An analysis of helium isotopes locked inside "super-deep" diamonds hundreds of kilometers below Earth's surface suggests that vast reservoirs of molten primordial source rock, perhaps nearly as old as the Earth, are present. The helium-bearing diamonds studied as part of the analysis provide the first and most direct record of the variation of helium isotope compositions below Earth's lithosphere, the authors say. Isotope compositions of volcanic rocks formed from magma once stored deep in the mantle are important; these compositions provide crucial information about the chemical reservoirs of Earth's interior. Of these, helium isotopes are one of the best tools for understanding the nature of the very deepest and oldest parts of Earth's mantle. Previous studies of helium isotopes have suggested that regions below the upper mantle may hold pristine reservoirs of primordial rock material. However, the preservation of such long-standing reservoirs has repeatedly been questioned based on disruptive deep-Earth dynamics. As well, there have been difficulties in accurately determining the provenance of isotopic signals from below the upper mantle using erupted basalts. Suzette Timmerman and colleagues located a set of diamonds that formed deep beneath the Earth in an area of Brazil known for its super-deep diamonds. As these diamonds formed in the transition zone of the mantle, a depth spanning 410 to 660 kilometers, helium and other elements became trapped in tiny fluid inclusions within the minerals. Timmerman et al. measured the isotopes of the captured elements and found extreme isotope variability, but also high 3He/4He ratios. The results indicate the existence of a deep, primordial source of helium that occasionally infiltrates the transition zone and mixes with subducting material from above, creating diverse isotopic compositions recorded in basalts.
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