Earth's original crust — lost quest?

Abstract

The quest for the earth's original crust, based upon the molten earth hypothesis, was traditionally directed toward the oldest granite batholiths. The search was abandoned many years ago when the principle was recognized that these granites are intrusive into still older geosynclinal sediments which, of course, can only be secondarily derived from some pre-existing crirst. The advent of the cold accretion model for the earth combined with the secondary origin of sial by mantle differentiation provides a new definition for the term original crust — its composition would be ultramafic and chondritic rather than sialic. Owing to the process of mantle thermal convection and of associated differentiation, it might be supposed that this crust, too, would have been completely destroyed. However, by the author's version of the geosynclinal mountain-building cycle, it is quite likely that fragments of the primitive crust were trapped in the oldest eugeosynclines and preserved to modern times. I suggest that eugeosynclines are deposited on sima and, upon their tectonic collapse, trap portions of the underlying ultramafic crust in the solid state. Accordingly there remains some hope that, among the ultramafic pods in the old nuclei of continents, undifferentiated fragments of the original crust may still be found. This may be termed a “raisin bread” concept for searching out the earth's original crust.