EARLY ARCHEAN OPHIOLITES AND THE COOL EARLY EARTH: CAN THEY BE EXPLAINED IN THE CONTEXT OF A TIDAL CAPTURE MODEL FOR THE ORIGIN OF THE MOON?

The discovery of a fairly complete ophiolite complex in the Paleoarchean Isua Complex (Furnes et al., 2007, Science, 315, p. 1704) suggests that something like plate tectonic processes were operating by about 3.8 Ga. Evidence from zircon crystals with dates of crystallization throughout most of the Hadean and Archean (Valley et al., 2002, Geology, 30, p. 351; Cavosie et al., 2005, EPSL, 235, p. 663) suggest that the primitive earth had some granitic crust and was cool enough to have ocean water on the surface during this time. A major question is what role the moon may have played in this early history of the planet? The Giant Impact Model (Canup, 2004, Icarus, 168, p. 433) appears to be incompatible with the concept of a Cool Early Earth because of the disruptive effects of earth tides raised by a lunar-sized body in a small circular orbit throughout this era. In contrast, the Tidal Capture Model (Malcuit et al., 1992, Proc. Vol., 3rd Archean Symp., p. 223) features capture into a highly elliptical orbit at about 3.9 Ga. The Tidal Capture Model is very compatible with the Cool Early Earth Model because the earth would be moonless for the first 650 Ma of earth history.

We have done a series of co-planar three-body numerical simulations of capture using a fourth-order Runge-Kutta integration procedure described in Malcuit et al. (1992). The important body parameters for the interacting bodies are the displacement Love number (h) and the specific dissipation factor (Q) for each body. Using reasonable h and Q values for the planetoid and planet, we find that well over 90% of the energy for capture must be dissipated in the planetoid. Earth tidal amplitudes for the capture encounter are about 20 km but decrease rapidly as the lunar orbit circularizes. By 5 Ka the rock tidal amplitudes are down to about 2 km in the equatorial zone. In general, the polar zones are sheltered from any severe rock tidal activity. Within 25 Ka the energy dissipation in earth is about equal to heat production due to radioactive element decay. Using the concept of Tidal Vorticity Induction (Bostrom, 2000, Oxford Univ. Press) we have developed a model showing how supra-subduction ophiolite complexes could be formed on a Cool Early Earth. The mainly horizontal movement of the lithosphere is powered by the unidirectional progression of rock tides in the equatorial zone of the planet.