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A deep mantle source for high 3He/4He ocean island basalts (OIB) inferred from Pacific near‐ridge seamount lavas

One of the most contentious issues in the debate on the origin of volcanic island and seamount chains is the significance of high 3He/4He ratios at such locations. The contemporary hotspot hypothesis calls for the high 3He/4He signature to be derived from a distinct source reservoir that lies deep in the mantle. The competing plate stress hypothesis claims that extreme isotopic signals, such as the high 3He/4He, come from dispersed crustal lithologies in the upper mantle. Here, we show that lavas from the East Pacific Rise–the ridge axis and near‐ridge seamounts, which have radiogenic isotope compositions overlapping with other Pacific OIB, do not have high 3He/4He ratios. This suggests that high 3He/4He is not associated with dispersed, heterogeneous lithologies embedded in the upper mantle. We conclude that the mantle source of high 3He/4He OIB is unique to volcanic island and seamount chains and likely resides at depth in the mantle.