Zr isotopes reveal long storage timescales of Archean magmas

Abstract

The storage timescales and thermal states of magma during the Archean are important factors affecting the growth and evolution of the continental crust. However, few reliable estimates exist for these parameters in ancient magmatic systems. Here, we constrain Archean magma storage timescales by using in-situ Zr isotope (δ94Zr) measurements in zircon (3.4 to 2.5 Ga) paired with disequilibrium crystal growth models. Our calculations suggest that fractional crystallization of zircon at isotopic equilibrium leads to small δ94Zr variations (< 0.4‰), indicating that the observed variations (-0.6 to + 0.4‰) are likely caused by disequilibrium crystal growth. The extents of isotopic disequilibrium require zircon growth rates of 0.0007–0.0415 µm/yr, which in turn yield (minimum) magma storage timescales of 5-193 kyr. Combined with evidence for zircon crystallization at high temperatures (730–910 ℃), these long timescales imply protracted periods of “warm storage” for Archean magmas, in stark contrast with predominantly shorter “cold storage” conditions observed in modern systems. Our results serve as independent evidence for higher geothermal gradients throughout the Archean, and the unique magmatic systems have cascading implications for the growth/emergence of continental crust, volcanic activity, and evolution of the Archean atmosphere.