Model versus measured detrital zircon age signatures of the early Earth

Abstract

The scarcity of Archean (4.0–2.5 Ga) rocks on Earth limits our ability to analyse when plate tectonics began and what tectonic mode may have preceded it. Proposed times for the initiation of plate tectonics range from the Hadean eon (>4 Ga) to the Neoproterozoic era (ca 1.0–0.5 Ga), before which the Earth may have been dominated by stagnant-lid regimes that cooled through volcanic advection, partial crustal convection, and/or conduction. Here, we use various 1D numerical simulations of potential geodynamic states of the early Earth to predict detrital zircon age patterns. We find that different Archean tectonic modes yield contrasting temporal differences between zircon crystallization ages and respective maximum depositional ages of the sediments from which the detrital zircons were sourced (we have termed such age differences ‘ΔtDZ’). We compare these predictions with a global compilation of detrital zircon age data from >400 Archean (meta)sedimentary units, including new data from six samples from the Pilbara Craton, Western Australia. Combined, the data show significantly increased contributions from high ΔtDZ detrital zircons after ca 3.4 Ga. For the Pilbara Craton, 90% of the detrital zircons in each of two >3.4 billion-year-old metasedimentary samples have crystallization ages that are ≤110 million years older than the maximum age of deposition (we use ‘ΔtDZ90’ to denote such age differences hereafter). The ΔtDZ90 of Pilbara strata formed at ca 3.29–3.27, 3.24–3.18 and ca 2.95 Ga show successively larger ΔtDZ90 values of ca 270, 290 and 570 million years, respectively. Globally, pre-3.4 billion-year-old samples show a low mean ΔtDZ90 of around 70 million years, but a mean of around 320 million years for 3.40–2.80 billion-year-old samples. Samples with >90% of zircons featuring >250 million year ΔtDZ values only occur at or after ca 3.23 Ga. These results are consistent with a significant slow-down in the rate of volcanic resurfacing, and increasing contributions of detritus from basins associated with exhumation and/or volcanic quiescence after ca 3.4 Ga. Compared with detrital zircon age predictions, our findings are best explained by a transition from volcanism-dominated stagnant-lid modes to one increasingly dominated by subduction over the period 3.4–3.2 Ga, with the emergence of plate tectonics within this period or at some time thereafter.