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

The progressive μ¹⁴²Nd decrease in early Archean rocks from +20 to 0 between 3.9 to 3.6 billions years (Gyr), with rocks younger than 3.5 Gyr showing no μ¹⁴²Nd anomalies, is thought to indicate the efficient remixing of the first primitive crust into the Archaean convecting mantle that ultimately produce a well-mixed present-day convecting mantle with μ¹⁴²Nd = 0. The implied long mixing time of approximately 1 Gyr from the Hadean to Archaean for the whole mantle is paradoxical on several levels. This is much longer than the rapid mixing time (<100 Myr) inferred for the Archean due to vigorous mantle convection related to Earth's hotter thermal regime, and similar to the mixing time inferred for the present-day Earth's mantle. Here we report a resolvable positive ¹⁴²Nd anomaly of μ¹⁴²Nd = +7 ± 3 ppm relative to the modern convecting mantle in a 2.7 Gyr old tholeiitic lava flow from the Abitibi Greenstone Belt in the Canadian Craton. Our result effectively extends the early Archean convective mixing time to approximately 1.8 Gyr, i.e. even longer than present-day mantle mixing timescale, despite a more vigorous convection expected in the Archean. Different hypotheses have been examined to explain such a protracted mixing in the Archean, such as mantle overturn, two-layer convection or the existence of a dense layer at the bottom of the mantle. We postulate that the requirement of a delayed mixing in a strongly convective mantle is best explained by long periods of stasis in the global plate system, with scarce episodes of subduction throughout the Hadean and Archean. Our numerical model confirms that in absence of continuous plate tectonics, the convective mantle mixing is relatively inefficient in erasing the chemical heterogeneities inherited from the primordial differentiation of the early Earth. This constrains the tectonic regime of the Hadean and Archean to a stagnant-lid regime with episodic subduction. In this case, the timing for the onset of continuous modern plate tectonics can only occur shortly before or after 2.7 Gyr.