. The fine-grained orthopyroxene aggregates and large poikilitic orthopyroxenes have low Cr2O3 and CaO contents, suggesting a secondary origin. Trace element compositions of orthopyroxene and/or amphibole in the main ultramafic rocks indicate that at least three types of metasomatic agents were required to form these minerals and the associated whole-rock chemical variations within the ultramafic body. Variations represent differences in the proportions of metasomatic orthopyroxene and/or amphibole and phlogopite added to a dunitic protolith. The main body of Ulamertoq ultramafics experienced metasomatism under granulite-facies. Retrograde cooling occurred, to 650◦C–850◦C and <1.8 GPa prior to local metasomatism via country-rock reaction. The presence of titanian clinohumite and its associated mineral assemblage in the least-metasomatised dunites suggest the possibility that the main ultramafic rocks went through a hydration/dehydration process at ∼800◦C–900◦C and <2 GPa prior to metasomatic modification. This study demonstrates that it is important to consider the effects of multi-stage metasomatism and metamorphism in order to elucidate the origin of the Archaean ultramafic rocks in Greenland and elsewhere.
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Science Advances, 2023
Moderately siderophile (e.g., Ni) and highly siderophile elements (HSEs) in the bulk silicate Ear... more Moderately siderophile (e.g., Ni) and highly siderophile elements (HSEs) in the bulk silicate Earth (BSE) are believed to be partly or near-completely delivered by late accretion after the depletion caused by metallic core formation. However, the extent and rate of remixing of late-accreted materials that equilibrated with Earth's prelate-veneer mantle have long been debated. Observing evidence of this siderophile element-depleted pre-lateveneer mantle would provide powerful confirmation of this model of early mantle evolution. We find that the mantle source of the~3.8-billion-year-old (Ga) Narssaq ultramafic cumulates from Southwest Greenland exhibits a subtle 60 Ni/ 58 Ni excess of~0.05 per mil and contains a clear HSE deficiency of~60% relative to the BSE. The intermediate Ni isotopic composition and HSE abundances of the~3.8-Ga Narssaq mantle mark a transitional Eoarchean snapshot as the poorly mixed 3.8-Ga mantle containing elements of prelate veneer mantle material transitions to modern Earth's mantle.
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Earth and Planetary Science Letters, 2024
Several ultramafic enclaves found within the Eoarchean Itsaq Gneiss Complex (IGC) in southern Wes... more Several ultramafic enclaves found within the Eoarchean Itsaq Gneiss Complex (IGC) in southern West Greenland have previously been interpreted as representing mantle relicts. However, as Archean ultramafic rocks are frequently overprinted by metamorphism and late-stage metasomatism, it can be difficult to distinguish between peridotites representing mantle residues or primitive crustal cumulates. Therefore, detailed evaluation of individual Eoarchean peridotite occurrences is required to better constrain their origin. Here, we present new petrological observations and geochemical data for the >3.8 Ga Narssaq Ultramafic Body (NUB) in the IGC of SW Greenland. The NUB ultramafic rocks have high FeOt and Cr contents with flat chondrite-normalized trace element patterns and positive Cr-Mg correlations, which distinguish them from mantle rocks. In addition, the variations in olivine and spinel compositions are consistent with a fractional crystallization process. The above geochemical features suggest that the ultramafic rocks of NUB cannot be explained as mantle residues. Instead, their major element compositions are related to the spatially associated tholeiitic amphibolites, indicating that these ultramafic rocks likely represent crustal cumulates derived from high-Mg magmas. This interpretation is supported by thermodynamic modeling using MELTS, which shows that the compositions of the ultramafic rocks in the NUB can be reproduced via fractional crystallization of local high-Mg tholeiitic basalts, involving the accumulation of olivine, spinel, and possibly clinopyroxene, in conjunction with trapped melt. The NUB ultramafic rocks can thus form as a natural consequence of fractional crystallization of regular highly magnesian tholeiitic magmas, making a cumulate origin the simplest explanation for Eoarchean peridotites in general. This also implies that the formation of the NUB did not involve refertilization of subduction-like components as previously assumed, and thus did not require subduction to have taken place in the Eoarchean.
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Lithos, 2023
Several studies focused on the ultramafic bodies of the Archaean continental crust in southern Gr... more Several studies focused on the ultramafic bodies of the Archaean continental crust in southern Greenland in order to gain information on early Earth petrogenetic, metamorphic and metasomatic processes. This research provides the first petrological dataset of the Miaggoq Ultramafic Complex (~1 km 2) in the Akia terrane, with a minimum age of 2997 ± 15 Ma. It comprises ultramafic (dunite, peridotite) and mafic (orthopyroxenite, norite) rocks along with chromitites and provides a window into Mesoarchaean mantle compositions. Field observations, such as chromitite bands, mineral layering, and orthopyroxenite oikocrysts in peridotites coupled with chemical analysis displaying high abundance of chromites in the dunitic rocks and high forsterite contents (Mg# 91 to 92.5) of the olivines, all point to a layered cumulate origin for the Miaggoq body. Pseudosection calculations along with geothermometry estimations reveal peak metamorphic conditions of 850-1100 • C at pressures of 0.7-1.25 GPa under anhydrous conditions followed by a possible metamorphic overprint at 650-800 • C and 0.7 GPa with relatively dry melting (0.025-0.125 wt% H 2 O). MELTS fractional crystallization coupled with cumulate modelling approximated the compositional trends with conditions on ~3 kbar with 1 wt% H 2 O. This research concludes that the Miaggoq body represents a layered cumulate complex that was derived by large degrees of partial melting of the mantle with possible assimilation (synonymous with contamination) of basalts in the crust. Overall, this study provides complementary data for the Mesoarchaean cumulate bodies of the Akia Terrane and their petrological processes.
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Geology, 2023
We present petrological data and seven Lu-Hf garnet-amphibole-whole rock ages obtained from a sin... more We present petrological data and seven Lu-Hf garnet-amphibole-whole rock ages obtained from a single garnet-hornblende-mica schist sample from the Isua supracrustal belt (West Greenland). Garnets grew during prograde metamorphism toward regional amphibolite-facies peak conditions, and a mylonitic foliation formed during and after garnet growth. Garnet crystals show typical prograde zoning with no visible traces of a relict garnet generation. They do show various degrees of retrogression. While some crystals are perfectly euhedral with only minor chemical alteration along cracks, others are elongated in the foliation and either grew in this shape or were deformed. Six garnet splits were separated from crushed single crystals and one from a crushed bulk sample. Individual three-point garnet-hornblende-whole rock ages scatter between 2.603 ± 0.018 Ga and 2.432 ± 0.059 Ga for single garnets. The garnet split from the bulk sample defines an age of 2.463 ± 0.031 Ga, the data point farthest from the regression line for all data points (2.551 ± 0.074 Ga, mean square of weighted deviates = 25). We interpret these data to indicate partial retrogression of a Neoarchean garnet population not significantly older than the oldest obtained three-point age. Well-preserved garnet zoning, regional peak temperatures well below the closing temperature of the Lu-Hf system, and the small scatter of Lu-Hf ages preclude an interpretation of the observed metamorphism and deformation as being Eoarchean in age.
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JGR Solid Earth, 2023
Ultra-depleted peridotites, which refer to peridotites that have undergone high degrees of partia... more Ultra-depleted peridotites, which refer to peridotites that have undergone high degrees of partial melting, are an important end-member component of Earth's mantle (e.g., Xu et al., 2021). Due to their refractory physiochemical characteristics, ultra-depleted peridotites have the potential to contribute to continent stabilization (Scott et al., 2019), lithospheric mantle processes during subduction initiation (Parkinson & Pearce, 1998) and craton formation (Pearson et al., 2021). The formation of ultra-depleted peridotites may play an important role in oceanic crust formation during the early stages of the subduction zone development. However, this mechanistic link between subduction initiation has mainly been reconstructed by studies on volcanic rocks, that is,
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Geochimica et Cosmochimica Acta, 2023
The extensive exposure of the Archean continental crust in southern West Greenland makes it an im... more The extensive exposure of the Archean continental crust in southern West Greenland makes it an important window into the tectonic evolution of early Earth. Here, we provide a comprehensive geochemical data set for tholeiitic amphibolites (meta-basalts), calc-alkaline leucoamphibolites (meta-andesites), and ultramafic rocks (meta-cumulates) for the Mesoarchean Bjørneøen Supracrustal Belt, Nuuk region, SW Greenland. This data helps constrain the geodynamic setting in which these rocks formed. The volcanic rocks display two distinct geochemical trends in terms of their La/Sm ratios. Nonetheless, both tholeiitic amphibolites and leucoamphibolites have negative Nb-Ta-Ti anomalies and thus geochemical features associated with apparent island arc or crust contamination processes. Uranium-lead zircon dating of a leucoamphibolite yields an age of 3077 ± 6 Ma, which is older than regional orthogneisses. A series of models for both major element variation (thermodynamics-based) and trace element variation (partition coefficient-based) implies that fractional crystallization of tholeiitic basalt can effectively produce the observed ultramafic rocks. Such ultramafic cumulates had low degrees of crystallinity, reflecting open system magmatic process at shallow depths likely representing magma conduits in a volcanic pile. The geochemical features of the andesites are distinct from the basalts and our modeling excludes a connection via fractional crystallization or crustal assimilation of the two suites. Instead, the andesites formed via high degrees of mixing between basaltic and felsic endmember magmas, requiring elevated temperatures in the mid-to lower-crust. The introduction of felsic components could be derived from partial melting of mafic lower crust, for example by mafic underplating or via some other process that achieves such anatexis, or alternatively by the addition of rhyolitic melt from extensive fractional crystallization. Mixing and homogenization of basaltic and felsic endmembers to produce andesites may occur in modern-style subduction environments, although this could also be feasible in other geodynamic settings in a hotter early Earth.
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Lithos, 2023
The Tasiilaq region in SE Greenland records the suturing of two major cratons during the 1.9 Ga N... more The Tasiilaq region in SE Greenland records the suturing of two major cratons during the 1.9 Ga Nagssugtoqidian orogeny, with the Rae Craton to the north subducting beneath the North Atlantic Craton (NAC) to the south. Supracrustal rocks that were deposited prior to the orogeny, and record it, can be used to put constraints on the composition and geodynamic relationships between these two cratons. Here, we present a detailed study of the pressure-temperature conditions and metamorphic history of the Kuummiut Terrane (Rae craton) using geothermobarometry and pseudosection modelling and compare it to the adjacent Schweizerland (Rae) and Isertoq terranes (NAC). We show that the supracrustal rocks of the Tasiilaq region record an orogenic pressure temperature path, typical of an accretionary wedge, similar to what is seen in modern orogenies, including the Variscides. The Kuummiut Terrane contains two peak mineral assemblages with distinct peak pressure and temperature at 8.7 to 9.8 kbar at 636 to 680 • C in the west and ~ 8.2 kbar at ~735 • C in the east. The suture zone between the Isertoq and Kuummiut terranes was intruded by the Ammassalik Intrusive Complex resulting in the formation of anatectic melts at ~790 • C and 6.3-8.0 kbar at 1910-1870 Ma, syn-to late-tectonically. This was followed by strike slip, crustal thickening, and regional shearing at 614 to 713 • C and 4.3 kbar to 7.4 kbar. This is the first event to affect all three terranes and juxtaposes blocks with differing peak metamorphism, bringing the Isertoq Supracrustal rocks to the same crustal level as the Kuummiut Supracrustal rocks and thrusting the Schweizerland Terrane on top of the Kuummiut Terrane. The nappe stacking kept upper to mid crustal rocks at close to peak temperatures, resulting in extensive melting, and the formation of pegmatites throughout the region. Post tectonic collapse began as early as 1820 Ma, which marks the end of decompression, with the intrusion of gabbro and granitic complexes at ~8 km depth. Finally, circulation of hydrothermal fluids at temperatures of <400 • C resulted in region wide retrogression.
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Chemical Geology, 2023
The oldest remnants of life on Earth from various localities in the Isua supracrustal belt in Gre... more The oldest remnants of life on Earth from various localities in the Isua supracrustal belt in Greenland date back to >3.7 billion years ago (Ga). They are in the form of graphite, whose biogenicity is controversial. Previous studies used the presence and isotopic composition of nitrogen in graphite from along the Isua belt to argue both for and against biogenicity. To determine if the nitrogen chemistry of graphite can indeed serve as a biosignature, we investigated a hydrothermal graphite deposit from southeast Greenland (1.87-1.82 Ga). We found indications that molar C/N ratios of hydrothermal graphite may be similar to those of biogenic graphite from the Archean rock record, meaning that the nitrogen content of graphite is itself perhaps not diagnostic of ancient life, requiring caution in future studies. However, the hydrothermal graphite deposit also revealed unusually low N concentrations in associated silicates, despite a wide range of K concentrations up to 5 wt%. Using a thermodynamic model of nitrogen speciation in the presence of graphite, paired with previously published partition coefficients for ammonium in K-silicates, we show that abiotic process can explain these low N-concentrations of around 1 μg/g in potassic silicates. Higher concentrations of >10 μg/g, such as those found in graphitic metapelites from the Isua supcracrustal belt, would, however, require an unusually ammonium-rich fluid. Such an ammonium-rich fluid is most easily derived from the breakdown of biomass within sediments prior to graphitization. We therefore conclude that potassic silicates associated with graphite can serve as an indirect biosignature. Our approach supports previous inferences of life on Earth back to at least 3.7 Ga.
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Geochemistry Geophysics Geosystems, 2022
We analyze a large database of abyssal peridotite clinopyroxene compositions using log-ratio tran... more We analyze a large database of abyssal peridotite clinopyroxene compositions using log-ratio transformation, principal component analysis (PCA) and k-means clustering, to better understand clinopyroxene compositional systematics in abyssal peridotites. We combine this analysis with open-system melting models to investigate the potential sources of compositional variation. PCA shows that 84% of the variation in clinopyroxene compositions can be represented using only 2-dimensional information. We use k-means clustering to classify clinopyroxene compositions into four clusters. Clusters 1–3, representing 85% of the data, show progressive depletions in LREE/HREE, and are associated with decreases in Na2O in clinopyroxene, and general increases in Cr# of spinel. We interpret peridotites with clinopyroxene compositions from clusters 1–3 to represent residues of partial melt extraction. The degree of melt extraction increases from cluster 1 to 3, and exerts a primary control on compositional variations. The presence or absence of garnet-field melting prior to spinel-field melting and the retained melt fraction during partial melting exert secondary controls on clinopyroxene compositions. Cluster 4 clinopyroxenes, representing show less fractionated LREE/HREE with low-HREE abundances, elevated Sr, and depleted signatures in their host peridotites. Clinopyroxene compositions in cluster 4 cannot be modeled by melt depletion alone. Instead, they are only reconstructed in our models where melt-rock interaction, suggesting that peridotites with cluster 4 clinopyroxenes have experienced both of these processes. Clusters 1–4 are observed in most ridges, reflecting compositional heterogeneity on each ridge. This variability reflects variations in the degree of partial melting, amount of garnet-field melting, retained melt fractions, and melt-rock interaction.
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Chemical Geology, 2022
Measurements of 142 Nd isotope signatures in Archean rocks are a powerful tool to investigate the... more Measurements of 142 Nd 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 142-143 Nd isotope systematics, by both TIMS and MC-ICP-MS. Our μ 142 Nd 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 142,143 Nd/ 144 Nd 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 142 Nd/ 144 Nd isotope compositions in the ambient mantle of SW Greenland that already started to commence by Eoarchean time, between ~3.8 Ga (μ 142 Nd = +13.0 ± 1.1) and ~ 3.72 Ga (μ 142 Nd = +9.8 ± 1.0). Even lower but homogeneous μ 142 Nd values of +3.8 ± 1.1 are found in ~3.4 Ga mantle-derived rocks from the Ameralik dike swarms. Our study reveals that ε 143 Nd (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 μ 142 Nd 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 147 Sm-143 Nd 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 142,143 Nd/ 144 Nd 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 μ 142 Nd 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 μ 142 Nd values as would be expected from our simple replenishment model. This argues for reworking of older Isua crustal material that carried elevated μ 142 Nd compositions.
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Earth and Planetary Science Letters, 2023
During the late Archaean, exotic juvenile continental (TTG) terranes assembled into stable craton... more During the late Archaean, exotic juvenile continental (TTG) terranes assembled into stable cratons leading to continental emergence and deposition of shallow-marine sedimentary sequences. This period of cratonisation coincided with crustal reworking and maturation driving the production of granites sensu stricto on most cratons, and may mark a final transition to mobile-lid tectonics. We investigate the relative timing of continental assembly, stabilization, emergence, and maturation, during the formation of the North Atlantic Craton (NAC) in West Greenland from its constituent terranes, using geochemical data from zircon and monazite extracted from its oldest mature metasedimentary unit, the Storø quartzite. Zircons form two U-Pb age groups: (i) an older > 2820 Ma group with juvenile (elevated) Hf(t) and δ 18 O, derived from weathering surrounding Mesoarchaean terranes; and (ii) a younger < 2700 Ma group with less radiogenic (lower) Hf(t) and elevated δ 18 O that record post-burial metamorphism peaking ca. 2620 Ma. The quartzite protolith has a maximum depositional age of ca. 2830 Ma, and was deposited after final TTG formation but prior to granite magmatism at ca. 2715 Ma, during which time terranes had sufficiently assembled, stabilized, and emerged to form a common watershed. Cratons form via lateral accretion which requires strong continental lithosphere, for which one agent is crustal reworking and maturation. However, for the NAC, terrane assembly and emergence commenced prior to granite formation, and crustal reworking may be a response to lithospheric thickening. Cratonisation involves a series of complex, intertwined processes operating over 100's of millions of years, which together lead to the development of thick, stable, continental lithosphere. Studies of ancient mature metasediments such as the Storø quartzite can help build timelines for these processes to ultimately better understand their choreography and co-dependencies, that together produced Earth's enduring cratons.
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Precambrian Research, 2022
The Tasiilaq region in SE Greenland contains Palaeoproterozoic supracrustal rocks, which record t... more The Tasiilaq region in SE Greenland contains Palaeoproterozoic supracrustal rocks, which record the collision and suturing of the Rae Craton and North Atlantic Craton during the 1.9 Ga Nagssugtoqidian orogeny. The original mineralogy and textures of the supracrustal rocks have been largely overprinted by amphibolite facies metamorphism, obscuring the original protoliths and complicating the interpretation of the early Earth surface environment in which these rocks formed. Here, we present new major and trace element data for the Schweizerland, Kuummiut and Isertoq Terranes, with the former two representing the Rae craton, and the latter the northern margin of the North Atlantic Craton. Supracrustal lithologies include metapelites, marbles, calcsilicate rocks and mafic to felsic volcanic rocks and/or shallow intrusive rocks. Pelites normalised to Post Archaean Australian shales have (sub)horizontal rare earth element patterns, with La/Sm CN : 1.61-23.91 and Eu/Eu* 0.34-1.83 and Th/Sc 1. The volcanic rocks are enriched relative to the primitive mantle (PM) with parallel to sub parallel PM-normalised heavy rare earth elements. They have a suprasubduction zone signature with distinct Nb-Ta troughs. The amphibolites can be further divided into those that interacted with the crust; back-arc basalts with subtle Nb-Ta troughs; and fore-arc basalts with distinct Nb-Ta and Zr-Hf troughs and negative Ti anomalies. The marbles consist of a Mg-rich dolomite group with δ 13 C 0.48 to 0.91 ‰ and δ 18 O − 6.13 to − 7.57 ‰; and Mg-poor calcitic carbonates with δ 13 C − 0.74 to 0.17 ‰ and δ 18 O − 10.81 to − 14.87 ‰. This isotopic composition suggests formation at ~ 2050 Ma during the low δ 13 C excursion following the Great Oxidation Event. We propose that the supracrustals of the Tasiilaq region were deposited at 2050 Ma in a series of interconnected basins during dual subduction convergence of the North Atlantic and Rae cratons, with development of an island arc with back arc basin in the north, and a continental arc in the south. These basins were inverted during continental collision, thrust, and folded into the Archaean Kuummiut and Isertoq TTGdominated terrane basement, before being subjected to the region-wide amphibolite facies metamorphism at ~ 1840 Ma during the Nagssugtoqidian orogeny.
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Geoscience Frontiers, 2022
Discontinuous chains of ultramafic rock bodies form part of the 3800–3700 Ma Isua Supracrustal Be... more Discontinuous chains of ultramafic rock bodies form part of the 3800–3700 Ma Isua Supracrustal Belt (ISB), hosted in the Itsaq Gneiss Complex of southwestern Greenland. These bodies are among the world’s oldest outcrops of ultramafic rocks and hence an invaluable geologic record. Ultramafic rocks from Lens B in the northwestern limb of ISB show characteristics of several stages of serpentinization and deserpentinization forming prograde and retrograde mineral assemblages. Ti-rich humite-group minerals such as titanian chondrodite (Ti-Chn) and titanian clinohumite (Ti-Chu) often occur as accessory phases in the metamorphosed ultramafic rocks. The Ti-rich humite minerals are associated with metamorphic olivine. The host olivine is highly forsteritic (Fo96-98) with variable MnO and NiO contents. The concentrations of the rare-earth elements (REE) and high-field strength elements (HFSE) of the metamorphic olivine are higher than typical mantle olivine. The textural and chemical characteristics of the olivine indicate metamorphic origin as a result of deserpentinization of a serpentinized ultramafic protolith rather than primary assemblage reflecting mantle residues from high-degrees of partial melting. The close association of olivine, antigorite and intergrown Ti-Chn and Ti-Chu suggests pressure condition between ∼1.3–2.6 GPa within the antigorite stability field (<700 °C). The overall petrological and geochemical features of Lens B ultramafic body within the Eoarchean ISB indicate that these are allochthonous ultramafic rocks that recorded serpentine dehydration at relatively lower temperature and reached eclogite facies condition during their complex metamorphic history similar to exhumed UHP ultramafic rocks in modern subduction zone channels.
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Earth and Planetary Science Letters, 2022
A critical component of our understanding of the evolution of Earth's mantle comes from rocks ide... more A critical component of our understanding of the evolution of Earth's mantle comes from rocks identified as direct mantle samples. Eoarchaean dunite lenses from the Isua Supracrustal Belt (ISB), North Atlantic Craton, Greenland, have been previously interpreted as depleted mantle wedge residues, complementary to arc-like volcanic rocks in the supracrustal sequence. This would place the ISB dunites among Earth's oldest mantle samples. We present new major element, platinum-group element (PGE) and Re-Os isotopic data for the ISB dunites, and critically assess the criteria previously used to invoke a mantle origin for the dunites. We find no evidence that uniquely supports a mantle origin. Instead, evidence of chromite and Os-Ir alloy fractionation, consistent Pt and Pd depletion, elevated Ni contents, and trace element systematics indicate that the dunites formed as olivine ± chromite cumulates with varying amounts of intercumulus melt. Their compositions indicate crystallisation from magmas represented by ISB volcanic rocks, and their Re-Os model ages overlap the ∼3720 Ma age of the volcanic sequence, consistent with the dunites representing magma chambers or conduits that fed the volcanic eruptions. Formation of the Isua dunites as cumulates removes an important line of evidence used to interpret the ISB as an ophiolite, and highlights the risks of using criteria that do not discriminate mantle residues from olivinerich cumulates. Extending this reasoning to other Eoarchaean crustal peridotites previously identified as mantle rocks suggests there may be no mantle residues anywhere in the Itsaq Gneiss Complex, and that the oldest mantle samples may only be found as xenoliths in volcanic rocks.
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Mineralogical Magazine, 2021
Whole-rock major-and trace-element data are presented on a sample collection from the >3 Ga Amiko... more Whole-rock major-and trace-element data are presented on a sample collection from the >3 Ga Amikoq Layered Complex (ALC), and hosting amphibolites within the Mesoarchean Akia terrane, SW Greenland. The lithologies range from leuconorite to melanorite/feldspathic orthopyroxenite, orthopyroxenite to harzburgite through to dunite, and tholeiitic basaltic-picritic mafic host rocks. The Amikoq Layered Complex samples are primitive (Mg#: 65-89) with elevated Ni and Cr contents. However, the absence of troctolitic lithologies and the presence of two orthopyroxene compositional trends, suggests that the successions might not be comagmatic. On the basis of trace-element cumulate models, relatively low Ni contents and minor negative Sr-Eu anomalies in some high-Ti ultramafic rocks, it is not possible to exclude a petrogenesis related to a melt similar to that of the mafic host-rocks. Ultramafic samples with U-shaped trace-element distribution patterns are petrogenetically related to the noritic sequences, either through cumulus mineral accumulation or melt-rock reactions. Assimilation-fractional-crystallisation modelling of melanorites nevertheless require the parental melt to have been contaminated/mixed with a component of island-arc-like tholeiite affinity. A boninite-like parental melt might have been derived from the subcontinental lithospheric mantle of the Akia terrane, or alternatively via assimilation of an ultramafic parental melt with island-arc-like tholeiite. Given the complex geological evolution and high-grade metamorphic overprint of the Amikoq Layered Complex, we are unable to differentiate between the two models.
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Ore Geology Reviews, 2021
We investigate the petrogenesis and tectonic significance of corundum (ruby)-bearing late Neoarch... more We investigate the petrogenesis and tectonic significance of corundum (ruby)-bearing late Neoarchean rocks from the Maniitsoq region of southern West Greenland using petrology, thermobarometry, phase equilibrium modelling, and carbon isotopes as well as U-Pb zircon and rutile geochronology. Corundum was generated by amphibolite-facies metamorphism of an aluminosilicate schist. Thermobarometry and phase equilibrium modelling constrain the P-T conditions of corundum growth to ~ 600-700 • C at ~ 4-7 kbar. The presence of graphite suggests corundum growth in the presence of a mixed hydrous-carbonic fluid; this fluid was responsible for the transfer of SiO 2 out of the precursor rock generating quartz-undersaturated conditions that enabled corundum growth. Carbon isotope values of graphite from corundum-bearing schist indicate a biogenic origin and are inconsistent with the fractionation of non-biogenic carbon. Zircon U-Pb geochronology documents regional metamorphism at c. 2.72-2.60 Ga. U-Pb rutile geochronology yields an upper intercept age of c. 2.5 Ga, which is interpreted as the best age estimate of corundum growth. The growth of corundum in southern West Greenland reflects heating associated with post-tectonic magmatism after the assembly of the North Atlantic Craton.
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Geochemical Perspective Letters, 2021
Oxygen isotopes provide a unique possibility to study Earth's late accretion phase from a lithoph... more Oxygen isotopes provide a unique possibility to study Earth's late accretion phase from a lithophile element perspective, because most carbonaceous chondritesmeteorites that likely resemble the composition of the terrestrial late veneerhave markedly different Δ 0 17 O values than the silicate Earth. Ultramafic rocks in the early Archean assemblage of southwest Greenland have not incorporated the full amount of late accreted materials, and therefore possibly record the Δ 0 17 O of the mantle before late accretion. We measured 17 O/ 16 O and 18 O/ 16 O ratios of olivine from these ultramafic rocks and compared them with olivine from post-Archean mantle peridotite. A missing late veneer component was not resolved. The missing component from the early Archean mantle is therefore restricted to ≤0.12 % of Earth's mass (M ⊕) for most carbonaceous chondrite-like materials, unless the missing component resembles CI chondritesthe only carbonaceous chondrites with Δ 0 17 O values similar to those of the silicate Earth. If the early Archean mantle had incorporated 60 % late veneer, the overall late accreted mass would be restricted to ≤0.3 % M ⊕ for most types of carbonaceous chondrites, with a more massive late veneer only possible for CI-like chondrites.
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Mineralium Deposita, 2024
northwards along the southwest coast of Greenland. The intrusions have previously been grouped in... more northwards along the southwest coast of Greenland. The intrusions have previously been grouped into the so-called "Greenland Norite Belt" (Nielsen 1976; Secher 1983; Garde et al. 2013), extending for ∼ 75 km in N-S, and 15 km in E-W direction. However, we will show in the following that most rocks in the belt contain significant clinopyroxene, i.e. are gabbronorites. True norites make up a smaller proportion, and pyroxenite and diorite are relatively rare (see also Waterton et al. 2020). It is possible if not likely that the pre-metamorphic mineral assemblage contained relatively less clinopyroxene, with relatively more calcic plagioclase. However, in order to reduce terminological ambiguity and to highlight the economic potential of the area, we will use the term "Maniitsoq Nickel Belt" for the studied intrusions. The Maniitsoq project area is underlain by the North Atlantic Craton. In western Greenland, the craton comprises six shear zone-bounded blocks, exposing granulite facies rocks in the north and prograde amphibolite facies rocks in the south (Windley and Garde 2009). From south to north, the blocks are: Ivittuut, Kvanefjord, Bjørnesund, Sermilik, Fiskefjord, and Maniitsoq. The Ni-bearing intrusions are hosted within the Fiskefjord block (Fig. 1A).
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Geochemical Perspectives Letters, 2024
The Ujaragssuit intrusion is thought to host the oldest chromitites (>3.8 Ga) on Earth, showing e... more The Ujaragssuit intrusion is thought to host the oldest chromitites (>3.8 Ga) on Earth, showing evidence of both Hadean mantle depletion events and nucleosynthetic isotopic heterogeneities. We set out to verify the age of the intrusion and identify the Os isotope composition of the Ujaragssuit mantle source. Here, we show that the only minimum age constraint is 2970 ± 8 Ma, provided by cross-cutting leucogranites. Concordant Re-Pt-Os isotope ages are consistent with formation of the intrusion from a chondritic primitive mantle source at 3246 ± 120 Ma; mean Pt-depletion ages of 3437 ± 587 Ma offer no direct evidence for Hadean mantle depletion. No nucleosynthetic Os isotopic anomalies could be identified, consistent with large scale Os homogeneity in the presolar nebular. The new 'young' age for Ujaragssuit means that nucleosynthetic anomalies occur repeatedly between ∼3.8 and >3.0 Ga in the NAC, suggesting its unique mantle source was repeatedly tapped over ∼600 Myr without significant mixing with the rest of Earth's mantle.
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JPet, 2022
Numerous ultramafic rocks occur as lens-shaped bodies in the Archaean continental crust exposed in... more Numerous ultramafic rocks occur as lens-shaped bodies in the Archaean continental crust exposed in southern West Greenland. As some of the oldest exposed ultramafic bodies, determining their origin, as mantle segments or magmatic cumulates, is an important yet controversial issue. The origin of these Archaean ultramafic rocks remains unclear, in-part because these rocks have undergone metasomatic modification since their formation, yet the effects of this metasomatism have so far not been assessed in detail, despite being crucial for understanding their geochemical evolution. Here, we examined the petrology, mineral and whole-rock chemistry of the largest ultramafic body located within the Mesoarchaean Akia terrane, known as the Ulamertoq ultramafic body, to elucidate the poly-metamorphic and metasomatic events that overprinted the protolith. Pronounced lithologic zoning from hydrous mineral-rich layers to orthopyroxene-rich ultramafic rocks at the boundaries between ultramafic rocks and the granitoid country rocks was formed locally by metasomatic reactions related to the granitoids. The main body of ultramafic rocks, far from the contacts, can be classified into four types based on mineral assemblage and chemistry. The fine-grained orthopyroxene aggregates and large poikilitic orthopyroxenes have low Cr2O3 and CaO contents, suggesting a secondary origin. Trace element compositions of orthopyroxene and/or amphibole in the main ultramafic rocks indicate that at least three types of metasomatic agents were required to form these minerals and the associated whole-rock chemical variations within the ultramafic body. Variations represent differences in the proportions of metasomatic orthopyroxene and/or amphibole and phlogopite added to a dunitic protolith. The main body of Ulamertoq ultramafics experienced metasomatism under granulite-facies. Retrograde cooling occurred, to 650◦C–850◦C and <1.8 GPa prior to local metasomatism via country-rock reaction. The presence of titanian clinohumite and its associated mineral assemblage in the least-metasomatised dunites suggest the possibility that the main ultramafic rocks went through a hydration/dehydration process at ∼800◦C–900◦C and <2 GPa prior to metasomatic modification. This study demonstrates that it is important to consider the effects of multi-stage metasomatism and metamorphism in order to elucidate the origin of the Archaean ultramafic rocks in Greenland and elsewhere.
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Science Advances, 2023
Moderately siderophile (e.g., Ni) and highly siderophile elements (HSEs) in the bulk silicate Ear... more Moderately siderophile (e.g., Ni) and highly siderophile elements (HSEs) in the bulk silicate Earth (BSE) are believed to be partly or near-completely delivered by late accretion after the depletion caused by metallic core formation. However, the extent and rate of remixing of late-accreted materials that equilibrated with Earth's prelate-veneer mantle have long been debated. Observing evidence of this siderophile element-depleted pre-lateveneer mantle would provide powerful confirmation of this model of early mantle evolution. We find that the mantle source of the~3.8-billion-year-old (Ga) Narssaq ultramafic cumulates from Southwest Greenland exhibits a subtle 60 Ni/ 58 Ni excess of~0.05 per mil and contains a clear HSE deficiency of~60% relative to the BSE. The intermediate Ni isotopic composition and HSE abundances of the~3.8-Ga Narssaq mantle mark a transitional Eoarchean snapshot as the poorly mixed 3.8-Ga mantle containing elements of prelate veneer mantle material transitions to modern Earth's mantle.
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Earth and Planetary Science Letters, 2024
Several ultramafic enclaves found within the Eoarchean Itsaq Gneiss Complex (IGC) in southern Wes... more Several ultramafic enclaves found within the Eoarchean Itsaq Gneiss Complex (IGC) in southern West Greenland have previously been interpreted as representing mantle relicts. However, as Archean ultramafic rocks are frequently overprinted by metamorphism and late-stage metasomatism, it can be difficult to distinguish between peridotites representing mantle residues or primitive crustal cumulates. Therefore, detailed evaluation of individual Eoarchean peridotite occurrences is required to better constrain their origin. Here, we present new petrological observations and geochemical data for the >3.8 Ga Narssaq Ultramafic Body (NUB) in the IGC of SW Greenland. The NUB ultramafic rocks have high FeOt and Cr contents with flat chondrite-normalized trace element patterns and positive Cr-Mg correlations, which distinguish them from mantle rocks. In addition, the variations in olivine and spinel compositions are consistent with a fractional crystallization process. The above geochemical features suggest that the ultramafic rocks of NUB cannot be explained as mantle residues. Instead, their major element compositions are related to the spatially associated tholeiitic amphibolites, indicating that these ultramafic rocks likely represent crustal cumulates derived from high-Mg magmas. This interpretation is supported by thermodynamic modeling using MELTS, which shows that the compositions of the ultramafic rocks in the NUB can be reproduced via fractional crystallization of local high-Mg tholeiitic basalts, involving the accumulation of olivine, spinel, and possibly clinopyroxene, in conjunction with trapped melt. The NUB ultramafic rocks can thus form as a natural consequence of fractional crystallization of regular highly magnesian tholeiitic magmas, making a cumulate origin the simplest explanation for Eoarchean peridotites in general. This also implies that the formation of the NUB did not involve refertilization of subduction-like components as previously assumed, and thus did not require subduction to have taken place in the Eoarchean.
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Lithos, 2023
Several studies focused on the ultramafic bodies of the Archaean continental crust in southern Gr... more Several studies focused on the ultramafic bodies of the Archaean continental crust in southern Greenland in order to gain information on early Earth petrogenetic, metamorphic and metasomatic processes. This research provides the first petrological dataset of the Miaggoq Ultramafic Complex (~1 km 2) in the Akia terrane, with a minimum age of 2997 ± 15 Ma. It comprises ultramafic (dunite, peridotite) and mafic (orthopyroxenite, norite) rocks along with chromitites and provides a window into Mesoarchaean mantle compositions. Field observations, such as chromitite bands, mineral layering, and orthopyroxenite oikocrysts in peridotites coupled with chemical analysis displaying high abundance of chromites in the dunitic rocks and high forsterite contents (Mg# 91 to 92.5) of the olivines, all point to a layered cumulate origin for the Miaggoq body. Pseudosection calculations along with geothermometry estimations reveal peak metamorphic conditions of 850-1100 • C at pressures of 0.7-1.25 GPa under anhydrous conditions followed by a possible metamorphic overprint at 650-800 • C and 0.7 GPa with relatively dry melting (0.025-0.125 wt% H 2 O). MELTS fractional crystallization coupled with cumulate modelling approximated the compositional trends with conditions on ~3 kbar with 1 wt% H 2 O. This research concludes that the Miaggoq body represents a layered cumulate complex that was derived by large degrees of partial melting of the mantle with possible assimilation (synonymous with contamination) of basalts in the crust. Overall, this study provides complementary data for the Mesoarchaean cumulate bodies of the Akia Terrane and their petrological processes.
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Geology, 2023
We present petrological data and seven Lu-Hf garnet-amphibole-whole rock ages obtained from a sin... more We present petrological data and seven Lu-Hf garnet-amphibole-whole rock ages obtained from a single garnet-hornblende-mica schist sample from the Isua supracrustal belt (West Greenland). Garnets grew during prograde metamorphism toward regional amphibolite-facies peak conditions, and a mylonitic foliation formed during and after garnet growth. Garnet crystals show typical prograde zoning with no visible traces of a relict garnet generation. They do show various degrees of retrogression. While some crystals are perfectly euhedral with only minor chemical alteration along cracks, others are elongated in the foliation and either grew in this shape or were deformed. Six garnet splits were separated from crushed single crystals and one from a crushed bulk sample. Individual three-point garnet-hornblende-whole rock ages scatter between 2.603 ± 0.018 Ga and 2.432 ± 0.059 Ga for single garnets. The garnet split from the bulk sample defines an age of 2.463 ± 0.031 Ga, the data point farthest from the regression line for all data points (2.551 ± 0.074 Ga, mean square of weighted deviates = 25). We interpret these data to indicate partial retrogression of a Neoarchean garnet population not significantly older than the oldest obtained three-point age. Well-preserved garnet zoning, regional peak temperatures well below the closing temperature of the Lu-Hf system, and the small scatter of Lu-Hf ages preclude an interpretation of the observed metamorphism and deformation as being Eoarchean in age.
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JGR Solid Earth, 2023
Ultra-depleted peridotites, which refer to peridotites that have undergone high degrees of partia... more Ultra-depleted peridotites, which refer to peridotites that have undergone high degrees of partial melting, are an important end-member component of Earth's mantle (e.g., Xu et al., 2021). Due to their refractory physiochemical characteristics, ultra-depleted peridotites have the potential to contribute to continent stabilization (Scott et al., 2019), lithospheric mantle processes during subduction initiation (Parkinson & Pearce, 1998) and craton formation (Pearson et al., 2021). The formation of ultra-depleted peridotites may play an important role in oceanic crust formation during the early stages of the subduction zone development. However, this mechanistic link between subduction initiation has mainly been reconstructed by studies on volcanic rocks, that is,
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Geochimica et Cosmochimica Acta, 2023
The extensive exposure of the Archean continental crust in southern West Greenland makes it an im... more The extensive exposure of the Archean continental crust in southern West Greenland makes it an important window into the tectonic evolution of early Earth. Here, we provide a comprehensive geochemical data set for tholeiitic amphibolites (meta-basalts), calc-alkaline leucoamphibolites (meta-andesites), and ultramafic rocks (meta-cumulates) for the Mesoarchean Bjørneøen Supracrustal Belt, Nuuk region, SW Greenland. This data helps constrain the geodynamic setting in which these rocks formed. The volcanic rocks display two distinct geochemical trends in terms of their La/Sm ratios. Nonetheless, both tholeiitic amphibolites and leucoamphibolites have negative Nb-Ta-Ti anomalies and thus geochemical features associated with apparent island arc or crust contamination processes. Uranium-lead zircon dating of a leucoamphibolite yields an age of 3077 ± 6 Ma, which is older than regional orthogneisses. A series of models for both major element variation (thermodynamics-based) and trace element variation (partition coefficient-based) implies that fractional crystallization of tholeiitic basalt can effectively produce the observed ultramafic rocks. Such ultramafic cumulates had low degrees of crystallinity, reflecting open system magmatic process at shallow depths likely representing magma conduits in a volcanic pile. The geochemical features of the andesites are distinct from the basalts and our modeling excludes a connection via fractional crystallization or crustal assimilation of the two suites. Instead, the andesites formed via high degrees of mixing between basaltic and felsic endmember magmas, requiring elevated temperatures in the mid-to lower-crust. The introduction of felsic components could be derived from partial melting of mafic lower crust, for example by mafic underplating or via some other process that achieves such anatexis, or alternatively by the addition of rhyolitic melt from extensive fractional crystallization. Mixing and homogenization of basaltic and felsic endmembers to produce andesites may occur in modern-style subduction environments, although this could also be feasible in other geodynamic settings in a hotter early Earth.
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Lithos, 2023
The Tasiilaq region in SE Greenland records the suturing of two major cratons during the 1.9 Ga N... more The Tasiilaq region in SE Greenland records the suturing of two major cratons during the 1.9 Ga Nagssugtoqidian orogeny, with the Rae Craton to the north subducting beneath the North Atlantic Craton (NAC) to the south. Supracrustal rocks that were deposited prior to the orogeny, and record it, can be used to put constraints on the composition and geodynamic relationships between these two cratons. Here, we present a detailed study of the pressure-temperature conditions and metamorphic history of the Kuummiut Terrane (Rae craton) using geothermobarometry and pseudosection modelling and compare it to the adjacent Schweizerland (Rae) and Isertoq terranes (NAC). We show that the supracrustal rocks of the Tasiilaq region record an orogenic pressure temperature path, typical of an accretionary wedge, similar to what is seen in modern orogenies, including the Variscides. The Kuummiut Terrane contains two peak mineral assemblages with distinct peak pressure and temperature at 8.7 to 9.8 kbar at 636 to 680 • C in the west and ~ 8.2 kbar at ~735 • C in the east. The suture zone between the Isertoq and Kuummiut terranes was intruded by the Ammassalik Intrusive Complex resulting in the formation of anatectic melts at ~790 • C and 6.3-8.0 kbar at 1910-1870 Ma, syn-to late-tectonically. This was followed by strike slip, crustal thickening, and regional shearing at 614 to 713 • C and 4.3 kbar to 7.4 kbar. This is the first event to affect all three terranes and juxtaposes blocks with differing peak metamorphism, bringing the Isertoq Supracrustal rocks to the same crustal level as the Kuummiut Supracrustal rocks and thrusting the Schweizerland Terrane on top of the Kuummiut Terrane. The nappe stacking kept upper to mid crustal rocks at close to peak temperatures, resulting in extensive melting, and the formation of pegmatites throughout the region. Post tectonic collapse began as early as 1820 Ma, which marks the end of decompression, with the intrusion of gabbro and granitic complexes at ~8 km depth. Finally, circulation of hydrothermal fluids at temperatures of <400 • C resulted in region wide retrogression.
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Chemical Geology, 2023
The oldest remnants of life on Earth from various localities in the Isua supracrustal belt in Gre... more The oldest remnants of life on Earth from various localities in the Isua supracrustal belt in Greenland date back to >3.7 billion years ago (Ga). They are in the form of graphite, whose biogenicity is controversial. Previous studies used the presence and isotopic composition of nitrogen in graphite from along the Isua belt to argue both for and against biogenicity. To determine if the nitrogen chemistry of graphite can indeed serve as a biosignature, we investigated a hydrothermal graphite deposit from southeast Greenland (1.87-1.82 Ga). We found indications that molar C/N ratios of hydrothermal graphite may be similar to those of biogenic graphite from the Archean rock record, meaning that the nitrogen content of graphite is itself perhaps not diagnostic of ancient life, requiring caution in future studies. However, the hydrothermal graphite deposit also revealed unusually low N concentrations in associated silicates, despite a wide range of K concentrations up to 5 wt%. Using a thermodynamic model of nitrogen speciation in the presence of graphite, paired with previously published partition coefficients for ammonium in K-silicates, we show that abiotic process can explain these low N-concentrations of around 1 μg/g in potassic silicates. Higher concentrations of >10 μg/g, such as those found in graphitic metapelites from the Isua supcracrustal belt, would, however, require an unusually ammonium-rich fluid. Such an ammonium-rich fluid is most easily derived from the breakdown of biomass within sediments prior to graphitization. We therefore conclude that potassic silicates associated with graphite can serve as an indirect biosignature. Our approach supports previous inferences of life on Earth back to at least 3.7 Ga.
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Geochemistry Geophysics Geosystems, 2022
We analyze a large database of abyssal peridotite clinopyroxene compositions using log-ratio tran... more We analyze a large database of abyssal peridotite clinopyroxene compositions using log-ratio transformation, principal component analysis (PCA) and k-means clustering, to better understand clinopyroxene compositional systematics in abyssal peridotites. We combine this analysis with open-system melting models to investigate the potential sources of compositional variation. PCA shows that 84% of the variation in clinopyroxene compositions can be represented using only 2-dimensional information. We use k-means clustering to classify clinopyroxene compositions into four clusters. Clusters 1–3, representing 85% of the data, show progressive depletions in LREE/HREE, and are associated with decreases in Na2O in clinopyroxene, and general increases in Cr# of spinel. We interpret peridotites with clinopyroxene compositions from clusters 1–3 to represent residues of partial melt extraction. The degree of melt extraction increases from cluster 1 to 3, and exerts a primary control on compositional variations. The presence or absence of garnet-field melting prior to spinel-field melting and the retained melt fraction during partial melting exert secondary controls on clinopyroxene compositions. Cluster 4 clinopyroxenes, representing show less fractionated LREE/HREE with low-HREE abundances, elevated Sr, and depleted signatures in their host peridotites. Clinopyroxene compositions in cluster 4 cannot be modeled by melt depletion alone. Instead, they are only reconstructed in our models where melt-rock interaction, suggesting that peridotites with cluster 4 clinopyroxenes have experienced both of these processes. Clusters 1–4 are observed in most ridges, reflecting compositional heterogeneity on each ridge. This variability reflects variations in the degree of partial melting, amount of garnet-field melting, retained melt fractions, and melt-rock interaction.
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Chemical Geology, 2022
Measurements of 142 Nd isotope signatures in Archean rocks are a powerful tool to investigate the... more Measurements of 142 Nd 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 142-143 Nd isotope systematics, by both TIMS and MC-ICP-MS. Our μ 142 Nd 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 142,143 Nd/ 144 Nd 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 142 Nd/ 144 Nd isotope compositions in the ambient mantle of SW Greenland that already started to commence by Eoarchean time, between ~3.8 Ga (μ 142 Nd = +13.0 ± 1.1) and ~ 3.72 Ga (μ 142 Nd = +9.8 ± 1.0). Even lower but homogeneous μ 142 Nd values of +3.8 ± 1.1 are found in ~3.4 Ga mantle-derived rocks from the Ameralik dike swarms. Our study reveals that ε 143 Nd (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 μ 142 Nd 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 147 Sm-143 Nd 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 142,143 Nd/ 144 Nd 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 μ 142 Nd 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 μ 142 Nd values as would be expected from our simple replenishment model. This argues for reworking of older Isua crustal material that carried elevated μ 142 Nd compositions.
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Earth and Planetary Science Letters, 2023
During the late Archaean, exotic juvenile continental (TTG) terranes assembled into stable craton... more During the late Archaean, exotic juvenile continental (TTG) terranes assembled into stable cratons leading to continental emergence and deposition of shallow-marine sedimentary sequences. This period of cratonisation coincided with crustal reworking and maturation driving the production of granites sensu stricto on most cratons, and may mark a final transition to mobile-lid tectonics. We investigate the relative timing of continental assembly, stabilization, emergence, and maturation, during the formation of the North Atlantic Craton (NAC) in West Greenland from its constituent terranes, using geochemical data from zircon and monazite extracted from its oldest mature metasedimentary unit, the Storø quartzite. Zircons form two U-Pb age groups: (i) an older > 2820 Ma group with juvenile (elevated) Hf(t) and δ 18 O, derived from weathering surrounding Mesoarchaean terranes; and (ii) a younger < 2700 Ma group with less radiogenic (lower) Hf(t) and elevated δ 18 O that record post-burial metamorphism peaking ca. 2620 Ma. The quartzite protolith has a maximum depositional age of ca. 2830 Ma, and was deposited after final TTG formation but prior to granite magmatism at ca. 2715 Ma, during which time terranes had sufficiently assembled, stabilized, and emerged to form a common watershed. Cratons form via lateral accretion which requires strong continental lithosphere, for which one agent is crustal reworking and maturation. However, for the NAC, terrane assembly and emergence commenced prior to granite formation, and crustal reworking may be a response to lithospheric thickening. Cratonisation involves a series of complex, intertwined processes operating over 100's of millions of years, which together lead to the development of thick, stable, continental lithosphere. Studies of ancient mature metasediments such as the Storø quartzite can help build timelines for these processes to ultimately better understand their choreography and co-dependencies, that together produced Earth's enduring cratons.
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Precambrian Research, 2022
The Tasiilaq region in SE Greenland contains Palaeoproterozoic supracrustal rocks, which record t... more The Tasiilaq region in SE Greenland contains Palaeoproterozoic supracrustal rocks, which record the collision and suturing of the Rae Craton and North Atlantic Craton during the 1.9 Ga Nagssugtoqidian orogeny. The original mineralogy and textures of the supracrustal rocks have been largely overprinted by amphibolite facies metamorphism, obscuring the original protoliths and complicating the interpretation of the early Earth surface environment in which these rocks formed. Here, we present new major and trace element data for the Schweizerland, Kuummiut and Isertoq Terranes, with the former two representing the Rae craton, and the latter the northern margin of the North Atlantic Craton. Supracrustal lithologies include metapelites, marbles, calcsilicate rocks and mafic to felsic volcanic rocks and/or shallow intrusive rocks. Pelites normalised to Post Archaean Australian shales have (sub)horizontal rare earth element patterns, with La/Sm CN : 1.61-23.91 and Eu/Eu* 0.34-1.83 and Th/Sc 1. The volcanic rocks are enriched relative to the primitive mantle (PM) with parallel to sub parallel PM-normalised heavy rare earth elements. They have a suprasubduction zone signature with distinct Nb-Ta troughs. The amphibolites can be further divided into those that interacted with the crust; back-arc basalts with subtle Nb-Ta troughs; and fore-arc basalts with distinct Nb-Ta and Zr-Hf troughs and negative Ti anomalies. The marbles consist of a Mg-rich dolomite group with δ 13 C 0.48 to 0.91 ‰ and δ 18 O − 6.13 to − 7.57 ‰; and Mg-poor calcitic carbonates with δ 13 C − 0.74 to 0.17 ‰ and δ 18 O − 10.81 to − 14.87 ‰. This isotopic composition suggests formation at ~ 2050 Ma during the low δ 13 C excursion following the Great Oxidation Event. We propose that the supracrustals of the Tasiilaq region were deposited at 2050 Ma in a series of interconnected basins during dual subduction convergence of the North Atlantic and Rae cratons, with development of an island arc with back arc basin in the north, and a continental arc in the south. These basins were inverted during continental collision, thrust, and folded into the Archaean Kuummiut and Isertoq TTGdominated terrane basement, before being subjected to the region-wide amphibolite facies metamorphism at ~ 1840 Ma during the Nagssugtoqidian orogeny.
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Geoscience Frontiers, 2022
Discontinuous chains of ultramafic rock bodies form part of the 3800–3700 Ma Isua Supracrustal Be... more Discontinuous chains of ultramafic rock bodies form part of the 3800–3700 Ma Isua Supracrustal Belt (ISB), hosted in the Itsaq Gneiss Complex of southwestern Greenland. These bodies are among the world’s oldest outcrops of ultramafic rocks and hence an invaluable geologic record. Ultramafic rocks from Lens B in the northwestern limb of ISB show characteristics of several stages of serpentinization and deserpentinization forming prograde and retrograde mineral assemblages. Ti-rich humite-group minerals such as titanian chondrodite (Ti-Chn) and titanian clinohumite (Ti-Chu) often occur as accessory phases in the metamorphosed ultramafic rocks. The Ti-rich humite minerals are associated with metamorphic olivine. The host olivine is highly forsteritic (Fo96-98) with variable MnO and NiO contents. The concentrations of the rare-earth elements (REE) and high-field strength elements (HFSE) of the metamorphic olivine are higher than typical mantle olivine. The textural and chemical characteristics of the olivine indicate metamorphic origin as a result of deserpentinization of a serpentinized ultramafic protolith rather than primary assemblage reflecting mantle residues from high-degrees of partial melting. The close association of olivine, antigorite and intergrown Ti-Chn and Ti-Chu suggests pressure condition between ∼1.3–2.6 GPa within the antigorite stability field (<700 °C). The overall petrological and geochemical features of Lens B ultramafic body within the Eoarchean ISB indicate that these are allochthonous ultramafic rocks that recorded serpentine dehydration at relatively lower temperature and reached eclogite facies condition during their complex metamorphic history similar to exhumed UHP ultramafic rocks in modern subduction zone channels.
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Earth and Planetary Science Letters, 2022
A critical component of our understanding of the evolution of Earth's mantle comes from rocks ide... more A critical component of our understanding of the evolution of Earth's mantle comes from rocks identified as direct mantle samples. Eoarchaean dunite lenses from the Isua Supracrustal Belt (ISB), North Atlantic Craton, Greenland, have been previously interpreted as depleted mantle wedge residues, complementary to arc-like volcanic rocks in the supracrustal sequence. This would place the ISB dunites among Earth's oldest mantle samples. We present new major element, platinum-group element (PGE) and Re-Os isotopic data for the ISB dunites, and critically assess the criteria previously used to invoke a mantle origin for the dunites. We find no evidence that uniquely supports a mantle origin. Instead, evidence of chromite and Os-Ir alloy fractionation, consistent Pt and Pd depletion, elevated Ni contents, and trace element systematics indicate that the dunites formed as olivine ± chromite cumulates with varying amounts of intercumulus melt. Their compositions indicate crystallisation from magmas represented by ISB volcanic rocks, and their Re-Os model ages overlap the ∼3720 Ma age of the volcanic sequence, consistent with the dunites representing magma chambers or conduits that fed the volcanic eruptions. Formation of the Isua dunites as cumulates removes an important line of evidence used to interpret the ISB as an ophiolite, and highlights the risks of using criteria that do not discriminate mantle residues from olivinerich cumulates. Extending this reasoning to other Eoarchaean crustal peridotites previously identified as mantle rocks suggests there may be no mantle residues anywhere in the Itsaq Gneiss Complex, and that the oldest mantle samples may only be found as xenoliths in volcanic rocks.
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Mineralogical Magazine, 2021
Whole-rock major-and trace-element data are presented on a sample collection from the >3 Ga Amiko... more Whole-rock major-and trace-element data are presented on a sample collection from the >3 Ga Amikoq Layered Complex (ALC), and hosting amphibolites within the Mesoarchean Akia terrane, SW Greenland. The lithologies range from leuconorite to melanorite/feldspathic orthopyroxenite, orthopyroxenite to harzburgite through to dunite, and tholeiitic basaltic-picritic mafic host rocks. The Amikoq Layered Complex samples are primitive (Mg#: 65-89) with elevated Ni and Cr contents. However, the absence of troctolitic lithologies and the presence of two orthopyroxene compositional trends, suggests that the successions might not be comagmatic. On the basis of trace-element cumulate models, relatively low Ni contents and minor negative Sr-Eu anomalies in some high-Ti ultramafic rocks, it is not possible to exclude a petrogenesis related to a melt similar to that of the mafic host-rocks. Ultramafic samples with U-shaped trace-element distribution patterns are petrogenetically related to the noritic sequences, either through cumulus mineral accumulation or melt-rock reactions. Assimilation-fractional-crystallisation modelling of melanorites nevertheless require the parental melt to have been contaminated/mixed with a component of island-arc-like tholeiite affinity. A boninite-like parental melt might have been derived from the subcontinental lithospheric mantle of the Akia terrane, or alternatively via assimilation of an ultramafic parental melt with island-arc-like tholeiite. Given the complex geological evolution and high-grade metamorphic overprint of the Amikoq Layered Complex, we are unable to differentiate between the two models.
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Ore Geology Reviews, 2021
We investigate the petrogenesis and tectonic significance of corundum (ruby)-bearing late Neoarch... more We investigate the petrogenesis and tectonic significance of corundum (ruby)-bearing late Neoarchean rocks from the Maniitsoq region of southern West Greenland using petrology, thermobarometry, phase equilibrium modelling, and carbon isotopes as well as U-Pb zircon and rutile geochronology. Corundum was generated by amphibolite-facies metamorphism of an aluminosilicate schist. Thermobarometry and phase equilibrium modelling constrain the P-T conditions of corundum growth to ~ 600-700 • C at ~ 4-7 kbar. The presence of graphite suggests corundum growth in the presence of a mixed hydrous-carbonic fluid; this fluid was responsible for the transfer of SiO 2 out of the precursor rock generating quartz-undersaturated conditions that enabled corundum growth. Carbon isotope values of graphite from corundum-bearing schist indicate a biogenic origin and are inconsistent with the fractionation of non-biogenic carbon. Zircon U-Pb geochronology documents regional metamorphism at c. 2.72-2.60 Ga. U-Pb rutile geochronology yields an upper intercept age of c. 2.5 Ga, which is interpreted as the best age estimate of corundum growth. The growth of corundum in southern West Greenland reflects heating associated with post-tectonic magmatism after the assembly of the North Atlantic Craton.
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Geochemical Perspective Letters, 2021
Oxygen isotopes provide a unique possibility to study Earth's late accretion phase from a lithoph... more Oxygen isotopes provide a unique possibility to study Earth's late accretion phase from a lithophile element perspective, because most carbonaceous chondritesmeteorites that likely resemble the composition of the terrestrial late veneerhave markedly different Δ 0 17 O values than the silicate Earth. Ultramafic rocks in the early Archean assemblage of southwest Greenland have not incorporated the full amount of late accreted materials, and therefore possibly record the Δ 0 17 O of the mantle before late accretion. We measured 17 O/ 16 O and 18 O/ 16 O ratios of olivine from these ultramafic rocks and compared them with olivine from post-Archean mantle peridotite. A missing late veneer component was not resolved. The missing component from the early Archean mantle is therefore restricted to ≤0.12 % of Earth's mass (M ⊕) for most carbonaceous chondrite-like materials, unless the missing component resembles CI chondritesthe only carbonaceous chondrites with Δ 0 17 O values similar to those of the silicate Earth. If the early Archean mantle had incorporated 60 % late veneer, the overall late accreted mass would be restricted to ≤0.3 % M ⊕ for most types of carbonaceous chondrites, with a more massive late veneer only possible for CI-like chondrites.
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