Deciphering the formation of Earth’s earliest continental crust is central to understanding its geodynamic evolution and the establishment of long-term planetary habitability. Archean Tonalite-Trondhjemite-Granodiorites (TTGs) are the preserved remnants of juvenile crust, yet the composition of their protoliths and the tectonic settings in which they formed remain debated.

This talk presents an integrated approach combining titanium (Ti) stable isotopes and trace element systematics from natural samples with high-pressure/high-temperature experiments to constrain TTG petrogenesis. Titanium isotopes are an ideal tool for tracing continental crustformation due to their sensitivity to rutile, ilmenite, amphibole, and (to a lesser extent), clinopyroxene and garnet – the main phases involved in TTG petrogenesis via partial melting of metabasalts.

We focus on Eoarchean metabasalts and TTGs from the Isua supracrustal belt (ISB) in southwest Greenland. Isua metabasalts are enriched in MgO and depleted in Al₂O₃ and TiO₂ compared to modern oceanic basalts. Partial melting experiments conducted at 1–1.8 GPa on synthetic Isua-like metabasalts demonstrate that such high-Mg, low-Al basalts are suitable protoliths capable of producing TTG-like melts.

The resulting experimental melts and mineral phases allow quantification of trace element and Ti isotopic fractionation during partial melting of hydrated mafic crust. Such data is critical for use in thermodynamic modelling of crust formation. Ti isotope data from Archean rocks combined with thermodynamic modelling supports a two-stage model for Archean crust formation: (1) generation of primitive tonalitic melts via polybaric partial melting of low TiO₂ metabasalts (~0.8–1.8 GPa), followed by; (2) fractional crystallisation to yield more evolved TTGs with heavier Ti isotopic compositions. These Ti isotope trends, combined with major and trace element proxies, are consistent with melting under variable pressure–temperature conditions, possibly within a ‘proto’-subduction zone environment.

Together, these results highlight the power of combining experimental petrology with novel isotopic tracers to reconstruct early crust-forming processes and assess their implications for Earth’s early geochemical evolution and planetary habitability.

Short-lived isotope systems such as ¹⁸²Hf-¹⁸²W (t_1/2 = 8.9Ma) and ¹⁴⁶Sm-¹⁴²Nd (t_1/2 ≈ 100 Ma) are pivotal tools in understanding Hadean and early Archean geodynamic processes, including core formation, silicate differentiation, and crust formation. However, there is no consensus on the average crustal composition of ¹⁸²W and ¹⁴²Nd, because excesses, deficits, and bulk silicate Earth (BSE)-like µ¹⁸²W and µ¹⁴²Nd values have been interpreted as regional rather than global average crustal compositions [1, 2].

To resolve this discrepancy, we analysed the µ¹⁸²W and µ¹⁴²Nd composition of Archean to Proterozoic (3.2 Ga to 1.2 Ga) siliciclastic rocks from the Kaapvaal Craton (South Africa) and the Yangtze Block (South China). These sedimentary rocks, integrating isotope signals from broad source regions, best represent the average upper continental crust composition. Trace element ratios (e.g., W/Th, La/Yb, Th/Sc) and long-lived radiogenic isotope compositions (¹⁴³Nd/¹⁴⁴Nd, ¹⁷⁶Hf/¹⁷⁷Hf) infer a mixture of dominantly felsic, but also mafic juvenile igneous source rocks. Except for one shale with ¹⁸²W deficits, all samples exhibit BSE-like µ¹⁸²W and µ¹⁴²Nd values. Consequently, our µ¹⁸²W and µ¹⁴²Nd data set represents the best current estimate for the composition of the upper continental crust between 3.2 and 1.2 Ga. The absence of resolvable µ¹⁸²W and µ¹⁴²Nd anomalies also constrains the corresponding mantle source compositions. This suggests that anomalous µ¹⁸²W and µ¹⁴²Nd compositions [1,3] likely originated from locally restricted and isolated mantle sources.

[1] Mundl et al. 2018; Chem.Geol.

[2] Boyet et al. 2021; GCA

[3] Willbold et al. 2011; Nature

The modern oceans’ Zr/Hf ratio (150-300¹) is significantly fractionated from the chondritic value (34.4²). This deviation is driven by the higher particle reactivity of Hf relative to Zr, resulting in preferential sorption onto (oxide)-particles in estuaries. The ratio increases from the coastline towards the open ocean, leading to variability between water masses¹. The short residence time of Hf and Zr makes this ratio a powerful tracer for ocean circulation patterns and sources affecting seawater.

The Zr/Hf of ancient seawater is poorly understood, but has great potential to extend this proxy as a paleoceanographic tracer. Banded iron formations (BIFs) may have preserved the Zr/Hf of ancient seawater and could serve as a seawater archive. We present high-precision Zr-Hf data of Precambrian BIFs complemented by available literature data to test this proxy as a paleoceanographic water mass tracer.

Archean BIFs display near-chondritic Zr/Hf, while super-chondritic ratios in individual BIF layers first appear ~2.51 Ga. The locations’ averages predominantly remain near-chondritic until ~2.0 Ga, whereas younger BIFs show mostly super-chondritic ratios. The shift from chondritic towards super-chondritic Zr/Hf ratios of BIFs records changing conditions throughout the Precambrian seawater affecting the fractionation of Zr and Hf. Potential causes include increased occurrence of (Fe)-Mn(oxide)-particles and enhanced riverine discharge. Regional differences among coeval formations suggest varying depositional settings and distinct water mass circulation in the Precambrian ocean. The Zr/Hf ratio measured in BIFs potentially traces the occurrence of Mn-oxides, hence, oxygenated water masses.

¹Godfrey et al., 1996, GCA 60

²Weyer et al., 2002, Chem.Geo. 187

The mechanisms that preserved the microbial mats of the Paleoarchean Moodies Group sandstones (c. 3.22 Ga) remain unclear. These kerogenous, microbially laminated structures occur abundantly in medium- to coarse- grained tidal-flat sandstones. They are readily mappable along a 15 km-long ridge of silicified strata in the central Barberton Greenstone Belt, South Africa. Silicification is thought to be attributable to the Moodies-age Lomati River Sill (LRS), a large mafic laccolith, c. 1 km below. Tidal-flat sandstones overlying the LRS feature abundant, isolated or clustered fluid-escape structures (FES) up to 6 m high. µ-XRF-generated elemental maps of nine slabbed and polished FES samples show enrichment of Fe, Cr, Ti and V along their central conduits and margins. S-Fe maps show thick pyrite rinds overgrowing detrital grains, possibly attributable to sulfur-reducing bacterial processes, magma degassing or breakdown of minerals. We speculate that the FES represent discrete pathways of potentially hot and silica-rich fluids migrating within the halo of the LRS and erupting into sand volcanoes, in which sand grains, primarily of volcanic provenance, underwent local argillic alteration. Precompaction surficial silicification (or carbonatization) by mineralized fluids may have preserved composition and texture perfectly. Current challenges to our research include modifications by Archean aggressive weathering, regional greenschist-facies metamorphism, recent weathering, lack of recent analogs, and reset of the original isotopic systems, all of which blur detailed insights.

The Volyn biota from the Precambrian Korosten pluton (Western Ukraine) represents an exceptional assemblage of (micro)fossils. Their significance lies in the extraordinary three- dimensional preservation as well as their unique mode of occurrence – found at depth of ≥100 m within miarolitic vacuoles in granitic pegmatites. Previously, we arrived at a minimum age of ~1.5 Ga for the Volyn biota, (Franz et al. doi.org/10.5194/bg-20-1901-2023), which was challenged by Popov (doi.org/10.3390/geosciences13100297). Head et al. (doi.org/10.5194/bg-21-1773-2024) doubted the fossil nature of the Volyn biota.

While confident in the classification as genuine fossils, we acknowledged that our minimum age estimate relies solely on one mineral (muscovite) and one isotope system (⁴⁰Ar/³⁹Ar; Franz et al. doi.org/10.5194/bg-21-4119-2024) and hence demands validation. Here, we present new geochronology data based on in situ Rb/Sr dating of igneous alkalifeldspar and Li-mica, post-igneous muscovite and hydrothermal buddingtonite together with in situ U/Pb dating of black opal (with organic matter). The latter acts as a cement within the pegmatites and therefore represents the latest mineral formation (see poster in session 01.21 Advances in Geochronology).

The Li-mica confirmed the previously known igneous event of ~1.7 Ga and records a hydrothermal event at ~1.5 Ga. The muscovite Rb-Sr data is in agreement with the previous ⁴⁰Ar/³⁹Ar age. Buddingtonite, which did not yield reliable ⁴⁰Ar/³⁹Ar ages, gave a reliable Rb/Sr isochron at ~520 Ma. Alkalifeldspar recorded the igneous event and several hydrothermal events at ~1.5 Ga, ~500 Ma, and ~200 Ma). Opal records the latter event and a Jurassic overprint at ~160 Ma.

Mars Exploration Rover Spirit landed in Gusev crater on Mars in 2004. The crater was selected as the landing site because delta-like deposits at the mouth of a large inflow channel, Ma’adim Vallis, suggest that it once held a lake. However, Spirit landed on top of lava flows obscuring any potential lake sediments. The Columbia Hills are located near the center of the crater and are embayed by these lava flows, thus representing older material. Though not lake deposits, the hills reveal evidence for substantial aqueous alteration including fine-grained, layered outcrop containing the iron hydroxide goethite at the West Spur (Morris et al. 2006); the Comanche outcrop containing ~26% magnesium/iron carbonate (Morris et al. 2010); and opaline silica deposits (Squyres et al. 2008) indicating an ancient hot spring (Ruff and Farmer 2016) in the Inner Basin. The hot spring deposits resemble biosignatures found in hot springs on Earth (Ruff and Farmer 2016). Here we present evidence that the mineralogy at the West Spur as maeasured with Spirit’s Mössbauer (Morris et al. 2006) and miniature Thermal Emission Spectrometer (mini-TES; Ruff et al. 2006; Wang et al. 2006) is consistent with serpentinization. Furthermore, serpentinization at West Spur, carbonates at the Comanche outcrop and the opaline silica hotspring deposits in the Inner Basin can be linked via an evolving aqueous fluid. The process of serpentinization adds to the Columbia Hills’ habitability because it can result in the formation of organic molecules via Fischer-Tropsch synthesis, and release hydrogen, which could support microbial metabolisms.