Investigating the state of silicate Earth differentiation at 3.5 Ga using Mo stable isotopes

• Applicant: Rachel Bezard, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 548157416

The continental crust (CC) stands as one of Earth’s unique attributes within the Solar System. Its formation has profoundly influenced both the deep (mantle) and surface (hydrosphere, atmosphere) layers of the Earth, with significant ramifications for the evolution of life. Understanding the timing and rate of CC development is therefore key to unravel the singular geological and biological evolution of our planet. Yet, the growth of CC has been a long-standing debate with numerous models, predominantly relying on chemical and radiogenic isotope data, proposed over the past 50 years. While some models suggest the existence of large CC volumes (>= 50% of today´s volume) by the end of the early Earth (~4.6 to ~3.5Ga), others support a near absence of CC at the time. In the last few years, non-traditional stable isotopes have been increasingly used to test these models. Among those, the Mo stable isotope system emerged as a powerful tool for the task. Current constraints on the Mo isotope composition (delta98/95Mo) of the modern silicate Earth reservoirs suggest a complementarity between the CC and the depleted MORB mantle (DMM), relative to the primitive mantle. In other words, the extraction of CC seemingly changes the delta98/95Mo of the mantle, and this can be used to assess the volume of CC isolated from the mantle by the end of the early Earth, as recorded by the delta98/95Mo of mantle-derived ultramafic/mafic volcanic rocks. The volume of CC isolated from the mantle at ~3.5 Ga, in turn, enables the discrimination between endmember CC growth models for the first billion year of the Earth. Currently, a settled case on the degree of silicate Earth differentiation at 3.5 Ga using Mo isotopes is hampered by the lack of precise constraints on: 1) the delta98/95Mo of the 3.5 Ga accessible mantle; 2) the behavior of Mo isotopes during early Earth mantle partial melting; and 3) the effects of early Earth evolved crust formation and crustal recycling on the delta98/95Mo of the mantle. This proposal aims at addressing these shortcomings using samples from the Barberton Granitoid-Greenstone Terrain (South Africa), with the overarching goal of shedding light on the volume of CC isolated from the mantle at 3.5 Ga and assessing the validity of existing crustal growth models.

• DFG Programme: Research Grants
• International Connection: Norway, South Africa
• Cooperation Partners: Professor Axel Hofmann, Ph.D.; Jaganmoy Jodder, Ph.D.