The early thermal history of the earth

Abstract

The early thermal history of the Earth is reconsidered with the constraint that core formation occur before the emplacement of the oldest known rock possessing remanent magnetism. The thermal consequences of core formation are such that it must antedate the oldest known surface rock. Arbitrary accretion models are constructed and radioactive abundances are assumed to effect this condition. For all cases, the Earth must accrete in a period of 500 000 years or less (4.5 billion years ago) to fulfill this constraint. Partial melting of silicates upon accretion is implied; core formation and large-scale differentiation of the Earth at that time are a distinct possibility. Several lines of evidence invalidate the chondritic abundances and the terrestrial abundances, calibrated on the basis of present-day heat flow values, as applicable to a homogeneous Earth. These values of U, Th, and K are probably too high. Thermal history models using recent determinations of U, Th, and K from oceanic lherzolites and evidence from continental heat flow imply that the Earth accreted in 200 000 years or less and that large-scale differentiation of the Earth upon accretion seems most likely. The above conclusion vindicates the hot origin hypothesis. One consequence is early enrichment of the radioactive elements in the outer regions of the Earth. The temperatures in the deep interior of the Earth reflect residual accretional energy, including that of core formation, and are not primarily a result of radioactive heating. A Mars model, with the same accretion rate and composition as the Earth model, is colder and undifferentiated due to its smaller mass. Venus is considered to have overall composition and gross thermal history similar to the Earth's.

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