Terrestrial planets fractionated synchronously with accretion, but Earth progressed through subsequent internally dynamic stages whereas Venus and Mars have been inert for more than 4 billion years
Published:October 01, 2015
Warren B. Hamilton, 2015. "Terrestrial planets fractionated synchronously with accretion, but Earth progressed through subsequent internally dynamic stages whereas Venus and Mars have been inert for more than 4 billion years", The Interdisciplinary Earth: A Volume in Honor of Don L. Anderson, Gillian R. Foulger, Michele Lustrino, Scott D. King
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Popular models of slow unidirectional evolution of each planet are based on dogmatized 1970s–1980s speculations that Earth has a perpetually hot core that drives narrow vertical plumes of unfractionated mantle which produce volcanoes, propel lithosphere plates, and are compensated by subduction. Long-lasting hot cores, plumes, and minimal fractionation were dogmatized also for Venus and Mars, by analogy, but with a different stagnant-lid conjecture, rather by disrupted-lid plate tectonics, for each. Physics, empirical data, and planetary imagery disprove all three mutually incompatible models. Radiogenic heat, ~5× greater than now, forced synaccretionary magma-ocean fractionation of each planet before 4.5 Ga. This produced thick mafic protocrusts, concentrated radioactivity at shallow depths, and permanently depleted lower mantles. On Earth, the protocrust lay directly above refractory dunite, in turn above denser fractionates. The shallow concentrations of radioactivity allowed deep interiors to cool quickly. Venus and Mars have never since had hot cores or asthenospheres, and their “volcanoes” and other features popularly attributed to plumes are products of bolide impacts on internally inactive planets.
Only Earth had enough radioactivity to remain warmer and to generate partial melts from protocrust to make Archean, and possibly Hadean, felsic crust. Dense garnet-rich residues of protocrust delaminated, sank through the low-density dunite, and began upper-mantle re-enrichment. Archean cratons stabilized where sinking of residua left derivative felsic crust directly upon sterile buoyant dunite. Where some protocrust remained, Proterozoic crustal activity ensued. This was mostly in the form of basin filling atop Archean felsic crust, commonly followed by radioactive heating, partial melting of basement plus fill, and structural inversion. Top-down enrichment of the upper mantle by evolving processes reached the critical level needed for plate tectonics only ca. 0.6 Ga. Plate motions are driven by subduction, which rights the density inversion due to top-down cooling of asthenosphere to lithosphere. Circulation is closed within the upper mantle.
Primary fractionation was hot and dry. The inner planets may have received most of their water in a barrage of icy bolides, centered ca. 4.1 or 4.0 Ga, best dated on Mars and Venus but in accord with terrestrial and, possibly, lunar data. Earth's new water may have enabled formation of Archean tonalite-trondhjemite-granodiorite from protocrust. Increasing downward cycling of volatiles into Earth's upper mantle ever since has been essential for continuing tectonism and magmatism.