Source evolution controls the molybdenum isotope composition of Archean continental crust

Vandenburg, Eric D., Nebel, Oliver, Spencer, Laura M., Millet, Marc-Alban ORCID: https://orcid.org/0000-0003-2710-5374, Smithies, R. Hugh, Cawood, Peter A., Churchus, Alexandra, Liang, Wenli, Bruand, Emilie, Moyen, Jean-François, McCoy-West, Alex and Chowdhury, Priyardarshi 2026. Source evolution controls the molybdenum isotope composition of Archean continental crust. Geochimica et Cosmochimica Acta 433 , pp. 71-90. 10.1016/j.gca.2026.10.009

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Abstract

Earth’s continental crust is its most chemically evolved and diverse reservoir. Numerous studies have investigated its stable metal isotope character to understand its formation and secular evolution. Molybdenum (Mo) isotopes are useful tracers of petrogenetic processes in Phanerozoic igneous rocks, but Archean granitoids, which make up much of Earth’s early continental crust, remain poorly characterized. Here, we report high-precision whole-rock Mo isotope data for 28 Archean granitoids from Australia’s Pilbara and Yilgarn cratons, with crystallization ages from 3.52 to 2.63 Ga. The samples cover a spectrum of magmatic series, including tonalite-trondhjemite-granodiorite (TTG), TTG hybrid, sanukitoid, potassic, and potassic hybrid magmas. After assessing the effects of alteration, crustal contamination, and fluid exsolution, we interpret the observed δ98/95Mo range of −0.93 to +0.25‰ as primarily reflecting magmatic source characteristics. TTG and TTG hybrid granitoids, together with samples derived from unenriched meta-mafic or mantle-like sources, have mean and median δ98/95Mo values close to, or slightly higher than, chondrite and depleted MORB mantle (DMM). In contrast, potassic, potassic hybrid, and sanukitoid series granitoids, as well as samples derived from enriched meta-mafic or felsic crustal sources, extend to lower, sub-mantle δ98/95Mo values. Elemental proxy relationships, major- and trace-element modeling, and phase-equilibrium constraints indicate that source composition and residual mineralogy during partial melting primarily control Mo isotope variability in these rocks. In crustally-derived granitoids, lower δ98/95Mo values are associated with more mature source compositions and with changes in the abundance and composition of residual feldspar. Amphibole, garnet, and biotite may also have subordinate effects. These relationships suggest that Mo isotopes track the progressive reworking of Archean crust. Juvenile crust-derived TTG magmatism is characterized by near- to suprachondritic δ98/95Mo, whereas later sanukitoid and potassic granitoid magmatism, linked respectively to mantle metasomatism and reworking of TTG-dominated gray gneiss, shifts toward lower δ98/95Mo values. The observed trends imply that intracrustal reworking alone can push continental averages to sub-mantle values, with ramifications for mass-balance models. Although TTGs are generally suprachondritic, their δ98/95Mo values are not high enough to easily support models requiring large volumes of Hadean to Paleoarchean continental extraction from a chondritic mantle to offset the subchondritic Mo isotope composition inferred for the depleted mantle from Archean komatiites. The strong Mo isotope contrast between continental crust and upper mantle may therefore have developed after 2.6 Ga, once crustal recycling became more extensive.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Schools > Earth and Environmental Sciences Schools > Physical, Chemical & Environmental Sciences
Publisher:	Elsevier
ISSN:	0016-7037
Date of First Compliant Deposit:	22 October 2026
Date of Acceptance:	6 October 2026
Last Modified:	22 Oct 2026 09:30
URI:	https://orca.cardiff.ac.uk/id/eprint/190365

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