A recent article in Nature1 presents credible evidence for a nearly 4500 million years (m.y.) long chemical isolation of a mantle pocket from convective disruption and mixing. Condensation of small, high-temperature mineral grains from the cooling solar nebula has been precisely dated at 4568 ± 1 Ma (Ma, one million years ago) as one of the earliest events known in the solar system. The accretion of such small grains of meteoritic material dispersed throughout the solar nebular cloud into a few large planets like the earth was quite rapid as inferred from the former presence of radionuclides with very short half-lives in early-formed solid objects. Specifically, the decay of 182Hf with a half-life of just 9 m.y. to 182W in the terrestrial silicatemantle shows that the earth not only accreted to almost its present size, but also was differentiated into a central iron core and a thick silicate slag floating over it within 30 m.y. after the above condensation event. This sets the age of the earth asnot younger than 4500 Ma. After the loss of much iron, the chemical make-up of this silicate slag (called the primitive mantle) would match that of the bulk earth only for silicate loving or preferring (lithophile) elements. So if, as currently believed, the bulk earth had the composition of the chondritic group of meteorites, the primitive mantle would retain exactly chondritic proportions, especially for the two neighbouring and chemically similar elements, samarium and neodymium belonging to the chemically coherent rare earth family.
展开▼
