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Toward a myth-free geodynamic history of Earth and its neighbors
- Hamilton, Warren B.
- Earth-science reviews 2019 pp. 102905
- Archean eon, Cambrian period, Proterozoic eon, animals, basins, fractionation, granite, ionization, isotopes, landscapes, liquids, magnetic fields, melting, models, oceans, radioactivity, recycling, sedimentary rocks, tectonics, volcanic activity
- Several defective assumptions have hindered understanding the evolution of Earth and its nearest neighbors. These include the claim that the Lu-Hf and Sm-Nd isotope systems can uniquely define oceanic rocks, acceptance of the “CHondritic Uniform Reservoir” (CHUR) model and a steadily depleting but fertile mantle, and belief that Proterozoic rocks exhibit features resembling those of Phanerozoic plate-tectonics. Earth's Archean was the era of internally mobile crust. In the period ~4.0–2.5 b.y. tonalite-trondhjemite-granodiorite (TTG) crust formed by hydrous partial melting of a mafic protocrust leaving dense, depleted, garnet-rich residue. This delaminated and sank to at least 200 km beginning top-down re-enrichment of the mantle. The remaining stabilized TTG crust directly underlay primordial low-density dunitic shallow mantle. Archean crust is granite and greenstone with no modern analogue. During the Proterozoic basins of volcanic and terrigenous sedimentary rocks formed on and between Archean shields. Where these basins thickened to ~40 km their deep regions partly melted by their own radioactivity and they were “inverted” by materials rich in highly evolved hydrous granites rising to mid-crustal level. This hydrous melting was enabled by a bombardment of icy bolides. Proterozoic dynamics were driven by vertical variations in density, reflect primarily the deposition and collapse of basins, and involved small horizontal motions only. Proterozoic paleomagnetic data cast doubt on the existence of a strong dipolar magnetic field at that time and there is no compelling evidence for Phanerozoic-like plate tectonics. Only near the end of the Proterozoic did downward recycling of fusible components enable a weak asthenosphere to develop over which lithospheric plates could slide. The Phanerozoic is the era of plate tectonics. Lithosphere motions are well documented by palaeomagnetism which suggests Earth's internal strong dipolar magnetic field may have developed at ~600 m.y. Organic evolution may have been enabled by these geodynamic changes. The Cambrian explosion of evolution produced all the phyla of modern animals within about 50 m.y., possibly because the newly-formed strong dipolar magnetic field provided a shield against ionized radiation. Multidisciplinary evidence indicates that Earth, Mars, Venus and the Moon thoroughly fractionated early–by 4.5–4.4 b.y.—to form cores, refractory mantles and thick mafic crusts. All were bombarded by bolides that saturated their surfaces with impact craters and pools of impact-melted mafic protocrust that fractionated into layered igneous complexes. Venus, Mars and the Moon retain their heavily impacted surfaces. Their upper mantles have been solid and strong subsequently, they lack asthenospheres and liquid cores, and cannot sustain plate tectonics or mantle plumes. Variants of plume theory have been inappropriately exported to Venus and Mars to explain circular features and volcanism. Martian “volcanoes” and Venusian “tessera plateaus” are impact-melt products. A long-lasting global magma ocean on the Moon are falsified by petrology and the preservation of extremely ancient landscapes. Volatiles were delivered to Earth, The Moon, Mars, and Venus in a barrage of icy bolides starting at ~4.0–3.9 b.y. which probably formed Earth's oceans and atmosphere. Oceans and their remnants survived for 2–3 b.y. on Venus and ~1 b.y. on Mars. Melts on the Moon were water-enriched for perhaps 1 b.y. and there may have been ancient transient liquid water there also. Only Earth was internally hot and active enough to circulate the volatiles downward enabling hydrous melting, slow re-enrichment of the upper mantle and, at ~600 m.y., plate tectonics and rapid biological evolution.