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Helium Isotopic Evidence for a Lower Mantle Component in Depleted Archean Komatiite
1D. Richard, B. Marty, M. Chaussidon, CNRS, Centre de Recherches Petrographiques et Geochimiques, Rue Notre-Dame des Pauvres, B.P. 20, 54501 Vandoeuvre Cedex, and Ecole Nationale Superieure de Geologie, 94 Avenue De Lattre de Tassigny, 54001 Nancy Cedex, France. N. Arndt, Geosciences, Unite Propre de Recherches 4661 CNRS, Universite de Rennes, 35042 Rennes Cedex, France.
Archean komatiites, volcanic rocks, suggest deep mantle sources. Helium isotope analysis reveals a lower mantle component, challenging previous upper mantle origin theories for these ancient rocks.
Area of Science:
- Geochemistry
- Petrology
- Earth Science
Background:
- Archean magnesium-rich komatiites suggest hot, deep mantle sources.
- Their trace-element and isotopic compositions resemble modern mid-ocean ridge basalts from the upper mantle.
Purpose of the Study:
- Investigate the mantle source of Archean komatiites.
- Determine the origin of their geochemical and isotopic signatures.
Main Methods:
- Analysis of helium isotopes (Helium-3 and Helium-4) in olivine from komatiites.
- Sequential crushing technique to extract helium.
Main Results:
- Helium isotopic composition varied significantly, indicating two distinct components.
- Enrichment in Helium-3 suggests a contribution from a deep, lower mantle reservoir.
Conclusions:
- Archean komatiites likely originated from a mixed mantle source, including a deep component.
- This finding reconciles the apparent contradiction between komatiite geochemistry and their presumed mantle origins.
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