From the LIPS of a serial killer: Endogenic retardation of biological evolution on unstable stagnant-lid planets

Abstract

Geological data imply plate tectonics was not active on the Archaean Earth pre-2.5 ​Ga. Key arguments include: the absence from the Archaean rock record of ophiolites and high-pressure metamorphic rocks; the rarity of Archaean andesites and lahars; the absence of arc-like, source-metasomatic, trace element signatures in Archaean calc-alkaline magmatic suites; and fundamental differences in overall structural style and constituent lithologies. Conversely, unstable stagnant-lid models better account for many features of Archaean geology. Thermo-mechanical models imply many unstable stagnant-lid planets would experience mobile-lid phases linked to periodic mantle overturns of 30–100 million years duration, separated by stable-lid phases lasting 100–300 My. Mantle overturn upwelling zones would be characterized by high magma fluxes that may have generated continental nuclei, and would have reworked and resurfaced tracts of pre-existing oceanic and continental lithosphere. Overturns would also have generated large-scale lateral mantle flow patterns that would have pushed against the sub-continental lithospheric mantle keels underlying continents, and so induced continental drift and orogenesis, allowing mobile-lid behaviour despite the absence of plate-boundary forces such as slab pull. Major resurfacing of Earth’s surface during overturns would have heated the hydrosphere and atmosphere, with many negative impacts on biota. The high magmatic fluxes associated with mantle overturn events are likely to have induced periodic mass extinctions that may have retarded biological evolution on Earth. Evolutionary progress towards more complex metazoan organisms may only have been possible after the more efficient plate tectonic cooling system helped create a stabler, more temperate planet; although causal relationships remain uncertain. Did the start of a plate tectonic, mobile-lid cooling system on Earth gradually end (or moderate) mantle overturn behaviour? Or was the world-girdling plate tectonic system only allowed to begin because ‘other factors’ gradually acted to suppress mantle overturns during the Proterozoic? These ‘other factors’ include: secular decay of radioactive isotopes, scavenging of radioactive elements into continents, and shrinkage of the fertile lower mantle reservoir that fed early overturns. When overturn behaviour ended on Earth is also uncertain. The 2.1–1.9 ​Ga magmatic and tectonic pulse on Earth is plausibly interpreted as an Archaean-style mantle overturn, but more research is needed to determine whether the Large Igneous Provinces (LIPs) of the Meso- and Neo-Proterozoic are as well. It is predicted that Earth-sized exoplanets with surface water and chondritic mantle compositions will spend at least the first 2 ​Ga of their evolution as unstable stagnant-lid planets, with periodic overturns preventing evolution of complex metazoan organisms. Many such planets could remain trapped in this cooling mode, with only rare cases transitioning into the more efficient plate tectonic cooling mode. If correct, this greatly decreases the probability that complex metazoans are present elsewhere in the universe.