Stagnant-lid tectonics in early Earth revealed by 142Nd variations in late Archean rocks

• A 142Nd anomaly is present in a 2.7 Ga old lava flow of the Abitibi Greenstone Belt.
• The terrestrial mantle, despite convecting, was not fully re-homogenized 2.7 Ga ago.
• Stagnant-lid regime can reduce convective mixing compared to mobile-lid regime.
• Archean Earth was characterized by a stagnant-lid regime, with limited subduction.
• A convective but poorly-mixed mantle in a stagnant-lid regime can be applied to Mars.

A major change in Earth's geodynamics occurred ~3 billion years (Ga) ago, likely related to the onset of modern and continuous plate tectonics. However, the question of how Earth functioned prior to this time is poorly constrained. Here, we find a resolvable positive 142Nd anomaly in a 2.7 Ga old tholeiitic lava flow from the Abitibi Greenstone Belt indicating that early-formed mantle heterogeneities persisted at least 1.8 Ga after Earth's formation. This result contradicts the expected rapid early (~0.1 Ga), as well as the slower present-day (~1 Ga) mixing rates in the convecting mantle. Using a numerical modeling approach, we show that convective mixing is inefficient in absence of mobile-lid plate tectonics. The preservation of a 142Nd anomaly until 2.7 Ga ago can be explained if throughout the Hadean and Archean, Earth was characterized by a stagnant-lid regime, possibly with sporadic and short subduction episodes. The major change in geodynamics observed around ~3 Ga ago can then reflect the transition from stagnant-lid plate tectonics to modern mobile-lid plate tectonics. Solving the paradox of a convective but poorly-mixed mantle has implications not only for Archean Earth, but also for other planets in the solar system such as Mars.