Element Partitioning And The Early Thermal History Of The Earth

Abstract

Abstract The partitioning of Ni, Co, Sc, and La between olivine and natural basaltic melt and between various subsolidus phases has been determined at 1800°C and 75 kbar. Aliquots of the mantle composition material KLB-1 were doped with 1-2 wt.% each of Ni, Co, and Sc, were compressed to high pressures, and heated in a uniaxial split-sphere anvil apparatus for approximately 1 hr. Successful run products typically consist of a subsolidus assemblage of olivine, orthopyroxene, clinopyroxene, spine!, and probably garnet at the cold end, and silicate melt containing quench crystals of olivine at the hot end. The liquidus boundary within the charge is defined by the appearance of sizable equant olivine crystals (instead of quench-textured olivine crystals, which are smaller and more elongate). Olivine/melt partition coefficients (D)at 75kbar and 1800°C, rounded to one significant figure, are D(Ni)= 2, D(Co)= 1, D(Sc)=0.1, and D(La) < 0.007. These partition coefficients may be used to test the hypothesis that the high Mg/ Si ratio in the upper mantle of the Earth relative to most chondritic meteorites results from the floating of olivine in a magma ocean, with subsequent mixing of that olivine into the upper mantle of the Earth. For example, the Ni/Co ratio inferred for the upper mantle is approximately chondritic. The experimentally determined partition coefficients imply that the addition of 30% olivine into the upper mantle to raise the Mg/Si ratio from CI chondritic to its present value yields a Ni/Co ratio 20-25% higher than its initial value. This result is inconsistent with the olivine flotation hypothesis as a means of explaining the elevated Mg/Si ratio of the upper mantle. The implication of these experiments and those of Kato et al. ( 1987, 1988a,b) is that minor and trace element abundances and ratios in the upper mantle of the Earth do not presently show the effects of extensive olivine, majorite garnet, or perovskite fractionation. One possibility is that the Earth was never substantially molten. If so, the accretional process must have delivered gravitational potential energy more slowly than current theory predicts, and an origin of the Moon in a giant impact would be unlikely. Alternatively, if the Earth were indeed substantially molten, then it is possible that minerals remained entrained in magma and were unable to segregate. In either case, the high Mg/Si ratio in the Earth relative to most classes of chondrites would be intrinsic to the Earth, implying that the accretional process did not mix material efficiently between 1 A.U. and 2-4 A.U. where most chondritic meteorites are presumed to originate.