The helium paradoxes

The ratio (3)He/(4)He (R) plays a central role in models of mantle evolution that propose an undegassed lower mantle, rich in the primordial isotope (3)He. A large primordial volatile-rich reservoir, a feature of recent models, is inconsistent with high-temperature accretion and with estimates of crustal and bulk Earth chemistry. High R can alternatively reflect high integrated (3)He/(U+Th) ratios or low (4)He abundances, as expected in refractory portions of the upper mantle. I show that high R materials are gas-poor and are deficient in radiogenic (4)He compared with midocean ridge basalts. The seemingly primitive (i.e., high R) signatures in “hotspot” magmas may be secondary, derived from CO(2)-rich gases, or residual peridotite, a result of differential partitioning of U and He into magmas. A shallow and low (3)He source explains the spatial variability and the temporal trends of R in ocean islands and is consistent with a volatile-poor planet. A shallow origin for the “primitive” He signature in ocean island basalts, such as at Loihi, reconciles the paradoxical juxtaposition of crustal, seawater, and atmospheric signatures with inferred “primitive” characteristics. High (238)U/(204)Pb components in ocean island basalts are generally attributed to recycled altered oceanic crust. The low (238)U/(3)He component may be in the associated depleted refractory mantle. High (3)He/(4)He ratios are due to low (4)He, not excess (3)He, and do not imply or require a deep or primordial or undegassed reservoir. (40)Ar in the atmosphere also argues against such models.