Helium–neon systematics and the structure of the mantle

We present here a comparison of helium and neon isotopes in the mantle. These two elements provide important constraints on the structure of the mantle and the exchange between the upper and the lower mantle. Using two low 4He/3He on-ridge hotspots located in the south Atlantic we observe that radiogenic He correlates with nucleogenic Ne. Using data from Loihi seamount and from the MORB glass 2ΠD43 (`popping rockʹ), we estimate the ratio of the [3He/22Ne] in the upper mantle to the ratio [3He/22Ne]LM in the lower mantle to be close to 1. This result is a strong argument (it is not a proof) in favour of the steady-state model for the upper mantle which suggests that all the primordial noble gases observed at the ridges come from the lower mantle.