Isotopic tales of ancient continents

Abstract

One of the features which makes the planet Earth a strange anomaly in the Solar System is the presence of continental crust, unknown on the other terrestrial-type planets. In contrast to the basaltic crust underlying the ocean basins, which is nowhere older than 200 Ma (million years) because of its continuous recycling through the mantle by plate-tectonic processes, the continental crust (with the average composition of diorite) is composed of rocks with ages ranging from zero to ~3,900 Ma. It is thus on the continents that 95% of the Earth's geologic record is to be found, including the origin of the unique geosphere-biosphere system. The mainstream of present-day opinion is that these ancient entities were derived from the mantle through magmatic processes, but conflicting views prevail with regard to their growth rates. In this lecture the view is taken that the continents have grown through geologic time by irreversible differentiation from the mantle. The task of gaining insight into the evolution of the continental crust has fallen mainly to the isotope geochemists. Application of radiogenic isotopes (Pb, Hf, Nd, Sr) provides reliable constraints on the age and temporal evolution of rock units, on the time of primary separation of continental material from the mantle, and on the assessment of the relative contributions of mantle and ancient continental crust to magma genesis. Recorded geologic history begins at ~3,900Ma with the oldest preserved continental crust. There is evidence that (some) continental crust was already in existence by ~4,300Ma ago. Most of the insight into the pre-recorded history of the Earth stems from studies of meteorites and other planets. One of the results of planetary exploration is the recognition of large-scale impact cratering by giant meteorites as a major, possibly even dominant geologic process in the early evolution of the Earth. The termination of the 'great bombardment' coincided approximately with the formation of the oldest preserved continental crust by-3,900Ma ago. Another major factor to be taken into account in the reconstruction of the early history of the Earth, is the much higher radiogenic heat production than today. Between 3,000 and 2,500Ma ago, the Earth acquired its modern appearance with the formation of huge volumes of juvenile continental crust from the mantle, and 50-80% of the present-day continental mass was in existence by ~2,500Ma ago. This fundamental change in the Earth's character marks the Archaean-Proterozoic transition. The growth of continental crust was not continuous through geologic time, but episodic, with five distinct periods of accelerated growth. After two successful decades of concentrated ocean-floor exploration, the target of international earth science in the 1980s has become the continents.