Brazil’s mantle rocks carry gas traces of ancient ocean-crust recycling
Noble gases in deep rock fragments from Brazil’s São Francisco Craton point to ancient oceanic crust and sediment entering the continental mantle. The peer-reviewed study compared several isotope systems rather than relying on a single gas. Its interpretation connects regional mantle differences to long-lived subduction, while the sampled rocks represent one part of the continent’s deep structure.
Science··Midday
Deep rock fragments retain several kinds of gas
Noble-gas measurements from Brazil’s São Francisco Craton, specifically rocks along its southwestern edge, point to material recycled from ancient oceanic crust and sediments. A craton is an old, relatively stable continental core. The single peer-reviewed study examined xenoliths, fragments of deep rock carried to the surface within magma. Comparing several gases allowed the researchers to examine mixtures within this part of the continental mantle.[1]
Helium, neon and xenon trace different contributions
Helium showed a strong contribution from radioactive decay. Neon combined signatures resembling mid-ocean-ridge mantle material with components from nuclear reactions. Xenon indicated varying mixtures of atmospheric gas, crustal fission products and mantle material. The researchers interpreted these contrasts as evidence of long-lived subduction, the descent of one tectonic plate beneath another, recycling hydrated oceanic crust and overlying sediments along the craton margin since the Archean.[1]
Sample preparation separates trapped gas from crystal contributions
Gas trapped in fluid and melt inclusions was released through crushing, while heating the remaining powders examined gas retained inside crystal lattices. The University of Tokyo provided mass-spectrometer measurements, accompanied by blank tests to assess contamination. Neon results affected by strong interference were excluded. The interpretation combines these isotope systems with the geological setting; a single gas does not independently date every recycling episode or characterize the entire mantle.[1]
