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TitleFrom the LIPS of a serial killer: endogenic retardation of biological evolution on unstable stagnant-lid planets
AuthorBédard, J H
SourcePlanetary and Space Science vol. 192, 105068, 2020 p. 1-10,
Alt SeriesNatural Resources Canada, Contribution Series 20200124
PublisherElsevier Ltd.
Mediapaper; on-line; digital
File formatpdf; html
Subjectstectonics; extraterrestrial geology; Science and Technology; Nature and Environment; geological history; plate tectonics; tectonic evolution; crustal evolution; models; lithosphere; mantle; convection; continental drift; orogenesis; evolution; extinctions, biotic; thermal history; Archean; Precambrian
Illustrationsschematic cross-sections
Released2020 08 16
Geological data imply that 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 suites; and fundamental differences in overall structural style and constituent lithologies. Conversely, an unstable stagnant-lid model better accounts for many features of Archaean geology. Thermo-mechanical models imply many unstable stagnant-lid planets experience 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 could have generated continental nuclei, and would have reworked and resurfaced large 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, 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 negative impacts on biota. It is predicted that high magmatic fluxes associated with mantle overturn events would have induced periodic mass extinctions that could have retarded biological evolution on Earth, and that evolutionary progress towards more complex metazoan organisms only began after the more efficient plate tectonic cooling system helped create a stabler, more temperate planet. Causal relationships remain uncertain, however. Did the start of a plate tectonic, mobile-lid cooling system on Earth gradually end (or moderate) overturn behaviour? Or was the world-girdling plate tectonic system only allowed to begin because other factors suppressed subsequent mantle overturns? 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 may be due to a mantle overturn, but the numerous Large Igneous Provinces (LIPs) of the Meso- and Neoproterozoic may have other root causes. 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 sentient life is present elsewhere in the universe.
Summary(Plain Language Summary, not published)
Evidence against a Plate Tectonic Archaean Earth is reviewed. The evidence favours an unstable stagnant lid model characterized by periodic mantle overturns. Overturns result in very Large Igneous Provinces that periodically caused mass extinctions. This retarded evolution, and it is only after Plate Tectonics stabilized Earth that metazoans were able to evolve.

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