CRYSTALLIZATION OF A HADEAN TERRESTRIAL MAGMA OCEAN

On the basis of compositional trends of lherzolite from Kilbourne Hole , New Mexico, USA (Carter, 1970), the composition of the primitive upp er mantle is calculated to have a strictly chondritic Ca/Al ratio of 1 .11, a value similar to but slightly lower than values calculated by o thers from xenolith data. This implies that the magma ocean that produ ced the upper mantle crystallized at equilibrium or experienced only a small amount of fractionation a conclusion consistent with recent flu id dynamic considerations. The non-chondritic Mg/Si, Mg/Ca, and Si/Ca ratios of the; mantle are attributed to volatilization that accompanie d a giant impact. A working model for the Hadean history of the Earth is developed that includes (1) accretion from chondritic parental mate rial, (2) a giant impact late in the accretion history that formed the Moon, completely melted the Earth, and caused partial devolatilizatio n, (3) segregation of the core and mantle by gravitational separation of immiscible iron and silicate liquids, (4) crystallization of the si licate magma ocean with at most only a minor amount of fractionation, and (5) partial melting of the upper mantle through geologic time to f orm the continental crust and an upper mantle depleted in Si/Mg and Fe /Mg relative to the lower mantle.