DATE2024.08.20 #Press Releases
Geochemical reservoirs hidden at the bottom of the mantle discovered
--Explains discrepancies in isotopic composition of Earth's mantle and primitive meteorites
Summary
A research group led by Keisuke Ozawa, a graduate student (at the time of the research), and Professor Kei Hirose of the Department of Earth and Planetary Science, Graduate School of Science, the University of Tokyo, used an isotope microscope system at Hokkaido University to determine the partitioning of trace elements (lanthanum, neodymium, samarium, lutetium and hafnium) between magma and minerals (bridgmanite) in coefficients were determined for the first time covering the wide pressure range of the Earth's lower mantle.
The hafnium and neodymium isotopic compositions of the entire mantle should match those of primitive meteorites. However, the hafnium-neodymium isotopic composition range observed at the surface (mantle array) is known to be discordant with that of primitive meteorites (Figure 1). This suggests that there is a “hidden geochemical reservoir” deep within the Earth that is not observed at the surface.
Modeling the crystallization and the evolution of the chemical composition of the basal magma ocean, which was formed during the formation of the Earth, using the partition coefficients obtained in this study, reveals that it has developed low hafnium isotopic anomalies and high neodymium isotopic anomalies with time. The highly crystallized magma of the basal magma oceans is iron-rich, has a high density, and is not entrained by the mantle upwelling that originates from the bottom of the mantle.
If this magma is a “hidden geochemical reservoir,” it could well explain the discrepancy between mantle array and primordial meteorite values. The "hidden geochemical reservoir" identified in this study may also store other elements, such as heat-source radioactive elements (uranium, thorium, potassium).
Figure : Diamond anvil cell apparatus realizing a deep Earth environment (left) and samples synthesized under high pressure and high temperature (right).
Links：Hokkaido University.（in Japanese)
Journals
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