We report new noble gas isotopic compositions of submarine basaltic glasses sampled from two seamounts discovered offshore of Chile, and inferred to have erupted as petit-spot volcanoes near the Juan Fernández hotspot. The samples have 3He/4He of 1–15 times atmosphere (Ra). Their neon isotope compositions are similar to those of the Hawaiian Islands and Réunion Island. Their 40Ar/36Ar range from atmospheric to 2300. Although the lavas are likely to be influenced by a hotspot-related component, the cause of the 3He/4He variation must be clarified to ascertain the mantle source. Variations in 3He/4He are not attributable to processes occurring at the Earth's surface such as degassing fractionation, mixing with atmosphere dissolved in seawater, or in-situ post-eruptive addition of 4He. A combination of the Ne-Ar isotope ratios corrected for atmospheric influence and He isotope ratios indicates that the noble gas isotopes of the lavas are a mixture of a hotspot magma, MORB-source, and radiogenic components. The lower 3He/4He are attributed to assimilation with the oceanic lithosphere, suggesting that the pristine 3He/4He of the lavas is hotspot-like. These features can be interpreted as indicating that part of the Juan Fernández plume infiltrated the lithosphere–asthenosphere boundary (LAB), and that the ponding magma has erupted as petit-spot volcanoes because of plate bending. The noble gas data indicate that LAB can be a reservoir for exotic melts, which might have lubricated plate tectonics.
- crustal assimilation
- Juan Fernández Islands
- lithosphere–asthenosphere boundary
- noble gas isotopic ratio