Conference Agenda

Multiple sulfur isotopes (³²S, ³³S, ³⁴S, ³⁶S) have become a powerful tool to track sulfur from surface reservoirs into the mantle and crust during the Archean. Here, we present bulk-rock multiple sulfur isotope data of >3.7 Ga rocks from different lithologies of the Isua Supracrustal belt (SW Greenland), well-characterized ultramafic rocks from the >3.7 Ga Nulliak assemblage (Labrador) [1], and from a pre-Uivak ultramafic body (Labrador) [1] to evaluate how widespread the mass independently fractionated sulfur (MIF-S) signature was incorporated. The metasedimentary rocks from the ISB yield δ³⁴S of +1.1-+2.7 and large Δ³³S anomalies of up to +3.1, overlapping with previous studies. In combination with published data [2-3], the ultramafic and mafic samples from Isua have δ³⁴S spanning from -0.18 to +2.7 and Δ³³S from -0.13 to +0.46. The δ³⁴S, D ³³S and Δ³⁶S data of the Nuliak suite and the pre-Uivak ultramafic body show incorporation of MIF-S, whereas the samples from Isua show more pronounced ^Δ33S signatures. These signatures reflect both hydrothermal sulfur processing and sedimentary pyrite sources. The coexistence of negative and positive δ³³S values in nearby but distinct rock types indicate a primary origin rather than a secondary redistribution. Direct U–Pb dating of sulfides with non-zero Δ³³S in >3.8 Ga peridotites south of the ISB supports an Eoarchean origin of the sulfur, suggesting early crust–mantle volatile cycling, potentially via subduction-like processes.

[1] Ishikawa et al. (2017) GCA 216, 286-211
[2] Siedenberg et al. (2016) Precamb.Res. 283, 1-12
[3] Lewis, et al. (2023) Chem.Geol. 633, 121568

Early mantle differentiaon is critcal for understanding the evolution of mantle reservoirs and their role in continental crust formation in Precambrian cratons. Primitive mantle-derived melts preserve the compositional diversity of their sources, melting conditions, and post-magmatic modification^[1]. This study investigates Archean komatiites, komatiitic basalts, and basalts (n = 22) from the Banasandra, Nagamangala, Holenarsipur, and Bababudan greenstone belts of the Western Dharwar Craton, southern India, using ¹⁷⁶Lu-¹⁷⁶Hf and ¹⁴⁷Sm-¹⁴³Nd isotope systematics, trace element abundances determined by ICP-MS, and high precision HFSE abundances obtained by isotope dilution.

The Al-rich, strongly depleted komatiites display flat rare earth element patterns, indicating that primary element ratios are largely preserved despite the loss of igneous textures. Trace element proxies, including Nb/Ta, Zr/Sm, and Zr/Hf, were used to evaluate mantle source characteristics and the effects of greenschist- to lower amphibolite-facies metamorphism. The komatiites and komatiitic basalts yielded ¹⁷⁶Lu-¹⁷⁶Hf and ¹⁴⁷Sm-¹⁴³Nd errorchrons of 3496 ± 280 Ma (MSWD = 64) and 3350 ± 490 Ma (MSWD = 187), respectively. The high MSWD values are consistent with post-magmatic isotopic disturbance and/or heterogeneous source signatures. Initial εHf and εNd values are highly radiogenic (εΗf = -2.3 - +72.0), and this result was verified through replicate sample preparation and isotope measurements.

Systematic HFSE variations, together with preserved trace element characteristics, suggest derivation from a depleted mantle source modified in an oceanic subduction-related environment. Alternatively, deep mantle processes capable of decoupling the Lu/Hf and Sm/Nd systems^[2] remain a possible explanation.

^[1] Puchtel et al. (2022)

^[2] Blichert-Toft et al. (2010)

The first fragments of sustainable proto-continental crust appeared during the Early Archean, but the exact mechanisms for Archean felsic crust formation remain elusive. Existing research hypothesized that remelting of primarily (seawater-origin) hydrated basalts was the dominant process of Archean felsic melts generation. Yet oxygen isotope ratio (δ¹⁸O) data used as a traditional proxy for deciphering melt provenance, suggest that Archean magmatism was mostly mantle-derived, with only a few weak traces of remelting of hydrated rocks. In order to better understand the meaning of the Archean δ¹⁸O record we analyze it in concert with the published tectonic amalgamation history of fifty Archean terranes. Our global δ¹⁸O database includes a thoroughly curated set of more than 200 published Archean felsic rock samples, complemented by new δ¹⁸O measurements of ca. 3.5 Ga old felsic volcanic rocks from the Kaapvaal and Singhbhum cratons. A comparison of the δ¹⁸O dataset with the Archean tectonic record shows two distinctive correlations. The oldest Archean terranes exhibit exclusively juvenile magmatism while the isotopic signature of seafloor remelting occurs during or after terrane collision. We infer that oxygen isotopes remain sensitive for tracing Archean tectonic processes and postulate that the first cratonic nuclei may have appeared during plume-induced thickening of primordial oceanic crust. Subsequent amalgamation of cratonic nuclei, driven by (yet to be determined) tectonic processes, led to burial and remelting of seafloor rocks that initiated a new stage of felsic magmatism. This process contributed towards the growth of continental crust that ultimately became the stable cratons.

The South American Atlantic margin comprises diverse crustal fragments associated with Neoproterozoic mobile belts. Among them, the Nico Pérez Terrane in Uruguay preserves the largest exposure of Archean rocks in southern South America, consisting of amphibolites, tonalitic to granitic gneisses, and migmatites. Geochronological studies reveal episodic Archean crustal growth during three main events: Paleoarchean (3.3–3.4 Ga), Mesoarchean (3.0–3.1 Ga), and Neoarchean (2.7–2.8 Ga). Lu–Hf isotopic signatures indicate reworking of an ancient paleocrust. Progressively more radiogenic ¹⁷⁶Hf/¹⁷⁷Hf values in younger samples, together with low isotopic dispersion, point to crustal homogenization through episodic juvenile inputs and/or anatexis rather than widespread new accretion. Th/U ratios predominantly indicate magmatic sources, with main metamorphic populations associated with the Mesoarchean and Neoarchean events. εHf_(t) vs. age diagrams indicate crustal or mixed signatures, with predominantly negative εHf_(t) values. KDE distributions of U–Pb and Hf–T_DM ages discriminate the main age populations and suggest a major mantle–crust differentiation event at ~3.7 Ga. Overall, the Archean evolution of the terrane was dominated by crustal reworking with episodic juvenile inputs following Eoarchean protocrust formation. The continuous reworking and isotopic homogenization indicated by the data, together with the TTG affinity proposed in previous studies, support processes such as melting of hydrated basaltic crust, sagduction, or early subduction during the evolution of the Uruguayan Archean crust. This evolution is comparable to that of other Archean blocks, such as the Kaapvaal Craton of southern Africa.

The production of felsic crust on the early Earth is widely attributed to partial melting of hydrated basaltic protoliths, generating tonalite–trondhjemite–granodiorite (TTG) magmas that form a major component of Archean continental nuclei. Amphibolites and hornblendites, although rarely preserved, are key archives of these melting processes, as hornblende-rich rocks may reflect mafic residues left after TTG generation. However, the origin of Archean hornblendites remains debated, as they may represent cumulates, secondary hydration products or refractory residues after melt extraction.

This study integrates field relationships, petrography, and phase-equilibrium modelling to constrain hornblendite formation in the Archean lower crust. The Paleoarchean Kaapvaal Craton preserves rare but well-exposed examples of hornblende-rich residues linked to in situ partial melting of amphibolites. In the Ancient Gneiss Complex (AGC, Eswatini) and the Inyoni Shear Zone (ISZ) of the Barberton granitoid–greenstone terrain (South Africa), amphibolites host felsic leucosomes with diffuse boundaries, bordered by coarse, monomineralic hornblendite domains. These zones are characterized by plagioclase and quartz depletion coupled with hornblende coarsening.

Phase-equilibrium modelling shows that fluid-absent melting at ~700°C produces negligible melt volumes (<5 mol.%) and fails to reproduce the observed mineral modes. In contrast, fluid-fluxed melting at the same temperatures generates ~20 mol.% melt. While concentrating hornblende in the residues matching the natural hornblendites. We therefore conclude that hornblendite formation in the AGC and ISZ reflects fluid-fluxed melting of mafic lower crust. These results provide evidence that some Archean hornblendites are residual products of fluid-fluxed melting, highlighting the central role of water in the generation of continental nuclei.

The breakup of the L-chondrite parent body at ~470 Ma is recorded by multiple chronometers, particularly ⁴⁰Ar/³⁹Ar and U–Pb–Pb ages. This event represents a major episode in the collisional evolution of the asteroid belt and has been widely used as a time marker for the formation of asteroid families and for dynamical models of main belt evolution.

High-precision ⁴⁰Ar/³⁹Ar ages, especially those derived from multiple isochron approaches, provide the most robust constraints on the timing of this event. After recalculation using updated decay constants, several well-constrained meteorites yield consistent ages, including Ghubara (469±8 Ma), Mbale (483±9 Ma), Tsarev (472±9 and 476±8 Ma), NWA6468 (457±10 Ma), and NWA091 (475±6 Ma). These data define an error-weighted mean age of 472.5±3.0 Ma (1σ).

U–Pb–Pb dating provides complementary constraints. Lower intercept ages from meteorites such as NWA11042, Novato, and Chateau Renard yield a combined estimate of 473±11 Ma. Additional data show a wider spread of ages, including values around ~460–500 Ma as well as significantly older ages (~650–700 Ma), suggesting that multiple impact events are recorded within L chondrites.

We report a new ⁴⁰Ar/³⁹Ar age of 471±7 Ma for the L6 chondrite Ramsdorf and a preliminary U–Pb–Pb lower intercept age of 641±108 Ma for the L5 chondrite Ghubara. As Ghubara also records a precise ~470 Ma ⁴⁰Ar/³⁹Ar age, it is uncertain if the older U–Pb–Pb age reflects a different impact event.

Together, these datasets confirm the robustness of the ~470 Ma breakup age while highlighting a more complex collisional history involving multiple impact events.