Concomitant occurrence of primordial and recycled noble gases in the Earth’s mantle

Helium and argon concentrations and isotopic compositions have been determined for a suite of fresh alpine-type peridotites from the Horoman ultramafic complex, Hokkaido, Japan. Crushing and heating gas extractions on the samples yielded quite uniform 3He/4He and 4He/40Arna(=non-atmospheric) ratios of 7.7–9.2 Ra and 1.5–3.4, respectively. These values are consistent with those suggested for a mantle source for mid-oceanic ridge basalts (MORBs) (∼8 Ra, 1.4–4.5, respectively). In contrast to these apparent affinities with a MORB component, the observed 40Ar/36Ar ratios (330–470) appeared to be significantly lower than the suggested MORB source value of >40 000, indicating that virtually all 36Ar resides in fluid inclusions should be of atmospheric origin. Intriguingly, the amounts of 36Ar in the samples showed a clear positive correlation with those of 3He (=the isotope which can safely be regarded as a primordial mantle component). The correlation between primordial and atmospheric components cannot be explained by shallow-level atmospheric contamination. Rather, we infer that an atmospheric component is of recycled origin which had introduced into the mantle by the subducting slab when the complex was at a mantle wedge setting. Infiltration of metasomatic fluid or melt produced in the mantle wedge above the subducting slab would result in fluid inclusions being formed with noble gas signatures like those observed in the samples. This would be the first compelling evidence that some fraction of recycled atmospheric noble gases derived from the subducting slab could have been preserved in the mantle without returning to the atmosphere through arc volcanism.