Earth's history trapped in the mantle

Kaneoka, Ichiro

'The isotopic compositions of noble gases in the mantle have been assumed to be the result of two processes: the incorporation of nuclides from various sources during the formation of the solar system, and the loss or addition, during the evolution of Earth, of radiogenic components that decay or accumulate at defined rates. If this assumption is correct, measurements of the isotopes of the noble gases should be able to constrain models of Earth's birth and evolution. Indeed, the ratios of isotopes in Earth's atmosphere are quite different from those in extraterrestrial materials. And so, if noble gases from Earth's atmosphere have been incorporated in the mantle-either early in Earth's history or over time the assumed starting composition of the mantle would change. There has been considerable debate whether the nonradiogenic noble gases in the mantle reflect mostly solar-like or mostly atmosphere-like compositions. As reported on page 1036 of this issue, Trieloff et al. (1) have analyzed typical oceanic island basalts from Loihi, Hawaii, and Iceland with high-precision mass spectrometry in an effort to settle this problem, with strikingly clear results that point to a terrestrial atmospheric origin for these trapped gases.' ' . . . . 'Trieloff et al.'s proposal would require that atmospheric noble gas components had been incorporated into the mantle by processes like subduction before the last homogenization of the two mantle reservoirs (within a few hundred million years after the formation of Earth) represented by mid- ocean ridge basalts and oceanic island basalts. It is not certain, however whether such complete homogenization might have been possible, even in early Earth. If terrestrial noble gas isotopic compositions had almost been established during accretion of planetesimals to form Earth, the difference of the atmospheric Ne (20Ne/22Ne = 9.80) from the mantle Ne needs to be explained by some later process that did not affect the isotopic compositions of heavier noble gases. Although Ne-B is suggested as a likely candidate for the pristine component in the mantle (1), additional confirmation is needed to settle this problem. In addition, Trieloff et al.'s data have added to the evidence indicating decoupling between the He and Ne isotopes. Hence, a detailed examination is required to reevaluate its importance. Our knowledge about the pristine noble gas isotopes in the mantle has been greatly increased by these new data, but many problems remain to be solved.'

Earth's history trapped in the mantle

Series