A COOL EARLY EARTH, RECYCLED ENRICHED CRUST AT ~3.95 GA, AND THE SUBDUCTION MECHANISMS ASSOCIATED WITH A TIDAL CAPTURE MODEL FOR THE ORIGIN OF THE MOON

Our views of the PRIMITIVE EARTH have been changing in recent years. When Preston Cloud (1972, AJS, 272, p. 537) proposed that the earliest era in earth history be called the “Hadean eon”, the common belief was that there was no interpretable rock and mineral record for that era. This concept changed significantly when Valley et al. (2002, Geology, 30, p. 351) interpreted the oxygen isotope information from the Jack Hills zircon crystals as indicating that the Earth was cool enough to have ocean water on the surface during most of that era.

Zeh et al. (2008, GCA, 72, p. 5304) suggest that a mafic Hadean protocrust with some tonalite-trondhjemite-granodiorite (TTG) complexes existed during the Cool Early Earth time. Bedard (2006, GCA, 70, p. 1188) has developed a geochemical model illustrating how such a mafic crust with TTG components can be developed from a basaltic volcanic complex on a “stagnant lid” planet. Amelin (2005, Science, 310, p. 1914), Zeh et al. (2008), and Bell et al. (2011, GCA, 75, p. 4816) suggest that this primitive crust was mainly or completely recycled into the mantle beginning at ~4 Ga.

The purpose of this paper is to demonstrate that the rock tidal processes associated with prograde gravitational capture of a lunar-like planetoid at ~3.95 Ga can cause a systematic, unidirectional subduction mechanism for recycling this primitive crust. In this capture model, the planetoid (Luna) is formed as a vulcanoid between Mercury and the Sun at ~4.55 Ga. Then over a period of ~600 Ma Luna’s orbit is perturbed, by a combination of the Sun, Mercury, and Venus, into an Earth-like orbit from which is can be gravitationally captured by tidal processes (Malcuit et al., 1992, Proc. Vol., 3^rd Int. Archean Symp., p. 223). Numerical simulations of the capture process suggest an early post-capture orbit with semi-major axis ~90 earth radii and eccentricity ~0.8. Rock tidal amplitudes on Earth would be up to 18 km during the early years of the capture episode but would diminish fairly rapidly as the lunar orbit circularizes (e. g., ~4 Ka, ~1 km and ~350 Ka, ~120 m). Since the tidal action is unidirectional, it is suggested that a series of tidal vorticity induction cells would form and that these cells could cause a succession of volcanic arcs to operate in the equatorial zone. This tidally forced subduction operation would be over at ~3.6 Ga.