Evolution of lithospheric thickness in Early Earth: Insights into tectonic regimes

Understanding lithospheric thickness during the Early Earth is crucial for unraveling tectonic modes (e.g., heat-pipe vs. stagnant-lid) and cooling mechanisms. The heat-pipe model likely produced a thick lithosphere early on, which thinned with declining volcanism before thickening again as stagnant-lid conduction became dominant. Conversely, the stagnant-lid mode involved a direct transition from a magma ocean to conduction, characterized by thin, weak lithospheres that gradually strengthened and thickened over time. Tracking the evolution of lithospheric thermal thickness provides a means to test these cooling mechanisms.

Basaltic rocks, the most abundant igneous rocks, offer critical insights into mantle conditions. Experimental evidence indicates that the oxides in primary basaltic melts are sensitive to melting pressure, making them effective proxies for lithospheric thickness. Using global geochemical datasets, such as GEOROC, we can infer lithospheric thickness from basaltic lithogeochemistry. This study evaluates lithospheric thickness during the Early Earth (4.0–3.0 Ga) using compiled basaltic lithogeochemical data. Despite their rarity, heat flow data, xenolith samples, and clinopyroxene thermobarometry were also used to validate findings. Basaltic lithogeochemistry indicates significant thinning from 120 km at ~3.65 Ga to 90 km by ~3.30 Ga, followed by subsequent thickening and eventual stabilization. Heat flow data, though craton-specific and with high age uncertainty, generally support a thinning trend from ~3.75 Ga to ~3.40 Ga, stabilizing and slightly thickening by ~3.20 Ga, with minimal fluctuations until ~3 Ga. Xenolith and clinopyroxene data, available only from ~3.60 Ga onward, indicate a stable lithospheric thickness between ~3.60 Ga and ~3.40 Ga, followed by thickening from ~3.40 Ga to ~3.20 Ga. These observations suggest an evolution from thick, cold lithospheres that initially thinned, likely transitioning to conductive cooling and thickening over time. This supports the probable viability of the heat-pipe model during the Early Earth and provides insights into the planet’s tectonic regimes and lithospheric evolution.