205-1 Insights into Archean Geodynamics from Diffusion Chronometry

Session: MSA 2025 Awards Lectures and Presidential Address

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Abstract:

The Archean Earth likely witnessed the onset of mobile-lid tectonics; however, these nascent plate interactions may have differed significantly from modern plate tectonics due to elevated mantle temperatures (Cawood et al., 2022; Herzberg et al., 2010). The nature of these early mobile-lid tectonic processes, therefore, remains elusive.

In this context, diffusion chronometry has emerged as a powerful tool for quantifying the durations and rates of metamorphic processes—such as cooling and exhumation rates. Recent work across various late Archean to early Proterozoic orogenic terranes—including the Southern Granulite Terrane and Coorg Block of India, as well as the Lewisian Complex of Scotland—have employed Fe–Mg diffusion modeling in coexisting phases like garnet and pyroxene. These investigations reveal that high-grade metamorphism during this interval occurred over protracted timescales, with post-peak cooling rates significantly slower than those typically observed in modern orogenic belts (Chowdhury and Chakraborty, 2019; Basak et al., 2023). Thus, by capturing the duration—not just the age—of processes, diffusion chronometry provides a unique temporal lens into the pace and thermal history of early Earth tectonic processes.

Integrating these timescale constraints with pressure–temperature (P–T) data argues for an orogenic model involving lithospheric delamination, shallow slab break-off, and prolonged heat flux. We propose the term peel-back orogenesis to describe this style of orogenic style (Chowdhury et al., 2017; 2020), which is characterized by asymmetric thermo-tectonic domains that generate paired metamorphism and crustal melting across multiple levels of the crust, as preserved in the rock record. The peel-back orogenic model echoes aspects of accretionary orogens consistent with the unique thermal and mechanical conditions of an early hot Earth.

Basak et al. J. Petrol. 64, egad026 (2023).

Cawood et al. Rev. Geophys. 60, e2022RG000789 (2022).

Chowdhury et al. Earth Planet. Sci. Lett. 538, 116224 (2020).

Chowdhury et al. Nat. Geosci. 10, 698–703 (2017).

Chowdhury & Chakraborty. J. Petrol. 60, 441–486 (2019).

Herzberg et al. Earth Planet. Sci. Lett. 292, 79–88 (2010).

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8731

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