Neon and helium isotopes as tracers of mantle reservoirs and mantle dynamics

He isotopes are usually used as tracers for the identification of mantle reservoirs and mantle dynamics. High 3He/4He ratios are thought to represent deep mantle material, whereas ratios around 8 RA (RA stands for the atmospheric 3He/4He ratio of 1.39  × 10− 6) are thought to be representative of the upper mantle. Based on He, Ne and Ar fusion data from fresh, submarine volcanic glasses of several Mid-Atlantic Ridge off-axis seamounts we show that He isotopes can be susceptible to interferences during melt formation and evolution resulting in the decoupling of He from other elements, such as e.g. Ne or Pb. All obtained He data are indistinguishable from that of the MORB range. In contrast, Ne isotopic compositions are much more primitive than MORB. Combined He, Ne and Ar systematics show that the source region of these seamounts has experienced a preferential loss of He compared to Ne and Ar. This He loss, combined with subsequent 4He production, resulted in the decoupling of the He isotope systematics from Ne and Pb. Thus, when considering He and Ne, only Ne has preserved the evidence that a primitive mantle component contributed to the formation of the investigated seamounts. As these seamounts are not fed from a mantle plume being derived from the deep mantle, the primitive Ne component resides within the upper mantle, implying that primitive noble gases are not necessarily indicative of deep mantle material.