The deep carbon cycle in the Archean is poorly constrained. Carbonate sedimentation only became an important reservoir for carbon from the late Archean onwards. It has been proposed that the transfer of carbon from the hydrosphere/atmosphere to the lithosphere mainly occurred during alteration of basalts of the oceanic crust [1]. While carbonation of ancient ultramafic rocks including komatiites has been described, it is often assumed that this carbonation occurred much later than the komatiite formation.

In this contribution, we investigate the role of carbonation of komatiites for the Archean deep carbon cycle. The Barberton Greenstone Belt of the Kaapvaal Craton, South Africa, provides a well-preserved pre-3 Ga terrestrial record and hosts hydrated komatiites that erupted ~3.48 Ga ago. We present data from samples of the ICDP drill core BARB1, transecting ultramafic lavas of the Komati Formation at depths of 108.77 to 112.73 meters. These komatiites have remained shielded from surface alteration throughout geological history and thus have not been affected by carbonate formation related to recent weathering.

The 3 m thick komatiite flow is covered by an andesitic volcaniclastic rock, where carbonate is intergrown with titanite in an albite-biotite-amphibole assemblage. A U-Pb age for titanite of 3266 ± 44 Ma demonstrates carbonate formation prior to this metamorphic overprint. In the uppermost 1.2 m of the komatiite flow only calcite is present, coexisting with chlorite, serpentine, tremolite, talc and magnetite. The volume of calcite decreases from 8-10 vol% in the first 40 cm to 2-6 vol% at 1 m depth where mainly spinifex textured komatiite is present. At 2-3 m depth, cumulate textures predominate, and the volume of calcite is always <2 vol%. Iron-bearing dolomite becomes the dominant carbonate with the volume increasing from 2 vol% to 9-13 vol% at the bottom of the flow. The carbonates display elevated Sr, Ba and B contents. The systematic change of carbonate minerals within the single komatiite flow and enrichment of these fluid-mobile elements indicate carbonation by seawater interaction directly after the emplacement of the flow rather than carbonate introduction during the later metamorphic event.

The intense carbonation of the komatiite lavas led to the incorporation of 1.5 to 6.5 wt% of CO₂, illustrating that secondary carbonate is an important sink for carbon. We performed phase equilibria modelling on a komatiite + 5 vol% calcite composition to determine whether such carbon can be recycled back into the mantle during burial of oceanic crust along expected Archean geotherms. During prograde metamorphism, calcite is replaced by dolomite, which is stable up to temperatures of only 700-750 °C [2]. Therefore, carbonate incorporated into altered komatiites will be entirely released during burial/subduction of oceanic crust in the Archean. This efficient recycling of carbon suggests that ingassing of C into the mantle was likely insignificant and might have helped to keep atmospheric CO₂ levels high in the Archean, an important aspect to explain the “faint young sun” paradox.

[1] Nakamura and Kato (2004): Geochem. Cosmochim. Acta 68 4595-4618.

[2] Tamblyn et al. (2023): Earth. Planet. Sci. Lett. 603, 117982

How to cite: Hermann, J., Vesin, C., Tamblyn, R., Hofmann, A., and Bolhar, R.: Carbonated komatiites and their importance for the Archean deep carbon cycle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6397, https://doi.org/10.5194/egusphere-egu26-6397, 2026.

A principal limitation of our understanding of the early Earth is the paucity of samples preserved from this time. Only around a dozen locations worldwide retain crust from the Eoarchaean period (>3.6 billion years, Ga), with controversy often surrounding the true age of these ancient vestiges of our planet. One of the best records of formation and alteration events is the mineral zircon. However, distinguishing between multiple igneous and metamorphic events experienced by a zircon population within an Archaean gneiss defines a critical question: how old is this rock?

Each new tract of ancient crust discovered carries a disproportionate significance in helping to shed light on the magmatic and geodynamic conditions shaping our planet's earliest evolution. Therefore, the utmost care must be taken when evaluating whether a terrane does host such ancient lithologies.  We present a new U-Pb, Hf isotope and trace element dataset of zircons from 16 Archaean basement gneisses from the Eokuk Uplift, northwestern Slave Craton. Preliminary work from Eokuk showed one tonalitic gneiss with a U-Pb zircon crystallisation age of 3.813 Ga and an initial εHf value of -2.5, indicating the presence of Eoarchean crust derived from the partial melting of long-lived Hadean (~4.2 Ga) protocrust (Stoian 2023, Unpublished Thesis). Further investigation by depth profiling on 53 zircons to target the rim and outer mantle of this apparently Eoarchaean sample has revealed the presence of younger 3.150 Ga igneous crystallised zones (83% with Th/U> 0.3), with 60% of depth profiles yielding only this age. This compares with 13% of zircon depth profiles with only ~3.8 Ga ages and 6% drilling through both age domains. Whilst this complicates the argument that this sample represents an unambiguously Eoarchaean rock, younger igneous recrystallisation rims on Eoarchaean zircons are frequent in lithologies interpreted to be Eoarchaean from other terranes. Of the 114 spot analyses on grain interiors from this rock, 62% are ~3.8 Ga, with just 4% being ~3.15 Ga, and the rest being too discordant for age determination. We therefore conclude that this lithology records a dominant Eoarchean-aged zircon population, with depth profiling proving a robust tool to identify subsequent recrystallisation events.  

Our geochronology study also reveals an additional four lithologies with Eoarchaean zircon cores (~3.6 – 3.7 Ga) from two distinct outcrops ~1km north of the preliminary study site. These rocks have additional igneous crystallisation ages at ~3.14, ~3.33, and ~3.43 Ga identified by combined textural and geochemical analysis. ~2.90 Ga metamorphic rims (Th/U <0.1) are identified in two of the four depth-profiled samples. These results further demonstrate that Eoarchaean crust in the Eokuk Uplift was continually reworked throughout the Palaeo- Mesoarchaean.  

Previously, Eoarchaean-aged crust was only identified in the Slave Craton from the Acasta Gneiss Complex, some 275km south of the Eokuk Uplift. This new discovery provides the strongest case to date that larger packages of Eoarchaean crust exist beyond Acasta in the northwest Slave Craton. Further exploration and detailed mapping are required to determine the extent of Earth’s most recently discovered Eoarchaean terrane. 

How to cite: Changleng, R., Schoonover, E. J., Stoian, C., Garber, J. M., Pearson, D. G., Luo, Y., and Reimink, J. R.: Beyond Acasta: A new Eoarchaean terrane identified in the Slave Craton , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20939, https://doi.org/10.5194/egusphere-egu26-20939, 2026.

Within the Aravalli-Delhi Fold Belt, NW India, sporadic granitic-gneisses outcrop are exposed in the Anasagar–Beawar region, whose age, origin, petrogenesis, tectonic setting, and stratigraphic status remain poorly constrained. Our zircon U–Pb geochronological data indicate that the protoliths of the Anasagar granite-gneiss (AGG) and Beawar granitic-gneiss (BGG) were emplaced at 1851 ± 20 Ma and 1130 ± 69 Ma, respectively. Field observations reveal that the AGG occurs as concordant, sheet-like bodies within the surrounding supracrustal rocks, giving a deceptive appearance of being the basement, whereas the BGG is exposed as isolated plutons surrounded by extensive soil cover. These relationships indicate that the Anasagar supracrustal sequence is older than 1.85 Ga—the crystallisation age of the AGG. Consequently, the AGG and its associated supracrustal rocks predate the majority of lithologies of the Delhi Supergroup. Therefore, these rocks are not part of the Delhi Supergroup but instead represent a small, isolated basin that developed during the closure of the Aravalli basin. Both AGG and BGG are megacrystic granitoids composed predominantly of quartz, K-feldspar, and plagioclase. They are calc-alkaline to shoshonitic, magnesian to ferroan, alkali-calcic to calcic, and are strongly peraluminous. The granitoids are characterised by high SiO₂, Na₂O + K₂O, low CaO and MgO, high FeO^t/(FeO^t + MgO), high Ga/Al ratios, and high average zircon saturation temperatures (~900°C for AGG and ~880°C for BGG), typical of A-type granitoids. Their enriched LREEs and LILEs, low Nb/Ta, and negative Eu anomalies indicate crustal sources. Both the granite-gneisses are classified as peraluminous A-type granitoids emplaced in a post-collisional extensional setting. The AGG exhibits εNd(t) values ranging from –6.4 to –4.6 and T_DM ages between 2.57 and 2.78 Ga, whereas the BGG shows εNd(t) values from –11.2 to –10.4 and T_DM ages between 2.53 and 2.81 Ga. Petrogenetic evidence suggests that both the granite-gneisses originated from the dehydration partial melting of Neoarchean crustal rocks, likely Banded Gneissic Complex (BGC) granitoids and Tonalite–Trondhjemite–Granodiorite (TTG) gneisses, under high-temperature and low-pressure conditions. The required heat flux was likely provided by large-scale mafic underplating or asthenosphere upwelling. Our findings suggest that the late Paleoproterozoic to early Mesoproterozoic tectonic evolution of the study area reflects two major events. The emplacement of the AGG represents the assembly of the Columbia supercontinent, whereas the BGG is not related to the assembly of Rodinia, as previously considered; instead, it more likely represents the pre-Rodinia assembly extension phase.

Keywords: NW India, Aravalli-Delhi Fold Belt, zircon U-Pb geochronology, A-type peraluminous granitoids, Sr-Nd isotopes, Crustal evolution

How to cite: Mohammed Khan, W. and Mondal, M. E. A.: Origin, petrogenesis, and tectonic implications of post-collisional A-type peraluminous granite-gneisses from the Aravalli–Delhi Fold Belt, NW India: Constraints from geochemistry, Sr–Nd isotopes, and zircon U–Pb geochronology, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-92, https://doi.org/10.5194/egusphere-egu26-92, 2026.

The Mid-Lithospheric Discontinuity (MLD) is a region of marked scattering of seismic waves which, according to several studies, is caused by mechanical weakness. It is located at an approximate depth of 100 km and has become a subject of intense research since it was identified through active-source seismology. The nature of the MLD is still a topic of intense debate. However, its contribution in destabilizing the continental lithosphere has often been invoked, and particularly in the destruction of cratons. Such a role is further enhanced when combined with other factors that may weaken the lithospheric mantle. Here we show the results of a 2D thermo-mechanical model, where we investigated the role of the MLD in the scenario of the interaction between long-lived mantle plumes and cratonic lithosphere. In this model, we implemented thermal and/or compositional plumes, with subsequent effects on their relative buoyancy with respect to the surrounding sublithospheric mantle. Our findings suggest that the combined effects of mantle plumes and MLD can effectively cause the destabilization and extensive delamination of cratonic lithosphere. However, mantle plumes must reach the MLD to trigger craton destabilization. For such a scenario, the presence of a weakened lithospheric mantle beneath the MLD is pivotal. This weak zone may be tectonic suture zone(s), or regions of melt and/or fluids percolation due to P-T variations in the plume during its ascent. We have verified that when plumes receive a constant material input from lower regions of the mantle, craton delamination can occur with very thin MLDs (< 10 km), and can be induced by cold and small compositional plumes, which are characterized by relatively low buoyancy.

How to cite: Lavecchia, A., Kovacs, I., Koptev, A., and Cloetingh, S.: Numerical modelling of plume-induced craton delamination: the role of the Mid-Lithospheric Discontinuity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2611, https://doi.org/10.5194/egusphere-egu26-2611, 2026.

The origin of Earth’s felsic protocrust remains enigmatic, with Iceland’s plume-thickened crust serving as a key analogue. We report Mg isotope systematics (δ²⁶Mg) across Hekla volcano’s basalt-rhyolite suite, revealing unprecedented variations from -0.20‰ to +0.77‰, distinctly heavier than oceanic igneous rocks (-0.47‰ to -0.06‰). While fractional crystallization explains δ²⁶Mg trends within basalt-andesite and dacite-rhyolite suites, the ~0.8‰ jump at intermediate compositions requires alternative processes. The exceptionally high δ²⁶Mg in dacites, coupled with Th/U, and O-Li isotope systematics, fingerprints melting of hydrothermally altered mafic crust, likely recycled via plume-driven isostatic subsidence. These findings demonstrate that supracrustal signatures found in felsic magmas can emerge without plate tectonics, reshaping our understanding of early continental crust formation.

How to cite: Han, Y., He, Y., Wu, H., Sigmarsson, O., Williams, H., and Ke, S.: The presence of supracrustal Mg isotope signature in plume-derived felsic magmas, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7240, https://doi.org/10.5194/egusphere-egu26-7240, 2026.

Tonalite-trondhjemite-granodiorites (TTGs) are a dominant lithology in many Archaean terrains and they retain a pivotal position in discussions of crust generation in the Archaean. Their major element compositions are consistent with partial melting of hydrated mafic source rocks, and yet their juvenile radiogenic isotope ratios indicate that they represent new continental crust.

This study links field evidence from the Lake Inari migmatite-granitoid terrain in northern Finland to petrogenetic models applicable to Archaean terranes. Lake Inari is part of the Meso- to Neoarchean TTG (tonalite-trondhjemite-granodiorite)-amphibolite terrains of Arctic Fennoscandia that form an extensive network of amphibolite metatexite-diatexite transitions controlled by melt proportions and syn-anatectic strain. The Lake Inari migmatite-granitoid terrain therefore provides a natural laboratory in which the bimodal association of felsic TTGs and their basaltic precursors are spatially and genetically linked, encouraging models in which the TTGs form directly through partial melting of the basalt. The zircons ages range over 300 Ma from 2.9-2.6 Ga (Joshi et al. 2024) and the geochemical data (Halla et al., 2024) confirm systematic trends supporting partial melting as the dominant TTG formation process. La/Sm increases from mafic rocks to TTGs, indicating progressive differentiation, but decreases at higher degrees of melting, defining a specific melting range. Th/Nb increases with La/Sm suggesting that negative Nb anomalies result from partial melting and differentiation. On average, Th/Nb increases from 0.17 in basalt to 0.96 in TTG (K₂O/Na₂O < 0.5). Co covaries with Ti in the TTG trending towards the mean Ti and Co values in the basalts, highlighting the role of ilmenite rather than rutile, and the REE variations indicate residual ampbibole rather than garnet. The average TTG was modelled as an 18% partial melt of basalt, assuming a bulk D-value of 0.01 for highly incompatible Th. The source mineralogy follows the thermodynamic model of Palin et al. (2016) for 20% melting at relatively shallow depths. While 20% represents an upper estimate, an 18% melting estimate yields bulk D-values of 0.4–0.5 for Rb, Sr, U, and Th; 1.37 for Nb and Ta; and 3.4–2.7 for Lu, Yb, and Y. Th/Nb increases with La/Sm in TTGs worldwide, highlighting its sensitivity to partial melting processes. The Lake Inari model is applied to other TTGs, allowing the distinction between TTGs derived from relatively high Th/Nb subduction-related sources and those formed in non-subduction settings, offering new insights into early continental growth. By linking field evidence with geochemical modelling, this study offers alternative insights into Archaean crustal evolution.

Halla et al (2024) Precam. Res. doi.org/10.1016/j.precamres.107407

Joshi et al (2024) Precam. Res. doi.org/10.1016/j.precamres.107418

Palin et al (2016) Precam. Res. doi.org/10.1016/j.precamres.11.001

How to cite: Hawkesworth, C., Halla, J., and Heilimo, E.: The Generation of Archaean TTG: insights from Lake Inari migmatites, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9716, https://doi.org/10.5194/egusphere-egu26-9716, 2026.

Precambrian iron formations (IF), which typically contain more than 15% Fe, are important economic sedimentary rocks for iron resources and constitute an archive for understanding the geochemical evolution and processes of the early Earth. While iron deposits are found throughout the geological record, the origin, depositional conditions, biogeochemical cycling, and sources of geochemical components in Precambrian iron formations remain unclear.

Here, for the first time, we present mineralogical, geochemical, and Sm-Nd isotope data from two drill holes in the 2.8-2.7 Ga Baniaka iron deposits, south-eastern Gabon. The IFs are underlain by gneiss, consisting of metamorphosed silicate IF (amphibolitic facies) and oxide IF facies, from the base to the top, with minor occurrences of interbedded gneiss occurring within the amphibolitic facies. The basal silicate IFs essentially comprise biotite, stilpnomelane, magnetite, and actinolite, while the upper oxide facies are dominated by hematite and goethite with traces of magnetite. Quartz is a key component of all the samples while kaolinite and smectite are present in some of the oxide IF and the upper part of silicate IF samples, respectively. Traces of chlorite and siderite are locally observed in a few samples., The IF samples, except the kaolinite-rich ones, are rich in Fe (~20-60 wt.%) and relatively low in Al (0.1-4.0 wt.%). The Si content ranges from 1.3 to 30 wt.%, while Ca, Ti, Mg, Mn, K, and Na occur in trace amounts. The absence of a significant correlation between Fe and detrital proxies (Al and Ti) suggests that Fe enrichment is not controlled by detrital flux, indicating the involvement of Fe cycling. 

The positive Eu anomalies in the chondrite- and shale-normalized rare earth element (REE) patterns, the slight depletion of light REE relative to heavy REE in the shale normalized patterns, and the chondritic to superchondritic Y/Ho values (27-48) demonstrate the influence of hydrothermal fluids and seawater mixing in an open ocean water depositional environment. The geochemical proxies of the incompatible elements (Th, Zr, Nb, and Sc), coupled with the positive εNd isotopic values (+0.34 to +9.75), are consistent with mantle-derived mafic materials from less-differentiated juvenile crust in a volcanic arc environment were significant source of the geochemical components during the deposition of the Archean Baniaka IF. Taken together, these results suggest that a significant proportion of the Fe likely derived from oceanic crust following hydrothermal alteration and seawater percolation within the Archean greenstone belts in southeastern Gabon.

How to cite: Aupissy, C. P. S., Bankole, O., Fontaine, C., and El Albani, A.: Mineralogical and geochemical constraints of Archean Baniaka Iron Formation: implication for origin and source of iron, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12496, https://doi.org/10.5194/egusphere-egu26-12496, 2026.

Two contrasting end-members of Archean crustal growth can be envisaged: one dominated by juvenile additions from a depleted mantle, and the other by extensive reworking and remelting of older, chemically evolved continental crust. Archean cratons preserve the earliest record of crust–mantle differentiation and extreme tectonics, yet the relative contributions of these two processes remain debated. We address this issue through an integrated field, petrographic, whole-rock geochemical, zircon U–Pb geochronological, and Lu–Hf isotopic study of Neoarchean high-K I-type granites from the Bundelkhand Craton, northern India. These metaluminous to weakly peraluminous granites (SiO₂ 64–76 wt.%) intrude Palaeo- to Mesoarchean tonalite–trondhjemite–granodiorite (TTG) gneisses and display LILE-enriched, Nb–Ti–Sr-depleted, arc-like trace-element signatures, consistent with generation in a convergent-margin or arc-related geodynamic setting. New zircon U–Pb ages of 2559 ± 16 Ma, 2530 ± 12 Ma, and 2520 ± 26 Ma define a ~30 Myr-long episode of Neoarchean felsic magmatism, marking a protracted period of late Archean thermal and magmatic activity in the Bundelkhand Craton. Zircon εHf(t) values ranging from –9.3 to –1.9, together with two-stage Hf model ages of ca. 3.0–3.4 Ga, indicate that these granites were generated predominantly by partial melting of Meso- to Palaeoarchean TTG and mafic lower crust, with only limited input from juvenile, depleted-mantle–derived magmas. Zircon solubility in silicate melts and Ti in Zircon thermometry yield crystallisation temperatures of ~720–800 °C, while pressure estimates indicate emplacement at shallow to mid-crustal levels (~3–19 km), reflecting thickening of an already stabilised cratonic root.

The Lu–Hf signatures of the Bundelkhand granites provide an important basis for comparison with other Neoarchean cratons. Coeval granitoid suites in the North China Craton typically show more juvenile, near-chondritic to positive εHf(t) values, reflecting substantial additions of depleted-mantle–derived magmas during Neoarchean crustal growth. In contrast, the strongly negative εHf(t) values obtained for the Bundelkhand granites closely resemble those reported from Neoarchean granitoids in the Zimbabwe Craton, where crustal reworking and remelting of Neoarchean–Mesoarchean crust dominate over juvenile additions. The Bundelkhand high-K I-type granites, therefore, represent a crustal reworking-dominated end-member of late Archean continental growth. Placed within a Kenorland framework, our results emphasise that parts of the Indian shield evolved through prolonged reworking of older continental lithosphere rather than large-scale juvenile accretion, and they provide first-order constraints on the geodynamic regimes that governed late Archean continental assembly.

Keywords
Archean cratons; Bundelkhand Craton; Neoarchean granite magmatism; zircon U–Pb geochronology; Lu–Hf isotopes; crustal reworking; North China Craton; Zimbabwe Craton; Kenorland.

How to cite: Bisht, B. P. S., Xie, H.-Q., and Nasipuri, P.: The role of high-K I-type granites in Neoarchean craton stabilisation: insights from the Bundelkhand Craton, India, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1020, https://doi.org/10.5194/egusphere-egu26-1020, 2026.

The Archean cratonic mantle formed as residues of extensive melt extraction, which is widely (but not universally) thought to have occurred in oceanic settings before being subducted and involved in the growth of the early continents. Despite its depleted nature, cratonic mantle peridotites, particularly from the Kaapvaal craton of southern Africa, often show geochemical and mineralogical evidence for intense secondary silica addition. Silica addition to the peridotitic cratonic mantle has been suggested to result from metasomatism by eclogite-derived silicic melts in subduction channels, serpentinization of oceanic protoliths, or, conversely, unrelated to oceanic protoliths and subduction tectonics, and results from interaction of the cratonic mantle with rising silica-enriched mantle-derived melts. 

Here we use the Si-O isotopic compositions of cratonic mantle peridotites to constrain the source of silica enrichment, and thus improve understanding of processes that operated during the formation of Earth's first continental lithosphere. Mineral separates from coarse, low-T (<1000°C), orthopyroxene-enriched peridotite xenoliths from the Kaapvaal craton (South Africa) have δ30Si values from -0.56 to +0.40 ‰ and δ¹⁸O from +3.7 to +5.6 ‰. Silica-enriched peridotites with mantle-like δ¹⁸O-δ³⁰Si indicate silica addition did not manifest in any isotopic change, in contrast to peridotites with high δ³⁰Si at low δ¹⁸O. Rising mantle-derived silica-enriched melts (formed by hydrous fluxing of harzburgite or wall rock assimilation) could be the culprits of silica enrichment, based on recent oxygen isotope and geochemical studies of lithospheric mantle peridotites, which reconciles with the mantle-like Si-O isotopic signatures observed here. Post-3.8 Ga granitoids are characterized by elevated δ³⁰Si, interpreted to be sourced from subduction-recycled Archean cherts with universally high δ³⁰Si. Since Archean siliceous sediments are generally characterized by δ¹⁸O>>5‰, this is likely not a feasible method to produce the elevated δ³⁰Si observed here. However, Precambrian ocean waters, with higher Si contents and δ³⁰Si > 0‰ could instead have facilitated the high δ³⁰Si with low δ¹⁸O observed for some Kaapvaal peridotites. However, both Si and O isotope disequilibrium observed in some of our samples raises questions regarding the timing of SiO₂-addition, suggesting that the Si-addition, and isotopic signatures, may be a post-Arcehan feature related to Proterozoic subduction-driven metasomatism.

How to cite: Smart, K., Moynier, F., Deng, Z., Harris, C., and Tappe, S.: Silicon isotopic evidence for post-Archean silica enrichment of the cratonic mantle lithosphere, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17565, https://doi.org/10.5194/egusphere-egu26-17565, 2026.

Emergence and formation of continental crust profoundly impacted Earth's evolution. Ensuing continental erosion enriched the oceans with terrestrial nutrients and set the stage for the evolution of complex life. Many crustal growth models indicate significant continental volume increases between 3.0–2.5 Ga, marking this period as critical for crustal evolution, possibly linked to the transition towards subduction tectonics.

Here we present a novel approach to trace continental emergence by reconstructing oceanic boron isotope composition from marine deposits (chert, iron formations, and shales). Boron enrichment in continental crust means runoff directly influences ocean boron concentration and isotopic composition, with continental runoff representing the largest modern source.

Our comprehensive marine B isotope record reveals a major compositional shift at 3.0 Ga: pre-3.0 Ga deposits show mean δ¹¹B values of -23.1 ± 2.7 ‰, whereas post-3.0 Ga sediments are more variable with mean δ¹¹B of -8.9 ± 3.1 ‰, projecting to seawater δ¹¹B = +16 ‰ at 2.4 Ga. This change reflects enhanced continental emergence and subaerial erosion after 3.0 Ga, substantially increasing boron flux and driving seawater towards higher δ¹¹B in the Proterozoic. A second elevation to modern values (δ¹¹B = +39.6 ‰) occurred throughout the Phanerozoic due to increased chemical weathering following land plant appearance, paralleling the increase in seawater δ⁷Li.

How to cite: Abbo, A., Marschall, H., and Gerdes, A.: Tracing Craton emergence with Boron isotopes in Archean–Proterozoic marine deposits, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7707, https://doi.org/10.5194/egusphere-egu26-7707, 2026.

Dyke swarms and sedimentary basins tend to appear in the late Archaean (Cawood et al., 2022), suggesting that the crust was cold and brittle enough to facilitate and preserve dyke swarms. However, the thermal and mechanical state of the Archaean lithosphere that facilitates extensive diking and fracturing remains unclear. 

Continental crust seems to have been relatively mafic in the Archaean (Dhuime at el., 2015, Hawkesworth and Jaupart, 2021), but over less than one billion years, it underwent internal differentiation largely driven by in-situ radiogenic heat production (Perry et al., 2006, Michaut and Jaupart, 2007). Magmatic rocks in greenstone belts show bimodal silica distribution (Kamber, 2015), and bulk crust was composed of ~30% felsic crust and ~70% depleted or dehydrated mafic crust (Hawkesworth and Jaupart, 2021). Tonalite-trondhjemite-granodiorites (TTGs) are consistent with internal differentiation, and are thought to have been produced via partial melting of hydrous metabasalts (Moyen and Martin, 2012). A preferred model of the Archaean lithosphere is to generate mafic crust until it was thick enough to melt and form TTGs and then to have a more felsic crust that ultimately stabilised sufficiently for dyke swarms. 

Here, we test the hypothesis that thickening and internal differentiation (felsification) of Archaean crust led to major cooling of the lithosphere allowing dyke swarms to be a feature of the late Archaean. We calculate thermal profiles for the Archaean lithosphere for different scenarios of internal differentiation between 3.5−2.5 Ga that are consistent with present-day observations (e.g., Michaut and Jaupart, 2007). These thermal profiles are then used to investigate melt transport in the lithosphere using a two-phase flow model that incorporates a new poro-viscoelastic–viscoplastic theory with a free surface (Li et al., 2023, Pusok et al., 2025), designed and validated as a consistent means to model dykes. Results show that a warmer, weaker crust facilitates formation of sills and smaller dikes, while a cold, brittle crust facilitates formation of larger dykes. Our results suggest that dyke swarms are evidence for a cooling geotherm and strengthening of crust, and that crustal differentiation was a necessary condition for crustal stability of Archean provinces. This threshold for dyke swarm formation could have implications for the onset of widespread subduction and plate tectonics.

References

Cawood et al. (2022) Rev. Geophys. DOI:10.1029/2022RG000789

Dhuime at el. (2015), Nat. Geosci. DOI:10.1038/ngeo2466

Hawkesworth and Jaupart (2021), EPSL DOI:10.1016/j.epsl.2021.117091

Kamber (2015), Precam. Res. DOI:10.1016/j.precamres.2014.12.007

Li et al. (2023), GJI DOI:10.1093/gji/ggad173

Moyen and Martin (2012), Lithos DOI:10.1016/j.lithos.2012.06.010

Michaut and Jaupart (2007), EPSL DOI:10.1016/j.epsl.2007.02.019

Perry et al. (2006), JGR DOI:10.1029/2005JB003893

Pusok et al. (2025), GRL DOI:10.1029/2025GL115228

How to cite: Pusok, A. E. and Hawkesworth, C.: Thickening and felsification of Archaean crust stabilised the geotherm allowing for dyke swarms in the late Archaean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5924, https://doi.org/10.5194/egusphere-egu26-5924, 2026.

Archaean lamprophyres and sanukitoids have been spatially and temporally linked to magmatic-hydrothermal gold deposits across the Yilgarn Craton of western Australia. Lamprophyres are considered to be the product of hydrous low-degree partial melting of metasomatic mantle source regions. Sanukitoids are relatively rare late Archaean mantle-derived hornblende-plagioclase porphyritic granitic complexes characterised by high MgO and relatively elevated concentrations of Ba and Sr. Therefore, sanukitoids exhibit mantle-derived (Mg, Ni, Cr) and incompatible element-enriched components (Sr and Ba) indicative of contributions from both mantle and crustal sources.  Cognate xenoliths within the sanukitoids are amphibole-rich and/or biotite-rich metabasites. One model proposed for explaining sanukitoid formation is through amphibole-dominated fractional crystallisation of a lamprophyric parental melt^[1].

Understanding whether the spatial relationship between gold systems and sanukitoid-lamprophyre magmatic systems is also genetic will be important for updating Archaean magmatic-hydrothermal gold deposit models. High precision trace gold analyses, with detection limits of 0.02 ppb, have been conducted on systematic samples of lamprophyres and sanukitoids to quantify the gold concentration variation during magmatic differentiation.

Here, we present mineral chemistry (amphibole, feldspar, mica) from sanukitoids associated with gold deposits and their cognate xenoliths in conjunction with whole-rock, and trace gold, geochemistry from the Yilgarn Craton. We present our results of fractional crystallisation modelling and our investigation into the behaviour of gold during mantle (primitive lamprophyres) and crust (evolved sanukitoid) transportation. We test whether the Archean lamprophyre-sanukitoid magmatic system is intrinsically elevated in gold or whether lamprophyre-sanukitoid magmas provide fertile fluid conduits for gold deposit formation.

^[1] Smithies et al., 2019. Nature Communications, 10(1), p.5559.

How to cite: Nash, I. J., Hollis, S. P., Hastie, A. R., Smithies, R. H., Verbeeten, A., Holder, D., and Stock, E.: Origin of sanukitoid magmas linked to Archaean intrusion-related Au deposits: Insights from the Yilgarn Craton, Australia, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13646, https://doi.org/10.5194/egusphere-egu26-13646, 2026.

The size and spacing of TTG batholiths is remarkably uniform within and amongst Archean granitoid-greenstone terrains. On craton-spanning maps, the average distance between centres of batholiths is, on average, twice the present-day depth to the Moho in the Zimbabwe, Ukraine, Superior and eastern Pilbara cratons, wherein the present erosional surface that reveals the granitoid-greenstone cellular pattern well corresponds to paleo-depths near the brittle-ductile rheologic transition (~520C). It is a fundamental principle of fluid dynamics that an array of convection cells in a horizontal layer convecting by Rayleigh-Bénard (temperature-induced) or Rayleigh-Taylor  (composition-induced) density instabilities has convection cell radii (half the distance between centres of cells) that scale near 1:1 with the thickness of the convecting layer, as is the case in all these granite-greenstone terrains. Heavy oxygen isotopic compositions of mafic granulites (intercalated with paragneisses) in the Archean lowermost crust exposed in the Kapuskasing Zone (south-central Superior), the Vredefort structure (Kaapvaal craton), and as Archean xenoliths in Tertiary diatremes in western Wyoming craton show them to be seafloor-altered metavolcanics that migrated to the base of the crust during the Archean. Metamorphic peak P-T measurements in supracrustal rocks spanning the prehnite-pumpellyite to garnet granulite facies in the Wawa-Kapuskasing crustal section preserve a precisely resolved, steep, conductive geotherm of ~40°C/km to ~12 km paleodepth and a much flatter gradient (~11°C/km) deflected toward an adiabatic gradient in convecting ductile rocks at ~12-40 km paleodepth. Metamorphic fluids released from supracrustal rocks that migrated to the base of the crust and underplated hotter overlying rocks lowered the solidus temperature of the fluid-metasomatised overlying rocks and induced production of silicic partial melts (TTGs) having spidergram spikes in relative abundance of water-soluble elements like spidergrams of Phanerozoic arc magma. The arc-like spidergram patterns are absent in most coeval greenstones, which have spidergram patterns resembling MORB and OIB melts of asthenospheric mantle.  Arc-like spidergram patterns in Archean TTGs can be well explained without plate tectonics. Craton-scale cellular arrays of greenstone belts and TTG batholiths are inconsistent with plate tectonics.

How to cite: Loucks, R.:  Crustal Convection Turned Out a Superior Craton—and Zimbabwe, Pilbara and Ukraine Archean Cratons Too, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15236, https://doi.org/10.5194/egusphere-egu26-15236, 2026.

Subaerial land today is mainly formed by continental crust, but before the stabilization of the first cratons, at ca. 3 Ga, volcanic structures (e.g., oceanic islands) may have been the first subaerial regions of the early Earth. Understanding the onset of felsic magmatism is crucial for constraining the formation of both continental crust and hypothetical early volcanic islands. Studies of ancient zircons suggest that subaerial land likely emerged at least by 3.5 Ga, but how long before that it began remains unknown. Our work examines circa 3.5 Ga old felsic volcanic rocks, the oldest known in the Kaapvaal (South Africa) and Singhbhum (India) cratons. We analyzed oxygen and Lu-Hf isotopes in zircon as they are effective proxies for distinguishing the melt source between mantle-derived and crustal (remelting of altered rocks and sediments). Oxygen isotopes ratios (δ¹⁸O) were measured by Secondary Ion Mass Spectrometry (SIMS) in coeval felsic units of the Kaapvaal Craton (i.e. Theespruit, Sandspruit, and Toggekry formations), and of the Singhbhum Craton (Daitari and Gorumahisani greenstone belts). This new data was compared with a newly compiled global Archean δ¹⁸O dataset (ca. 13,000 data points). Our felsic volcanic rocks display the averaged δ¹⁸O values ranging between 5.1 and 5.8 ± 0.24 ‰ (2 sd), which are purely mantle-like values. The only exception is a Toggekry formation sample (δ¹⁸O 3.9 ± 0.24 ‰), which reflects remelting of hydrothermally altered rocks. Published εHf values for the same rocks fall between CHUR and Depleted Mantle trends, implying juvenile melt signatures. In this context, we highlight the significance of the early Earth's first felsic rocks, whose formation is usually attributed to partial melting of a hydrated basaltic oceanic crust. In contrast, our data emphasizes the importance of purely mantle-derived felsic melts in the Archean. These felsic melts can be a result of extensive fractional crystallization (ca. 80%) of a stalled basaltic melt. Such relatively dry melting (possessing only juvenile water) requires elevated heat flow, and thick lithosphere. During the Archean, these conditions may have prevailed in a thick basaltic oceanic plateau setting. Reworking (i.e., melting) of such ancient oceanic plateaus could have led to the renewed generation of felsic melts producing buoyant silicic rocks and ultimately result in the consolidation and emergence of the earliest continental crust. The global Archean δ¹⁸O values compilation suggests that the mantle and seawater-altered rocks are both important sources of felsic melts during the Archean. This highlights the significance of global Archaean tectonic regimes that may have led to the formation of the first subaerial landmass in brief stints.

How to cite: Gromov, P., Jodder, J., Conrad, C. P., Torsvik, T. H., Agangi, A., Wiedenbeck, M., Couffignal, F., Glynn, S. M., and Gaina, C.: Widespread felsic volcanism as a possible step towards Archean subaerial landmass: Insights from combined oxygen and hafnium isotopes in zircon, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17446, https://doi.org/10.5194/egusphere-egu26-17446, 2026.

Geochemical Investigation of Paleoproterozoic Siliclastic Rocks: Implications for Evolution of North Indian Craton.

Sadia Khanam¹, Nurul Absar², Mohammad Adnan Quasim¹

• 1-  Aligarh Muslim University
• 2- Pondicherry University

The Paleoproterozoic was the most significant era in geological history, during which crustal stabilization occurred, and Earth began to develop an atmosphere habitable to life after the Great Oxygenation Event. Paleoproterozoic cratonic rocks preserve chemical signatures that provide insight into the onset of plate tectonics, supercontinent assembly, and the Great Oxidation Event, offering a unique archive for reconstructing Earth’s first billion years and guiding mineral resource exploration. The Indian subcontinent has a unique cratonic nucleus comprised of rocks of   the Paleoproterozoic, that is, the Aravalli craton. However, few studies have been conducted on the complete evolution of the Aravalli Craton. To this end, we investigated the Paleoproterozoic Rajgarh Formation of the Alwar sub-basin, North Delhi Fold Belt. This study provides critical insights into the provenance, depositional environment, and tectonic setting of the North Indian Craton (NIC) through petrographic, mineralogical, and geochemical analyses. The sandstones are quartzarenite to arkosic in nature and composed of monocrystalline quartz with undulose extinction, feldspars, micas, cordierite, and heavy minerals, including zircon, garnet, and tourmaline. Modal analysis and Qt–F–RF ternary plots indicate cratonic interior provenance. Chondrite-normalized REE patterns display fractionated LREE, flat HREE, and negative Eu anomalies (average Eu/Eu* = 0.76), consistent with felsic to intermediate sources such as granitoids, TTG gneisses, and granulites of the Banded Gneissic Complex and the Sandmata Complex. Trace element ratios (Th/Sc, La/Sc, La/Yb, and Cr/V) and discrimination diagrams (La–Th–Sc, Th–Sc–Zr/10, Ga with V and Sn) also suggest derivation from felsic crust in active to passive margin settings. Redox-sensitive proxies (U, V, Mo, Cd and EF of RSTE) and pyrite occurrence indicate deposition in oxic to sub-oxic shallow-marine environments with intermittent dysoxic phases. Collectively, the Rajgarh Formation of Alwar sub-basin records a complete tectonic cycle: intracratonic rifting and basin initiation, felsic-dominated sediment supply, shallow-marine rift deposition, and basin stabilization during the Mesoproterozoic Delhi Orogeny. Comparisons with other Purana basins (Vindhyan, Cuddapah, and Bayana) demonstrate that the NIC experienced widespread intracratonic extension during the Columbia supercontinent cycle, followed by stabilization during Rodinia accretion.

How to cite: khanam, S., Absar, N., and Quasim, M.: Geochemical Investigation of Paleoproterozoic Siliclastic Rocks: Implications for Evolution of North Indian Craton. , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1292, https://doi.org/10.5194/egusphere-egu26-1292, 2026.